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Geosynthetic reinforced segmental retaining walls
Geotextiles and Geomembranes 19 (2001) 359–386
Geosynthetic reinforced segmental retaining walls Robert M. Koernera,*, Te-Yang Soongb a
Geosynthetic Research Institute, Drexel University, Philadelphia, PA 19104, USA Earth Tech Consultants, Inc., 36133 Schoolcraft Road, Livonia, MI 48150 USA
b
Received 23 December 2000; received in revised form 9 March 2001; accepted 24 May 2001
Abstract Segmental retaining walls (SRWs) (primarily those with precast concrete block facing) reinforced by geogrids or geotextiles are in a period of enormous growth. Estimates are that 35,000 of these walls exist and that they cover the entire range of practical wall heights. This paper gives a perspective of the evolution of retaining walls in general, and follows with results of a recent cost survey. It is seen that geosynthetic reinforced walls are the least expensive of all wall categories and at all wall heights. Three design methods are then compared to one another with respect to their details and idiosyncrasies. This is followed by a numeric example showing that the modified Rankine method is the most conservative, the FHWA method is intermediate, and the NCMA method is the least conservative. A survey of the literature is included where it is seen that there have been approximately 26 walls which suffered either excessive deformation or actual collapse. The overwhelming causes for these cases of poor performance were (i) backfilling with improperly draining fine grained soil and (ii) contractors deficiencies which could have been avoided with proper quality control and inspection. The paper, which reflects North American practice, closes with a discussion of possible concerns most of which are under active investigation. Clearly, continued strong growth for geosynthetic reinforced SRWs is justified. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Segmental retaining walls; Modular block walls; Wall costs; Wall design; Field behavior
1. Introduction It is quite understandable that retaining wall design and construction has occupied a pivotal position in the historic development of geotechnical engineering. Retaining
*Corresponding author. Tel.: +1-610-522-8440; fax: +1-610-522-8441. E-mail address: [email protected] (R.M. Koerner). 0266-1144/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 6 - 1 1 4 4 ( 0 1 ) 0 0 0 1 2 - 7
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walls (along with earth dams) were among the first soil-related structures to be considered as both critical and permanent insofar as their service life was concerned. Along with this significance came a variety of retaining wall types, design methods, and related construction methodologies. Over time, the classical gravity retaining walls transitioned into reinforced concrete types, some with buttresses and counterforts. These were then followed by a variety of crib and bin-type walls. A paradigm shift occurred in the 1960s with the advent of mechanically stabilized earth (MSE) masses, i.e., reinforced layers of soil allowing for modular construction, which was clearly recognized as being advantageous in most situations. The reinforcement was initially steel straps, and subsequently welded wire mesh provided an alternative. Wall facings varied from metallic-to-reinforced concrete-to-segmental units of a variety of types and shapes. By the 1980s this MSE technology segued into polymeric reinforcement using geogrids, geotextiles and polymer straps. Thus, at the present time there exists four categories of wall types, each with subcategories. They are somewhat arbitrarily grouped as follows: 1. Rigid and/or gravity walls a. concrete cantilever b. concrete cantilever with buttresses/counterforts c. rubble masonry d. cylinder piles e. soldier piles and tiebacks 2. Prefabricated and compartmentalized gravity walls a. metal bins b. precast concrete bins c. precast concrete cribs d. gabions 3. MSEFwith metal reinforcement a. precast concrete facing panels b. cast-in-place facing c. segmental retaining walls (SRWs) (modular block facing) 4. MSEFwith geosynthetic reinforcement a. precast concrete facing panels b. cast-in-place facing c. segmental retaining walls (SRWs) (modular block facing) This paper focuses on the fourth category, namely MSE-walls with geosynthetic reinforcement. While a variation exists in the type of polymer reinforcement (e.g., geogrids, geotextile, and even polymer straps), it is the facing which is an everevolving transition. MSE geosynthetic reinforced wall facing are of the following types: * * *
wrap-around facing timber facing welded-wire mesh facing
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gabion facing precast full-height concrete facing cast-in-place full-height concrete facing precast panel wall facing units SRWs; also called modular concrete block walls
It is this last type of wall facing that is seeing the largest growth at the present time. This comes about for a number of reasons, among which are, dry-cast fabrication of the blocks, ease of placement using manual labor, ease of geosynthetic connection to the facing, conformance to essentially any variation in line and grade, good tolerance for irregularities, and (perhaps most importantly) outstanding aesthetics. The SRWs are being mainstreamed in large part, due to their pleasing appearance. In addition to a growing use by geotechnical consultants; building architects, landscape developers, golf and park superintendents, commercial and private property owners, etc., have readily accepted these wall systems and have utilized them accordingly. Accompanying this expansion in the shear number of walls, it is understandable that the height of the walls is also increasing. No longer confined to low and medium heights, MSE walls with geosynthetic reinforcement now compete with other wall types in all height categories. Twelve meter high walls, and above, exist at the present time. The largest (located in Taiwan) is approximately 38 m high. In the authors opinion, the profession finds itself in the midst of a massive transition from a bevy of wall types to a predominance of MSE-walls with geosynthetic reinforcement, and particularly of the SRW-type. Such walls are the topic of this paper. The paper (based largely on North American experience) will present a comparison of wall cost data, and then follow with a detailed comparison of three different design methodologies. It will then present a listing of poor performing wall case histories taken from the literature and from personal files, and finally present areas of concern where additional investigation is felt to be warranted.
2. Wall costs Retaining wall cost surveys have undoubtedly been conducted by many public agencies, private users, design engineers, contractors and manufacturers over the years. The perspective for this cost survey, however, begins in 1973. This date follows closely the early use of MSE walls using steel straps. Lee (1973) used categories of gravity walls and crib/bin and compared them to MSE walls with steel reinforcement. He furthermore subdivided the walls into high (HX9.0 m), medium (4.5oHo9.0) and low (Hp4.5 m) height categories. The unit prices, on the basis of dollars per square meter of wall face, are given in Table 1. Readily seen is that MSE walls with metallic reinforcement are the least expensive wall of the types surveyed at all heights. The Lee survey was followed by one conducted by the VSL Corporation in 1981 which included welded wire mesh in the MSE (metal reinforced) wall category. The
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Table 1 Comparison of past retaining wall costs with current (1998) survey results (units are U.S. dollars per square meter of wall facing)a Wall category
Wall height (relative)
Lee et al. (1973)
VSL Corporation (1981)
Yako and Christopher (1988)
Koerner et al. (1998)
Gravity
high medium low
300 190 190
570 344 344
570 344 344
760 573 455
Crib/Bin walls
high medium low
245 230 225
377 280 183
377 280 183
I/D 390 272
MSE walls (metal reinforced)
high medium low
140 100 70
300 280 172
300 280 172
385 381 341
MSE walls (geosynthetic reinforced)
high medium low
N/A N/A N/A
N/A N/A N/A
250 180 130
357 279 223
a
Note: I/D, Inadequate data; N/A, not available at time of survey.
cost data is also shown in Table 1. Here it is seen that all wall costs increased considerably in the 8-year interval between 1973 and 1981, and that MSE walls with both steel straps and welded wire mesh remained the least expensive. Yako and Christopher (1988) performed a survey seven-years after the VSL survey which focused on MSE walls with geosynthetic reinforcement. The data appears in Table 1 where it is seen that the data for the original three categories of walls was taken directly from the VSL study. The important part of the Yako and Christopher survey, however, is that the MSE walls with geosynthetic reinforcement have been added and are seen to be the least expensive of all wall categories. Ten years later, Koerner et al. (1998) conducted a survey which included all four wall categories so as to update the three earlier studies. The survey form was sent to all fifty U. S. Departments of Transportation. Of the 40-states responding, bid prices for the different wall types listed in the introduction were solicited and are included in the data base. As such, the data reflects a nation-wide study of thousands of walls which were publicly funded; as opposed to walls that were privately funded. Cost data on privately funded walls is just becoming available, but is seen to be still lower than the costs reported here, Koerner et al. (2001). The data from the 1998 survey appears in Table 1 and can be contrasted to the three earlier surveys. When graphing results from these four surveys the differences become even more apparent, see Fig. 1. Gravity walls are by far the most expensive, with crib/bin walls and MSE (metal) walls significantly less expensive. Note that crib/bin walls are rarely over 7-m in height. It is also obvious that MSE (geosynthetic) walls are the least expensive of all wall categories and over all wall heights. Convergence, however, seems to be
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Fig. 1. Mean values of various categories of retaining wall costs, after Koerner et al. (1998).
Table 2 Statistical data for retaining wall costs from Koerner et al. (1998) surveya Wall category
Wall height (m)
Wall cost in dollars(m2) Mean
Std. Dev.
Variance (%)
Gravity walls
>9.0 4.5–9.0 o4.5
760 573 455
180 224 166
24 39 37
Crib/bin walls
>9.0 4.5–9.0 o4.5
I/D 390 272
I/D 129 98
I/D 33 36
MSE (metal)
>9.0 4.5–9.0 o4.5
385 381 341
122 126 135
32 33 40
MSE (geosynthetic)
>9.0 4.5–9.0 o4.5
357 279 223
73 81 67
20 29 30
a
Note: I/D, Inadequate data.
occurring within the two different MSE types (metal and geosynthetics) in the high wall height category. This latter survey also generated statistical data in providing a mean value (which was plotted in Fig. 1), standard deviation and variance. This information is provided in Table 2, where it is seen that the standard deviation in data is highest with gravity walls, intermediate with crib/bin and MSE (metal) walls, and the least with MSE (geosynthetic) walls. Variance values, however, are similar in all wall categories.
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3. Elements of design and numeric example This section presents essential elements in the design of geosynthetic reinforced retaining walls. It is illustrated for SRWs, but is similar for other facing types as well. Embodied in the design are both external and internal considerations; *
*
external stability issues: * mass sliding on foundation soil * bearing capacity of foundation soil * overturning about the toe of the wall internal stability issues: * reinforcement spacing and tensile overstress * soil pullout length * facing connection overstress
These six considerations will be analyzed using (i) a modified Rankine approach as illustrated by Koerner (1998), (ii) the Federal Highway Administration approach, FHWA (1998) and (iii) the National Concrete Masonry Association approach, NCMA (1997). A numeric example of the design of a 7-m, high SRW will follow so as to illustrate the differences in results using the three methods. The design comparisons and numeric example will use symbols and definitions as illustrated in Fig. 2.
Fig. 2. Identification of terms used in the design of geosynthetic reinforced retaining walls.
