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Internal deformation behavior of geosynthetic-reinforced soil walls
ARTICLE IN PRESS
Geotextiles and Geomembranes 25 (2007) 10–22 www.elsevier.com/locate/geotexmem
Internal deformation behavior of geosynthetic-reinforced soil walls Myoung-Soo Won, You-Seong Kim Department of Civil Engineering, Chonbuk National University, 664-14 Deogjin-dong 1Ga, Deogjin-gu, Jeonju, Jeollabuk-do 561-756, South Korea Received 16 April 2006; received in revised form 29 September 2006; accepted 16 October 2006 Available online 29 November 2006
Abstract Local deformation of geosynthetics, such as geogrids, and nonwoven and woven geotextiles, was measured to analyze the stability of geosynthetic-reinforced soil (GRS) structures. To analyze the deformation behavior of geosynthetics applied to a reinforced soil structure, the tensile load–elongation properties of the geosynthetic and local deformation measurement data are required. However, local deformation of nonwoven geotextile (NWGT), which is permeable, is difficult to measure with strain gauges. This study proposes a new, more convenient, method to measure the deformation behavior of NWGTs using a strain gauge and examines its suitability via laboratory tests and field trials on two GRS walls. A wide-width tensile test, conducted under a confining pressure of 70 kPa, showed that local deformation of NWGT, measured with strain gauges of type AE-11-S80N-120-EL, was similar to total deformation measured with linear variable deformation transformer (LVDT). In field trials, NWGT showed a larger deformation range than woven geotextile or geogrid. However, the deformation patterns of the three materials were similar. The strain gauges attached to NWGT in the walls worked normally for 16 months. Therefore, the method proposed in this study for measuring NWGT deformation using a strain gauge was effective and valuable. Pore water pressure in the GRS wall can be ignored since the backfill remains unsaturated regardless of rainfall. However, it should be noted for design purposes that horizontal earth pressures at the wall face are greater at the bottom and top of the wall than at rest. r 2006 Elsevier Ltd. All rights reserved. Keywords: Geosynthetics; Nonwoven geoxtiles (NWGTs); Geogrid; Strain gauge; Reinforced soil; GRS wall
1. Introduction Reinforced walls have been the subject of considerable research, and a number of recent papers have examined different aspects of their design and behavior (Kazimierowicz-Frankowska, 2005; Al Hattamleh and Muhunthan, 2006; Nouri et al., 2006). Owing to the increasing need for clayey soil (CL) as backfill in reinforced soil walls, nonwoven geotextiles (NWGTs) with drainage capability have received attention. NWGTs have the merit of high drainage capability and low cost, but also have a drawback of low tensile stiffness and higher deformability than geogrids or woven geotextiles. To analyze the deformation behaviors of reinforcements, load–elongation properties and local deformation measurement data are needed; however, measuring local deformations of GRS walls in Corresponding author. Tel.: +82 11 9197 0125; fax: +82 63 270 2421.
E-mail address: [email protected] (M.-S. Won). 0266-1144/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2006.10.001
the field is problematic. Sluimer and Risseeuw (1982) and Leshchinsky and Fowler (1990) have suggested using silicon to attach strain gauges to the woven geotextile. Strain gauges can be attached directly to woven geotextiles and geogrids (Boyle, 1995; Koerner, 1996); however, it is not easy to measure deformation of NWGTs by direct attachment of strain gauges because the gauges separate from the surface of NWGTs to which they are attached as NWGTs are elongated by a tensile force. Huang (1998) used a method of attaching strain gauges to the surface of NWGT composed of a core layer of knitted textile and needle-punched double layer of NWGT, by using gauge cement (Kyowa, EC-30). This study examines an easy method, using an adhesive, to attach strain gauges to NWGTs, and the applicability of the technique via laboratory and field tests. To analyze deformation behavior of reinforcements within GRS walls, two 5-m walls were constructed on a weak, shallow-layered foundation and fitted with a compound arrangement of
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
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Table 1 Geosynthetic reinforcement properties Products
KOLON P5100 KOLON KM5001 AKILEN GRID5/3
Materials
Polyester Polyester Polyester
Description
Nonwoven needle-punched Woven multi-filament Geogrid coated with PVC resin
Thickness (mm)
5 0.25 0.5
Tensile stiffness (kN/m)
156.4 235.2 215.2
Tensile strength (kN/m) Manufacturer (KS K 0520)
Researchers (ASTM D 4595)
100 50 50
89.7 51.1 44.3
Nonwoven geotextile 80mm
Manufacturer
CAS Corporation
Type Gauge factor Transverse sensitivity Resistance
AE-11-S80N-120-EL 2.171.0% 1.20% 120 O70.2%
NWGTs/woven geotextiles and nonwoven/geogrids. Deformation behavior inside the GRS walls was analyzed using data collected from four earth-pressure gauges, four pore-water pressure gauges, and 124 strain gauges attached to NWGTs and woven geotextiles and geogrid reinforcements over a period of about 1.5 years.
30mm
Table 2 Strain-gauge properties
Lead line
Adhesive Strain gauge
Terminal strip V.M. Tape
N-1 Coating
Nonwoven geotextile
Adhesive
Fig. 1. Attachment of strain gauge to nonwoven geotextile (NWGT).
2. Strain-gauge attachment method Table 1 shows the characteristics of NWGTs and woven geotextiles and geogrid used in this study. As shown in Table 2, strain gauges of the type AE-11-S80N-120-EL (manufactured by Cas Co. Ltd.) were used. For woven geotextiles and geogrid, strain gauges were attached directly; for NWGT, strain gauges were attached as shown in Fig. 1. Details of the attachment method are as follows: (1) Draw a rectangle on the NWGT large enough to fit the strain gauge and the terminal strip. Make the longer side of the rectangle face the direction of tension. (2) Spread a sufficient amount of adhesive evenly on the rectangle. (3) About 3–5 min after applying the adhesive, use a smooth, flat, rubber board to press down for 10 min, so that the adhesive adheres flatly to the NWGT. (4) Place strain gauge and terminal on the area 24 h after application of adhesive (when it is completely cured). The strain gauge and terminal strip are then wired and soldered together, leaving a small loop of wires in between them. (5) Connect the lead line. Protect the strain gauge from damage by covering with a N1 coating and waterproof tape. Using adhesive to attach the strain gauge to the NWGT results in less resistance against tension stress, due to its
Fig. 2. Attachment of strain gauges to nonwoven geotextile (NWGT) used in this study.
ductile and elastic properties, and it also prevents the water rising from below. Therefore, deformation behavior of the NWGT can be effectively measured in actual working conditions. Fig. 2 shows the strain gauges attached to the NWGT in GRS walls as shown in Figs. 9–11, whose deformation behavior was measured using the method suggested in this study. Fig. 3 shows that the strain gauges
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
did not separate from the surface to which they were attached at large strains, and the stiffness of the adhesive placed over a certain area of the NWGT was small enough
to ensure that the strain was similar to that experienced by NWGT without the adhesive. Figs. 4 and 5 show an in-soil load–elongation apparatus, which is similar to the confined extension test device developed by Yuan et al. (1998) to discover the load– elongation properties of geosynthetics under confinement. A confinement box with internal dimensions of 500 mm 500 mm in plan and 200 mm deep, and an upper and lower air bladder loading system were installed on the apparatus to repeatedly simulate field density conditions when geosynthetics are confined and to apply normal stresses. Fig. 6 shows the results of a wide-width tensile test (ASTM D4595), which was conducted under a 70-kPa confining pressure to ascertain whether the deformation behavior of NWGT can be effectively measured in GRS
Fig. 3. Results of wide-width tensile test (ASTM D4595) for nonwoven geotextile(NWGT): (a) deformation of specimen after rupture; (b) zoom view of rupture.
LVDT
Rear Load Cell Clamp
150mm
Pressure Gauge 290mm
Confinement Box Upper Air Bladder 505mm
Steel Roller
Lower Air Bladder Supporting Table Specimen Length
Primer Clamp Speed Controller
Pressure Controller
1060mm Hydraulic Power Unit 1700mm Fig. 4. Schematic diagram of the in-soil load–elongation apparatus.
