- Email: [email protected]
Settlement predictions in the Anatolian Motorway, Turkey
Engineering Geology 67 (2002) 185 – 199 www.elsevier.com/locate/enggeo
Settlement predictions in the Anatolian Motorway, Turkey Su¨leyman Dalgıcß a,*, Orhan Sßimsßek b a
Istanbul University, Faculty of Engineering, 34850 Avcılar, Istanbul, Turkey b IC Consulenten, Zollhouse weg No. 1 Bergheim, Salzburg, Austria Received 19 February 2001; accepted 20 May 2002
Abstract The Anatolian Motorway through the Asarsuyu Valley passes across the landslides which have been extensively disturbed by past fault movements. The Asarsuyu Valley is the most important crossing of the motorway between Istanbul and Ankara route. Along the Asarsuyu Valley, about 7 km of the roadway is still under construction. In this study, the magnitude and the rate of the settlement over consolidated clays in lacustrine deposits within the Asarsuyu Valley were compared with each other. On the basis of field observations and laboratory test results, it was determined that the lacustrine deposits were eroded up to 15 m by the river in the valley bottom. As results of unloading and desiccation process, the clay layers are overconsolidated. Settlement calculations indicate that the amount of clay layers has caused the intolerable consolidation settlement under the concrete structures and motorway embankment. In this respect, preloading embankment on clay and silty clay deposits was projected and constructed. On the basis of evaluations, estimated values of settlement are lower than those realized. However, the predicted settlement quantities are found reliable and comparable to field measurements. On the other hand, significant differences were observed between calculated and measured rate of the settlement. The high rate of the settlement, which was measured during preloading, was caused by the viscoelastic strain due to the relatively high load and sandy pockets available in the clay layers, but was not detected by the drilling and the micro/macro texture of the clay layers. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Landslide; Clay; Consolidation; Settlement; Predictions; Measurements
1. Introduction This study was carried out in the Asarsuyu Valley crossing of the Anatolian Motorway (Fig. 1). The Asarsuyu crossing is located next to the Bolu Tunnel crossing (Dalgıcß , 2000). The engineering geological problems were observed both in the Asarsuyu crossing and in the Bolu Tunnel of the motorway. One of
*
Corresponding author. Fax: +90-212-5911997. E-mail address: [email protected] (S. Dalgıcß).
these geotechnical problems in the Asarsuyu Valley of the Anatolian Motorway is the overconsolidated clay layers, which are encountered in the recent fluvial deposits. Investigations reveal that the deposition of the clay layers is associated with the Bakacak landslide blocking the valley front. In the area, other huge landslides, which were caused by the North Anatolian Fault, such as the Bakacak landslide, are available. The horizontal and vertical extensions of the clay layers in the lake deposits were investigated by a total number of 16 boreholes. The natural unit weight, grain unit weight, grain size distribution, Atterberg
0013-7952/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 3 - 7 9 5 2 ( 0 2 ) 0 0 1 5 4 - 0
186
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Fig. 1. Location map.
Limits and Consolidation tests were performed on the undisturbed samples extracted from boreholes. These tests were conducted in accordance with ASTM standards in the laboratories of Astaldi, the contractor firm of Gu¨mu¨sß ova– Gerede Motorway, and the General Directorate of State Highways. According to settlement calculations performed by the authors on the basis of results of 16 consolidation tests, preloading embankment was found to be necessary and the contractor firm implemented it. Following the preloading fill, settlement measurements were controlled with 5 magnetic settlement columns and 15 settlement plates. This study indicated that there is an 88% consistency between the settlement values determined by calculation and in situ measurement. However, some differences were observed in rate of the consolidation. The reason for this is the viscoelastic strain due to the relatively high load and the micro and macro textures of the clay that cannot be determined
easily in the laboratory conditions, and also clay layers containing some sandy pockets in the field. Most of the literatures about the consolidation properties of the clays are for the determination of the relations between compression index and index ¨ nalp, 1996; properties (Ansal, 1987; Gu¨ndu¨z and O Bowles, 1979; Herrero, 1980). In addition, estimation of the compression index, depending on the mineralogic composition of the soils and the geographical position, changes one region to another. In this study, therefore, the value for the compression index of the clays was evaluated with respect to results obtained from odometer tests.
2. Lithology of the Asarsuyu Valley The Asarsuyu Valley route passes through landslides formed in weak zones related to paleotectonic
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
thrust faults and the neotectonic North Anatolian Fault Zone (Dalgıcß, 1998a). The Yedigo¨ller formation consists of metamorphic rocks and is the oldest unit present at the Asarsuyu Valley. It is tectonically overlain by the metamorphic I˙kizoluk formation of Devonian age. Above these strata the upper Cretaceous to upper Eocene sedimentary units are encoun-
187
tered. These formations are overlain by alluvium, colluvium and lacustrine deposits (Figs. 2 and 3). Alluvium deposits are generally composed of rounded, subrounded, pebbly sand, blocky pebbly sand and blocky pebbles derived from weakly altered, intermediately soft—very compact amphibolite, metagranite and other rocks. Thickness of these deposits at
Fig. 2. Evaluation of the Asarsuyu Valley before the clay deposits.
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Fig. 3. Longitudinal cross-section of the Asarsuyu Valley.
188
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
the downstream of Asarsuyu river is about 20 m. Investigation boreholes drilled at the upstream part of the river indicate that thickness of alluvium lowers to 6 –8 m. Grain size of the deposits decreases from upstream to downstream. Among the alluvium deposits, there are lens or wedge-shaped, weakly plastic, intermediately compact, green silt and sandy silt together with clay, silt and silty clay deposits with a total thickness between 3 and 5 m. Within the alluvium deposits, there are also plastic, brownish grey coloured, overconsolidated, cohesive, fissured clay and silty clays with thickness of 2 –11 m. There are two suggestions for the occurrence of these deposits (Dalgıcß, 1994). They are: (1) alluvial deposits; (2) lacustrine deposits formed as a result of blocking of the Asarsuyu Valley by the Bakacak landslide. The first suggestion indicates that, due to geotechnical characteristics, these deposits cannot be fluvial sediments. As will be explained further in detail, these deposits are composed of overconsolidated, cohesive, fissured, plastic clay and silty clay (CH). The second suggestion is based on blocking of
189
Asarsuyu Valley by the Bakacak landslide, which formed a lake behind the valley. Findings supporting this suggestion are given below.
Cutting of landslide material in exploratory drill holes. The motion of Bakacak landslide at south has changed Asarsuyu riverbed. Remnant of the landslide material encountered at northern side of the valley. Soil characteristics of the clayey deposits. Considering the field characteristics given above, it was believed that these sediments were deposited in the lake that was formed behind the landslide dam.
3. Slope failure in Asarsuyu Valley The Bakacak landslide, which gave rise to the formation of lacustrine deposits in the study area, has a length of 4 – 5 km and a width of 1.5 km and is still active (Fig. 4). As the toe of this landslide is
Fig. 4. Landslides areas in the Asarsuyu Valley.
190
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Table 1 Sieve analysis results of clay layer Grain size (%)
Min.
Max.
