DESIGN, CONSTRUCTION AND PERFORMANCE OF HIGH EMBANKMENT ON SOFT CLAY DEPOSITS

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1 IGC 2009, Guntur, INDIA DESIGN, CONSTRUCTION AND PERFORMANCE OF HIGH EMBANKMENT ON SOFT CLAY DEPOSITS Avik Kumar Mandal AGM, Geotechnical Engineering, M/s. LEA Associates South Asia Pvt. Ltd., New Delhi , India. Pradyot Biswas CGM, M/s. LEA Associates South Asia Pvt. Ltd., New Delhi , India. P. Jagannatha Rao Consulting Engineer, Faridabad , India. ABSTRACT: National Highway 5A (Chandikhol Paradip in Orissa) cuts across river delta and embankments of upto m height were to be built on as much as m deep soft clay. The strength of soft clay was improved to the required level using PVDs and stage loading. Maximum 4 stages loading were used and the waiting periods varied from 110 days to 35 days, depending on the height of embankment and properties and depth of soft clay. Dissipation of pore pressures and progress of settlements were monitored. Increase in shear strength after each stage was monitored. These data were used to control the progress of embankment construction. Sand core embankments were adopted for construction of the embankment fill, utilizing the abundantly available river bed sand. 1. INTRODUCTION The stretch of highway from Chandikhol to Paradip (NH 5A) is located in the Mahanadi Brahmani combined delta. For much of the length along the alignment, the subsurface formation consists of soft clays. As the highway cuts across the streams and rivers, high approach embankments were required at such locations. On soft clay subsoil layers, these were prone to be unsafe in bearing capacity, rotational stability as well as will experience large settlements. Ground improvement was essential for building the stable embankments with permissible settlement on soft sub soils. The paper presents the case history of ground improvement technique adopted in the project, covering design, construction and performance monitoring. Embankments of height ranging from 6.00 m to m are situated at locations of 11 structures and at each structure there are two approach embankments, one on either side. Of the 11 locations, soft clay sub soil, ranging in depth from 5.00 m to m was met with at the six locations and ground improvement was required at these 6 locations i.e. at 12 approach embankments. At the remaining 5 locations, although the embankment height ranged from 7.00 m to m, no ground improvement was found necessary, as the subsoil was mostly loose to medium dense sand/silty sand. 1.1 Typical Cross Section of Embankment Large quantities of fill materials are required for building the high embankments. The local soil along the alignment is clay of high plasticity and is not well suited as embankment fill. To overcome this problem of fill material, innovative concept of sand core embankment was developed and adopted for the construction of high approach embankments in the project. Sand from the large number of streams cutting the alignment was used for building the embankments which has provided an economical option. Since sand is granular material without cohesion, embankment configuration shown in Figure 1 was adopted, to keep the outer slope surface in place and protect the same against local failures. This consisted of a core of well compacted sand with outer lining of locally available cohesive moorum. The thickness of side cover moorum layer was 1.25 m up to 7.00 m high embankments and 1.50 m for embankments higher than 7.00 m. The outer cover protects the sand layer from local slumps and rainwater erosion. To keep the sand core well drained, 1.00 m wide window of sand in the intermediate moorum layers and 450 mm thick well compacted gravel blanket at embankment base have been provided. The side slopes for the approach embankments have been provided depending on the height of embankments, available land width, subsoil condition, overall safety and economics, at the given location. In general, side slopes adopted for the embankments ranged from 1v : 2h to 1v : 3h for the embankment heights up to 6.00 m. For the height of embankment beyond 6.00 m and up to m, the side slope adopted was varying between 1v : 3.5h and 1v: 5h. 798