1000mm
Front Load Cell Hydraulic Cylinder
Fig. 5. Wide-width tensile test process under confinement.
230mm 185mm
12
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
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a 15 Local strain measured by strain gauges(%)
13.5%
12 10.9% Nonwoven Geotextile Strain gauge 1 Strain gauge 2
9
5.7%
6
3.7% 3
48%
38% 0 0
10
20 30 Total strain measured by LVDT(%)
40
50
b 100 Strain gauge 2 LVDT Strain gauge 1 1 Strain gauge 1 2 Strain gauge 2 20cm
60
Reinforced with primer resin 7.5cm 40
10cm
Tensiile force (kN/m)
80
5.5cm
7.5cm 1
2
4.5cm
20 Reinforced with primer resin Tensile force direction
38%
48%
0 0
10
20
30
40
50
Strain (%) Fig. 6. Load–elongation properties of nonwoven geotextile (NWGT) under 70 kPa confining pressure.
walls under field conditions. In the figure, the local deformation measured with strain gauges was similar to the total deformation measured with linear variable deformation transformer (LVDT). In particular, repeated loading–unloading cycles showed the same pattern at a total deformation strain of 38–48%. This indicates that our method was effective in measuring deformation behavior of NWGT. Strain gauge 2 showed a larger local deformation than strain gauge 1, presumably because it was placed 1 cm closer to a tensile loading plate, as shown in Fig. 6. According to Boyle (1995), deformation measurements of geosynthetics obtained by using a strain gauge are affected
by the degree of flatness of the material, condition of the fibers and filament bundles in the strain direction, Poisson’s ratio and confining pressure during the test. Kim and Won (2001) reported that specimen size, strain rate, and confining pressure affect the strain-gauge measurements of geosynthetic deformation. The above factors, which are related to geosynthetic characteristics and external elements such as confinement, strain ratio, and adhesive, are thought to be unreliable when analyzing measurement with strain gauges. Fig. 7 shows the relationships between local and total deformation, with respect to the specimen size, strain rate,
ARTICLE IN PRESS
c
Geogrid
Local strain (%)
20 15 10
Average Line
5 0 0
5
b
10 15 20 25 30 35 40 45 50 55 60 Woven Geotextile
Local strain (%)
20 15
40
Nonwoven geotextile
35 30 25
Confining pressure: 0kPa
20 Confinging pressure: 70kPa
15 10 5 0 0
5
10
15 20 25 30 Total strain (LVDT) (%)
35
40
Fig. 8. Relationship between local deformation and total deformation of nonwoven geotextile (NWGT) with and without confining pressures of 70 kPa.
Average Line
10
Local strain (strain gauge) / Total strain (LVDT) (%)
M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
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5
Table 3 Results of field boring log
0 0
5
a
Fill
Silty clay
Gravel
Soft rock
Thickness (m) Depth (m)
1.4 0–1.4
3.4 1.4–4.8
0.7 4.8–5.5
SPT N value
—
3–4
50
2.5 5.5–8.0 (end boring) —
10 15 20 25 30 35 40 45 50 55 60 Nonwoven Geotextile
Local strain (%)
20 15 Average Line
10
3. Field GRS wall construction and the analysis of the deformation behavior of geosynthetics
5
3.1. Field GRS wall construction
0 0
5
10 15 20 25 30 35 40 45 50 55 60 Total strain measured by LVDT(%)
Fig. 7. Relationship between local deformation and total deformation for (a) nonwoven geotextile, (b) woven geotextile, and (c) geogrid (Kim and Won, 2001).
and confining pressure, from the study of Kim and Won (2001). Fig. 8 shows the relationships between local and total deformation of NWGTs from a wide-width tensile test under in-air and -soil conditions. In this figure, the local deformation measured under a confining pressure of 70 kPa appeared to be approximately half of the local deformation measured in air. Although there is a possibility to overestimating NWGT deformation if the average line in Fig. 7(a) is used to convert from local to total deformation, in this study, using the average curve for each reinforcement material in Fig. 7, the local deformation, measured with strain gauges, was converted into total deformation because it proved to be difficult to qualitatively analyze the deformation of geosynthetics measured with strain gauges.
In this study, low plasticity, CL from a nearby field was used as backfill, and nonwoven, woven, and geogrid materials were used as reinforcement. As shown in Table 3, the GRS wall was constructed on a shallow layer of a weak foundation with an average N value of 4 from ground surface to approximately 5 m in depth, as shown in Figs. 9 and 10. Table 4 shows the backfill properties used in the study. The friction angle and cohesion in Table 4 were determined from the consolidated drained direct shear tests. In accordance with assembly type of reinforcement materials, the GRS walls were divided into sections as follows: Section I (NWGT and woven geotextile) and Section II (NWGT and geogrid), then subdivided into sections A, B, C, and D, as shown in Figs. 9 and 10. Subsections A and B and C and D were the same mix of geosynthetics. To place geosynthetics as shown in Figs. 9–11 would presumably be expected to optimize the reinforcement effects by combining the relatively greater stiffness woven geotextiles and geogrids compared with NWGTs and NWGTs with drainage capacity. To find out whether or not there is a reinforced concrete (rigid wall) face effect on load application on the top surface of GRS walls,
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
15
8.0m
9m
SECTION I (Nonwoven & Geogrid)
A
0.3m
H=5.0m
3.
SECTION II (Nonwoven & Geogrid)
C
1.5m
B
D
H=0.5m
9m
3.
Geogrid
Nonwoven geotextile Geosynthetics Woven geotextile
2.7m Gabion
Fig. 9. Schematic diagram of the geosynthetic-reinforced soil (GRS) walls.
Fig. 10. The complete view of the geosynthetic-reinforced soil (GRS) walls after construction.
Table 4 Backfill properties Parameter
Value
Unit weight (kN/m3) Cohesion (kN/m2) Friction angle (deg.) Specific gravity Liquid limit (%) Plastic limit (%) Water content (%) Coefficient of permeability (cm/s) Unified classification Proctor maximum dry density (kN/m3) Proctor optimum water content (%)
18.5 54.8 29.1 2.67 30.6 22.3 17 3.157 106 CL 18.0 15.8
back-to-back type walls, in which the distance of unreinforced soil defined by the ends of reinforcement from either side of the wall was rather narrow, were constructed in this study. The wall surfaces of sections B and D were reinforced with concrete placement 18 months after construction. GRS walls were constructed in a step-by-step method. Initially, the gravel-filled gabion was heaped at the front. Using a mini vibrating plate compactor which weighed 88 kg, each 30 cm layer was compacted twice at a spacing of 15 cm, and GRS walls with a total height of 5 m were built by repeating the above method. The average degree of compaction in the backfill material was 88%. During compaction, soil and reinforcement were incorporated by allowing the horizontal deformation of walls faces. To allow the horizontal deformation during the construction walls probably helps to prevent the occurance of deformations at wall faces after completion of the walls. The GRS walls were completed in 57 days. As Fig. 11 shows, measuring instruments were placed on the wall of Sections I and II. The notations used in Fig. 7 (such as AN1 and BW4) are defined as follows: the first letter (A or B) indicates the subsection of wall, as shown in Fig. 9; the second letter (N, W, or G) indicates the sort of geosynthetics (N: NWGT; W: woven geotextile; G: geogrid); and the numbers indicate the layer of reinforcement from the bottom and the location of strain gauges on the geosynthetics. The behavior of these instruments on the GRS walls was automatically recorded using a data-logger. 3.2. Analysis of the deformation behaviors of geosynthetics As explained in the previous section, local deformation of geosynthetics was measured with strain gauges attached
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
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SECTION I (Nonwoven & Woven) UNIT = m Reinforced concrete wall
Gabion AN17
1
2
3
1
4
3
2
AN12
BN12 3
4
4
BW10
AW10
4
1
3
2
1.0
3 0.25 BW14
3 4
4
0.25 AW14
2 2
4
1 1
2 4
3
2 1 1.2
5.0
1.5
1
BN17
3
1.2
2
1.7
1
AN7
BN7 3
1
2
3
4
1
2
3
4
4
4
3
1
1.5
2
1
0.25
0.25 AW4 AN3
BN4 BN3 0.5
0.5 3 1.2
1.0
1.0
4
3
2
1
4
3
2
1
BN1
AN1 0.3
2
1.2
2
3
2 2.7
1 0.4
1.8
1
LEGEND Strain gauge
Pore water pressure cell
Horizontal earth pressure cell
Fig. 11. Installed instruments at Section I.