Average value (x)
Standard error (SE)
Standard deviation (SD)
Gravel (%) Sand (%) Silt (%) Clay (%)
0 0.1 11 60
0 0.3 39 88
0 1.15 34.15 65.16
0 0.53 9.89 9.56
0 0.55 10.83 10.47
crossed by a section of the Anatolian Motorway where a 2.6-km-long viaduct has been built, ensuring the stability of the slide is important for the long-term integrity of the structure. The lithologies of the Ikizoluk and Yedigoller formations in the Asarsuyu Valley have been both extensively disintegrated as a result of tectonic disturbance and further weakened by weathering. As a result, the slides are related to the
fault structures as opposed to the normal discontinuity-controlled instabilities observed in strong rock masses, which contain distinct joint systems. The Bakacak landslide at Zekidag˘ occurs mainly in the altered material along the Asarsuyu Fault. Within these highly disturbed tectonic zones, failure occurs relatively easily when sudden earthquake movements take place (Dalgıcß, 1998b).
Fig. 5. The variation of Atterberg limits and water content of cohesive soil with depth.
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
191
4. Seismicity of the Asarsuyu Valley
Table 2 Consistency limits (Atterberg limits) of clay layer
The magnitude of the earthquakes causing the fault movements in the Asarsuyu Valley is estimated to be around 7.0 (Dalgıcß, 1994). It can therefore be assumed that earthquakes in the region have contributed to several of the landslides when the horizontal and vertical earth accelerations have affected slopes in a critical condition (Dalgıcß, 1998b). Likewise, during the 12 November 1999 Du¨zce earthquake of M = 7.2, a landslide occurred in the Asarsuyu Valley triggered by the earthquake. In addition, Ambraseys (1988) records that the 1957 Abant earthquake triggered several landslides in the region. These events support the blocking of the Asarsuyu Valley by the Bakacak landslide and the formation of the lake deposits.
Consistency limits
Min.
Water content (%) Liquid limit [LL] (%) Plastic limit [PL] (%) Plasticity index [PI] (%) Liquidity index [LI] (%) Consistency index [CI] (%)
27.07 89.42 45.71
12.4
13.8
49.6
94.0
74.77
13.79
14.32
24.0
36.8
30.75
4.61
4.78
24.9
67.2
44.72
11.48
11.91
5. Geotechnical properties of the lake deposits Clayey layers in the lake deposits were evaluated with the use of the data from 16 geotechnical investigation bore holes drilled by Astaldi, the main contractor of the Asarsuyu passage in the Gu¨mu¨sWova –Gerede Motorway. In addition, in accordance with ASTM (1985) standards, natural unit weight (cn), grain unit weight ( Gs), grain size distribution, and Atterberg limits of 16 undisturbed samples from the boreholes were determined. The consolidation properties were conducted by odometer tests and the shear strengths of the soil have also been measured.
6. Index properties The grain size distribution of the disturbed and undisturbed clayey samples from the lake deposits of the Asarsuyu Valley was determined by sieving from number 4, 10, 40, and 200 sieves and, silt and clay size remaining from the 200 sieves was determined by the hydrometer test (Table 1). On the basis of results, 60 –80% of the lake sediments are made of clay and silt. The remaining part (11% and 39%) is composed of silt, and a little part is sand. However, there are also sites that have not been sampled during drillings. Therefore, it can be stated that the sand ratio
Max. Average Standard Standard value (x) error (SE) deviation (SD)
0.12
0.92
0.34
0.25
0.27
0.07
0.87
0.64
0.25
0.27
of these deposits is high. Moreover, standard error and standard deviation values given in Table 1 are also high. This may indicate that grains are of different sizes. The specific gravity of the clays does not vary to any great extent, ranging between 2.60 and 2.67, with a mean value of 2.62. Test results from the examination of the clay layers are that an average value of the water content, the degree of saturation and unit weight are 45%, 79%, 1800 kg/m3, respectively. The average values of liquid limit, plastic limit and plasticity index for these clays are 75%, 31%, and 45%, respectively (Table 2). The liquid limit varies from 49.6% to 94%, the plastic limit from 24% to 37% and the plasticity index from 25% to 67%. The liquidity index is always very low indicating that the value of natural moisture content is never above that of the plastic limit. This is typical of highly overconsolidated clays (Fig. 5). The range of liquidity index varies from 0.12 to 0.92 with an average value of 0.34. The consistency indices suggest that this is stiff clay (Table 2).
7. Undrained shear strength Triaxial tests with unconsolidated –undrained shear parameters were determined on 16 clay and silt samples. The range of undrained cohesion varied from 40 to 219 kN/m2 with an average value of 83 kN/m2. Variation of test results with respect to depth is shown
192
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Fig. 6. Undrained cohesion for cohesive soil.
in Fig. 6. Results show that unconsolidated – undrained parameters of clay and silty clays increase from top to bottom. Changing of values may be explained by overconsolidation of upper layers. However, strength values obtained from SPT tests indicate no significant difference between surface and bottom sections (Fig. 7).
8. Consolidation properties Consolidation properties of the clays were performed on 16 undisturbed samples, taken from the surface to a depth of 11.25 m, using the odometer device based on ASTM (1985) standards. Using the
data obtained from tests, graphics of pressure (logarithmic) – void ratio and settlement-time relation of clays were drawn. Consolidation coefficient (Cv), volumetric compression coefficient (Mv) and compression indices (Cc and Cr) were determined from these graphics. Also using the relation between preconsolidation pressure ( Pc) and initial effective vertical stress ( Po), overconsolidation ratio (OCR) was computed. Preconsolidation pressure and compression indices (Cc = compression index and Cr = recompression index) were graphically determined (Table 3). The preconsolidation pressure has been determined from the laboratory curves by the procedure proposed by Casagrande. The preconsolidation char-
Fig. 7. Variation of N SPT values with depth.
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
193
Table 3 Consolidation parameters Sample
Chainage (km)
Depth (m)
Po
Pc
OCR
eo
Cr
Cc
M345-UD1 M345-UD5 M346-UD2 M346-UD4 M346-UD9 M346-UD11 M304-UD2 M304-UD3 M304-UD4 M347-UD2 M347-UD4 M347-UD9 M347-UD11 M305-UD1 M344-UD2 M306-UD2
9 + 650 to 9 + 800
2.4 4.2 2.5 3.7 7.7 9.3 5.0 9.0 10.75 3.25 4.75 8.75 10.25 9.25 9.25 11.25
19.2 33.6 20.0 29.6 61.6 74.4 40.0 72.0 86.0 26.0 38.0 70.0 82.0 74.0 74.0 90.0
240 250 280 300 300 320 250 280 400 200 250 300 320 280 350 280
12.5 7.44 14 10.13 4.87 4.30 6.25 3.88 4.65 7.69 6.57 4.28 3.90 3.78 4.72 3.11
1.326 1.116 1.148 1.275 1.043 0.955 1.245 1.009 0.900 1.234 1.044 1.147 1.039 0.838 1.385 1.155
0.087 0.072 0.044 0.080 0.090 0.055 0.064 0.049 0.062 0.029 0.038 0.082 0.072 0.072 0.12 0.060
0.62 0.52 0.34 0.45 0.66 0.35 0.61 0.24 0.46 0.36 0.27 0.52 0.48 0.42 0.54 0.31
9 + 800 to 9 + 860
9 + 860 to 9 + 900
9 + 900 to 10 + 000
10 + 00 to 10 + 060
Po = effective pressure (kN/m2); Pc = preconsolidation pressure (kN/m2); OCR = overconsolidation ratio; Cc = compression index; Cr = recompression index; eo = void ratio.
acteristics of the lake deposit profile are expressed in terms of the consolidation ratio (OCR) versus depth plot, as shown in Fig. 8. Evaluations reveal that Pc values in first meters are extremely high in comparison to Po. Overconsolidation ratio ( Pc/Po) values in silty clays drop from 14 to 3 from the surface to a depth of 12 m. These values indicate that the upper
clay and silty clays above are more overconsolidated in comparison to those underlying. The coefficients of consolidation (Cv) obtained from the laboratory tests for various sublayers are shown in Fig. 9. For a pressure interval of 1– 4 kg/ cm2, an average (Cv) value for consolidation settlement time is taken as 0.003 cm2/s. The Cv values are
Fig. 8. The variation of the overconsolidation ratio with depth.