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3.1. External stability issue Table 3 presents the calculation methods for determining the coefficient of active earth pressure from which the active earth pressure force is calculated, i.e., * *
for soil: P ¼ 0:5g H 2 Ka for surcharge: P ¼ qHKa
where P is the active earth pressure (kN/m), g the unit weight of backfill soil (kN/m3), H the height of wall (m) and q the surcharge behind wall (kN/m2). Table 3 also presents the inclination angle at which P acts on the reinforced soil zone. Note that in all methods ‘‘P’’ acts at H/3 above the foundation soil for soil pressure, and H/2 for surcharge pressure. Rankine’s analysis is the simplest but also the most restrictive. It can only include a horizontal thrust (which is probably not accurate for soil versus soil) so it is modified in Table 3 to include an inclination angle. The FHWA and NCMA methods use the Coulomb analysis which can handle earth pressure inclination angles other than horizontal. Importantly, the Coulomb analysis can include backslopes and batter walls. Table 3 illustrates the wide range of wall variations that are included in the FHWA and NCMA design methods. Table 4 presents the various calculation methods to arrive at a factor-of-safety (FS) value for sliding of the entire MSE mass (facing, drainage soil zone and reinforced soil zone) along the foundation soil or rock. A horizontal force summation is used in the process. All three calculation methods require the resulting FS to equal or exceed a value of 1.5. Not illustrated in Table 4 is the possibility of sliding within the MSE soil mass, e.g., along an individual geogrid or geotextile layer and exiting between adjacent facing elements. The situation is handled in the same manner but with obvious differences in the interface friction values and the respective forces. Table 5 presents the various methods to calculate load eccentricity on the basis of the drainage/reinforced soil zone where it interfaces with the foundation soil. This load eccentricity is then used in the calculation of the MSE-mass bearing pressure (BP), as well as the foundation soil bearing capacity (BC). The result of this ratio is the FS-value which must exceed, or equal, either 2.0 or 2.5 using the various methods illustrated. Table 6 presents the various design methods used for the calculation of a FS-value against overturning of the MSE mass about the toe of the wall. It uses the earth pressures at their respective inclinations and locations to obtain the overturning moment. When compared to the resulting or stabilizing moment the ratio results in a FS-value. This value must exceed, or equal, 2.0, except for FHWA which feels this mode of failure is unlikely and that the calculation is not necessary. 3.2. Internal stability issues Table 7 presents the various design methodologies to obtain the spacing of the geosynthetic layers so as not to overstress them. In each calculation a design
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Modified Rankine
FHWA
NCMA
Vertical wall (i.e., oC0) (1) Horizontal backslope (i.e., b ¼ 0)
Vertical wall (i.e., op101) (1) Horizontal backslope (i.e., b ¼ 0)
Vertical wall (i.e., op101) (1) Horizontal backslope (i.e., b ¼ 0)
f Ka ¼ tan2 45 i 2
f Ka ¼ tan2 45 i 2
Ka ¼
Inclination (deg.)=dpfi
Inclination (deg.)=01
Inclination (deg.)=01
(2) Inclined backslope
(2) Inclined backslope
" pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi# cos b cos2 b cos2 fi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ka ¼ cos b cos b þ cos2 b cos2 fi
" pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi# cos b cos2 b cos2 fi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ka ¼ cos b cos b þ cos2 b cos2 fi
Inclination (deg.)=b
Inclination (deg.)=b
1 sin fi ¼ tan2 ð45 fi Þ 1 þ sin fi
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Table 3 Coefficient of active earth pressure and its inclination angle
Batter wall (i.e., o > 101)
sin2 ðy þ fÞ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 sin ðfi þ dÞ sin ðfi bÞ sin2 y sin ðy þ dÞ 1 þ sin ðf dÞ sin ðy þ bÞ
Inclination (deg.)=d þ 90 y
Ka ¼
sin2 ðf þ oÞ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 sin ðfi þ dÞ sin ðfi bÞ sin2 o sin ðo dÞ 1 þ sin ðo dÞ sin ðo þ bÞ
Inclination (deg.)=d o
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Ka ¼
Batter Wall (i.e., o > 101)
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Table 4 Sliding against foundation soil or rock FHWA
NCMA
General situation
(1) Horizontal backslope
General situation
(2) Inclined backslope
FS ¼
F X1:5 PH
where F ¼ Wm; m ¼min. of (tan ff ; tan fr or tan r), ff the friction angle of foundation soil, fr the friction angle of reinforced soil, r the friction angle of GS-to-soil and PH ¼ Ps þ Pq
FS ¼
F X1:5 PH
where F ¼ ðW1 þ W2 þ P sin bÞm; m=min. of (tan ff ; tan fr or tan r), ff the friction angle of foundation soil, fr the friction angle of reinforced soil, r the friction angle of GS-to-soil
FS ¼
F X1:5 PH
If reinforced soil controls F ¼ Cds ðW1 þ W2 þ qd Lb Þtan fr If drainage soil controls F ¼ Cds ðW1 þ W2 þ qd Lb Þtan fd If foundation soil controls
F ¼ Cds cf L þ ðW1 þ W2 þ qd Lb Þtan ff PH ¼ P cos ðd oÞ
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Modified Rankine
Table 5 Eccentricity and foundation soil bearing capacity Modified Rankine
Mov L o W þ qL 6
where Mov ¼ H
FS ¼
P ðcos bÞ e¼
Ps Pq þ 3 2
L eo in soil 6
Bearing capacity; BC X2:0 Bearing pressure; BP
where BC ¼ cf Nc þ 0:5ðL 2eÞgf Ng W þ qL BP ¼ L 2e
NCMA
FS ¼
h L L P ðsin bÞ W2 3 2 6 W1 þ W2 þ P sin b
L L L W2 X2 qd Lb Xq PsðHÞ Ys þ PqðHÞ Yq W1 X1 2 2 2 e¼ Wrð1Þ þ Wrð2Þ þ qd Lb
L eo in rock 4
Bearing capacity; BC X2:5 Bearing pressure; BP
where BC ¼ cf Nc þ 0:5ðL 2eÞgf Ng W1 þ W2 P sin b BP ¼ L 2e
Bearing capacity; BC X2:0 Bearing pressure; BP
where BC ¼ cf Nc þ 0:5gf ðL 2eÞNg þ gf Hemb Nq BP ¼
Wrð1Þ þ Wrð2Þ þ ðq1 þ qd ÞLb L 2e
369
Note: FSX2.0 might be acceptable if there is good geotechnical report available.
FS ¼
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e¼
FHWA
370
Modified Rankine
FHWA
NCMA
1. The resultant of vertical forces has to be within the middle one-third of the base. Otherwise longer reinforcement is necessary 2. LX0.7 H (unless there is good geotechnical report available) Note: This mode of failure is not felt to be a concern, i.e., there have been no exsisting problems or failures
FS ¼
Ms X2:0 Mov
Ms X2:0 Mov
where
where WL 2 Ps Pq þ ¼H 3 2
MS ¼ Mov
FSov ¼
MS ¼ W1 X1 þ W2 X2 þ qd Lb Xq Mov ¼ PsðHÞ Ys þ PqðHÞ Yq
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Table 6 Overturning about the toe of the MSE wall
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Table 7 Spacing of geosynthetic reinforcement layers Modified Rankine
Sv ¼
Tdes Cr sH
Sv ¼
NCMA
Tdes p0:8 m sH
where
where Tdes ¼
FHWA
Tult ðPRFÞðFSÞ
Cr ¼ coverage ratio sH ¼ Kar ðgz þ qÞ f Kar ¼ tan 45 r 2
Vertical spacing should be selected based on Nmin and Hu where
Tdes p Tallow Cr Cr ¼ coverage ratio sH ¼ Kar sv þ Dsh
Nmin ¼ min: number of layers PsðHÞ þ PqðHÞ ¼ Tdes Tdes ¼
sv ¼ gr Z þ s2 þ q þ Dsv
2
s2 ¼
gi L tan b 2 ðthe sloped backfillÞ
Tult ðPRFÞðFSÞ
Hu ¼ height of facing unit
Dsv ; Dsh ¼ Vertical and horizontal stress due to concentrated surcharge load For the nth layer of reinforcement: Tdes > FRðnÞ Lateral earth pressure coefficient, Kar : For vertical wall (oo101) with horizontal backfill and no wall friction f0 Kar ¼ tan2 45 r 2 For wall face batter f0 o > 101; y ¼ 45 þ r 2 Kar ¼
sin2 ðy þ f0r Þ 2 sin f0r sin3 y 1 þ sin y
where FgðnÞ ¼ Applied force in the nth layer ¼ Kar ðgr Zn þ qÞAcðnÞ cos ðd oÞ
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(material) strength is required. This value contains various reduction factors which are applied to the reinforcement geosynthetic’s ultimate strength which is typically obtained by an index test method. Such reduction factors as installation damage, long-term creep, degradation, etc., are handled somewhat differently by the respective design methods. It should be noted that, as inferred by the NCMA design, the spacing is often controlled by the facing block height or other facing controlled idiosyncrasy. Table 8 presents the various methods of calculating the geosynthetic reinforcement length within the soil mass behind the hypothetical failure plane. Note that this length constitutes the tail end of the reinforced soil zone. The design methods produce a FS-value for a given or assumed length of embedment. This length is often assumed to be one meter and it is the typical default value. Note that all three methods assume a Rankine failure plane rising at a angle of ‘‘45 þ f=2’’ with the horizontal at the inside toe of the wall facing and then extending linearly through the soil backfill mass. There is a distinct possibility that this failure plane can transition into a logarithmic spiral for high walls. This would save a considerable length of reinforcement for such high-wall situations. Table 9 presents the various methods to calculate the design (or required) facing stress where the geosynthetic reinforcement layers exit the masonry block or other facing anchorage into the drainage zone. The required connection strength is based not on the theoretical earth pressure applied, but on the tensile strength of the geosynthetic material that is selected. As such, there is no listing in the modified Rankine column, however, both FHWA and NCMA have a recommended procedure. The value is felt by the authors to be very subjective and largely controlled by the installation contractors methodology, see Soong and Koerner (1997). The two design methods shown in Table 9 vary widely in both concept and level of detail in the resulting numeric values to be resisted by the connection. Not only is the design strength at issue, the results of laboratory connection testing is also difficult to interpret, particularly with friction connections where sliding of the reinforcement is observed. This typically occurs at low normal stresses, i.e., near the top of the wall. Note that NCMA limits this slippage to 19 mm at which point the corresponding load is assumed for Tconn. The procedure for connection assessment is different from others in that a FSvalue is not explicitly obtained. Rather, a FS-value is assumed (usually it is 1.5) and a required connection strength is calculated. This value can vary with the wall height and has the effect of shifting the design burden to the laboratory testing of the various facing systems vis-"a-vis the particular type of reinforcement being utilized. 3.3. Numeric example The comparisons of the three design methodologies given in the previous sections, i.e., in Tables 3–9, are admittedly complex and somewhat tedious. So as to illustrate their relative impact on a practical problem the following SRW design example is offered. Fig. 3 illustrates a 7-m high geosynthetic reinforced wall consisting of 0.25-m high modular facing blocks. The reinforced soil properties are given as
Modified Rankine
FS ¼
2Le Ci Cr sv tanfir X1:5 Sv sh
where FS is the safety factor against pullout, Le the embedment length (min. 1.0 m), Ci the interaction coefficient, Cr the coverage ratio
FHWA
FS ¼
NCMA
2Le F * Cr agZp X1:5 Sv sh
where FS is the safety factor against pullout, F the pullout resistance factor, Cr the coverage ratio, a the scale correction factor (0.6B1.0)
FS ¼
ACn X1:5 FgðnÞ
where FS is the safety factor against pullout, FgðnÞ the applied force in the nth layer, ACn the anchorage capacity of the nth layer ¼ 2LaðnÞ Ci ðdn gr þ qd Þtan fr ; Ci ¼interaction coefficient
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Table 8 Soil pullout resistance behind potential failure plane
373
374
Table 9 Facing (or anchorage) stress FHWA
NCMA
Not applicable (see text for discussion)
Tconn is the lesser of:
(1) Tdes ; design strenght of the reinforcement where Tdes ¼
Tconn X1:5Tdes ðstrengthÞ - or -
Tult ðPRFÞð1:5Þ
(2) The reduced ultimate connection strength based on connection/seam strenght, i.e., Trupt ¼ where CRu ¼
Tult ðCRu Þ 1:5ðRFD ÞðRFCR Þ Tultc and Tultc ¼peak strength Tlot
where rupture is the mode of failure, Tlot ¼ultimate strength of the lot used for the connection strength testing (3) The reduced ultimate connection strength based on pullout, i.e., Tanchorage ¼
Tult ðCRs Þ 1:5
Tsc Tlot and Tsc =peak strength where pullout is the mode of failure
where CRs ¼
Tconn at 19 mm X1:0 Tdes ðserviceabilityÞ
where Tdes ¼
Tult ðPRFÞðFSÞ
Tconn at 19 mm ¼connection strength at 19 mm deformation
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Modified Rankine
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Fig. 3. Numeric example used to illustrate differences in design methodologies present in this paper.