C SECTION, 3RD LAYER (Nonwoven Geotextile) 2.5
End of the Construction of the Wall
2.0 CN3-4(Strain gage 4)
CN3-3(Strain gage 3)
Strain (%)
1.5
CN3-2(Straing gage 2)
1.0 CN3-1(Strain gage 1) CN3-1 CN3-2 CN3-3 CN3-4
0.5
0.0 0
50
100
150
200 250 300 Elapsed time (days)
350
400
450
500
Fig. 12. Deformation behavior of nonwoven geotextile (NWGT) measured in the third reinforcement layer of subsection C.
on NWGTs and woven geotextiles, and geogrid within the GRS walls. Figs. 12–14 show local deformations converted into total deformation with the use of Fig. 7. The nonwoven material had a larger range of variance than the woven or geogrid; however, this does not pose a serious difficulty in understanding the general deformation behavior of NWGT. The figures show that, up to 25 days after construction of the walls, deformation of the reinforcement materials increased. This increase seems to have been
caused by residual stress from compaction and a storm that resulted in a rainfall of 180 mm/day on day 13. The heavy rainfall seemed to have little effect on pore water pressure because the variation of pre-water pressures when it is rained about 180 mm in a day was less than 3 kPa. However, the extent of 10 cm unequal settlements on the ground surface at the top of the walls after the storm resulted from the heavy rainfall. The storm forced an increase in the external deformation of GRS walls and the
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
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A SECTION, 4TH LAYER(Woven Geotextile) 2.5 AW4-1 AW4-2 AW4-3
2.0
Strain (%)
1.5
1.0
AW4-3(Strain gage 3)
End of the Construction of the Wall
0.5 AW4-2(Strain gage 2)
0.0
AW4-1(Strain gage 1) 0
50
100
150
200 250 300 Elapsed time (days)
350
400
450
500
Fig. 13. Deformation behavior of woven geotextile measured in the fourth reinforcement layer of subsection A.
D SECTION, 4TH LAYER(Geogrid) 2.5 DG4-1 DG4-3 DG4-4
Strain (%)
2.0
1.5 End of the Construction of the Wall DG4-3(Strain gage 3)
1.0
0.5
DG4-1(Strain gage 1)
DG4-4(Strain gage 4)
0.0 0
50
100
150
200 250 300 Elapsed time (days)
350
400
450
500
Fig. 14. Deformation behavior of geogrid measured the fourth reinforcement layer of subsection D.
deformation presumably compelled geosynthetics to deform. Figs. 15 and 16 show the deformation behavior of geosynthetics in Sections I and II with elapsed time. The deformation of the nonwoven material was larger than that of woven material or geogrid, and the difference increased in the upper part of the wall. In the 7th, 12th, and 17th reinforcement layers, NWGTs were laid to cut across the wall’s length, resulting in relatively heavier vertical soil pressures on the reinforcement materials and, thus, a greater influence of unequal settlement. The length of geosynthetics, except for layers 1, 7, 12, and 17 from the bottom of the wall, was 1.5 m, and 4th
strain gauges were attached 25 cm from the rear end of geosynthetics, as shown in Fig. 11. The maximum deformation, measured during construction to 10 days after completion of the walls, was as follows: NWGT: 2.94%; woven geotextile: 0.65%; geogrid: 1.07%. The maximum deformation measured over 16 months was as follows: NWGT: 9.05%; woven geotextile: 2.92%; geogrid: 2.33%. However, the maximum deformation of nonwoven reinforcement occurred in the 17th layer of Section I. Since this value is affected by outside elements, such as the impact caused by people walking on the wall, it was considered an exception, and the maximum deformation of NWGT was 6.05%.
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
18
9.054% 5.0
5
N
5.0
Layer 17
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
3.0
2.0
1.5
1.0
0.5
0.0
10 days after construction 100 days after construction 365 days after construction maximum value
N: nonwoven geotextiles W: woven geotexties 5.0
4
2.5
W
5.0
Layer 14
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 5.0
Layer 12
N
2.5
0.0
3
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
4.0
W
2.915%
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 5.0
Layer 10
2.5
2.5 0.650% 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
2
N
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
5.0
Layer 7
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
1
4.0
Strain (%)
Height above base of wall : h (m)
2.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
5.0
Layer 4
W
2.5
2.5
5.0
5.0 2.5
N
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
2.5
Layer 3
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 5.0
Layer 1
N 2.5
2.5
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Horizontal distance from wall faces (m) A - B section, ε (%) Fig. 15. Deformation behavior of the geosynthetics measured in Section I.
4. Pore water pressure and horizontal pressure 4.1. Pore water pressure This study used CL as backfill for GRS walls and NWGT, which is permeable, as reinforcement. Since excessive pore water pressure might affect the deformation of the GRS wall body, a total of four pore-water pressure cells were installed in subsection of Section I, as shown in Fig. 11, to examine the behavior of excessive pore water pressure in relation to rainfall intensity.
Figs. 17 and 18 show the changes in pore water pressure and rainfall intensity with elapsed time. Pore water pressure ranged within 3 kPa when affected by a heavy rainfall of 180 mm/day, which occurred 13 days after the construction of the wall. Generally, however, the change in pore water pressure ranges 5 to 10 kPa, regardless of rainfall intensity. The backfill material is probably unsaturated regardless of rainfall intensity for the following reasons: (1) pore-water pressure cells were installed the embankment, which was 17% of water content in soil; (2) the period of measurement in this study showed negative
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22 5.0
5
5.0
Layer 17
N
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
10 days after construction 100 days after construction 365 days after construction maximum value
N: nonwoven geotextiles G: geogrids 5.0
G
2.326%
4
19
5.0
Layer 14
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
6.052% 5.0
5.0
Layer 12
N
2.5
0.0
3
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
4.0
G
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 5.0
Layer 10
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
2
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
5.0
Layer 7
N
Strain (%)
Height above base of wall : h (m)
2.5
2.5
2.5 2.938% 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
4.0
G
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 5.0
Layer 4 1.067%
2.5
2.5
5.0
5.0 0.0
1
0.5
1.0
1.5
2.0
2.5
3.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5 N
4.0
3.5
4.0
5.0
N
3.5 3.0 Layer 3 3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 2.5
2.5
2.0
1.5
1.0
0.5
0.0
5.0
Layer 1
2.5
2.5
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Horizontal distance from wall faces (m) C - D section, ε (%) Fig. 16. Deformation behavior of the geosynthetics measured in Section II.
pore-water pressure; (3) there was no sign of drainage through NWGTs from the backfill material. Therefore, no drainage effect by NWGTs with rainfall was observed, and this means that the pore water pressure inside the wall was hardly affected by rainfall. 4.2. Horizontal earth pressure Fig. 19 illustrates the horizontal earth pressure distribution on the GRS wall’s face with elapsed time. As shown, horizontal earth pressure, affected by a greater residual horizontal stress due to compaction and rainfall, increased up to 30 days after the construction, maintained a steady state, and then decreased slightly thereafter. Horizontal
pressures were measured with E1 and E3 earth-pressure cells installed at 1 and 3.4 m, respectively, below the surface. After 87 days, the horizontal pressure of E1 was higher than that of E3, which was installed 2.4 m deeper than E1. Possible explanations are as follows: (a) the GRS wall was constructed on a shallow, weak foundation; (b) a CL, having high compressibility, was used as backfill instead of sandy soil; and (c) the wall surface was constructed so that multi-layered reinforcement hugged the gabion. These factors presumably contributed to the deformation of the wall, as shown in Fig. 20, resulting in passive earth pressure at the upper part of the soil wall. Unfortunately, the external deformation of GRS walls had not been measured over a time period. However, it could
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
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PORE WATER PRESSURE (B SECTION) 5 End of the construction
P4 P3 P2 P1
Pore water pressures (kPa)
0
Rainfall: 180mm/day
−5
P1
−10 P3 −15
−20
P4
0
50
100
150
P2
200 250 300 Elapsed time (days)
350
400
450
500
Fig. 17. Pore water pressure distribution with elapsed time.