194
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Fig. 9. The variation of consolidation coefficient (Cv) with applied pressure.
within the range of 0.0015– 0.014 cm2/s and do not show any relationship with depth.
9. Geological evolution and preconsolidation pressure In the case of the preconsolidation pressure ( Pc) being higher than the present overburden pressure, one of the following conditions may be the cause: (a) Thicker soil overburden, which has since been removed or eroded (b) Change in the groundwater level (Kenny, 1964) (c) Desiccation of soil (increasing of Pc with respect to Po). Besides the general conditions mentioned above, cementing, a change in the ion concentration, oxidation (Bjerrum, 1972), depositional conditions, and mineralogic composition are the other important factors increasing the preconsolidation pressure ( Pc). At normal conditions, consolidated clay and silty clay are expected in recent fluvial deposits. However,
considering the geologic model of the valley (Figs. 2, 3 and 4), the Bakacak landslide closed this part of valley and gave rise to the formation of clay and silty clay layers. By the erosion of landslide materials in front of the lake deposits, the part of the geologic load was unloaded from the upper part of the clayey deposits and, as a consequence, the clay layers were overconsolidated. After the erosional phenomena, desiccation processes also took place to contribute to the overconsolidation of clay layers. Preconsolidation pressure ( Pc) values (200 – 400 kN/m2) also indicate that erosion is about 15 m. This is also supported by uneroded remnants of the landslide in the valley.
10. Settlement calculation Accurate determination of Pc values of the clay is vital for the accurate settlement analyses and for determining the geological evolution. In the situations where (1) Pc = Po (normal consolidation) (2) If P + Po < Pc (overconsolidated clay) (3) If P + Po>Pc (overconsolidated clay).
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
As stated above, clay and silty clays in the study area are overconsolidated. The sum of embankment load (DP) and the overburden load ( Po) exceed preconsolidation pressure ( Pc), and therefore, the third case is valid. A small part of the calculated and measured settlements support the second case. Consolidation settlements of clay and silty layers under the embankment were computed at 19 points. Primary settlements were taken as the total of each sublayer’s settlement. The following equation was used in this approach (Das, 1983).
S¼H
Cr Pc Cc DP þ Po log þH log 1 þ eo Po 1 þ eo Pc
ð1Þ
where S = total settlement; H = thickness of clay layer; Cr = recompression index; eo = initial void ratio; DP = applied load; Pc = preconsolidation pressure; Cc = compression index of normal consolidated clay. Summary of these results are given in Table 4. The consolidation settlement time (t90%) is calculated in accordance with the Terzahgi consolidation theory.
195
The time factor (Tv) was established for consolidation degree U = 90%. Where tð90%Þ ¼ Tv ð1=2h2 ÞCv 11. Settlement monitoring Preloading embankment was constructed at heights between 12 and 19 m in order to provide preconsolidation for solid clay and silty clay encountered under motorway embankment and concrete structures. The height of preloading fill was projected on the basis of calculated total settlement, settlement amount tolerable for concrete structure (15 cm) and 90% settlement time. In situ settlement measurements were implemented with 15 settlement plates and 5 magnetic settlement columns. Settlement plates measure total settlement from the soil surface while magnetic settlement columns measure settlements on different levels. Settlement was taken for an observation period lasting 380 days. Measurements taken with settlement columns and settlement plates in this period are consistent. Minimum and maximum values obtained for the
Table 4 Predicted and monitored settlement rate Chainage (km)
Thickness of comp. layer (m)
Height of preloading embankment (m) as constructed
Predicted settlement (a) (cm)
Monitored settlement (b) (cm)
a/b = rate (%)
9 + 650 9 + 675 9 + 700 9 + 730 9 + 760 9 + 790 9 + 820 9 + 850 9 + 880 9 + 920 9 + 935 9 + 965 9 + 990 10 + 015 10 + 045 10 + 075 10 + 095 10 + 130 10 + 152
3 4 5 5.5 5 5 8 10 8 11.5 11.5 8 8 8 5 4.5 5 4.5 5
14 16 16 14 13 13 13 13 13 12 13 10 13 16.8 18.5 19 19 19 19
21 29 35 32 26 26 24 31 24 31 37 16 25 40 28 28 28 20 21
21 20 28 26 28 24 14 32 22 25 33 17 21 36 24 25 26 26 18
100 70 80 82 100 92 60 100 91 80 89 100 80 90 85 89 93 100 85
196
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Fig. 10. Predicted and monitored settlement along preloading embankment.
Fig. 11. Typical theoretic and monitored settlements curve in the embankment.
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
whole preloading embankment area are 140 and 360 mm, respectively, averaging as 245 mm. The settlement predictions based on statistical average of laboratory-determined compressibility parameters are compared with settlements measured in an instrumented preloading embankment. The predicted magnitudes of the settlements are found reliable and comparable to field measurements (Fig. 10). The predicted versus measured settlement ratio is 88%. According to the results of the measurements, most of the consolidation settlements were rapidly progressed during the preloading, but its rate was reduced gradually in time and decelerated after a certain time period. These data indicate that the measured settlement rate is higher than expected (Fig. 11). This difference is likely the results of the viscoelastic behaviour of soil due to the relatively high loading. As can be seen in Fig. 11, the time– settlement graphic is similar to the viscoelastic graphic. In addition, the average ratio of field to laboratory Cv is found in the order of 0.3 to 0.5. This difference between the predicted and measured parameters can be attributed to a more efficient than anticipated subsurface drainage system substantiated by the presence of thin permeable sublayers. The inclusion of permeable sublayers into the low permeable clay layers of lake deposit serves to accelerate the consolidation of the finer grained strata and to significantly reduce the drainage paths. Also, routine sampling and testing techniques are not adequate for heterogeneous and complex subsoil profiles to represent the in situ conditions (Dhowian et al., 1987). The pore water pressures measured at the piezometer elevations are shown in Fig. 12. The pore water pressures increase rapidly and reached the maximum
197
values approximately at the end of the preloading of 10 m high. Maximum pore water pressure measured at the end of the 10-m high preloading is approximately 1.6 kg/cm2. The expected behaviour of the peak values of pore water pressures is comparable with the increase in the total vertical stress due to the embankment loading. After a certain period, pore water pressure drops down to 0.2 kg/cm2 and it does not increase during the final stage of loading. This behaviour is expected due to the behaviour of the viscoelastic strain and the developed macro fabric, pockets of fine sand. These permeable sublayers allow spontaneous dissipation of the pore pressures during the construction period (Shibata and Sekiguchi, 1984; Jamiolkowqki and Lancellotta, 1984).