g ¼ 18 kN/m3, fr ¼ 321 and c ¼ 0; i.e., it is a sand. Consider this to include the drainage soil zone as well (although it should be gravel) since its properties will not be significantly different. The retained soil is at a unit weight of 17 kN/m3, with fi ¼ 301 and a conservatively estimated c ¼ 0: The friction angle of the reinforced soil with the foundation soil is ff ¼ 301 and the foundation soil has a unit weight of 17 kN/m3. Its bearing capacity is 690 kN/m2. The proposed geosynthetic reinforcement has an ultimate wide width strength of 160 kN/m and with cumulative reduction factors of 4.0 (1.33 1.20 2.5) results in an allowable strength of 40 kN/m. When coupled with a global FS ¼ 1:5 for design uncertainties, this results in a design strength of 26.7 kN/m. The coverage ratio of the reinforcement is 0.80, i.e., 80% of the backfill surface at each layer of reinforcement is covered with reinforcement. The interaction coefficient with the soil in the reinforced zone is 0.75. Table 10 presents the results of the various elements of the design for the three respective design methods given in the previous subsection. Regarding the external stability considerations, the modified Rankine method is seen to be most conservative for foundation sliding, bearing capacity and overturning (i.e., the lowest FS-values). The FHWA method is intermediate in its FS-values. For all three external considerations, the NCMA is the least conservative method (i.e., highest FS-values). Regarding the internal stability considerations, the FS-values on reinforcement strength was calculated at each layer. Here it is seen that the modified Rankine and FHWA methods are the most conservative throughout all layers, with the NCMA method being the least conservative, but only nominally so. Generally, the same trend is seen in the FS-values for soil pullout resistance. At the upper 2-layers,
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Table 10 Results of numeric example illustrated in Fig. 3 for three design methods (a) External stability considerations Item
Modified Rankine
FHWA
NCMA
FSFoundation sliding Eccentricity (m) FSBearing capacity FSOverturning
2.07 0.64 3.59 3.43
2.30 0.65 3.59 n/a
2.87 0.42 5.35 4.93
(b) Internal stability consideration reinforcement strength FS-Values Layer number
Elevation (m)
11 10 9 8 7 6 5 4 3 2 1
6.25 5.25 4.25 3.75 3.25 2.75 2.25 1.75 1.25 0.75 0.25
Soil pullout resistance 11 10 9 8 7 6 5 4 3 2 1
6.25 5.25 4.25 3.75 3.25 2.75 2.25 1.75 1.25 0.75 0.25
Modified Rankine
FHWA
NCMA
2.97 2.28 2.19 2.88 2.57 2.31 2.11 1.93 1.79 1.66 1.55
3.18 2.25 2.34 2.84 2.53 2.28 2.08 1.91 1.76 1.64 1.53
3.26 2.30 2.29 2.91 2.60 2.34 2.13 1.96 1.81 1.68 1.57
1.32 3.26 6.30 10.87 12.27 13.66 15.04 16.41 17.78 19.15 20.51
3.04 4.85 8.58 13.76 15.12 16.47 17.82 19.18 20.53 21.88 23.23
1.56 4.16 9.01 15.41 17.77 20.14 22.50 24.87 27.23 29.59 31.96
Required connection resistancea Required connection resistance (kN/m) Layer number
Elevation (m)
Modified Rankine
FHWA
NCMA
11 10 9 8 7 6 5 4 3 2 1
6.25 5.25 4.25 3.75 3.25 2.75 2.25 1.75 1.25 0.75 0.25
n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a
10.08 14.29 14.35 11.29 12.67 14.06 15.44 16.82 18.21 19.59 20.97
8.40 11.90 11.95 9.41 10.56 11.71 12.86 14.01 15.17 16.32 17.47
a
Note: n/a, not addressed.
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however, the NCMA method gives relatively low values. This is particularly the case for the uppermost layer. For the required facing connection resistance the modified Rankine method is not applicable and the FHWA method is slightly more conservative than the NCMA method. The general trends in this numeric example are to be expected. It is well known that the Coulomb method produces lower earth pressures than the Rankine method for various practical conditions. Thus the fact that the modified Rankine method results are the most conservative in all design elements is not surprising. Had the wall facing been at a batter, the differences would have been even greater. Also to be expected is that the FHWA method is somewhat more conservative than the NCMA method. The public-ownership section (via FHWA) has traditionally been more conservative than the private ownership section (e.g., via NCMA and others). These trends were clearly illustrated in the selected numeric example.
4. Case histories of inadequate wall performance It has been estimated by the National Concrete Masonry Association that there are 25,000 MSE walls of the segmental retaining wall (masonry block) type in the United States. Of this total, at least half are geosynthetically reinforced. In addition, there are many other facing types in addition to masonry blocks, e.g., segmental concrete panels, wrap-around, prefabricated panels, cast-in place panels and timber. Considering the worldwide situation there could easily be 35,000 walls which are geosynthetically reinforced using geogrids (mainly), geotextiles (less frequently) and polymer straps (rarely). The vast majority of these walls have been successful insofar as their performance is concerned. Hundreds of papers in the literature attest to this very positive performance record. Designers for private owners and public agencies repeatedly use geosynthetic reinforced walls and the numbers in each ownership group are rapidly growing. Yet, there have been wall problems with respect to both serviceability (excessive deformation) and actual failures (collapse). The number of each is very small, however, lessons can be learned by assembling these case histories and analyzing them as a group to see if trends exist. Hopefully, the common situations can be identified and avoided in the future. From the open literature and our own files, twenty-six (26) case histories of problems with geosynthetic reinforced walls are presented in Tables 11 and 12. Twelve are of the serviceability type, wherein excessive deformation of the bottom, top or entire wall occurred. Fourteen are failures, wherein a portion of the wall actually collapsed. This is not meant to suggest that more walls fail than excessively deform; we believe that the opposite is much more likely. The numbers only appear to indicate that people tend to investigate and publish failures (vis-"a-vis serviceability problems) due to the obvious finality of the situation. Whether the deformation case histories will eventually lead to failures is not known.
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No.
Facing
GS-type
Year
Service time
Cause
Problem
Soil backfill (reinforced zone)
Weatherb
Reference
S1 S2
WAW WAW
GG GG
1984 1990
3-yr ?
design design
clay (ML-CL) sandy gravel
cold climate n/a
Burwash and Frost (1991) Christopher(1993)
S3 S4
SRW SRW
GG GG
1990 1994
? DC
design design
sand n/a
unknown rain
Bathurst and Simac (1994) Sandri (1997)
S5
SRW
GG
1994
DC
contractor
n/a
n/a
Sandri (1997)
S6 S7 S8 S9
SRW SRW SRW SRW
GG GG GG GT
1994 1994 1994 1994
DC DC DC 2-yr
contractor contractor contractor contractor
n/a n/a n/a clay (CL)
n/a n/a n/a unknown
Sandri (1997) Sandri (1997) Sandri (1997) Gassner and James (1998)
S10 S11 S12
SRW PPW SRW
GG GG GG
1995 1995 1998
6-mo 4-yr 8-mo
design design design
wall rotation bulge at bottom (surcharge) bulge at bottom bulge at top (poor drainage) bulge at top (heavy rollers) poor alignment poor block placement footing too shallow midwall bulge (wrong spacing) deformed throughout bulge at top bulge at top
clay clayey silt silty clay
rain n/a int. rain
authors authors authors
a Notes: WAW, wrap around wall; SRW, segmental retaining wall; PPW, precast panel wall; GG, geogrid; GT, geotextile; DC, during construction; n/a, not applicable to problem. b weather refers to climate during or shortly before incident occurred.
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Table 11 Case histories of wall serviceability problems, i.e., excessive deformationa
No.
Facing
GS-Type
Year
Lifetime
Cause
Reason
Soil backfill (reinforced zone)
Weatherb
Reference
F1 F2 F3 F4 F5
Timber SRW SRW WAW WAW
GT GG GG GG GG
1987 1990 1992 1992 1992
3-mo 6-mo 2-mo ? ?
contractor contractor design design design
no compaction clay sand silty clay silty clay
n/a n/a dry heavy rain heavy rain
Richardson and Behr (1998) Leonards et al. (1994) Berg and Meyers (1997) Huang (1994) Huang (1994)
F6
SCP
GG
1992
?
design
silty clay
heavy rain
Huang (1994)
F7 F8 F9 F10
SRW SRW SRW SRW
GT GT GG GG
1993 1994 1994 1996
3-yr 2-yr ? 12-mo
design contractor design design
silty clay clay poor drainage clay
heavy rain heavy rain rainy heavy rain
Gassner and James (1998) Gassner and James (1998) Sandri (1997) authors
F11 F12 F13 F14
SRW SRW SRW SRW
GG GG GG GG
1997 1998 1998 1998
12 mo 18 mo 8 mo 12 mo
design design design design
connection omitted GG-omitted global/compound hydrostatic hydrostatic (GG pullout) hydrostatic (GG break) hydrostatic GT omitted hydrostatic hydrostatic (poor drainage) hydrostatic hydrostatic hydrostatic hydrostatic
clay clay (ML-SP) silty clay clayey silt
rainy rainy int. rain int. rain
authors authors authors authors
a Notes: SRW, segmental retaining wall; WAW, wrap around wall; SCP, segmental concrete panel; GT, geotextile; GG, geogrid; n/a, not applicable to problem. b weather refers to climate during or shortly before incident occurred.
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Table 12 Case histories of wall failures, i.e., actual collapsea
379
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In the serviceability case histories assembled in Table 11, it is seen that large-scale excessive deformation can be at the top, bottom or throughout the wall and can even be rotational. Within the group of seven design-related case histories, five had finegrained backfill soils in the reinforced zone and two had granular soils throughout. The other five case histories experienced individual block, or localized, distortion. All of these appear to have been caused by contractors activities during construction. The message in both situations appear to be clear in that (i) fine-grained silts and clays should be questioned insofar as backfill soils are concerned, and (ii) contractors operations must be monitored and inspected in addition to adequate construction quality control (CQC). In the failure (or collapse) case histories assembled in Table 12, it is seen that hydrostatic pressure arising from lack of drainage from fine-grained soil backfills in the reinforced zone was the overriding reason for the failures. This occurred in 10 of the 14 case histories and all are listed as design related causes. It supports the concern over silt and clay backfills seen in the serviceability case histories. In three other failure case histories, contractor deficiencies were observed, again supporting the findings of the serviceability case histories insofar as lack of inspection and installation quality control are concerned. Interestingly, in only one of the 26-case histories presented in Tables 11 and 12 is the problem not fine-grained soil backfill or inadequate construction/inspection procedures. This is case history ‘‘F3’’ which experienced a global failure behind and beneath the entire wall structure carrying it and a much larger body of soil downslope in a large-scale failure. Once again, it was a fine-grained soil problem albeit not in the reinforced soil zone, per se.