2500
200 Accumulated Daily
Daily rainfall (mm/day)
160
2000
120
1500
80
1000
40
500
0 0
50
100
150
200 250 300 Elapsed time (day)
350
400
450
0 500
Fig. 18. Rainfall with elapsed time.
be observed visually that an arch-shaped settlement and unequal settlements of 10 cm in size occurred on the earthern surface at top of the GRS wall. E2 consistently showed low horizontal pressures from the start of measurements, possibly because either the earth-pressure cell itself malfunctioned or it was pushed back by deformation of the wall body. Fig. 21 illustrates the horizontal earth pressure measured at the wall surface 10, 100, 200, 300, and 400 days after construction, and Rankine’s active earth pressure and Jaky’s earth pressure at rest. The equations used to
calculate earth pressures are as follows: Rankine’s active pressure; Jaky’s earth pressure at rest;
sa ¼ sv tan
2
f 45 , 2
sh ¼ sv ð1 sin fÞ,
in which sv is the vertical total stress (kPa) and f the friction angle (1). The horizontal earth pressures measured 10 and 300 days after construction of the wall were higher than the active earth pressure and lower than the earth pressure at
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21
HORIZONTAL EARTH PRESSURE (B SECTION)
Horizontal earth pressures (kPa)
80
End of the construction
60
E4
40
E4 E3 E2 E1
E1 20 E3
E1 0 E2 0
50
100
150
200 250 300 Elapsed time (days)
350
400
450
500
Fig. 19. Horizontal earth pressure on geosynthetic-reinforced soil (GRS) wall’s face with elapsed time.
UNIT = m 1.0
Before the deformation of the wall
After the deformation of the wall
1.2
E1
E3 1.2
Horizontal earth pressure cell
1.2
E2
0.4
E4
Fig. 20. Estimation of geosynthetic-reinforced soil (GRS) wall’s exterior deformation.
rest in the lower part of the wall. However, in other parts of the wall face, the horizontal earth pressure was smaller than the active earth pressure. Similar horizontal earthpressure distributions have been reported by Andrawes et al. (1990), Ho (1993), and Ho and Rowe (1996). Ho and Rowe (1996) reported that when the length of the reinforcement is smaller than 0.7H (H ¼ wall height), the horizontal earth pressure at the wall’s face increases due to the influence of horizontal earth pressure working at the rear of the reinforcement backfill. The increased horizontal earth pressure induces a moment originating from the GRS wall’s toe, and vertical and horizontal stresses at the toe may be higher than corresponding theoretical values. The horizontal earth pressure measured 100 and 200 days after the construction of the GRS wall was higher than earth
pressure at rest in the lower and upper parts of the wall face. The horizontal earth pressure distribution measured 100 and 200 days after construction was similar to the value obtained by Skinner and Rowe (2005) through numerical analyses of a hypothetical 6-m high geosynthetic-reinforced soil (GRS) wall supporting a bridge abutment and approach road constructed on a 10-m thick yielding, CL deposit. Horizontal earth pressures at the wall face are related to the wall’s safety against settlement failures caused by excessive forward tilt due to a high toe pressure, failures at wall joints, and pullout failures. Thus, when a GRS wall with a flexible wall face is constructed on a shallow, weak foundation, the design must consider the possibility that horizontal earth pressure may be larger than earth pressure at rest at the bottom of the wall. 5. Conclusions A laboratory wide-width tensile test conducted under a confining pressure of 70 kPa showed that the pattern of local deformation on NWGT measured with strain gauges resembled that of the total deformation measured with LVDT. In GRS walls, NWGT showed a larger deformation range than the woven geotextile or geogrid. However, deformation patterns of these three reinforcement materials were similar and the strain gauges attached to the geosynthetics functioned normally for 16 months. Therefore, the method of measuring a NWGT deformation by using a strain gauge, as suggested by this study, was effective. The backfill material probably remained unsaturated regardless of rainfall because there were no signs of drainage through NWGT from the backfill, and the pore water pressures throughout the measurement period showed negative values. Therefore, pore water pressures
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5 400 days after the constroction of the wall 300 days after the constroction of the wall 200 days after the constroction of the wall 100 days after the constroction of the wall 10 days after the constroction of the wall Earth pressures at rest (K0) Active earth Pressues (Ka)
Height above base of wall (m)
4
3
2
1
0 0
10 20 30 40 Horizontal earth pressures behind wall face (kPa)
50
Fig. 21. Comparison between measured and theoretical horizontal earth pressures.
in the wall can be ignored. However, horizontal earth pressures at the wall face were larger at the bottom and top of the wall than earth pressures at rest. Therefore, when a GRS wall with a flexible wall face is constructed on a shallow, weak foundation, as in this study, precautions must be taken during the design and construction of the wall, since the horizontal earth pressure can be larger than earth pressure at rest at the bottom of the wall. Acknowledgments This work was supported by the Research Center of Industrial Technology at Chonbuk National University, South Korea. References Al Hattamleh, O., Muhunthan, B., 2006. Numerical procedures for deformation calculations in the reinforced soil walls. Geotextiles and Geomembranes 24 (1), 52–57. Andrawes, K.Z., Loke, K.H., Yeo, K.C., Murray, R.T., 1990. Application of boundary yielding concept to full scale reinforced and unreinforced soil walls. Performance of Reinforced Soil Structures. British Geotechnical Society, pp. 79–83. Boyle, S.R., 1995. Deformation prediction of geosynthetic reinforced soil retaining walls Ph.D. dissertation, University of Washington, p. 391.
Ho, S.K., 1993. A numerical investigation into the behaviour of reinforced soil walls. Ph.D. dissertation, University of Western Ontario, London, Canada, 408pp. Ho, S.K., Rowe, R.K., 1996. Effect of wall geometry on the behaviour of reinforced soil walls. Geotextiles and Geomembranes 14, 521–541. Huang, C.-C., 1998. Investigation of the local strains in a geosynthetic composite. Geotextiles and Geomembranes 16, 175–193. Kazimierowicz-Frankowska, K., 2005. A case study of a geosynthetic reinforced wall with wrap-around facing. Geotextiles and Geomembranes 23 (1), 107–116. Kim, Y.-S., Won, M.-S., 2001. An experimental study on the deformation properties assessment method of geosynthetic reinforcements. KSCE Journal of Civil Engineering 9 (5), 363–369 (in Korean). Koerner, R.M., 1996. The state-of-the-practice regarding in-situ monitoring of geosynthetics. In: Proceedings of the First European Geosynthetics Conference, The Netherlands, pp. 77–86. Leshchinsky, D., Fowler, J., 1990. Laboratory measurement of load– elongation relationship of high-strength geotextiles. Geotextiles and Geomembranes 9 (2), 145–164. Nouri, H., Fakher, A., Jones, C.J.F.P., 2006. Development of horizontal slice method for seismic stability analysis of reinforced slopes and walls. Geotextiles and Geomembranes 24 (3), 175–187. Skinner, G.D., Rowe, R.K., 2005. Design and behaviour of a geosynthetic reinforced retaining wall and bridge abutment on a yielding foundation. Geotextiles and Geomembranes 23, 234–260. Sluimer, G., Risseeuw, P., 1982. A strain-gauge technique for measuring deformation in geotextile. In: Proceedings of the Second International Conference on Geotextiles, Las Vegas, USA, pp. 835–838. Yuan, Z., Swan, R.H., Bachus, R.C., Elias, V., 1998. Soil confinement effect on stress–strain properties of geosynthetics. In: Sixth International Conference on Geosynthetics, vol. 2, pp. 523–528.