12. Evaluation In order to predict consolidation settlement of clayey soils accurately, suitable modelling of soil behaviour, realistic determination of soil parameters and using of appropriate numeric methods are required (Ladd et al., 1977). Meanwhile, development of a soil model that is valid at all effective stress levels and drainage conditions is not available yet. Uniaxial consolidation theory, methods based on probability theory and numeric methods yield accurate results in some application cases (Balasubramaniam and Brenner, 1981). Creep type secondary settlement on very soft clays and peat soils makes consolidation modelling harder. In order to determine primary consolidation settlement realistically, it is necessary to know the time-dependent change of void ratio at a certain effective stress. However, develop-
Fig. 12. The pore pressures recorded at piezometer levels.
198
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
ment of a time-dependent consolidation process requires a material method relevant to water flux in soil voids (such as Darcy Law), an equation for permeability –void ratio and a stress-unit deformation equation for the grain phase of the soil. Some assumptions in the Terzaghi uniaxial consolidation theory are not realistic and are inconsistent with application cases. At unstable conditions, depending on the effective stress level, soil characteristics may change in a nonlinear shape (Barden, 1969; Ladd et al., 1977). As a result, considering the settlement – time relation determined from odometer tests conducted in the laboratory, if a geotechnic design is implemented for important projects, different results might be obtained in practice (Sag˘lamer and Yılmaz, 1998). However, close agreement between observed and predicted behaviour was obtained when field parameters were used in the analyses. Although substantially overestimating the time required for completion of primary settlements, the finite element predictions of the magnitude and the rate of settlements are considered satisfactory (Al-Shamrani and Dhowian, 1996). Investigations reveal that consolidation in the study area was rapidly developed when compared to that in the laboratory. Fissured structure in the clay layer compressible during the loading, which could not be determined sensitively during the field investigations, silt and sand levels together with the size of consolidation device, that is unsuitable to reflect the field conditions, accelerated the consolidation process. Because drainage lengths are not sufficiently known, consolidation coefficients (Cv) indirectly calculated from the field data are not meaningful. Instead, t90 or t50 values should be respected.
prevailed in a later stage as also shown by a 15-m eroded part in the valley. Consolidation tests conducted on these deposits reveal that settlement values tolerable for concrete structures may be exceeded. In this respect, the soil improvement by preloading is found to be necessary. Preloading embankment of height ranging from 12 to 19 m has been constructed on clay and silty clay deposits and in situ settlement measurements were checked with the use of 5 magnetic rings and 15 settlement plates. The measured settlements compare well with those predicted by using consolidation parameters averaged from laboratory data. Investigations indicate that there is an 88% consistency between the settlement magnitude calculated on the basis of field and laboratory data. Settlement rates determined in the field are higher than those obtained in the laboratory. This could be attributed to the viscoelastic strain and the insufficient size of tested samples determined in the laboratory, which could not adequately represent micro and macro texture of the clays and clay laminations and associated sandy layers that could not be sufficiently determined with drilling techniques. In order to examine this negative effect, consolidation tests should be conducted on bigger samples or Cv should be evaluated in the field.
Acknowledgements The authors acknowledge the help of Astaldi, the main contractor of the Gu¨mu¨sWova – Gerede Motorway and Yu¨ksel-Rendel, the Control Engineer on behalf of the client, for their help in the preparation of this manuscript.
13. Conclusions Overconsolidated clay layers were encountered within recent lake deposits in the Asarsuyu Valley pass at the Bolu mountain part of the Anatolian Motorway. Field and laboratory investigations indicate that overconsolidated clay was deposited in a lake environment, which is formed as a result of blocking of the valley by the Bakacak landslide. It was determined that these clay deposits gained an overconsolidated character with erosion, and drying processes
References Al-Shamrani, M., Dhowian, W.A., 1996. Preloading for reduction of compressibility characteristics of Sabkha soil profiles. Engineering Geology 48 (1 – 2), 19 – 41. Ambraseys, N.N., 1988. Engineering seismology. Earthquake Eng. Struct. Dyn. 17, 1 – 105. Ansal, M.A., 1987. Statistical analysis for consolidation properties. Second Turkish Congress on Soil Mechanics and Foundation Engineering, Istanbul, Turkey, 15 – 28 (in Turkish).
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199 ASTM, American Society for Testing and Material, 1985. Annual Book of ASTM Standards Soil and Rock, Building Stones, Section-4, Construction, V.04.08, ASTM Publication, 972, 166 – 194. Balasubramaniam, A.S., Brenner, B.P., 1981. Consolidation and settlement of soft clay. Soft Clay Engineering. Developments in Geotechnical Engineering. Elsevier, p. 20. Barden, L., 1969. Time depend deformation of normally consolidated clays and peats. Journal of the Soil Mechanics and Foundation Engineering Division, ASCE 95, 1 – 31. Bjerrum, L., 1972. Embankments on soft ground. Proc. Speciality Conf. Perform. Earth and Earth-Supported Struct. ASCE, pp. 1 – 54. Bowles, J.E., 1979. Physical and Geotechnical Properties of Soils. McGraw-Hill, New York, 410 pp. Dalgıcß, S., 1994. Engineering geology of the passage of the Anatolian Motorway through the Bolu mountains. Unpublished PhD Thesis, Department of Geological Engineering, Istanbul University, Turkey, 213 pp., in Turkish. Dalgıcß , S., 1998a. Selecting of crushed rock quarries for the construction of Anatolian Motorway. Environmental and Engineering Geoscience IV (4), 511 – 518. Dalgıcß, S., 1998b. Slope stability problems of the weak rocks in the Asarsuyu pass of the Anatolian Motorway. Bulletin of the International Association for Engineering Geology and the Environment 57, 199 – 206. Dalgıcß , S., 2000. Comparison of geological conditions predicted from boreholes and found in situ: a case study of a tunnel in Turkey. Bulletin of the International Association for Engineering Geology and the Environment 58 (3), 115 – 123.
199
Das, B.M., 1983. Principles of Foundation Engineering, Brooks. Cool, Engineering Division, p. 134. Dhowian, A.W., Erol, A.O., Sultan, S., 1987. Settlement predictions in complex Sabkha soil profiles. Bulletin of the International Association of Engineering Geology (36), 11 – 21. ¨ nalp, A., 1996. The relationship among the compresGu¨ndu¨z, Z., O sion indices and physical properties of clays. Sixth Turkish Congress on Soil Mechanics and Foundation Engineering, I˙zmir, Turkey, pp. 82 – 88 (in Turkish). Herrero, O.R., 1980. Universal compression index equation. Journal of the Geotechnical Engineering Division, Proceedings of the American Society of Civil Engineers 106 (GT 11), 1179 – 1200. Jamiolkowqki, M., Lancellotta, R., 1984. Embankment on vertical drains—pore pressures during construction. International Conference on Case Histories in Geotechnical Engineering 1, 275 – 278. Kenny, T.C., 1964. Sea level movements and the geologic histories of the Post-Glacial soils at Boston, Nicolet, Ottowa and Oslo. Geotechnique 14, 203 – 204. Ladd, C.C., Foott, R., Ishihara, K., Schlosser, F., Poulos, H.G., 1977. Stress deformation and strength characteristics. Proceedings of the 9th International Conference on Soil Mechanics and Foundation Engineering, Tokyo 2, 421 – 494. Sag˘lamer, A., Yılmaz, E., 1998. Soil improvement by pre-loading at Samsun – Cßarsß amba airport. Seventh Turkish Congress on Soil Mechanics and Foundation Engineering, Yıldız Technical University, Istanbul, Turkey, pp. 512 – 521 (in Turkish). Shibata, T., Sekiguchi, H., 1984. Performance of trial embankment on soft clay. International Conference on Case Histories in Geotechnical Engineering 2, 599 – 608.