5. Concerns As mentioned in the previous section of this paper there have been problems with MSE-geosynthetic reinforced walls concerning both serviceability issues and actual collapses. While the percentages are miniscule in light of the number of walls built, forensic insight into causes leads to identifiable concerns. These concerns, coupled with other perceived concerns, are the subject of this section. 5.1. Low permeability backfill The concern over low permeability backfill soils (see Mitchell and Zornberg, 1995) in the reinforced zone is a major issue leading to the largest number of serviceability problems and actual failures listed in Tables 11 and 12. Obviously, the use of low permeability backfill soil contributed greatly toward an inexpensive wall system, but it can also lead to excessive deformations and/or failure. Table 13 compares recommendations for soil gradations in the reinforced backfill zone by three different references. When the above data is plotted, as it is in Fig. 4, the tremendous differences can be better appreciated. The NCMA recommendations are extremely broad, with up to
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35% passing the No. 200 sieve being allowed. FHWA is somewhat more restrictive with up to 15% being allowed. While NCMA places no restrictions on the characteristics of the material passing the No. 200 sieve, e.g., on clay content, plasticity index or activity, FHWA does say that the material passing the No. 200 sieve should have a plasticity index less than 6.0, should be free of organics and should not exceed 19 mm in size so as to minimize installation damage. Conversely to FHWA and NCMA, Koerner recommends that zero particles pass the No. 200 sieve. The criterion ensures a clean sand as being the finest material allowed. If fines (silts and/or clays) are allowed for the reinforced zone backfill soil, any possible water in front, behind and beneath the reinforced zone must be carefully collected, transmitted, and discharged. Proper filtration and drainage control is
Table 13 Reinforced soil zone gradation requirements Sieve size
F No. No. No. No. No.
4 10 40 100 200
Particle size (mm)
100 4.8 2.0 0.42 0.15 0.075
Percent passing requirement Koerner (1998)
FHWA (1998)
NCMA (1997)
F 100 90–100 0–60 0–5 0
F 100 F 0–60 F 0–15
75–100 20–100 F 0–60 F 0–35
Fig. 4. Limiting gradation requirements within the reinforced zone of MSE retaining walls with geosynthetic reinforcement.
382
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absolutely critical in this regard. Furthermore, the top of the zone should be waterproofed, e.g., by a geomembrane or a geosynthetic clay liner, to prevent water from entering the backfill zone from the surface. Surface water drainage as well as drainage from the retained earth zone is obviously of concern with respect to potential buildup of pore water pressures behind or within the reinforced soil zone. See Koerner and Soong (2000) for SRW drainage system designs in this regard. 5.2. Various design issues There are the obvious design differences noted in the previous section of this paper which give rise to variations as noted in the example problem. While these differences are felt to be substantive, others not mentioned are equally pressing. Some commentary follows in this regard. 5.2.1. Seismic design Seismic design was not addressed in this paper and is obviously of concern in areas of such activity. While too early in the history of MSE-geosynthetic reinforced walls to accumulate significant performance during earthquakes, the flexibility of such walls should prove to be an asset as compared to rigid walls. Reports by Tatsuoko et al. (1998) during the Kobe, Japan earthquake lend credibility to this statement, but clearly additional performance over time is necessary and particularly for SRWs. A recent workshop was very significant in this regard, Ling and Leshchinsky (2000). The nature and behavior of friction connections of the geosynthetic reinforcement to the wall facing is under active investigation by several groups, see Bathurst and Simac (1994). When properly constructed such friction connections appear to be adequate, but the design concerning both required strength and allowable strength needs additional investigation, see Berg and Nelson (2000). 5.2.2. Design details The design details of penetrations in the drainage and backfill zones (lightpole stantions, guard post piling, utility pole anchors, catch basins and manholes, drainage outlets, etc.) are difficult to handle from a quantitative design perspective. Factors such as coverage ratio and interaction coefficients are difficult to ascribe values to, and one tends to be extremely conservative. This obviously affects the economics of the final design. 5.2.3. Durability of both the facing and the reinforcement Durability of both the facing and the reinforcement is important in the context of the typical 75 to 100 year lifetime expected of reinforced wall systems. Regarding the durability of the facing itself, masonry blocks are being investigated by NCMA. The issue of the proper cement and additives in light of the manufacturing process is under investigation. For dry cast masonry block facing, the major unresolved issues are freeze-thaw behavior in cold climates and wet-dry behavior in all climates. Other facings should have similar, or comparable, challenges insofar as the required durability research and development is concerned. Regarding the durability of the
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reinforcement, FHWA has a number of significant research programs completed, Salman et al. (1997, 1998), and ongoing, Hsuan and Koerner (1999). As these studies are completed they are being regularly embodied in AASHTO (1997) specifications. 5.3. Quality control and quality assurance Noted in Table 11 was that the lack of CQC and (CQA) resulting in a number of serviceability problems. Whether an unsightly aligned wall leads to eventual collapse is doubtful, but aesthetics play a major role in the acceptance of these walls and they should obviously be constructed properly, and in accordance with the site-specific design plans and specifications. Perhaps the typical practice of a manufacturer giving a few hours instruction at the beginning of a project on proper construction practice is insufficient, but this is not known to be a fact. What is observed by the authors is a general lack of monitoring and inspection on the part of the owner (both public and private) or by the certifying wall designer. CQA of the walls under construction, which are both critical in their performance and permanent in their service life, should be inspected on a full-time basis. 5.4. Maintenance On a regular basis, during the service lifetime of geosynthetic reinforced MSE walls, a site visit should be made by the owner or owner’s consultant for the purpose of visually observing the condition of the wall. Deformations are the obvious target and if observed, a monitoring program, e.g., surveying of wall movements and/or backfill settlements, should be instituted. If serious enough, deformation gages and inclinometers can be installed and the movement can be quantified over time. Cracking of blocks is an indication of distortion of some type, perhaps foundation settlement, and should be investigated accordingly. Low-cost masonry crack monitoring gages can be used in this regard. Damage to individual facing blocks, e.g., by traffic accidents, or dislodged cap blocks should be repaired and/or replaced. Grouting behind the blocks may be required in extreme cases. If surface water drainage is not in accordance with the original design, or if it has been changed subsequent to the construction of the wall, its implications must be addressed. Remediation may be warranted. An important indication of nonequlibrium conditions in the backfilled soil (drainage, reinforcement or retained earth zones) is loss of soil from the spaces between blocks. Lost backfill soil has been a precursor to collapse in at least one wall, Bernardi (1998).
6. Summary and conclusions The use of SRWs that are geosynthetic reinforced is growing at an enormous rate. This growth is justified on the basis of excellent performance-to-date, superb aesthetics, relative ease-of-construction, and overall low cost.
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Several design methodologies are available and this paper compared and contrasted three of these methods which are commonly used in North American practice. Results on a typical wall example were seen to be least conservative by NCMA, intermediate by FHWA, and most conservative using modified Rankine. Technically, however, all three approaches are felt to be sound and the ultimate choice is project-specific. It is important to note that the tacit assumption in all cases is that adequately draining backfill soils are used in the reinforcement zone. As was seen in the case histories this assumption is often disregarded. A literature survey was presented describing 12-serviceability problems and 14wall failures. Of the total, 17 of the cases had low permeability backfill soils in the reinforced zone and 8 had uncontrolled or inadequate CQC/CQA in the construction of the walls. Only one of the case histories was due to conditions other than the above two reasons. In light of the above performance, which is excellent considering that as many as 35,000 of these walls exist, a series of concerns were presented. These included the two issues just raised (low permeability backfill soils and the lack of field quality control and inspection) in addition to selected design issues and long-term maintenance issues. In conclusion, it is felt that SRWs that are geosynthetically reinforced are completely justified in their growth at present rates, and when augmented by the worldwide market will be the choice of all wall systems in the future and will be nothing short of phenomenal.
Acknowledgements The financial assistance of the member organizations of the Geosynthetic Institute and its related institutes (Research, Information, Education, Accreditation and Certification) is sincerely appreciated. Appreciations is also extended to Drs. Dov Leshchinsky and Barry R. Christopher for review of the design tables of Section 3.0 of the paper.
References AASHTO, 1997. Standard Specifications for Highway Bridges, Div. 1, Sect. 5, Retaining Walls, Washington, DC, 89pp. Bathurst, R.J., Simac, M.R., 1994. Geosynthetic reinforced segmental retaining wall structures in North America. In: Proceedings of the Fifth International Geosynthetics Conference, Singapore, SEAC-IGA, Keynote Lecture Volume, pp. 29–54. Berg, R.R., Nelson, B., 2000. Practical implications of MBW unit-geosynthetic connection strength requirements. In: Proceedings of the 14th GRI Conference. GAI Publications. Folsom, PA, pp. 307–322. Berg, R.R., Meyers, M.S., 1997. Analysis of the collapse of a 6.7 m high geosynthetic-reinforced wall structure. In: Proceedings of the Geosynthetics ’97. IFAI, Roseville, MN, pp. 85–104.