Geotextiles and Geomembranes 25 (2007) 10–22 www.elsevier.com/locate/geotexmem
Internal deformation behavior of geosynthetic-reinforced soil walls Myoung-Soo Won, You-Seong Kim Department of Civil Engineering, Chonbuk National University, 664-14 Deogjin-dong 1Ga, Deogjin-gu, Jeonju, Jeollabuk-do 561-756, South Korea Received 16 April 2006; received in revised form 29 September 2006; accepted 16 October 2006 Available online 29 November 2006
Abstract Local deformation of geosynthetics, such as geogrids, and nonwoven and woven geotextiles, was measured to analyze the stability of geosynthetic-reinforced soil (GRS) structures. To analyze the deformation behavior of geosynthetics applied to a reinforced soil structure, the tensile load–elongation properties of the geosynthetic and local deformation measurement data are required. However, local deformation of nonwoven geotextile (NWGT), which is permeable, is difficult to measure with strain gauges. This study proposes a new, more convenient, method to measure the deformation behavior of NWGTs using a strain gauge and examines its suitability via laboratory tests and field trials on two GRS walls. A wide-width tensile test, conducted under a confining pressure of 70 kPa, showed that local deformation of NWGT, measured with strain gauges of type AE-11-S80N-120-EL, was similar to total deformation measured with linear variable deformation transformer (LVDT). In field trials, NWGT showed a larger deformation range than woven geotextile or geogrid. However, the deformation patterns of the three materials were similar. The strain gauges attached to NWGT in the walls worked normally for 16 months. Therefore, the method proposed in this study for measuring NWGT deformation using a strain gauge was effective and valuable. Pore water pressure in the GRS wall can be ignored since the backfill remains unsaturated regardless of rainfall. However, it should be noted for design purposes that horizontal earth pressures at the wall face are greater at the bottom and top of the wall than at rest. r 2006 Elsevier Ltd. All rights reserved. Keywords: Geosynthetics; Nonwoven geoxtiles (NWGTs); Geogrid; Strain gauge; Reinforced soil; GRS wall
1. Introduction Reinforced walls have been the subject of considerable research, and a number of recent papers have examined different aspects of their design and behavior (Kazimierowicz-Frankowska, 2005; Al Hattamleh and Muhunthan, 2006; Nouri et al., 2006). Owing to the increasing need for clayey soil (CL) as backfill in reinforced soil walls, nonwoven geotextiles (NWGTs) with drainage capability have received attention. NWGTs have the merit of high drainage capability and low cost, but also have a drawback of low tensile stiffness and higher deformability than geogrids or woven geotextiles. To analyze the deformation behaviors of reinforcements, load–elongation properties and local deformation measurement data are needed; however, measuring local deformations of GRS walls in Corresponding author. Tel.: +82 11 9197 0125; fax: +82 63 270 2421.
E-mail address: [email protected] (M.-S. Won). 0266-1144/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2006.10.001
the field is problematic. Sluimer and Risseeuw (1982) and Leshchinsky and Fowler (1990) have suggested using silicon to attach strain gauges to the woven geotextile. Strain gauges can be attached directly to woven geotextiles and geogrids (Boyle, 1995; Koerner, 1996); however, it is not easy to measure deformation of NWGTs by direct attachment of strain gauges because the gauges separate from the surface of NWGTs to which they are attached as NWGTs are elongated by a tensile force. Huang (1998) used a method of attaching strain gauges to the surface of NWGT composed of a core layer of knitted textile and needle-punched double layer of NWGT, by using gauge cement (Kyowa, EC-30). This study examines an easy method, using an adhesive, to attach strain gauges to NWGTs, and the applicability of the technique via laboratory and field tests. To analyze deformation behavior of reinforcements within GRS walls, two 5-m walls were constructed on a weak, shallow-layered foundation and fitted with a compound arrangement of
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Table 1 Geosynthetic reinforcement properties Products
KOLON P5100 KOLON KM5001 AKILEN GRID5/3
Materials
Polyester Polyester Polyester
Description
Nonwoven needle-punched Woven multi-filament Geogrid coated with PVC resin
Thickness (mm)
5 0.25 0.5
Tensile stiffness (kN/m)
156.4 235.2 215.2
Tensile strength (kN/m) Manufacturer (KS K 0520)
Researchers (ASTM D 4595)
100 50 50
89.7 51.1 44.3
Nonwoven geotextile 80mm
Manufacturer
CAS Corporation
Type Gauge factor Transverse sensitivity Resistance
AE-11-S80N-120-EL 2.171.0% 1.20% 120 O70.2%
NWGTs/woven geotextiles and nonwoven/geogrids. Deformation behavior inside the GRS walls was analyzed using data collected from four earth-pressure gauges, four pore-water pressure gauges, and 124 strain gauges attached to NWGTs and woven geotextiles and geogrid reinforcements over a period of about 1.5 years.
30mm
Table 2 Strain-gauge properties
Lead line
Adhesive Strain gauge
Terminal strip V.M. Tape
N-1 Coating
Nonwoven geotextile
Adhesive
Fig. 1. Attachment of strain gauge to nonwoven geotextile (NWGT).
2. Strain-gauge attachment method Table 1 shows the characteristics of NWGTs and woven geotextiles and geogrid used in this study. As shown in Table 2, strain gauges of the type AE-11-S80N-120-EL (manufactured by Cas Co. Ltd.) were used. For woven geotextiles and geogrid, strain gauges were attached directly; for NWGT, strain gauges were attached as shown in Fig. 1. Details of the attachment method are as follows: (1) Draw a rectangle on the NWGT large enough to fit the strain gauge and the terminal strip. Make the longer side of the rectangle face the direction of tension. (2) Spread a sufficient amount of adhesive evenly on the rectangle. (3) About 3–5 min after applying the adhesive, use a smooth, flat, rubber board to press down for 10 min, so that the adhesive adheres flatly to the NWGT. (4) Place strain gauge and terminal on the area 24 h after application of adhesive (when it is completely cured). The strain gauge and terminal strip are then wired and soldered together, leaving a small loop of wires in between them. (5) Connect the lead line. Protect the strain gauge from damage by covering with a N1 coating and waterproof tape. Using adhesive to attach the strain gauge to the NWGT results in less resistance against tension stress, due to its
Fig. 2. Attachment of strain gauges to nonwoven geotextile (NWGT) used in this study.
ductile and elastic properties, and it also prevents the water rising from below. Therefore, deformation behavior of the NWGT can be effectively measured in actual working conditions. Fig. 2 shows the strain gauges attached to the NWGT in GRS walls as shown in Figs. 9–11, whose deformation behavior was measured using the method suggested in this study. Fig. 3 shows that the strain gauges
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did not separate from the surface to which they were attached at large strains, and the stiffness of the adhesive placed over a certain area of the NWGT was small enough
to ensure that the strain was similar to that experienced by NWGT without the adhesive. Figs. 4 and 5 show an in-soil load–elongation apparatus, which is similar to the confined extension test device developed by Yuan et al. (1998) to discover the load– elongation properties of geosynthetics under confinement. A confinement box with internal dimensions of 500 mm 500 mm in plan and 200 mm deep, and an upper and lower air bladder loading system were installed on the apparatus to repeatedly simulate field density conditions when geosynthetics are confined and to apply normal stresses. Fig. 6 shows the results of a wide-width tensile test (ASTM D4595), which was conducted under a 70-kPa confining pressure to ascertain whether the deformation behavior of NWGT can be effectively measured in GRS
Fig. 3. Results of wide-width tensile test (ASTM D4595) for nonwoven geotextile(NWGT): (a) deformation of specimen after rupture; (b) zoom view of rupture.
LVDT
Rear Load Cell Clamp
150mm
Pressure Gauge 290mm
Confinement Box Upper Air Bladder 505mm
Steel Roller
Lower Air Bladder Supporting Table Specimen Length
Primer Clamp Speed Controller
Pressure Controller
1060mm Hydraulic Power Unit 1700mm Fig. 4. Schematic diagram of the in-soil load–elongation apparatus.