Settlement predictions in the Anatolian Motorway, Turkey Su¨leyman Dalgıcß a,*, Orhan Sßimsßek b a
Istanbul University, Faculty of Engineering, 34850 Avcılar, Istanbul, Turkey b IC Consulenten, Zollhouse weg No. 1 Bergheim, Salzburg, Austria Received 19 February 2001; accepted 20 May 2002
Abstract The Anatolian Motorway through the Asarsuyu Valley passes across the landslides which have been extensively disturbed by past fault movements. The Asarsuyu Valley is the most important crossing of the motorway between Istanbul and Ankara route. Along the Asarsuyu Valley, about 7 km of the roadway is still under construction. In this study, the magnitude and the rate of the settlement over consolidated clays in lacustrine deposits within the Asarsuyu Valley were compared with each other. On the basis of field observations and laboratory test results, it was determined that the lacustrine deposits were eroded up to 15 m by the river in the valley bottom. As results of unloading and desiccation process, the clay layers are overconsolidated. Settlement calculations indicate that the amount of clay layers has caused the intolerable consolidation settlement under the concrete structures and motorway embankment. In this respect, preloading embankment on clay and silty clay deposits was projected and constructed. On the basis of evaluations, estimated values of settlement are lower than those realized. However, the predicted settlement quantities are found reliable and comparable to field measurements. On the other hand, significant differences were observed between calculated and measured rate of the settlement. The high rate of the settlement, which was measured during preloading, was caused by the viscoelastic strain due to the relatively high load and sandy pockets available in the clay layers, but was not detected by the drilling and the micro/macro texture of the clay layers. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Landslide; Clay; Consolidation; Settlement; Predictions; Measurements
1. Introduction This study was carried out in the Asarsuyu Valley crossing of the Anatolian Motorway (Fig. 1). The Asarsuyu crossing is located next to the Bolu Tunnel crossing (Dalgıcß , 2000). The engineering geological problems were observed both in the Asarsuyu crossing and in the Bolu Tunnel of the motorway. One of
*
Corresponding author. Fax: +90-212-5911997. E-mail address: [email protected] (S. Dalgıcß).
these geotechnical problems in the Asarsuyu Valley of the Anatolian Motorway is the overconsolidated clay layers, which are encountered in the recent fluvial deposits. Investigations reveal that the deposition of the clay layers is associated with the Bakacak landslide blocking the valley front. In the area, other huge landslides, which were caused by the North Anatolian Fault, such as the Bakacak landslide, are available. The horizontal and vertical extensions of the clay layers in the lake deposits were investigated by a total number of 16 boreholes. The natural unit weight, grain unit weight, grain size distribution, Atterberg
0013-7952/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 3 - 7 9 5 2 ( 0 2 ) 0 0 1 5 4 - 0
186
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Fig. 1. Location map.
Limits and Consolidation tests were performed on the undisturbed samples extracted from boreholes. These tests were conducted in accordance with ASTM standards in the laboratories of Astaldi, the contractor firm of Gu¨mu¨sß ova– Gerede Motorway, and the General Directorate of State Highways. According to settlement calculations performed by the authors on the basis of results of 16 consolidation tests, preloading embankment was found to be necessary and the contractor firm implemented it. Following the preloading fill, settlement measurements were controlled with 5 magnetic settlement columns and 15 settlement plates. This study indicated that there is an 88% consistency between the settlement values determined by calculation and in situ measurement. However, some differences were observed in rate of the consolidation. The reason for this is the viscoelastic strain due to the relatively high load and the micro and macro textures of the clay that cannot be determined
easily in the laboratory conditions, and also clay layers containing some sandy pockets in the field. Most of the literatures about the consolidation properties of the clays are for the determination of the relations between compression index and index ¨ nalp, 1996; properties (Ansal, 1987; Gu¨ndu¨z and O Bowles, 1979; Herrero, 1980). In addition, estimation of the compression index, depending on the mineralogic composition of the soils and the geographical position, changes one region to another. In this study, therefore, the value for the compression index of the clays was evaluated with respect to results obtained from odometer tests.
2. Lithology of the Asarsuyu Valley The Asarsuyu Valley route passes through landslides formed in weak zones related to paleotectonic
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
thrust faults and the neotectonic North Anatolian Fault Zone (Dalgıcß, 1998a). The Yedigo¨ller formation consists of metamorphic rocks and is the oldest unit present at the Asarsuyu Valley. It is tectonically overlain by the metamorphic I˙kizoluk formation of Devonian age. Above these strata the upper Cretaceous to upper Eocene sedimentary units are encoun-
187
tered. These formations are overlain by alluvium, colluvium and lacustrine deposits (Figs. 2 and 3). Alluvium deposits are generally composed of rounded, subrounded, pebbly sand, blocky pebbly sand and blocky pebbles derived from weakly altered, intermediately soft—very compact amphibolite, metagranite and other rocks. Thickness of these deposits at
Fig. 2. Evaluation of the Asarsuyu Valley before the clay deposits.
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Fig. 3. Longitudinal cross-section of the Asarsuyu Valley.
188
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
the downstream of Asarsuyu river is about 20 m. Investigation boreholes drilled at the upstream part of the river indicate that thickness of alluvium lowers to 6 –8 m. Grain size of the deposits decreases from upstream to downstream. Among the alluvium deposits, there are lens or wedge-shaped, weakly plastic, intermediately compact, green silt and sandy silt together with clay, silt and silty clay deposits with a total thickness between 3 and 5 m. Within the alluvium deposits, there are also plastic, brownish grey coloured, overconsolidated, cohesive, fissured clay and silty clays with thickness of 2 –11 m. There are two suggestions for the occurrence of these deposits (Dalgıcß, 1994). They are: (1) alluvial deposits; (2) lacustrine deposits formed as a result of blocking of the Asarsuyu Valley by the Bakacak landslide. The first suggestion indicates that, due to geotechnical characteristics, these deposits cannot be fluvial sediments. As will be explained further in detail, these deposits are composed of overconsolidated, cohesive, fissured, plastic clay and silty clay (CH). The second suggestion is based on blocking of
189
Asarsuyu Valley by the Bakacak landslide, which formed a lake behind the valley. Findings supporting this suggestion are given below.
Cutting of landslide material in exploratory drill holes. The motion of Bakacak landslide at south has changed Asarsuyu riverbed. Remnant of the landslide material encountered at northern side of the valley. Soil characteristics of the clayey deposits. Considering the field characteristics given above, it was believed that these sediments were deposited in the lake that was formed behind the landslide dam.
3. Slope failure in Asarsuyu Valley The Bakacak landslide, which gave rise to the formation of lacustrine deposits in the study area, has a length of 4 – 5 km and a width of 1.5 km and is still active (Fig. 4). As the toe of this landslide is
Fig. 4. Landslides areas in the Asarsuyu Valley.
190
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Table 1 Sieve analysis results of clay layer Grain size (%)
Min.
Max.