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Bernardi, M., 1998. Necessity of proper site assessment using SRWs as a case history. In: Proceedings of the GRI-12 Conference on Lessons Learned from Geosynthetics Case Histories. GSI, Folsom, PA, pp. 58–65. Burwash, W.J., Frost, J.D., 1991. Case history of a 9-m high geogrid reinforced wall backfilled with cohesive soil. In: Proceedings of the Geosynthetics ’91. IFAI, Roseville, MN, pp. 485–493. Christopher, B.R., 1993. Deformation Response and Wall Stiffness in Relation to Reinforced Soil Wall Design. Doctoral Thesis to Purdue University, West Lafayette, Indiana, 354pp. Federal Highway Administration (FHWA), Elias, V., Christopher, B.R., 1998. Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines. FHWA-SA-96-071, Washington, DC, September, 371pp. Gassner, F.W., James, G.M., 1998. Failure of two fabric reinforced segmental block walls in South Africa. In: Proceedings of the sixth International Conference on Geosynthetics. IFAI, Roseville, MN, pp. 559–564. Hsuan, Y.G., Koerner, G.R., 1999. Report to Federal Highway Administration on Field Measured Parameters Effecting the Durability of Geosynthetics (to appear). Huang, C.C., 1994. Report on three unsuccessful reinforced walls. In: Tatsuoka, F., Leshchinsky, D., (Eds.), Recent Case Histories of Permanent Geosynthetic-Reinforced Soil Retaining Walls. Balkema, Rotterdam, pp. 219–222. Koerner, R.M., 1998. Designing with Geosynthetics, 4th Edition. Prentice Hall Publication Co., Englewood Cliffs, NJ, 761pp. Koerner, J., Soong, T.-Y., Koerner, R.M., 1998. Earth Retaining Wall Costs in the USA. Geosynthetic Institute, Folsom, PA. Koerner, R.M., Soong, T.-Y., 2000. Design of Drainage Systems for Segmental Retaining Walls, In: Proceedings of the 18th ASCE/PennDOT Geotechnical Conference. Hershey, PA, November 1–3, pp. 1–38. Koerner, R.M., Koerner, J., Soong, T.-Y., 2001. Earth retaining wall costs in the USA. In: Proceedings of the Geosynthetics 2001 Conference. IFAI, Roseville, MN (to be published). Lee, K.L., Adams, B.D., Vagneron, J.M.J., 1973. Reinforced earth retaining walls. Journal of the Soil Mechanics and Foundation Division, ASCE 99(SM10), 745–764. Leonards, G.A., Frost, J.D., Bray, J.D., 1994. Collapse of geogrid reinforced retaining structure. Journal of Performance of Constructed Facilities 8 (4), 274–292. Ling, H.I., Leshchinksy, D., 2000. In: Proceedings of International Workshop on Seismic Performance of Geosynthetic-Reinforced Soil Structures. Columbia University, New York, NY, October 30–31, approx. 300pp. Mitchell, J.K., Zornberg, J.G., 1995. Reinforced Soil Structures with Poorly Draining Backfills. Geosynthetics Int. IFAI 2 (1), 265–299. National Concrete Masonry Association (NCMA), 1997. Design Manual for Segmental Retaining Walls. Collin, J.G. (Ed.), Herndon, VA, 289pp. Richardson, G.N., Behr Jr., L.H., 1998. Geotextile reinforced wall: failure and remedy. GFR 6 (4), 14–18. Salman, A., Elias, V., Juran, I., Lu, S., Pearce, E. 1997. Durability of geosynthetics based on accelerated laboratory testing. In: Proceedings of the Geosynthetics ’97. IFAI, Roseville, MN, pp. 217–251. Salman, A., Elias, V., DiMillio, A., 1998. The effect of oxygen pressure, temperature and manufacturing processes on laboratory degradation of polypropylene geosynthetics. In: Proceedings of the sixth International Conference on Geosynthetics. IFAI, Roseville, MN, pp. 683–690. Sandri, D., 1997. Problems in Constructing Segmental Retaining Walls. In: Proceedings of the GRI-11 Conference on Field Installation of Geosynthetics. GII, Folsom, PA, pp. 23–40. Soong, T.-Y., Koerner, R.M., 1997. On the required connection strength of geosynthetically reinforced walls. Journal of Geotextiles and Geomembranes 15, 377–393. Tatsuoko, F., Koseki, J., Tateyama, M., Munaf, Y., Horii, K., 1998. Seismic stability against high seismic loads of geosynthetic reinforced soil retaining structures. In: Proceedings of the sixth International Geosynthetics Conference. IFAI, St. Paul, MN, pp. 103–142.
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VSL Corporation, 1981. VSL Retained Earth, Technical Data, Rock and Soil Stabilization Systems. Los Gatos, CA. Yako, M.A., Christopher, B.R., 1988. Polymerically Reinforced Retaining Walls and Slopes in North America. In: Jarrett, P.M., McGown, A. (Eds.), The Application of Polymeric Reinforcement in Soil Retaining Structures. Kluwer Academic Publishers, Dordicht, pp. 239–283.
Geosynthetic reinforced segmental retaining walls Robert M. Koernera,*, Te-Yang Soongb a
Geosynthetic Research Institute, Drexel University, Philadelphia, PA 19104, USA Earth Tech Consultants, Inc., 36133 Schoolcraft Road, Livonia, MI 48150 USA
b
Received 23 December 2000; received in revised form 9 March 2001; accepted 24 May 2001
Abstract Segmental retaining walls (SRWs) (primarily those with precast concrete block facing) reinforced by geogrids or geotextiles are in a period of enormous growth. Estimates are that 35,000 of these walls exist and that they cover the entire range of practical wall heights. This paper gives a perspective of the evolution of retaining walls in general, and follows with results of a recent cost survey. It is seen that geosynthetic reinforced walls are the least expensive of all wall categories and at all wall heights. Three design methods are then compared to one another with respect to their details and idiosyncrasies. This is followed by a numeric example showing that the modified Rankine method is the most conservative, the FHWA method is intermediate, and the NCMA method is the least conservative. A survey of the literature is included where it is seen that there have been approximately 26 walls which suffered either excessive deformation or actual collapse. The overwhelming causes for these cases of poor performance were (i) backfilling with improperly draining fine grained soil and (ii) contractors deficiencies which could have been avoided with proper quality control and inspection. The paper, which reflects North American practice, closes with a discussion of possible concerns most of which are under active investigation. Clearly, continued strong growth for geosynthetic reinforced SRWs is justified. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Segmental retaining walls; Modular block walls; Wall costs; Wall design; Field behavior
1. Introduction It is quite understandable that retaining wall design and construction has occupied a pivotal position in the historic development of geotechnical engineering. Retaining
*Corresponding author. Tel.: +1-610-522-8440; fax: +1-610-522-8441. E-mail address: [email protected] (R.M. Koerner). 0266-1144/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 6 - 1 1 4 4 ( 0 1 ) 0 0 0 1 2 - 7
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walls (along with earth dams) were among the first soil-related structures to be considered as both critical and permanent insofar as their service life was concerned. Along with this significance came a variety of retaining wall types, design methods, and related construction methodologies. Over time, the classical gravity retaining walls transitioned into reinforced concrete types, some with buttresses and counterforts. These were then followed by a variety of crib and bin-type walls. A paradigm shift occurred in the 1960s with the advent of mechanically stabilized earth (MSE) masses, i.e., reinforced layers of soil allowing for modular construction, which was clearly recognized as being advantageous in most situations. The reinforcement was initially steel straps, and subsequently welded wire mesh provided an alternative. Wall facings varied from metallic-to-reinforced concrete-to-segmental units of a variety of types and shapes. By the 1980s this MSE technology segued into polymeric reinforcement using geogrids, geotextiles and polymer straps. Thus, at the present time there exists four categories of wall types, each with subcategories. They are somewhat arbitrarily grouped as follows: 1. Rigid and/or gravity walls a. concrete cantilever b. concrete cantilever with buttresses/counterforts c. rubble masonry d. cylinder piles e. soldier piles and tiebacks 2. Prefabricated and compartmentalized gravity walls a. metal bins b. precast concrete bins c. precast concrete cribs d. gabions 3. MSEFwith metal reinforcement a. precast concrete facing panels b. cast-in-place facing c. segmental retaining walls (SRWs) (modular block facing) 4. MSEFwith geosynthetic reinforcement a. precast concrete facing panels b. cast-in-place facing c. segmental retaining walls (SRWs) (modular block facing) This paper focuses on the fourth category, namely MSE-walls with geosynthetic reinforcement. While a variation exists in the type of polymer reinforcement (e.g., geogrids, geotextile, and even polymer straps), it is the facing which is an everevolving transition. MSE geosynthetic reinforced wall facing are of the following types: * * *
wrap-around facing timber facing welded-wire mesh facing
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361
gabion facing precast full-height concrete facing cast-in-place full-height concrete facing precast panel wall facing units SRWs; also called modular concrete block walls
It is this last type of wall facing that is seeing the largest growth at the present time. This comes about for a number of reasons, among which are, dry-cast fabrication of the blocks, ease of placement using manual labor, ease of geosynthetic connection to the facing, conformance to essentially any variation in line and grade, good tolerance for irregularities, and (perhaps most importantly) outstanding aesthetics. The SRWs are being mainstreamed in large part, due to their pleasing appearance. In addition to a growing use by geotechnical consultants; building architects, landscape developers, golf and park superintendents, commercial and private property owners, etc., have readily accepted these wall systems and have utilized them accordingly. Accompanying this expansion in the shear number of walls, it is understandable that the height of the walls is also increasing. No longer confined to low and medium heights, MSE walls with geosynthetic reinforcement now compete with other wall types in all height categories. Twelve meter high walls, and above, exist at the present time. The largest (located in Taiwan) is approximately 38 m high. In the authors opinion, the profession finds itself in the midst of a massive transition from a bevy of wall types to a predominance of MSE-walls with geosynthetic reinforcement, and particularly of the SRW-type. Such walls are the topic of this paper. The paper (based largely on North American experience) will present a comparison of wall cost data, and then follow with a detailed comparison of three different design methodologies. It will then present a listing of poor performing wall case histories taken from the literature and from personal files, and finally present areas of concern where additional investigation is felt to be warranted.
2. Wall costs Retaining wall cost surveys have undoubtedly been conducted by many public agencies, private users, design engineers, contractors and manufacturers over the years. The perspective for this cost survey, however, begins in 1973. This date follows closely the early use of MSE walls using steel straps. Lee (1973) used categories of gravity walls and crib/bin and compared them to MSE walls with steel reinforcement. He furthermore subdivided the walls into high (HX9.0 m), medium (4.5oHo9.0) and low (Hp4.5 m) height categories. The unit prices, on the basis of dollars per square meter of wall face, are given in Table 1. Readily seen is that MSE walls with metallic reinforcement are the least expensive wall of the types surveyed at all heights. The Lee survey was followed by one conducted by the VSL Corporation in 1981 which included welded wire mesh in the MSE (metal reinforced) wall category. The
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Table 1 Comparison of past retaining wall costs with current (1998) survey results (units are U.S. dollars per square meter of wall facing)a Wall category
Wall height (relative)
Lee et al. (1973)
VSL Corporation (1981)
Yako and Christopher (1988)
Koerner et al. (1998)
Gravity
high medium low
300 190 190
570 344 344
570 344 344
760 573 455
Crib/Bin walls
high medium low
245 230 225
377 280 183
377 280 183
I/D 390 272
MSE walls (metal reinforced)
high medium low
140 100 70
300 280 172
300 280 172
385 381 341
MSE walls (geosynthetic reinforced)
high medium low
N/A N/A N/A
N/A N/A N/A
250 180 130
357 279 223
a
Note: I/D, Inadequate data; N/A, not available at time of survey.
cost data is also shown in Table 1. Here it is seen that all wall costs increased considerably in the 8-year interval between 1973 and 1981, and that MSE walls with both steel straps and welded wire mesh remained the least expensive. Yako and Christopher (1988) performed a survey seven-years after the VSL survey which focused on MSE walls with geosynthetic reinforcement. The data appears in Table 1 where it is seen that the data for the original three categories of walls was taken directly from the VSL study. The important part of the Yako and Christopher survey, however, is that the MSE walls with geosynthetic reinforcement have been added and are seen to be the least expensive of all wall categories. Ten years later, Koerner et al. (1998) conducted a survey which included all four wall categories so as to update the three earlier studies. The survey form was sent to all fifty U. S. Departments of Transportation. Of the 40-states responding, bid prices for the different wall types listed in the introduction were solicited and are included in the data base. As such, the data reflects a nation-wide study of thousands of walls which were publicly funded; as opposed to walls that were privately funded. Cost data on privately funded walls is just becoming available, but is seen to be still lower than the costs reported here, Koerner et al. (2001). The data from the 1998 survey appears in Table 1 and can be contrasted to the three earlier surveys. When graphing results from these four surveys the differences become even more apparent, see Fig. 1. Gravity walls are by far the most expensive, with crib/bin walls and MSE (metal) walls significantly less expensive. Note that crib/bin walls are rarely over 7-m in height. It is also obvious that MSE (geosynthetic) walls are the least expensive of all wall categories and over all wall heights. Convergence, however, seems to be
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363
Fig. 1. Mean values of various categories of retaining wall costs, after Koerner et al. (1998).