1000mm
Front Load Cell Hydraulic Cylinder
Fig. 5. Wide-width tensile test process under confinement.
230mm 185mm
12
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13
a 15 Local strain measured by strain gauges(%)
13.5%
12 10.9% Nonwoven Geotextile Strain gauge 1 Strain gauge 2
9
5.7%
6
3.7% 3
48%
38% 0 0
10
20 30 Total strain measured by LVDT(%)
40
50
b 100 Strain gauge 2 LVDT Strain gauge 1 1 Strain gauge 1 2 Strain gauge 2 20cm
60
Reinforced with primer resin 7.5cm 40
10cm
Tensiile force (kN/m)
80
5.5cm
7.5cm 1
2
4.5cm
20 Reinforced with primer resin Tensile force direction
38%
48%
0 0
10
20
30
40
50
Strain (%) Fig. 6. Load–elongation properties of nonwoven geotextile (NWGT) under 70 kPa confining pressure.
walls under field conditions. In the figure, the local deformation measured with strain gauges was similar to the total deformation measured with linear variable deformation transformer (LVDT). In particular, repeated loading–unloading cycles showed the same pattern at a total deformation strain of 38–48%. This indicates that our method was effective in measuring deformation behavior of NWGT. Strain gauge 2 showed a larger local deformation than strain gauge 1, presumably because it was placed 1 cm closer to a tensile loading plate, as shown in Fig. 6. According to Boyle (1995), deformation measurements of geosynthetics obtained by using a strain gauge are affected
by the degree of flatness of the material, condition of the fibers and filament bundles in the strain direction, Poisson’s ratio and confining pressure during the test. Kim and Won (2001) reported that specimen size, strain rate, and confining pressure affect the strain-gauge measurements of geosynthetic deformation. The above factors, which are related to geosynthetic characteristics and external elements such as confinement, strain ratio, and adhesive, are thought to be unreliable when analyzing measurement with strain gauges. Fig. 7 shows the relationships between local and total deformation, with respect to the specimen size, strain rate,
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c
Geogrid
Local strain (%)
20 15 10
Average Line
5 0 0
5
b
10 15 20 25 30 35 40 45 50 55 60 Woven Geotextile
Local strain (%)
20 15
40
Nonwoven geotextile
35 30 25
Confining pressure: 0kPa
20 Confinging pressure: 70kPa
15 10 5 0 0
5
10
15 20 25 30 Total strain (LVDT) (%)
35
40
Fig. 8. Relationship between local deformation and total deformation of nonwoven geotextile (NWGT) with and without confining pressures of 70 kPa.
Average Line
10
Local strain (strain gauge) / Total strain (LVDT) (%)
M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
14
5
Table 3 Results of field boring log
0 0
5
a
Fill
Silty clay
Gravel
Soft rock
Thickness (m) Depth (m)
1.4 0–1.4
3.4 1.4–4.8
0.7 4.8–5.5
SPT N value
—
3–4
50
2.5 5.5–8.0 (end boring) —
10 15 20 25 30 35 40 45 50 55 60 Nonwoven Geotextile
Local strain (%)
20 15 Average Line
10
3. Field GRS wall construction and the analysis of the deformation behavior of geosynthetics
5
3.1. Field GRS wall construction
0 0
5
10 15 20 25 30 35 40 45 50 55 60 Total strain measured by LVDT(%)
Fig. 7. Relationship between local deformation and total deformation for (a) nonwoven geotextile, (b) woven geotextile, and (c) geogrid (Kim and Won, 2001).
and confining pressure, from the study of Kim and Won (2001). Fig. 8 shows the relationships between local and total deformation of NWGTs from a wide-width tensile test under in-air and -soil conditions. In this figure, the local deformation measured under a confining pressure of 70 kPa appeared to be approximately half of the local deformation measured in air. Although there is a possibility to overestimating NWGT deformation if the average line in Fig. 7(a) is used to convert from local to total deformation, in this study, using the average curve for each reinforcement material in Fig. 7, the local deformation, measured with strain gauges, was converted into total deformation because it proved to be difficult to qualitatively analyze the deformation of geosynthetics measured with strain gauges.
In this study, low plasticity, CL from a nearby field was used as backfill, and nonwoven, woven, and geogrid materials were used as reinforcement. As shown in Table 3, the GRS wall was constructed on a shallow layer of a weak foundation with an average N value of 4 from ground surface to approximately 5 m in depth, as shown in Figs. 9 and 10. Table 4 shows the backfill properties used in the study. The friction angle and cohesion in Table 4 were determined from the consolidated drained direct shear tests. In accordance with assembly type of reinforcement materials, the GRS walls were divided into sections as follows: Section I (NWGT and woven geotextile) and Section II (NWGT and geogrid), then subdivided into sections A, B, C, and D, as shown in Figs. 9 and 10. Subsections A and B and C and D were the same mix of geosynthetics. To place geosynthetics as shown in Figs. 9–11 would presumably be expected to optimize the reinforcement effects by combining the relatively greater stiffness woven geotextiles and geogrids compared with NWGTs and NWGTs with drainage capacity. To find out whether or not there is a reinforced concrete (rigid wall) face effect on load application on the top surface of GRS walls,
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8.0m
9m
SECTION I (Nonwoven & Geogrid)
A
0.3m
H=5.0m
3.
SECTION II (Nonwoven & Geogrid)
C
1.5m
B
D
H=0.5m
9m
3.
Geogrid
Nonwoven geotextile Geosynthetics Woven geotextile
2.7m Gabion
Fig. 9. Schematic diagram of the geosynthetic-reinforced soil (GRS) walls.
Fig. 10. The complete view of the geosynthetic-reinforced soil (GRS) walls after construction.
Table 4 Backfill properties Parameter
Value
Unit weight (kN/m3) Cohesion (kN/m2) Friction angle (deg.) Specific gravity Liquid limit (%) Plastic limit (%) Water content (%) Coefficient of permeability (cm/s) Unified classification Proctor maximum dry density (kN/m3) Proctor optimum water content (%)
18.5 54.8 29.1 2.67 30.6 22.3 17 3.157 106 CL 18.0 15.8
back-to-back type walls, in which the distance of unreinforced soil defined by the ends of reinforcement from either side of the wall was rather narrow, were constructed in this study. The wall surfaces of sections B and D were reinforced with concrete placement 18 months after construction. GRS walls were constructed in a step-by-step method. Initially, the gravel-filled gabion was heaped at the front. Using a mini vibrating plate compactor which weighed 88 kg, each 30 cm layer was compacted twice at a spacing of 15 cm, and GRS walls with a total height of 5 m were built by repeating the above method. The average degree of compaction in the backfill material was 88%. During compaction, soil and reinforcement were incorporated by allowing the horizontal deformation of walls faces. To allow the horizontal deformation during the construction walls probably helps to prevent the occurance of deformations at wall faces after completion of the walls. The GRS walls were completed in 57 days. As Fig. 11 shows, measuring instruments were placed on the wall of Sections I and II. The notations used in Fig. 7 (such as AN1 and BW4) are defined as follows: the first letter (A or B) indicates the subsection of wall, as shown in Fig. 9; the second letter (N, W, or G) indicates the sort of geosynthetics (N: NWGT; W: woven geotextile; G: geogrid); and the numbers indicate the layer of reinforcement from the bottom and the location of strain gauges on the geosynthetics. The behavior of these instruments on the GRS walls was automatically recorded using a data-logger. 3.2. Analysis of the deformation behaviors of geosynthetics As explained in the previous section, local deformation of geosynthetics was measured with strain gauges attached
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SECTION I (Nonwoven & Woven) UNIT = m Reinforced concrete wall
Gabion AN17
1
2
3
1
4
3
2
AN12
BN12 3
4
4
BW10
AW10
4
1
3
2
1.0
3 0.25 BW14
3 4
4
0.25 AW14
2 2
4
1 1
2 4
3
2 1 1.2
5.0
1.5
1
BN17
3
1.2
2
1.7
1
AN7
BN7 3
1
2
3
4
1
2
3
4
4
4
3
1
1.5
2
1
0.25
0.25 AW4 AN3
BN4 BN3 0.5
0.5 3 1.2
1.0
1.0
4
3
2
1
4
3
2
1
BN1
AN1 0.3
2
1.2
2
3
2 2.7
1 0.4
1.8
1
LEGEND Strain gauge
Pore water pressure cell
Horizontal earth pressure cell
Fig. 11. Installed instruments at Section I.