Average value (x)
Standard error (SE)
Standard deviation (SD)
Gravel (%) Sand (%) Silt (%) Clay (%)
0 0.1 11 60
0 0.3 39 88
0 1.15 34.15 65.16
0 0.53 9.89 9.56
0 0.55 10.83 10.47
crossed by a section of the Anatolian Motorway where a 2.6-km-long viaduct has been built, ensuring the stability of the slide is important for the long-term integrity of the structure. The lithologies of the Ikizoluk and Yedigoller formations in the Asarsuyu Valley have been both extensively disintegrated as a result of tectonic disturbance and further weakened by weathering. As a result, the slides are related to the
fault structures as opposed to the normal discontinuity-controlled instabilities observed in strong rock masses, which contain distinct joint systems. The Bakacak landslide at Zekidag˘ occurs mainly in the altered material along the Asarsuyu Fault. Within these highly disturbed tectonic zones, failure occurs relatively easily when sudden earthquake movements take place (Dalgıcß, 1998b).
Fig. 5. The variation of Atterberg limits and water content of cohesive soil with depth.
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
191
4. Seismicity of the Asarsuyu Valley
Table 2 Consistency limits (Atterberg limits) of clay layer
The magnitude of the earthquakes causing the fault movements in the Asarsuyu Valley is estimated to be around 7.0 (Dalgıcß, 1994). It can therefore be assumed that earthquakes in the region have contributed to several of the landslides when the horizontal and vertical earth accelerations have affected slopes in a critical condition (Dalgıcß, 1998b). Likewise, during the 12 November 1999 Du¨zce earthquake of M = 7.2, a landslide occurred in the Asarsuyu Valley triggered by the earthquake. In addition, Ambraseys (1988) records that the 1957 Abant earthquake triggered several landslides in the region. These events support the blocking of the Asarsuyu Valley by the Bakacak landslide and the formation of the lake deposits.
Consistency limits
Min.
Water content (%) Liquid limit [LL] (%) Plastic limit [PL] (%) Plasticity index [PI] (%) Liquidity index [LI] (%) Consistency index [CI] (%)
27.07 89.42 45.71
12.4
13.8
49.6
94.0
74.77
13.79
14.32
24.0
36.8
30.75
4.61
4.78
24.9
67.2
44.72
11.48
11.91
5. Geotechnical properties of the lake deposits Clayey layers in the lake deposits were evaluated with the use of the data from 16 geotechnical investigation bore holes drilled by Astaldi, the main contractor of the Asarsuyu passage in the Gu¨mu¨sWova –Gerede Motorway. In addition, in accordance with ASTM (1985) standards, natural unit weight (cn), grain unit weight ( Gs), grain size distribution, and Atterberg limits of 16 undisturbed samples from the boreholes were determined. The consolidation properties were conducted by odometer tests and the shear strengths of the soil have also been measured.
6. Index properties The grain size distribution of the disturbed and undisturbed clayey samples from the lake deposits of the Asarsuyu Valley was determined by sieving from number 4, 10, 40, and 200 sieves and, silt and clay size remaining from the 200 sieves was determined by the hydrometer test (Table 1). On the basis of results, 60 –80% of the lake sediments are made of clay and silt. The remaining part (11% and 39%) is composed of silt, and a little part is sand. However, there are also sites that have not been sampled during drillings. Therefore, it can be stated that the sand ratio
Max. Average Standard Standard value (x) error (SE) deviation (SD)
0.12
0.92
0.34
0.25
0.27
0.07
0.87
0.64
0.25
0.27
of these deposits is high. Moreover, standard error and standard deviation values given in Table 1 are also high. This may indicate that grains are of different sizes. The specific gravity of the clays does not vary to any great extent, ranging between 2.60 and 2.67, with a mean value of 2.62. Test results from the examination of the clay layers are that an average value of the water content, the degree of saturation and unit weight are 45%, 79%, 1800 kg/m3, respectively. The average values of liquid limit, plastic limit and plasticity index for these clays are 75%, 31%, and 45%, respectively (Table 2). The liquid limit varies from 49.6% to 94%, the plastic limit from 24% to 37% and the plasticity index from 25% to 67%. The liquidity index is always very low indicating that the value of natural moisture content is never above that of the plastic limit. This is typical of highly overconsolidated clays (Fig. 5). The range of liquidity index varies from 0.12 to 0.92 with an average value of 0.34. The consistency indices suggest that this is stiff clay (Table 2).
7. Undrained shear strength Triaxial tests with unconsolidated –undrained shear parameters were determined on 16 clay and silt samples. The range of undrained cohesion varied from 40 to 219 kN/m2 with an average value of 83 kN/m2. Variation of test results with respect to depth is shown
192
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Fig. 6. Undrained cohesion for cohesive soil.
in Fig. 6. Results show that unconsolidated – undrained parameters of clay and silty clays increase from top to bottom. Changing of values may be explained by overconsolidation of upper layers. However, strength values obtained from SPT tests indicate no significant difference between surface and bottom sections (Fig. 7).
8. Consolidation properties Consolidation properties of the clays were performed on 16 undisturbed samples, taken from the surface to a depth of 11.25 m, using the odometer device based on ASTM (1985) standards. Using the
data obtained from tests, graphics of pressure (logarithmic) – void ratio and settlement-time relation of clays were drawn. Consolidation coefficient (Cv), volumetric compression coefficient (Mv) and compression indices (Cc and Cr) were determined from these graphics. Also using the relation between preconsolidation pressure ( Pc) and initial effective vertical stress ( Po), overconsolidation ratio (OCR) was computed. Preconsolidation pressure and compression indices (Cc = compression index and Cr = recompression index) were graphically determined (Table 3). The preconsolidation pressure has been determined from the laboratory curves by the procedure proposed by Casagrande. The preconsolidation char-
Fig. 7. Variation of N SPT values with depth.
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
193
Table 3 Consolidation parameters Sample
Chainage (km)
Depth (m)
Po
Pc
OCR
eo
Cr
Cc
M345-UD1 M345-UD5 M346-UD2 M346-UD4 M346-UD9 M346-UD11 M304-UD2 M304-UD3 M304-UD4 M347-UD2 M347-UD4 M347-UD9 M347-UD11 M305-UD1 M344-UD2 M306-UD2
9 + 650 to 9 + 800
2.4 4.2 2.5 3.7 7.7 9.3 5.0 9.0 10.75 3.25 4.75 8.75 10.25 9.25 9.25 11.25
19.2 33.6 20.0 29.6 61.6 74.4 40.0 72.0 86.0 26.0 38.0 70.0 82.0 74.0 74.0 90.0
240 250 280 300 300 320 250 280 400 200 250 300 320 280 350 280
12.5 7.44 14 10.13 4.87 4.30 6.25 3.88 4.65 7.69 6.57 4.28 3.90 3.78 4.72 3.11
1.326 1.116 1.148 1.275 1.043 0.955 1.245 1.009 0.900 1.234 1.044 1.147 1.039 0.838 1.385 1.155
0.087 0.072 0.044 0.080 0.090 0.055 0.064 0.049 0.062 0.029 0.038 0.082 0.072 0.072 0.12 0.060
0.62 0.52 0.34 0.45 0.66 0.35 0.61 0.24 0.46 0.36 0.27 0.52 0.48 0.42 0.54 0.31
9 + 800 to 9 + 860
9 + 860 to 9 + 900
9 + 900 to 10 + 000
10 + 00 to 10 + 060
Po = effective pressure (kN/m2); Pc = preconsolidation pressure (kN/m2); OCR = overconsolidation ratio; Cc = compression index; Cr = recompression index; eo = void ratio.
acteristics of the lake deposit profile are expressed in terms of the consolidation ratio (OCR) versus depth plot, as shown in Fig. 8. Evaluations reveal that Pc values in first meters are extremely high in comparison to Po. Overconsolidation ratio ( Pc/Po) values in silty clays drop from 14 to 3 from the surface to a depth of 12 m. These values indicate that the upper
clay and silty clays above are more overconsolidated in comparison to those underlying. The coefficients of consolidation (Cv) obtained from the laboratory tests for various sublayers are shown in Fig. 9. For a pressure interval of 1– 4 kg/ cm2, an average (Cv) value for consolidation settlement time is taken as 0.003 cm2/s. The Cv values are
Fig. 8. The variation of the overconsolidation ratio with depth.