Table 2 Statistical data for retaining wall costs from Koerner et al. (1998) surveya Wall category
Wall height (m)
Wall cost in dollars(m2) Mean
Std. Dev.
Variance (%)
Gravity walls
>9.0 4.5–9.0 o4.5
760 573 455
180 224 166
24 39 37
Crib/bin walls
>9.0 4.5–9.0 o4.5
I/D 390 272
I/D 129 98
I/D 33 36
MSE (metal)
>9.0 4.5–9.0 o4.5
385 381 341
122 126 135
32 33 40
MSE (geosynthetic)
>9.0 4.5–9.0 o4.5
357 279 223
73 81 67
20 29 30
a
Note: I/D, Inadequate data.
occurring within the two different MSE types (metal and geosynthetics) in the high wall height category. This latter survey also generated statistical data in providing a mean value (which was plotted in Fig. 1), standard deviation and variance. This information is provided in Table 2, where it is seen that the standard deviation in data is highest with gravity walls, intermediate with crib/bin and MSE (metal) walls, and the least with MSE (geosynthetic) walls. Variance values, however, are similar in all wall categories.
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3. Elements of design and numeric example This section presents essential elements in the design of geosynthetic reinforced retaining walls. It is illustrated for SRWs, but is similar for other facing types as well. Embodied in the design are both external and internal considerations; *
*
external stability issues: * mass sliding on foundation soil * bearing capacity of foundation soil * overturning about the toe of the wall internal stability issues: * reinforcement spacing and tensile overstress * soil pullout length * facing connection overstress
These six considerations will be analyzed using (i) a modified Rankine approach as illustrated by Koerner (1998), (ii) the Federal Highway Administration approach, FHWA (1998) and (iii) the National Concrete Masonry Association approach, NCMA (1997). A numeric example of the design of a 7-m, high SRW will follow so as to illustrate the differences in results using the three methods. The design comparisons and numeric example will use symbols and definitions as illustrated in Fig. 2.
Fig. 2. Identification of terms used in the design of geosynthetic reinforced retaining walls.
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365
3.1. External stability issue Table 3 presents the calculation methods for determining the coefficient of active earth pressure from which the active earth pressure force is calculated, i.e., * *
for soil: P ¼ 0:5g H 2 Ka for surcharge: P ¼ qHKa
where P is the active earth pressure (kN/m), g the unit weight of backfill soil (kN/m3), H the height of wall (m) and q the surcharge behind wall (kN/m2). Table 3 also presents the inclination angle at which P acts on the reinforced soil zone. Note that in all methods ‘‘P’’ acts at H/3 above the foundation soil for soil pressure, and H/2 for surcharge pressure. Rankine’s analysis is the simplest but also the most restrictive. It can only include a horizontal thrust (which is probably not accurate for soil versus soil) so it is modified in Table 3 to include an inclination angle. The FHWA and NCMA methods use the Coulomb analysis which can handle earth pressure inclination angles other than horizontal. Importantly, the Coulomb analysis can include backslopes and batter walls. Table 3 illustrates the wide range of wall variations that are included in the FHWA and NCMA design methods. Table 4 presents the various calculation methods to arrive at a factor-of-safety (FS) value for sliding of the entire MSE mass (facing, drainage soil zone and reinforced soil zone) along the foundation soil or rock. A horizontal force summation is used in the process. All three calculation methods require the resulting FS to equal or exceed a value of 1.5. Not illustrated in Table 4 is the possibility of sliding within the MSE soil mass, e.g., along an individual geogrid or geotextile layer and exiting between adjacent facing elements. The situation is handled in the same manner but with obvious differences in the interface friction values and the respective forces. Table 5 presents the various methods to calculate load eccentricity on the basis of the drainage/reinforced soil zone where it interfaces with the foundation soil. This load eccentricity is then used in the calculation of the MSE-mass bearing pressure (BP), as well as the foundation soil bearing capacity (BC). The result of this ratio is the FS-value which must exceed, or equal, either 2.0 or 2.5 using the various methods illustrated. Table 6 presents the various design methods used for the calculation of a FS-value against overturning of the MSE mass about the toe of the wall. It uses the earth pressures at their respective inclinations and locations to obtain the overturning moment. When compared to the resulting or stabilizing moment the ratio results in a FS-value. This value must exceed, or equal, 2.0, except for FHWA which feels this mode of failure is unlikely and that the calculation is not necessary. 3.2. Internal stability issues Table 7 presents the various design methodologies to obtain the spacing of the geosynthetic layers so as not to overstress them. In each calculation a design
366
Modified Rankine
FHWA
NCMA
Vertical wall (i.e., oC0) (1) Horizontal backslope (i.e., b ¼ 0)
Vertical wall (i.e., op101) (1) Horizontal backslope (i.e., b ¼ 0)
Vertical wall (i.e., op101) (1) Horizontal backslope (i.e., b ¼ 0)
f Ka ¼ tan2 45 i 2
f Ka ¼ tan2 45 i 2
Ka ¼
Inclination (deg.)=dpfi
Inclination (deg.)=01
Inclination (deg.)=01
(2) Inclined backslope
(2) Inclined backslope
" pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi# cos b cos2 b cos2 fi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ka ¼ cos b cos b þ cos2 b cos2 fi
" pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi# cos b cos2 b cos2 fi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ka ¼ cos b cos b þ cos2 b cos2 fi
Inclination (deg.)=b
Inclination (deg.)=b
1 sin fi ¼ tan2 ð45 fi Þ 1 þ sin fi
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Table 3 Coefficient of active earth pressure and its inclination angle
Batter wall (i.e., o > 101)
sin2 ðy þ fÞ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 sin ðfi þ dÞ sin ðfi bÞ sin2 y sin ðy þ dÞ 1 þ sin ðf dÞ sin ðy þ bÞ
Inclination (deg.)=d þ 90 y
Ka ¼
sin2 ðf þ oÞ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 sin ðfi þ dÞ sin ðfi bÞ sin2 o sin ðo dÞ 1 þ sin ðo dÞ sin ðo þ bÞ
Inclination (deg.)=d o
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Ka ¼
Batter Wall (i.e., o > 101)
367
368
Table 4 Sliding against foundation soil or rock FHWA
NCMA
General situation
(1) Horizontal backslope
General situation
(2) Inclined backslope
FS ¼
F X1:5 PH
where F ¼ Wm; m ¼min. of (tan ff ; tan fr or tan r), ff the friction angle of foundation soil, fr the friction angle of reinforced soil, r the friction angle of GS-to-soil and PH ¼ Ps þ Pq
FS ¼
F X1:5 PH
where F ¼ ðW1 þ W2 þ P sin bÞm; m=min. of (tan ff ; tan fr or tan r), ff the friction angle of foundation soil, fr the friction angle of reinforced soil, r the friction angle of GS-to-soil
FS ¼
F X1:5 PH
If reinforced soil controls F ¼ Cds ðW1 þ W2 þ qd Lb Þtan fr If drainage soil controls F ¼ Cds ðW1 þ W2 þ qd Lb Þtan fd If foundation soil controls
F ¼ Cds cf L þ ðW1 þ W2 þ qd Lb Þtan ff PH ¼ P cos ðd oÞ
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Modified Rankine
Table 5 Eccentricity and foundation soil bearing capacity Modified Rankine
Mov L o W þ qL 6
where Mov ¼ H
FS ¼
P ðcos bÞ e¼
Ps Pq þ 3 2
L eo in soil 6
Bearing capacity; BC X2:0 Bearing pressure; BP
where BC ¼ cf Nc þ 0:5ðL 2eÞgf Ng W þ qL BP ¼ L 2e
NCMA
FS ¼
h L L P ðsin bÞ W2 3 2 6 W1 þ W2 þ P sin b
L L L W2 X2 qd Lb Xq PsðHÞ Ys þ PqðHÞ Yq W1 X1 2 2 2 e¼ Wrð1Þ þ Wrð2Þ þ qd Lb
L eo in rock 4
Bearing capacity; BC X2:5 Bearing pressure; BP
where BC ¼ cf Nc þ 0:5ðL 2eÞgf Ng W1 þ W2 P sin b BP ¼ L 2e
Bearing capacity; BC X2:0 Bearing pressure; BP
where BC ¼ cf Nc þ 0:5gf ðL 2eÞNg þ gf Hemb Nq BP ¼
Wrð1Þ þ Wrð2Þ þ ðq1 þ qd ÞLb L 2e
369
Note: FSX2.0 might be acceptable if there is good geotechnical report available.