C SECTION, 3RD LAYER (Nonwoven Geotextile) 2.5
End of the Construction of the Wall
2.0 CN3-4(Strain gage 4)
CN3-3(Strain gage 3)
Strain (%)
1.5
CN3-2(Straing gage 2)
1.0 CN3-1(Strain gage 1) CN3-1 CN3-2 CN3-3 CN3-4
0.5
0.0 0
50
100
150
200 250 300 Elapsed time (days)
350
400
450
500
Fig. 12. Deformation behavior of nonwoven geotextile (NWGT) measured in the third reinforcement layer of subsection C.
on NWGTs and woven geotextiles, and geogrid within the GRS walls. Figs. 12–14 show local deformations converted into total deformation with the use of Fig. 7. The nonwoven material had a larger range of variance than the woven or geogrid; however, this does not pose a serious difficulty in understanding the general deformation behavior of NWGT. The figures show that, up to 25 days after construction of the walls, deformation of the reinforcement materials increased. This increase seems to have been
caused by residual stress from compaction and a storm that resulted in a rainfall of 180 mm/day on day 13. The heavy rainfall seemed to have little effect on pore water pressure because the variation of pre-water pressures when it is rained about 180 mm in a day was less than 3 kPa. However, the extent of 10 cm unequal settlements on the ground surface at the top of the walls after the storm resulted from the heavy rainfall. The storm forced an increase in the external deformation of GRS walls and the
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A SECTION, 4TH LAYER(Woven Geotextile) 2.5 AW4-1 AW4-2 AW4-3
2.0
Strain (%)
1.5
1.0
AW4-3(Strain gage 3)
End of the Construction of the Wall
0.5 AW4-2(Strain gage 2)
0.0
AW4-1(Strain gage 1) 0
50
100
150
200 250 300 Elapsed time (days)
350
400
450
500
Fig. 13. Deformation behavior of woven geotextile measured in the fourth reinforcement layer of subsection A.
D SECTION, 4TH LAYER(Geogrid) 2.5 DG4-1 DG4-3 DG4-4
Strain (%)
2.0
1.5 End of the Construction of the Wall DG4-3(Strain gage 3)
1.0
0.5
DG4-1(Strain gage 1)
DG4-4(Strain gage 4)
0.0 0
50
100
150
200 250 300 Elapsed time (days)
350
400
450
500
Fig. 14. Deformation behavior of geogrid measured the fourth reinforcement layer of subsection D.
deformation presumably compelled geosynthetics to deform. Figs. 15 and 16 show the deformation behavior of geosynthetics in Sections I and II with elapsed time. The deformation of the nonwoven material was larger than that of woven material or geogrid, and the difference increased in the upper part of the wall. In the 7th, 12th, and 17th reinforcement layers, NWGTs were laid to cut across the wall’s length, resulting in relatively heavier vertical soil pressures on the reinforcement materials and, thus, a greater influence of unequal settlement. The length of geosynthetics, except for layers 1, 7, 12, and 17 from the bottom of the wall, was 1.5 m, and 4th
strain gauges were attached 25 cm from the rear end of geosynthetics, as shown in Fig. 11. The maximum deformation, measured during construction to 10 days after completion of the walls, was as follows: NWGT: 2.94%; woven geotextile: 0.65%; geogrid: 1.07%. The maximum deformation measured over 16 months was as follows: NWGT: 9.05%; woven geotextile: 2.92%; geogrid: 2.33%. However, the maximum deformation of nonwoven reinforcement occurred in the 17th layer of Section I. Since this value is affected by outside elements, such as the impact caused by people walking on the wall, it was considered an exception, and the maximum deformation of NWGT was 6.05%.
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
18
9.054% 5.0
5
N
5.0
Layer 17
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
3.0
2.0
1.5
1.0
0.5
0.0
10 days after construction 100 days after construction 365 days after construction maximum value
N: nonwoven geotextiles W: woven geotexties 5.0
4
2.5
W
5.0
Layer 14
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 5.0
Layer 12
N
2.5
0.0
3
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
4.0
W
2.915%
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 5.0
Layer 10
2.5
2.5 0.650% 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
2
N
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
5.0
Layer 7
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
1
4.0
Strain (%)
Height above base of wall : h (m)
2.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
5.0
Layer 4
W
2.5
2.5
5.0
5.0 2.5
N
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
2.5
Layer 3
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 5.0
Layer 1
N 2.5
2.5
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Horizontal distance from wall faces (m) A - B section, ε (%) Fig. 15. Deformation behavior of the geosynthetics measured in Section I.
4. Pore water pressure and horizontal pressure 4.1. Pore water pressure This study used CL as backfill for GRS walls and NWGT, which is permeable, as reinforcement. Since excessive pore water pressure might affect the deformation of the GRS wall body, a total of four pore-water pressure cells were installed in subsection of Section I, as shown in Fig. 11, to examine the behavior of excessive pore water pressure in relation to rainfall intensity.
Figs. 17 and 18 show the changes in pore water pressure and rainfall intensity with elapsed time. Pore water pressure ranged within 3 kPa when affected by a heavy rainfall of 180 mm/day, which occurred 13 days after the construction of the wall. Generally, however, the change in pore water pressure ranges 5 to 10 kPa, regardless of rainfall intensity. The backfill material is probably unsaturated regardless of rainfall intensity for the following reasons: (1) pore-water pressure cells were installed the embankment, which was 17% of water content in soil; (2) the period of measurement in this study showed negative
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22 5.0
5
5.0
Layer 17
N
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
10 days after construction 100 days after construction 365 days after construction maximum value
N: nonwoven geotextiles G: geogrids 5.0
G
2.326%
4
19
5.0
Layer 14
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
6.052% 5.0
5.0
Layer 12
N
2.5
0.0
3
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
4.0
G
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 5.0
Layer 10
2.5
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
2
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
5.0
Layer 7
N
Strain (%)
Height above base of wall : h (m)
2.5
2.5
2.5 2.938% 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
5.0
4.0
G
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 5.0
Layer 4 1.067%
2.5
2.5
5.0
5.0 0.0
1
0.5
1.0
1.5
2.0
2.5
3.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5 N
4.0
3.5
4.0
5.0
N
3.5 3.0 Layer 3 3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0 2.5
2.5
2.0
1.5
1.0
0.5
0.0
5.0
Layer 1
2.5
2.5
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Horizontal distance from wall faces (m) C - D section, ε (%) Fig. 16. Deformation behavior of the geosynthetics measured in Section II.
pore-water pressure; (3) there was no sign of drainage through NWGTs from the backfill material. Therefore, no drainage effect by NWGTs with rainfall was observed, and this means that the pore water pressure inside the wall was hardly affected by rainfall. 4.2. Horizontal earth pressure Fig. 19 illustrates the horizontal earth pressure distribution on the GRS wall’s face with elapsed time. As shown, horizontal earth pressure, affected by a greater residual horizontal stress due to compaction and rainfall, increased up to 30 days after the construction, maintained a steady state, and then decreased slightly thereafter. Horizontal
pressures were measured with E1 and E3 earth-pressure cells installed at 1 and 3.4 m, respectively, below the surface. After 87 days, the horizontal pressure of E1 was higher than that of E3, which was installed 2.4 m deeper than E1. Possible explanations are as follows: (a) the GRS wall was constructed on a shallow, weak foundation; (b) a CL, having high compressibility, was used as backfill instead of sandy soil; and (c) the wall surface was constructed so that multi-layered reinforcement hugged the gabion. These factors presumably contributed to the deformation of the wall, as shown in Fig. 20, resulting in passive earth pressure at the upper part of the soil wall. Unfortunately, the external deformation of GRS walls had not been measured over a time period. However, it could
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
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PORE WATER PRESSURE (B SECTION) 5 End of the construction
P4 P3 P2 P1
Pore water pressures (kPa)
0
Rainfall: 180mm/day
−5
P1
−10 P3 −15
−20
P4
0
50
100
150
P2
200 250 300 Elapsed time (days)
350
400
450
500
Fig. 17. Pore water pressure distribution with elapsed time.