194
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Fig. 9. The variation of consolidation coefficient (Cv) with applied pressure.
within the range of 0.0015– 0.014 cm2/s and do not show any relationship with depth.
9. Geological evolution and preconsolidation pressure In the case of the preconsolidation pressure ( Pc) being higher than the present overburden pressure, one of the following conditions may be the cause: (a) Thicker soil overburden, which has since been removed or eroded (b) Change in the groundwater level (Kenny, 1964) (c) Desiccation of soil (increasing of Pc with respect to Po). Besides the general conditions mentioned above, cementing, a change in the ion concentration, oxidation (Bjerrum, 1972), depositional conditions, and mineralogic composition are the other important factors increasing the preconsolidation pressure ( Pc). At normal conditions, consolidated clay and silty clay are expected in recent fluvial deposits. However,
considering the geologic model of the valley (Figs. 2, 3 and 4), the Bakacak landslide closed this part of valley and gave rise to the formation of clay and silty clay layers. By the erosion of landslide materials in front of the lake deposits, the part of the geologic load was unloaded from the upper part of the clayey deposits and, as a consequence, the clay layers were overconsolidated. After the erosional phenomena, desiccation processes also took place to contribute to the overconsolidation of clay layers. Preconsolidation pressure ( Pc) values (200 – 400 kN/m2) also indicate that erosion is about 15 m. This is also supported by uneroded remnants of the landslide in the valley.
10. Settlement calculation Accurate determination of Pc values of the clay is vital for the accurate settlement analyses and for determining the geological evolution. In the situations where (1) Pc = Po (normal consolidation) (2) If P + Po < Pc (overconsolidated clay) (3) If P + Po>Pc (overconsolidated clay).
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
As stated above, clay and silty clays in the study area are overconsolidated. The sum of embankment load (DP) and the overburden load ( Po) exceed preconsolidation pressure ( Pc), and therefore, the third case is valid. A small part of the calculated and measured settlements support the second case. Consolidation settlements of clay and silty layers under the embankment were computed at 19 points. Primary settlements were taken as the total of each sublayer’s settlement. The following equation was used in this approach (Das, 1983).
S¼H
Cr Pc Cc DP þ Po log þH log 1 þ eo Po 1 þ eo Pc
ð1Þ
where S = total settlement; H = thickness of clay layer; Cr = recompression index; eo = initial void ratio; DP = applied load; Pc = preconsolidation pressure; Cc = compression index of normal consolidated clay. Summary of these results are given in Table 4. The consolidation settlement time (t90%) is calculated in accordance with the Terzahgi consolidation theory.
195
The time factor (Tv) was established for consolidation degree U = 90%. Where tð90%Þ ¼ Tv ð1=2h2 ÞCv 11. Settlement monitoring Preloading embankment was constructed at heights between 12 and 19 m in order to provide preconsolidation for solid clay and silty clay encountered under motorway embankment and concrete structures. The height of preloading fill was projected on the basis of calculated total settlement, settlement amount tolerable for concrete structure (15 cm) and 90% settlement time. In situ settlement measurements were implemented with 15 settlement plates and 5 magnetic settlement columns. Settlement plates measure total settlement from the soil surface while magnetic settlement columns measure settlements on different levels. Settlement was taken for an observation period lasting 380 days. Measurements taken with settlement columns and settlement plates in this period are consistent. Minimum and maximum values obtained for the
Table 4 Predicted and monitored settlement rate Chainage (km)
Thickness of comp. layer (m)
Height of preloading embankment (m) as constructed
Predicted settlement (a) (cm)
Monitored settlement (b) (cm)
a/b = rate (%)
9 + 650 9 + 675 9 + 700 9 + 730 9 + 760 9 + 790 9 + 820 9 + 850 9 + 880 9 + 920 9 + 935 9 + 965 9 + 990 10 + 015 10 + 045 10 + 075 10 + 095 10 + 130 10 + 152
3 4 5 5.5 5 5 8 10 8 11.5 11.5 8 8 8 5 4.5 5 4.5 5
14 16 16 14 13 13 13 13 13 12 13 10 13 16.8 18.5 19 19 19 19
21 29 35 32 26 26 24 31 24 31 37 16 25 40 28 28 28 20 21
21 20 28 26 28 24 14 32 22 25 33 17 21 36 24 25 26 26 18
100 70 80 82 100 92 60 100 91 80 89 100 80 90 85 89 93 100 85
196
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
Fig. 10. Predicted and monitored settlement along preloading embankment.
Fig. 11. Typical theoretic and monitored settlements curve in the embankment.
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
whole preloading embankment area are 140 and 360 mm, respectively, averaging as 245 mm. The settlement predictions based on statistical average of laboratory-determined compressibility parameters are compared with settlements measured in an instrumented preloading embankment. The predicted magnitudes of the settlements are found reliable and comparable to field measurements (Fig. 10). The predicted versus measured settlement ratio is 88%. According to the results of the measurements, most of the consolidation settlements were rapidly progressed during the preloading, but its rate was reduced gradually in time and decelerated after a certain time period. These data indicate that the measured settlement rate is higher than expected (Fig. 11). This difference is likely the results of the viscoelastic behaviour of soil due to the relatively high loading. As can be seen in Fig. 11, the time– settlement graphic is similar to the viscoelastic graphic. In addition, the average ratio of field to laboratory Cv is found in the order of 0.3 to 0.5. This difference between the predicted and measured parameters can be attributed to a more efficient than anticipated subsurface drainage system substantiated by the presence of thin permeable sublayers. The inclusion of permeable sublayers into the low permeable clay layers of lake deposit serves to accelerate the consolidation of the finer grained strata and to significantly reduce the drainage paths. Also, routine sampling and testing techniques are not adequate for heterogeneous and complex subsoil profiles to represent the in situ conditions (Dhowian et al., 1987). The pore water pressures measured at the piezometer elevations are shown in Fig. 12. The pore water pressures increase rapidly and reached the maximum
197
values approximately at the end of the preloading of 10 m high. Maximum pore water pressure measured at the end of the 10-m high preloading is approximately 1.6 kg/cm2. The expected behaviour of the peak values of pore water pressures is comparable with the increase in the total vertical stress due to the embankment loading. After a certain period, pore water pressure drops down to 0.2 kg/cm2 and it does not increase during the final stage of loading. This behaviour is expected due to the behaviour of the viscoelastic strain and the developed macro fabric, pockets of fine sand. These permeable sublayers allow spontaneous dissipation of the pore pressures during the construction period (Shibata and Sekiguchi, 1984; Jamiolkowqki and Lancellotta, 1984).
12. Evaluation In order to predict consolidation settlement of clayey soils accurately, suitable modelling of soil behaviour, realistic determination of soil parameters and using of appropriate numeric methods are required (Ladd et al., 1977). Meanwhile, development of a soil model that is valid at all effective stress levels and drainage conditions is not available yet. Uniaxial consolidation theory, methods based on probability theory and numeric methods yield accurate results in some application cases (Balasubramaniam and Brenner, 1981). Creep type secondary settlement on very soft clays and peat soils makes consolidation modelling harder. In order to determine primary consolidation settlement realistically, it is necessary to know the time-dependent change of void ratio at a certain effective stress. However, develop-
Fig. 12. The pore pressures recorded at piezometer levels.