FS ¼
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e¼
FHWA
370
Modified Rankine
FHWA
NCMA
1. The resultant of vertical forces has to be within the middle one-third of the base. Otherwise longer reinforcement is necessary 2. LX0.7 H (unless there is good geotechnical report available) Note: This mode of failure is not felt to be a concern, i.e., there have been no exsisting problems or failures
FS ¼
Ms X2:0 Mov
Ms X2:0 Mov
where
where WL 2 Ps Pq þ ¼H 3 2
MS ¼ Mov
FSov ¼
MS ¼ W1 X1 þ W2 X2 þ qd Lb Xq Mov ¼ PsðHÞ Ys þ PqðHÞ Yq
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Table 6 Overturning about the toe of the MSE wall
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Table 7 Spacing of geosynthetic reinforcement layers Modified Rankine
Sv ¼
Tdes Cr sH
Sv ¼
NCMA
Tdes p0:8 m sH
where
where Tdes ¼
FHWA
Tult ðPRFÞðFSÞ
Cr ¼ coverage ratio sH ¼ Kar ðgz þ qÞ f Kar ¼ tan 45 r 2
Vertical spacing should be selected based on Nmin and Hu where
Tdes p Tallow Cr Cr ¼ coverage ratio sH ¼ Kar sv þ Dsh
Nmin ¼ min: number of layers PsðHÞ þ PqðHÞ ¼ Tdes Tdes ¼
sv ¼ gr Z þ s2 þ q þ Dsv
2
s2 ¼
gi L tan b 2 ðthe sloped backfillÞ
Tult ðPRFÞðFSÞ
Hu ¼ height of facing unit
Dsv ; Dsh ¼ Vertical and horizontal stress due to concentrated surcharge load For the nth layer of reinforcement: Tdes > FRðnÞ Lateral earth pressure coefficient, Kar : For vertical wall (oo101) with horizontal backfill and no wall friction f0 Kar ¼ tan2 45 r 2 For wall face batter f0 o > 101; y ¼ 45 þ r 2 Kar ¼
sin2 ðy þ f0r Þ 2 sin f0r sin3 y 1 þ sin y
where FgðnÞ ¼ Applied force in the nth layer ¼ Kar ðgr Zn þ qÞAcðnÞ cos ðd oÞ
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(material) strength is required. This value contains various reduction factors which are applied to the reinforcement geosynthetic’s ultimate strength which is typically obtained by an index test method. Such reduction factors as installation damage, long-term creep, degradation, etc., are handled somewhat differently by the respective design methods. It should be noted that, as inferred by the NCMA design, the spacing is often controlled by the facing block height or other facing controlled idiosyncrasy. Table 8 presents the various methods of calculating the geosynthetic reinforcement length within the soil mass behind the hypothetical failure plane. Note that this length constitutes the tail end of the reinforced soil zone. The design methods produce a FS-value for a given or assumed length of embedment. This length is often assumed to be one meter and it is the typical default value. Note that all three methods assume a Rankine failure plane rising at a angle of ‘‘45 þ f=2’’ with the horizontal at the inside toe of the wall facing and then extending linearly through the soil backfill mass. There is a distinct possibility that this failure plane can transition into a logarithmic spiral for high walls. This would save a considerable length of reinforcement for such high-wall situations. Table 9 presents the various methods to calculate the design (or required) facing stress where the geosynthetic reinforcement layers exit the masonry block or other facing anchorage into the drainage zone. The required connection strength is based not on the theoretical earth pressure applied, but on the tensile strength of the geosynthetic material that is selected. As such, there is no listing in the modified Rankine column, however, both FHWA and NCMA have a recommended procedure. The value is felt by the authors to be very subjective and largely controlled by the installation contractors methodology, see Soong and Koerner (1997). The two design methods shown in Table 9 vary widely in both concept and level of detail in the resulting numeric values to be resisted by the connection. Not only is the design strength at issue, the results of laboratory connection testing is also difficult to interpret, particularly with friction connections where sliding of the reinforcement is observed. This typically occurs at low normal stresses, i.e., near the top of the wall. Note that NCMA limits this slippage to 19 mm at which point the corresponding load is assumed for Tconn. The procedure for connection assessment is different from others in that a FSvalue is not explicitly obtained. Rather, a FS-value is assumed (usually it is 1.5) and a required connection strength is calculated. This value can vary with the wall height and has the effect of shifting the design burden to the laboratory testing of the various facing systems vis-"a-vis the particular type of reinforcement being utilized. 3.3. Numeric example The comparisons of the three design methodologies given in the previous sections, i.e., in Tables 3–9, are admittedly complex and somewhat tedious. So as to illustrate their relative impact on a practical problem the following SRW design example is offered. Fig. 3 illustrates a 7-m high geosynthetic reinforced wall consisting of 0.25-m high modular facing blocks. The reinforced soil properties are given as
Modified Rankine
FS ¼
2Le Ci Cr sv tanfir X1:5 Sv sh
where FS is the safety factor against pullout, Le the embedment length (min. 1.0 m), Ci the interaction coefficient, Cr the coverage ratio
FHWA
FS ¼
NCMA
2Le F * Cr agZp X1:5 Sv sh
where FS is the safety factor against pullout, F the pullout resistance factor, Cr the coverage ratio, a the scale correction factor (0.6B1.0)
FS ¼
ACn X1:5 FgðnÞ
where FS is the safety factor against pullout, FgðnÞ the applied force in the nth layer, ACn the anchorage capacity of the nth layer ¼ 2LaðnÞ Ci ðdn gr þ qd Þtan fr ; Ci ¼interaction coefficient
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Table 8 Soil pullout resistance behind potential failure plane
373
374
Table 9 Facing (or anchorage) stress FHWA
NCMA
Not applicable (see text for discussion)
Tconn is the lesser of:
(1) Tdes ; design strenght of the reinforcement where Tdes ¼
Tconn X1:5Tdes ðstrengthÞ - or -
Tult ðPRFÞð1:5Þ
(2) The reduced ultimate connection strength based on connection/seam strenght, i.e., Trupt ¼ where CRu ¼
Tult ðCRu Þ 1:5ðRFD ÞðRFCR Þ Tultc and Tultc ¼peak strength Tlot
where rupture is the mode of failure, Tlot ¼ultimate strength of the lot used for the connection strength testing (3) The reduced ultimate connection strength based on pullout, i.e., Tanchorage ¼
Tult ðCRs Þ 1:5
Tsc Tlot and Tsc =peak strength where pullout is the mode of failure
where CRs ¼
Tconn at 19 mm X1:0 Tdes ðserviceabilityÞ
where Tdes ¼
Tult ðPRFÞðFSÞ
Tconn at 19 mm ¼connection strength at 19 mm deformation
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Modified Rankine
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375
Fig. 3. Numeric example used to illustrate differences in design methodologies present in this paper.
g ¼ 18 kN/m3, fr ¼ 321 and c ¼ 0; i.e., it is a sand. Consider this to include the drainage soil zone as well (although it should be gravel) since its properties will not be significantly different. The retained soil is at a unit weight of 17 kN/m3, with fi ¼ 301 and a conservatively estimated c ¼ 0: The friction angle of the reinforced soil with the foundation soil is ff ¼ 301 and the foundation soil has a unit weight of 17 kN/m3. Its bearing capacity is 690 kN/m2. The proposed geosynthetic reinforcement has an ultimate wide width strength of 160 kN/m and with cumulative reduction factors of 4.0 (1.33 1.20 2.5) results in an allowable strength of 40 kN/m. When coupled with a global FS ¼ 1:5 for design uncertainties, this results in a design strength of 26.7 kN/m. The coverage ratio of the reinforcement is 0.80, i.e., 80% of the backfill surface at each layer of reinforcement is covered with reinforcement. The interaction coefficient with the soil in the reinforced zone is 0.75. Table 10 presents the results of the various elements of the design for the three respective design methods given in the previous subsection. Regarding the external stability considerations, the modified Rankine method is seen to be most conservative for foundation sliding, bearing capacity and overturning (i.e., the lowest FS-values). The FHWA method is intermediate in its FS-values. For all three external considerations, the NCMA is the least conservative method (i.e., highest FS-values). Regarding the internal stability considerations, the FS-values on reinforcement strength was calculated at each layer. Here it is seen that the modified Rankine and FHWA methods are the most conservative throughout all layers, with the NCMA method being the least conservative, but only nominally so. Generally, the same trend is seen in the FS-values for soil pullout resistance. At the upper 2-layers,
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Table 10 Results of numeric example illustrated in Fig. 3 for three design methods (a) External stability considerations Item
Modified Rankine
FHWA
NCMA
FSFoundation sliding Eccentricity (m) FSBearing capacity FSOverturning
2.07 0.64 3.59 3.43
2.30 0.65 3.59 n/a
2.87 0.42 5.35 4.93
(b) Internal stability consideration reinforcement strength FS-Values Layer number
Elevation (m)
11 10 9 8 7 6 5 4 3 2 1
6.25 5.25 4.25 3.75 3.25 2.75 2.25 1.75 1.25 0.75 0.25
Soil pullout resistance 11 10 9 8 7 6 5 4 3 2 1
6.25 5.25 4.25 3.75 3.25 2.75 2.25 1.75 1.25 0.75 0.25
Modified Rankine
FHWA
NCMA
2.97 2.28 2.19 2.88 2.57 2.31 2.11 1.93 1.79 1.66 1.55
3.18 2.25 2.34 2.84 2.53 2.28 2.08 1.91 1.76 1.64 1.53
3.26 2.30 2.29 2.91 2.60 2.34 2.13 1.96 1.81 1.68 1.57
1.32 3.26 6.30 10.87 12.27 13.66 15.04 16.41 17.78 19.15 20.51
3.04 4.85 8.58 13.76 15.12 16.47 17.82 19.18 20.53 21.88 23.23
1.56 4.16 9.01 15.41 17.77 20.14 22.50 24.87 27.23 29.59 31.96
Required connection resistancea Required connection resistance (kN/m) Layer number
Elevation (m)
Modified Rankine
FHWA
NCMA
11 10 9 8 7 6 5 4 3 2 1
6.25 5.25 4.25 3.75 3.25 2.75 2.25 1.75 1.25 0.75 0.25
n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a
10.08 14.29 14.35 11.29 12.67 14.06 15.44 16.82 18.21 19.59 20.97
8.40 11.90 11.95 9.41 10.56 11.71 12.86 14.01 15.17 16.32 17.47
a
Note: n/a, not addressed.
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377
however, the NCMA method gives relatively low values. This is particularly the case for the uppermost layer. For the required facing connection resistance the modified Rankine method is not applicable and the FHWA method is slightly more conservative than the NCMA method. The general trends in this numeric example are to be expected. It is well known that the Coulomb method produces lower earth pressures than the Rankine method for various practical conditions. Thus the fact that the modified Rankine method results are the most conservative in all design elements is not surprising. Had the wall facing been at a batter, the differences would have been even greater. Also to be expected is that the FHWA method is somewhat more conservative than the NCMA method. The public-ownership section (via FHWA) has traditionally been more conservative than the private ownership section (e.g., via NCMA and others). These trends were clearly illustrated in the selected numeric example.
4. Case histories of inadequate wall performance It has been estimated by the National Concrete Masonry Association that there are 25,000 MSE walls of the segmental retaining wall (masonry block) type in the United States. Of this total, at least half are geosynthetically reinforced. In addition, there are many other facing types in addition to masonry blocks, e.g., segmental concrete panels, wrap-around, prefabricated panels, cast-in place panels and timber. Considering the worldwide situation there could easily be 35,000 walls which are geosynthetically reinforced using geogrids (mainly), geotextiles (less frequently) and polymer straps (rarely). The vast majority of these walls have been successful insofar as their performance is concerned. Hundreds of papers in the literature attest to this very positive performance record. Designers for private owners and public agencies repeatedly use geosynthetic reinforced walls and the numbers in each ownership group are rapidly growing. Yet, there have been wall problems with respect to both serviceability (excessive deformation) and actual failures (collapse). The number of each is very small, however, lessons can be learned by assembling these case histories and analyzing them as a group to see if trends exist. Hopefully, the common situations can be identified and avoided in the future. From the open literature and our own files, twenty-six (26) case histories of problems with geosynthetic reinforced walls are presented in Tables 11 and 12. Twelve are of the serviceability type, wherein excessive deformation of the bottom, top or entire wall occurred. Fourteen are failures, wherein a portion of the wall actually collapsed. This is not meant to suggest that more walls fail than excessively deform; we believe that the opposite is much more likely. The numbers only appear to indicate that people tend to investigate and publish failures (vis-"a-vis serviceability problems) due to the obvious finality of the situation. Whether the deformation case histories will eventually lead to failures is not known.
378
No.
Facing
GS-type
Year
Service time
Cause
Problem
Soil backfill (reinforced zone)
Weatherb
Reference
S1 S2
WAW WAW
GG GG
1984 1990
3-yr ?
design design
clay (ML-CL) sandy gravel
cold climate n/a
Burwash and Frost (1991) Christopher(1993)
S3 S4
SRW SRW
GG GG
1990 1994
? DC
design design
sand n/a
unknown rain
Bathurst and Simac (1994) Sandri (1997)
S5
SRW
GG
1994
DC
contractor
n/a
n/a
Sandri (1997)
S6 S7 S8 S9
SRW SRW SRW SRW
GG GG GG GT
1994 1994 1994 1994
DC DC DC 2-yr
contractor contractor contractor contractor
n/a n/a n/a clay (CL)
n/a n/a n/a unknown
Sandri (1997) Sandri (1997) Sandri (1997) Gassner and James (1998)
S10 S11 S12
SRW PPW SRW
GG GG GG
1995 1995 1998
6-mo 4-yr 8-mo
design design design
wall rotation bulge at bottom (surcharge) bulge at bottom bulge at top (poor drainage) bulge at top (heavy rollers) poor alignment poor block placement footing too shallow midwall bulge (wrong spacing) deformed throughout bulge at top bulge at top
clay clayey silt silty clay
rain n/a int. rain
authors authors authors
a Notes: WAW, wrap around wall; SRW, segmental retaining wall; PPW, precast panel wall; GG, geogrid; GT, geotextile; DC, during construction; n/a, not applicable to problem. b weather refers to climate during or shortly before incident occurred.