2500
200 Accumulated Daily
Daily rainfall (mm/day)
160
2000
120
1500
80
1000
40
500
0 0
50
100
150
200 250 300 Elapsed time (day)
350
400
450
0 500
Fig. 18. Rainfall with elapsed time.
be observed visually that an arch-shaped settlement and unequal settlements of 10 cm in size occurred on the earthern surface at top of the GRS wall. E2 consistently showed low horizontal pressures from the start of measurements, possibly because either the earth-pressure cell itself malfunctioned or it was pushed back by deformation of the wall body. Fig. 21 illustrates the horizontal earth pressure measured at the wall surface 10, 100, 200, 300, and 400 days after construction, and Rankine’s active earth pressure and Jaky’s earth pressure at rest. The equations used to
calculate earth pressures are as follows: Rankine’s active pressure; Jaky’s earth pressure at rest;
sa ¼ sv tan
2
f 45 , 2
sh ¼ sv ð1 sin fÞ,
in which sv is the vertical total stress (kPa) and f the friction angle (1). The horizontal earth pressures measured 10 and 300 days after construction of the wall were higher than the active earth pressure and lower than the earth pressure at
ARTICLE IN PRESS M.-S. Won, Y.-S. Kim / Geotextiles and Geomembranes 25 (2007) 10–22
21
HORIZONTAL EARTH PRESSURE (B SECTION)
Horizontal earth pressures (kPa)
80
End of the construction
60
E4
40
E4 E3 E2 E1
E1 20 E3
E1 0 E2 0
50
100
150
200 250 300 Elapsed time (days)
350
400
450
500
Fig. 19. Horizontal earth pressure on geosynthetic-reinforced soil (GRS) wall’s face with elapsed time.
UNIT = m 1.0
Before the deformation of the wall
After the deformation of the wall
1.2
E1
E3 1.2
Horizontal earth pressure cell
1.2
E2
0.4
E4
Fig. 20. Estimation of geosynthetic-reinforced soil (GRS) wall’s exterior deformation.
rest in the lower part of the wall. However, in other parts of the wall face, the horizontal earth pressure was smaller than the active earth pressure. Similar horizontal earthpressure distributions have been reported by Andrawes et al. (1990), Ho (1993), and Ho and Rowe (1996). Ho and Rowe (1996) reported that when the length of the reinforcement is smaller than 0.7H (H ¼ wall height), the horizontal earth pressure at the wall’s face increases due to the influence of horizontal earth pressure working at the rear of the reinforcement backfill. The increased horizontal earth pressure induces a moment originating from the GRS wall’s toe, and vertical and horizontal stresses at the toe may be higher than corresponding theoretical values. The horizontal earth pressure measured 100 and 200 days after the construction of the GRS wall was higher than earth
pressure at rest in the lower and upper parts of the wall face. The horizontal earth pressure distribution measured 100 and 200 days after construction was similar to the value obtained by Skinner and Rowe (2005) through numerical analyses of a hypothetical 6-m high geosynthetic-reinforced soil (GRS) wall supporting a bridge abutment and approach road constructed on a 10-m thick yielding, CL deposit. Horizontal earth pressures at the wall face are related to the wall’s safety against settlement failures caused by excessive forward tilt due to a high toe pressure, failures at wall joints, and pullout failures. Thus, when a GRS wall with a flexible wall face is constructed on a shallow, weak foundation, the design must consider the possibility that horizontal earth pressure may be larger than earth pressure at rest at the bottom of the wall. 5. Conclusions A laboratory wide-width tensile test conducted under a confining pressure of 70 kPa showed that the pattern of local deformation on NWGT measured with strain gauges resembled that of the total deformation measured with LVDT. In GRS walls, NWGT showed a larger deformation range than the woven geotextile or geogrid. However, deformation patterns of these three reinforcement materials were similar and the strain gauges attached to the geosynthetics functioned normally for 16 months. Therefore, the method of measuring a NWGT deformation by using a strain gauge, as suggested by this study, was effective. The backfill material probably remained unsaturated regardless of rainfall because there were no signs of drainage through NWGT from the backfill, and the pore water pressures throughout the measurement period showed negative values. Therefore, pore water pressures
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22
5 400 days after the constroction of the wall 300 days after the constroction of the wall 200 days after the constroction of the wall 100 days after the constroction of the wall 10 days after the constroction of the wall Earth pressures at rest (K0) Active earth Pressues (Ka)
Height above base of wall (m)
4
3
2
1
0 0
10 20 30 40 Horizontal earth pressures behind wall face (kPa)
50
Fig. 21. Comparison between measured and theoretical horizontal earth pressures.
in the wall can be ignored. However, horizontal earth pressures at the wall face were larger at the bottom and top of the wall than earth pressures at rest. Therefore, when a GRS wall with a flexible wall face is constructed on a shallow, weak foundation, as in this study, precautions must be taken during the design and construction of the wall, since the horizontal earth pressure can be larger than earth pressure at rest at the bottom of the wall. Acknowledgments This work was supported by the Research Center of Industrial Technology at Chonbuk National University, South Korea. References Al Hattamleh, O., Muhunthan, B., 2006. Numerical procedures for deformation calculations in the reinforced soil walls. Geotextiles and Geomembranes 24 (1), 52–57. Andrawes, K.Z., Loke, K.H., Yeo, K.C., Murray, R.T., 1990. Application of boundary yielding concept to full scale reinforced and unreinforced soil walls. Performance of Reinforced Soil Structures. British Geotechnical Society, pp. 79–83. Boyle, S.R., 1995. Deformation prediction of geosynthetic reinforced soil retaining walls Ph.D. dissertation, University of Washington, p. 391.
Ho, S.K., 1993. A numerical investigation into the behaviour of reinforced soil walls. Ph.D. dissertation, University of Western Ontario, London, Canada, 408pp. Ho, S.K., Rowe, R.K., 1996. Effect of wall geometry on the behaviour of reinforced soil walls. Geotextiles and Geomembranes 14, 521–541. Huang, C.-C., 1998. Investigation of the local strains in a geosynthetic composite. Geotextiles and Geomembranes 16, 175–193. Kazimierowicz-Frankowska, K., 2005. A case study of a geosynthetic reinforced wall with wrap-around facing. Geotextiles and Geomembranes 23 (1), 107–116. Kim, Y.-S., Won, M.-S., 2001. An experimental study on the deformation properties assessment method of geosynthetic reinforcements. KSCE Journal of Civil Engineering 9 (5), 363–369 (in Korean). Koerner, R.M., 1996. The state-of-the-practice regarding in-situ monitoring of geosynthetics. In: Proceedings of the First European Geosynthetics Conference, The Netherlands, pp. 77–86. Leshchinsky, D., Fowler, J., 1990. Laboratory measurement of load– elongation relationship of high-strength geotextiles. Geotextiles and Geomembranes 9 (2), 145–164. Nouri, H., Fakher, A., Jones, C.J.F.P., 2006. Development of horizontal slice method for seismic stability analysis of reinforced slopes and walls. Geotextiles and Geomembranes 24 (3), 175–187. Skinner, G.D., Rowe, R.K., 2005. Design and behaviour of a geosynthetic reinforced retaining wall and bridge abutment on a yielding foundation. Geotextiles and Geomembranes 23, 234–260. Sluimer, G., Risseeuw, P., 1982. A strain-gauge technique for measuring deformation in geotextile. In: Proceedings of the Second International Conference on Geotextiles, Las Vegas, USA, pp. 835–838. Yuan, Z., Swan, R.H., Bachus, R.C., Elias, V., 1998. Soil confinement effect on stress–strain properties of geosynthetics. In: Sixth International Conference on Geosynthetics, vol. 2, pp. 523–528.