Proceedings of Indian Geotechnical Conference December 15-17,2011, Kochi (Paper No. S-191)
UNDRAINED SHEAR STRENGTH GAIN WITH CONSOLIDATION AT SOFT AND SENSITIVE SOIL SITES Debasis Roy, Associate Professor, Dept. of Civil Engrg., IIT Kharagpur, Email:
[email protected] Raghvendra Singh, Research Student, Dept. of Civil Engrg., IIT Kharagpur Email:
[email protected] ABSTRACT: Staged construction and ground improvements are two strategies adopted in designing high embankments at sites underlain by soft and sensitive foundation soils. Usually the gain in shear strengths are estimated from poorly documented local experience database and an elaborate scheme of construction monitoring is implemented to ensure the pace of construction does not lead to remolding and progressive failure. A procedure has been developed in this study for estimation of undrained shear strength as a function of embankment height and rate of construction. The procedure has been calibrated using two embankment failure case histories in eastern India reported by Roy and Singh [1] and the monitoring data obtained during the reconstruction activities of one of these embankments. The proposed procedure was then utilized in predicting the embankment response at two soft soil sites (Chung et al. [2]). Use of the undrained shear strengths in limit equilibrium slope stability assessments was found to provide results that were in qualitative agreement with the observed embankment behavior at the two sites.
INTRODUCTION In staged construction embankments on soft soil sites the rate of filling is governed by the increase in soil strength as a result of consolidation so as to preclude instability at any stage of construction. Sometimes vertical drains are used to accelerate the consolidation process. The undrained shear strength of foundation soils often increased substantially with consolidation due to staged construction and ongoing consolidation. If the shear strength gain is not appropriately accounted for in the design, the design is unlikely to be economical or even feasible. However, a simple frame work for assessing the gain in undrained shear strength due to staged construction and ongoing consolidation is rare. The objective of this study is to develop a simple method that accounts for staged construction and the degree of consolidation to estimate the strength gain due to staged increase of embankment height or rate of construction for assessing the stability of published case histories reporting performance of embankments constructed over soft fine grained foundation soils. The results of the stability analyses have been compared against observed embankment performance. LAYOUT OF TEXT To estimate the fictitious settlement, the rate of settlement of the embankments was obtained from the monitoring data collected from the reconstruction k18 and k26 sites (Roy and Singh [1]). The detailed soil investigation including the standard penetration tests (SPT), vane shear tests (VST), undrained unconsolidated triaxial tests (UU), one dimensional consolidation tests and monitoring data to estimate the change of pore water pressure after failure of these k18 and k26 sites were carried out to support estimation of fictitious settlement and for the validation of performance of the embankments.
ESTIMATION OF SETTLEMENT The total vertical stress at the center of layer prior to embankment construction, and stress increment due to embankment construction, are calculated at the centre of the soil layers. The embankment is simulated by a surcharge applied over an area equal to the footprint of the embankment. Newmark’s chart was used to estimate. The coefficient of consolidation, Cv and the compression index, Cc are estimated from incrementally loaded one dimensional consolidation tests conducted in the laboratory and/or borehole permeability testing. The initial void ratio, e0 is estimated from insitu moisture content information obtained from field investigation. The total settlement of the soil layer for a particular overburden pressure was calculated using: '
Sult
Cc V 'V u H u log v ' v 1 e0 Vv
(1)
where H is the thickness of the consolidating layer. The settlement, St, at time, t, is calculated using St=Sult*U, where U is the degree of consolidation. Estimation of Incremental Overburden Pressure The fictitious surcharge is obtained by equating the ultimate settlement due to the fictitious surcharge to the estimated settlement at a given time after the beginning of embankment construction due to the actual earthwork fill placement. From the stress increment due to the fictitious surcharge at the center of the consolidating layer was obtained using the Newmark’s chart. Estimation of OCR If the overconsolidation ratio estimated in this manner exceeds the overconsolidation ratio obtained in the laboratory from incrementally loaded one dimensional consolidation testing of undisturbed samples, then the overconsolidation ratio representative of the consolidating layer is assumed to
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Debasis Roy & Raghvendra Singh be equal to the estimated overconsolidation ratio. Otherwise,
the overconsolidation ratio obtained from laboratory test is assumed to be applicable for the consolidating layer. Estimation of Undrained Shear Strength Ratio The values obtained from field vane shear testing or laboratory UU or CU tests conducted in the laboratory on undisturbed or high quality soil samples are plotted against the corresponding overconsolidation ratios obtained from laboratory one dimensional incrementally loaded consolidation test on undisturbed or high quality soil samples after correcting the laboratory curve for sampling disturbance according to Casagrande [3] procedure. Using the relationship and the overconsolidation ratio for a given stage of embankment construction obtained as outlined in the preceding paragraph, the undrained shear strength ratio for that stage of embankment construction is estimated. COMPARISON WITH SHANSEP MODEL
The SHANSEP framework is presented in Figure 1. The estimates of undrained strength ratio, from case studies are plotted against the values of the over consolidation ratio, OCR obtained from case studies shown in Figure 2. Also shown on Figure 2 are the data presented by Ladd et al. [4] from six soft soil sites in support of the SHANSEP framework.