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Table 11 Case histories of wall serviceability problems, i.e., excessive deformationa
No.
Facing
GS-Type
Year
Lifetime
Cause
Reason
Soil backfill (reinforced zone)
Weatherb
Reference
F1 F2 F3 F4 F5
Timber SRW SRW WAW WAW
GT GG GG GG GG
1987 1990 1992 1992 1992
3-mo 6-mo 2-mo ? ?
contractor contractor design design design
no compaction clay sand silty clay silty clay
n/a n/a dry heavy rain heavy rain
Richardson and Behr (1998) Leonards et al. (1994) Berg and Meyers (1997) Huang (1994) Huang (1994)
F6
SCP
GG
1992
?
design
silty clay
heavy rain
Huang (1994)
F7 F8 F9 F10
SRW SRW SRW SRW
GT GT GG GG
1993 1994 1994 1996
3-yr 2-yr ? 12-mo
design contractor design design
silty clay clay poor drainage clay
heavy rain heavy rain rainy heavy rain
Gassner and James (1998) Gassner and James (1998) Sandri (1997) authors
F11 F12 F13 F14
SRW SRW SRW SRW
GG GG GG GG
1997 1998 1998 1998
12 mo 18 mo 8 mo 12 mo
design design design design
connection omitted GG-omitted global/compound hydrostatic hydrostatic (GG pullout) hydrostatic (GG break) hydrostatic GT omitted hydrostatic hydrostatic (poor drainage) hydrostatic hydrostatic hydrostatic hydrostatic
clay clay (ML-SP) silty clay clayey silt
rainy rainy int. rain int. rain
authors authors authors authors
a Notes: SRW, segmental retaining wall; WAW, wrap around wall; SCP, segmental concrete panel; GT, geotextile; GG, geogrid; n/a, not applicable to problem. b weather refers to climate during or shortly before incident occurred.
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Table 12 Case histories of wall failures, i.e., actual collapsea
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In the serviceability case histories assembled in Table 11, it is seen that large-scale excessive deformation can be at the top, bottom or throughout the wall and can even be rotational. Within the group of seven design-related case histories, five had finegrained backfill soils in the reinforced zone and two had granular soils throughout. The other five case histories experienced individual block, or localized, distortion. All of these appear to have been caused by contractors activities during construction. The message in both situations appear to be clear in that (i) fine-grained silts and clays should be questioned insofar as backfill soils are concerned, and (ii) contractors operations must be monitored and inspected in addition to adequate construction quality control (CQC). In the failure (or collapse) case histories assembled in Table 12, it is seen that hydrostatic pressure arising from lack of drainage from fine-grained soil backfills in the reinforced zone was the overriding reason for the failures. This occurred in 10 of the 14 case histories and all are listed as design related causes. It supports the concern over silt and clay backfills seen in the serviceability case histories. In three other failure case histories, contractor deficiencies were observed, again supporting the findings of the serviceability case histories insofar as lack of inspection and installation quality control are concerned. Interestingly, in only one of the 26-case histories presented in Tables 11 and 12 is the problem not fine-grained soil backfill or inadequate construction/inspection procedures. This is case history ‘‘F3’’ which experienced a global failure behind and beneath the entire wall structure carrying it and a much larger body of soil downslope in a large-scale failure. Once again, it was a fine-grained soil problem albeit not in the reinforced soil zone, per se.
5. Concerns As mentioned in the previous section of this paper there have been problems with MSE-geosynthetic reinforced walls concerning both serviceability issues and actual collapses. While the percentages are miniscule in light of the number of walls built, forensic insight into causes leads to identifiable concerns. These concerns, coupled with other perceived concerns, are the subject of this section. 5.1. Low permeability backfill The concern over low permeability backfill soils (see Mitchell and Zornberg, 1995) in the reinforced zone is a major issue leading to the largest number of serviceability problems and actual failures listed in Tables 11 and 12. Obviously, the use of low permeability backfill soil contributed greatly toward an inexpensive wall system, but it can also lead to excessive deformations and/or failure. Table 13 compares recommendations for soil gradations in the reinforced backfill zone by three different references. When the above data is plotted, as it is in Fig. 4, the tremendous differences can be better appreciated. The NCMA recommendations are extremely broad, with up to
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35% passing the No. 200 sieve being allowed. FHWA is somewhat more restrictive with up to 15% being allowed. While NCMA places no restrictions on the characteristics of the material passing the No. 200 sieve, e.g., on clay content, plasticity index or activity, FHWA does say that the material passing the No. 200 sieve should have a plasticity index less than 6.0, should be free of organics and should not exceed 19 mm in size so as to minimize installation damage. Conversely to FHWA and NCMA, Koerner recommends that zero particles pass the No. 200 sieve. The criterion ensures a clean sand as being the finest material allowed. If fines (silts and/or clays) are allowed for the reinforced zone backfill soil, any possible water in front, behind and beneath the reinforced zone must be carefully collected, transmitted, and discharged. Proper filtration and drainage control is
Table 13 Reinforced soil zone gradation requirements Sieve size
F No. No. No. No. No.
4 10 40 100 200
Particle size (mm)
100 4.8 2.0 0.42 0.15 0.075
Percent passing requirement Koerner (1998)
FHWA (1998)
NCMA (1997)
F 100 90–100 0–60 0–5 0
F 100 F 0–60 F 0–15
75–100 20–100 F 0–60 F 0–35
Fig. 4. Limiting gradation requirements within the reinforced zone of MSE retaining walls with geosynthetic reinforcement.
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absolutely critical in this regard. Furthermore, the top of the zone should be waterproofed, e.g., by a geomembrane or a geosynthetic clay liner, to prevent water from entering the backfill zone from the surface. Surface water drainage as well as drainage from the retained earth zone is obviously of concern with respect to potential buildup of pore water pressures behind or within the reinforced soil zone. See Koerner and Soong (2000) for SRW drainage system designs in this regard. 5.2. Various design issues There are the obvious design differences noted in the previous section of this paper which give rise to variations as noted in the example problem. While these differences are felt to be substantive, others not mentioned are equally pressing. Some commentary follows in this regard. 5.2.1. Seismic design Seismic design was not addressed in this paper and is obviously of concern in areas of such activity. While too early in the history of MSE-geosynthetic reinforced walls to accumulate significant performance during earthquakes, the flexibility of such walls should prove to be an asset as compared to rigid walls. Reports by Tatsuoko et al. (1998) during the Kobe, Japan earthquake lend credibility to this statement, but clearly additional performance over time is necessary and particularly for SRWs. A recent workshop was very significant in this regard, Ling and Leshchinsky (2000). The nature and behavior of friction connections of the geosynthetic reinforcement to the wall facing is under active investigation by several groups, see Bathurst and Simac (1994). When properly constructed such friction connections appear to be adequate, but the design concerning both required strength and allowable strength needs additional investigation, see Berg and Nelson (2000). 5.2.2. Design details The design details of penetrations in the drainage and backfill zones (lightpole stantions, guard post piling, utility pole anchors, catch basins and manholes, drainage outlets, etc.) are difficult to handle from a quantitative design perspective. Factors such as coverage ratio and interaction coefficients are difficult to ascribe values to, and one tends to be extremely conservative. This obviously affects the economics of the final design. 5.2.3. Durability of both the facing and the reinforcement Durability of both the facing and the reinforcement is important in the context of the typical 75 to 100 year lifetime expected of reinforced wall systems. Regarding the durability of the facing itself, masonry blocks are being investigated by NCMA. The issue of the proper cement and additives in light of the manufacturing process is under investigation. For dry cast masonry block facing, the major unresolved issues are freeze-thaw behavior in cold climates and wet-dry behavior in all climates. Other facings should have similar, or comparable, challenges insofar as the required durability research and development is concerned. Regarding the durability of the
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reinforcement, FHWA has a number of significant research programs completed, Salman et al. (1997, 1998), and ongoing, Hsuan and Koerner (1999). As these studies are completed they are being regularly embodied in AASHTO (1997) specifications. 5.3. Quality control and quality assurance Noted in Table 11 was that the lack of CQC and (CQA) resulting in a number of serviceability problems. Whether an unsightly aligned wall leads to eventual collapse is doubtful, but aesthetics play a major role in the acceptance of these walls and they should obviously be constructed properly, and in accordance with the site-specific design plans and specifications. Perhaps the typical practice of a manufacturer giving a few hours instruction at the beginning of a project on proper construction practice is insufficient, but this is not known to be a fact. What is observed by the authors is a general lack of monitoring and inspection on the part of the owner (both public and private) or by the certifying wall designer. CQA of the walls under construction, which are both critical in their performance and permanent in their service life, should be inspected on a full-time basis. 5.4. Maintenance On a regular basis, during the service lifetime of geosynthetic reinforced MSE walls, a site visit should be made by the owner or owner’s consultant for the purpose of visually observing the condition of the wall. Deformations are the obvious target and if observed, a monitoring program, e.g., surveying of wall movements and/or backfill settlements, should be instituted. If serious enough, deformation gages and inclinometers can be installed and the movement can be quantified over time. Cracking of blocks is an indication of distortion of some type, perhaps foundation settlement, and should be investigated accordingly. Low-cost masonry crack monitoring gages can be used in this regard. Damage to individual facing blocks, e.g., by traffic accidents, or dislodged cap blocks should be repaired and/or replaced. Grouting behind the blocks may be required in extreme cases. If surface water drainage is not in accordance with the original design, or if it has been changed subsequent to the construction of the wall, its implications must be addressed. Remediation may be warranted. An important indication of nonequlibrium conditions in the backfilled soil (drainage, reinforcement or retained earth zones) is loss of soil from the spaces between blocks. Lost backfill soil has been a precursor to collapse in at least one wall, Bernardi (1998).
6. Summary and conclusions The use of SRWs that are geosynthetic reinforced is growing at an enormous rate. This growth is justified on the basis of excellent performance-to-date, superb aesthetics, relative ease-of-construction, and overall low cost.
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Several design methodologies are available and this paper compared and contrasted three of these methods which are commonly used in North American practice. Results on a typical wall example were seen to be least conservative by NCMA, intermediate by FHWA, and most conservative using modified Rankine. Technically, however, all three approaches are felt to be sound and the ultimate choice is project-specific. It is important to note that the tacit assumption in all cases is that adequately draining backfill soils are used in the reinforcement zone. As was seen in the case histories this assumption is often disregarded. A literature survey was presented describing 12-serviceability problems and 14wall failures. Of the total, 17 of the cases had low permeability backfill soils in the reinforced zone and 8 had uncontrolled or inadequate CQC/CQA in the construction of the walls. Only one of the case histories was due to conditions other than the above two reasons. In light of the above performance, which is excellent considering that as many as 35,000 of these walls exist, a series of concerns were presented. These included the two issues just raised (low permeability backfill soils and the lack of field quality control and inspection) in addition to selected design issues and long-term maintenance issues. In conclusion, it is felt that SRWs that are geosynthetically reinforced are completely justified in their growth at present rates, and when augmented by the worldwide market will be the choice of all wall systems in the future and will be nothing short of phenomenal.
Acknowledgements The financial assistance of the member organizations of the Geosynthetic Institute and its related institutes (Research, Information, Education, Accreditation and Certification) is sincerely appreciated. Appreciations is also extended to Drs. Dov Leshchinsky and Barry R. Christopher for review of the design tables of Section 3.0 of the paper.
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