198
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199
ment of a time-dependent consolidation process requires a material method relevant to water flux in soil voids (such as Darcy Law), an equation for permeability –void ratio and a stress-unit deformation equation for the grain phase of the soil. Some assumptions in the Terzaghi uniaxial consolidation theory are not realistic and are inconsistent with application cases. At unstable conditions, depending on the effective stress level, soil characteristics may change in a nonlinear shape (Barden, 1969; Ladd et al., 1977). As a result, considering the settlement – time relation determined from odometer tests conducted in the laboratory, if a geotechnic design is implemented for important projects, different results might be obtained in practice (Sag˘lamer and Yılmaz, 1998). However, close agreement between observed and predicted behaviour was obtained when field parameters were used in the analyses. Although substantially overestimating the time required for completion of primary settlements, the finite element predictions of the magnitude and the rate of settlements are considered satisfactory (Al-Shamrani and Dhowian, 1996). Investigations reveal that consolidation in the study area was rapidly developed when compared to that in the laboratory. Fissured structure in the clay layer compressible during the loading, which could not be determined sensitively during the field investigations, silt and sand levels together with the size of consolidation device, that is unsuitable to reflect the field conditions, accelerated the consolidation process. Because drainage lengths are not sufficiently known, consolidation coefficients (Cv) indirectly calculated from the field data are not meaningful. Instead, t90 or t50 values should be respected.
prevailed in a later stage as also shown by a 15-m eroded part in the valley. Consolidation tests conducted on these deposits reveal that settlement values tolerable for concrete structures may be exceeded. In this respect, the soil improvement by preloading is found to be necessary. Preloading embankment of height ranging from 12 to 19 m has been constructed on clay and silty clay deposits and in situ settlement measurements were checked with the use of 5 magnetic rings and 15 settlement plates. The measured settlements compare well with those predicted by using consolidation parameters averaged from laboratory data. Investigations indicate that there is an 88% consistency between the settlement magnitude calculated on the basis of field and laboratory data. Settlement rates determined in the field are higher than those obtained in the laboratory. This could be attributed to the viscoelastic strain and the insufficient size of tested samples determined in the laboratory, which could not adequately represent micro and macro texture of the clays and clay laminations and associated sandy layers that could not be sufficiently determined with drilling techniques. In order to examine this negative effect, consolidation tests should be conducted on bigger samples or Cv should be evaluated in the field.
Acknowledgements The authors acknowledge the help of Astaldi, the main contractor of the Gu¨mu¨sWova – Gerede Motorway and Yu¨ksel-Rendel, the Control Engineer on behalf of the client, for their help in the preparation of this manuscript.
13. Conclusions Overconsolidated clay layers were encountered within recent lake deposits in the Asarsuyu Valley pass at the Bolu mountain part of the Anatolian Motorway. Field and laboratory investigations indicate that overconsolidated clay was deposited in a lake environment, which is formed as a result of blocking of the valley by the Bakacak landslide. It was determined that these clay deposits gained an overconsolidated character with erosion, and drying processes
References Al-Shamrani, M., Dhowian, W.A., 1996. Preloading for reduction of compressibility characteristics of Sabkha soil profiles. Engineering Geology 48 (1 – 2), 19 – 41. Ambraseys, N.N., 1988. Engineering seismology. Earthquake Eng. Struct. Dyn. 17, 1 – 105. Ansal, M.A., 1987. Statistical analysis for consolidation properties. Second Turkish Congress on Soil Mechanics and Foundation Engineering, Istanbul, Turkey, 15 – 28 (in Turkish).
S. Dalgıcß, O. Sßimsßek / Engineering Geology 67 (2002) 185–199 ASTM, American Society for Testing and Material, 1985. Annual Book of ASTM Standards Soil and Rock, Building Stones, Section-4, Construction, V.04.08, ASTM Publication, 972, 166 – 194. Balasubramaniam, A.S., Brenner, B.P., 1981. Consolidation and settlement of soft clay. Soft Clay Engineering. Developments in Geotechnical Engineering. Elsevier, p. 20. Barden, L., 1969. Time depend deformation of normally consolidated clays and peats. Journal of the Soil Mechanics and Foundation Engineering Division, ASCE 95, 1 – 31. Bjerrum, L., 1972. Embankments on soft ground. Proc. Speciality Conf. Perform. Earth and Earth-Supported Struct. ASCE, pp. 1 – 54. Bowles, J.E., 1979. Physical and Geotechnical Properties of Soils. McGraw-Hill, New York, 410 pp. Dalgıcß, S., 1994. Engineering geology of the passage of the Anatolian Motorway through the Bolu mountains. Unpublished PhD Thesis, Department of Geological Engineering, Istanbul University, Turkey, 213 pp., in Turkish. Dalgıcß , S., 1998a. Selecting of crushed rock quarries for the construction of Anatolian Motorway. Environmental and Engineering Geoscience IV (4), 511 – 518. Dalgıcß, S., 1998b. Slope stability problems of the weak rocks in the Asarsuyu pass of the Anatolian Motorway. Bulletin of the International Association for Engineering Geology and the Environment 57, 199 – 206. Dalgıcß , S., 2000. Comparison of geological conditions predicted from boreholes and found in situ: a case study of a tunnel in Turkey. Bulletin of the International Association for Engineering Geology and the Environment 58 (3), 115 – 123.
199
Das, B.M., 1983. Principles of Foundation Engineering, Brooks. Cool, Engineering Division, p. 134. Dhowian, A.W., Erol, A.O., Sultan, S., 1987. Settlement predictions in complex Sabkha soil profiles. Bulletin of the International Association of Engineering Geology (36), 11 – 21. ¨ nalp, A., 1996. The relationship among the compresGu¨ndu¨z, Z., O sion indices and physical properties of clays. Sixth Turkish Congress on Soil Mechanics and Foundation Engineering, I˙zmir, Turkey, pp. 82 – 88 (in Turkish). Herrero, O.R., 1980. Universal compression index equation. Journal of the Geotechnical Engineering Division, Proceedings of the American Society of Civil Engineers 106 (GT 11), 1179 – 1200. Jamiolkowqki, M., Lancellotta, R., 1984. Embankment on vertical drains—pore pressures during construction. International Conference on Case Histories in Geotechnical Engineering 1, 275 – 278. Kenny, T.C., 1964. Sea level movements and the geologic histories of the Post-Glacial soils at Boston, Nicolet, Ottowa and Oslo. Geotechnique 14, 203 – 204. Ladd, C.C., Foott, R., Ishihara, K., Schlosser, F., Poulos, H.G., 1977. Stress deformation and strength characteristics. Proceedings of the 9th International Conference on Soil Mechanics and Foundation Engineering, Tokyo 2, 421 – 494. Sag˘lamer, A., Yılmaz, E., 1998. Soil improvement by pre-loading at Samsun – Cßarsß amba airport. Seventh Turkish Congress on Soil Mechanics and Foundation Engineering, Yıldız Technical University, Istanbul, Turkey, pp. 512 – 521 (in Turkish). Shibata, T., Sekiguchi, H., 1984. Performance of trial embankment on soft clay. International Conference on Case Histories in Geotechnical Engineering 2, 599 – 608.