VALIDATION The proposed procedure was utilized in predicting the embankment response at two soft soil sites (Chung et al. [2]). The detailed description of these case history and the analyses is given in following subsections. West Breakwater Site The west breakwater in the new Busan port is founded on thick normally consolidated clay layers (Chung et al. [2]). A normally consolidated clay deposit, 35-55 m thick, underlies the breakwater, and therefore the upper layer of the deposit was excavated and replaced with sand in order to increase the stability of breakwater. For the breakwater site, the consistency of the clay varied from very soft to firm as the depth increased and was separated from an irregular thickness of hard clay by a thin sand layer of maximum thickness 4 m. The bed rock was granite for the west breakwater site and the depth of the sea bed was approximately 9 m for the west breakwater site below the approximate lowest low water level, which was adopted as sea level datum for the design.
Fig. 3 Stability analysis West Breakwater site The total thickness of soft and firm clay layers at the failed part of the west breakwater was 36.5 m. With the help of SANSHAP model, the undrained shear strength profiles of the West breakwater site are approximated by as shown in Figure 3.
Fig. 1 Strength ratio-OCR SHANSEP Model In spite of the scatter in the data, an approximate agreement between the data from case studies and data presented by Ladd et al. [4] is apparent from the plot for all sites.
Fig. 2 Strength ratio-OCR Comparison
East Breakwater Site The east breakwater in the new Busan port is founded on thick normally consolidated clay layers (Chung et al. 2006). A normally consolidated clay deposit, 35-55 m thick, underlies the breakwater, and therefore the upper layer of the deposit was excavated and replaced with sand in order to increase the stability of breakwater. For the breakwater site, the consistency of the clay varied from very soft to firm as the depth increased and was separated from an irregular thickness of hard clay by a thin sand layer of maximum thickness 4 m. The clay deposit at the east site was thinner than at the west site, and the hard clay layer did not exist at the east breakwater site. The bed rock was andesine for the east breakwater site the depth of the sea bed was 11.4 m for the east breakwater site below the approximate lowest low water level, which was adopted as sea level datum for the
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Undrained Shear Strength Gain with Consolidation at Soft and Sensitive Soil Sites design. The total thickness of soft and firm clay layers at the failed part of the east site was 27.5 m. The undrained shear strength profiles of the East breakwater site are approximated by as shown in Figure 4. The stability analysis of the East breakwater was conducted using the Computer Software package XSTABL Version 5.1 [5]. The stability analysis shown in Figure 4 indicates that the East breakwater was safe with a factor of safety of 1.532. This analysis is compared with the analysis which was conducted by Chung et al. [2].
increasing surcharge primarily to provide a simple tool to the geotechnical engineers for developing a reasonable schedule for constructing high embankments at sites underlain by soft and sensitive fine grained soils. The procedure is calibrated using two embankment failure case histories in eastern India and employed in assessing the stability of two published embankment performance case histories. The results of these computations were found to be in agreement with observed embankment behavior. REFERENCES 1. Roy, D. and Singh, R. (2008), Failure of mechanically
2. 3. 4.
5.
Fig. 4 Stability analysis East Breakwater site CONCLUSIONS A procedure has been proposed in the paper for assessing the increase in the undrained shear strength as a function of
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stabilized earth embankments at two soft and sensitive soil sites, Journal of Performance of constructed facilities, ASCE, 22, 373-380. Chung, S.G. Kim, S.K. Kang, Y.J. and Im, J.C. and Nagendra Prasad, K. (2006), Failure of breakwater founded on thick normally consolidated clay layer. Geotechnique, 56, 393-409.
Casagrande, G. (1946), Ladd, C.C. Foott, R. Ishihara, K. Schlosser, F. and Poulos, H.G. (1977), Stress-deformation and strength characteristics, Proc., 9th Int. Conf. on Soil Mech. and Found. Engrg., Tokyo, 412-494. Interactive Software Design Inc. (1994), XSTABL User’s Manual. Moscow, ID, USA.