2 HARD SHOULDER (CARRIAGEWAY) PAVED SHOULDER HARD SHOULDER GSB STONE PITCHING 1 n AC DBM WMM GSB 1500 THICK SAND FILL MOORUM SELECTECD SUBGRADE n A 1000 WIDE WINDOW OF C/C SPACING THICK INTERMEDIATE MOORUM LAYER MOORUM SIDE COVER OF THICKNESS 1250 TO THICK SAND FILL COMPACTED GRAVEL LAYER AT BASE T1 SOFT CLAY LAYER THICK SAND BLANKET SPACING = S (TYP) GEOFABRIC T2 SOFT CLAY LAYER mm X 5 mm GEOSYNTHETIC BAND DRAIN `S' mm c/c IN TRIANGULAR ARRANGEMENT MEDIUM DENSE TO DENSE COHSIONLESS SOIL LAYER OR STIFF TO VERY STIFF COHSIVE SOIL LAYER Fig. 1: Typical Cross Section for Approach Embankment at Bridges (Ground Improvement by Geosynthetic Vertical Band Drain (PVD) and Stage Construction) 1.2 Properties of Soft Clay High approach embankments have been provided at the location of 6 structures as stated in Table 2 and thus there are 12 approach embankments, as stated in para 1.1, where deep clay subsoil layers have been found to exist. At each location, the soft clay deposits were found to be in layers of varying strength and other properties. Up to 3 4 different layers were observed in these deposits of soft clay. Within the limited space of this paper, only one embankment out of total 12 approaches, viz. Pattamundai Canal Chandikhol Side Approach at km 14/1 has been chosen. Total thickness of soft clay at this location is m and is in 2 layers of different properties. Table 2 summarizes the index properties, shear strength properties and consolidation properties of soft clay subsoil at this location. Layer No. Table 1: Properties of Soft Clay Subsoil: Location 14/1 Thickness LL PI NMC C u e 0 C v (m 2/ day ) C r C c P c From the above table, it is observed that the Liquid Limits (LL) of the clayey subsoil are very high and natural moisture contents are close to the LL, which clearly indicates the softness of the clayey soil. The low shear strength, high void ratio with compressibility truly exhibits the weak subsoil for the high approach embankments. The properties of soft clay deposits at other locations are sometimes close to the above values or differ by significant amount. 1.3 Expected Behavior of Embankment in Original Ground Condition The heights of embankments, thickness soft compressible soil and corresponding factors of safety for the embankments, in rotational stability and bearing capacity are given below, in Table 2 for the case of no ground improvement. Total settlements due to consolidation of soft clay layers and time required for 90% degree of consolidation are also given here. Table 2: Details of Analysis of High Approach Embankment over Soft Clay (without Ground Improvement) Structure No. 14/1 34/2 34/3 36/4 50/3 65/1 Maximum Height (m) Total Thickness of Soft Compressible Layer (m) F.O.S in Rotational Stability F.O.S in Bearing Capacity Total Estimated Settlement (m) Time for 90% settlement in Yrs > 50 > 50 > 50 > 10 > 20 > 5 Rotational stability was evaluated using software HED of IRC, and as per Bishop s simplified method of analysis. Bearing capacity was calculated using Pilot s Charts, which takes into account the failure by squeezing that occurs in case of wide embankment or where thickness of soft clay is such that ratio of base width to thickness is high. Under such conditions, the bearing capacity factor Nc works 799

3 out to be higher than given by Terzaghi's formula (Stabilenka Manual 1987). Total settlement and degree of consolidation were estimated using the software HED of IRC. 2. GROUND IMPROVEMENT TECHNIQUE ADOPTED Stage construction along with Prefabricated Vertical Band Drains (PVDs) was adopted as the ground improvement technique at this site. The methodology of the ground improvement involves: (a) laying of non-woven geotextile fabric separator directly over soft ground, (b) laying of sand blanket over geotextile separator, (c) installation of PVDs at designed spacing. Normally PVDs have been extended to the complete depth of the soft clay layer, (d) placing of 1 st stage of fill. The height of fill placed depends on the initial strength of the clay and is chosen such that the fill placed is safe against failure in rotation stability and bearing capacity, (e) providing a waiting period, normally to achieve 90% combined radial and vertical consolidation. (f) The strength of the subsoil clay layer increases due to the first stage loading. (g) Taking advantage of the strength gain from 1 st stage loading, second stage loading is placed. The cycle from (d) to (g) is continued till the final height of embankment is reached, a waiting period as per calculation, being provided at each stage of embankment construction. Table 3: Details of Stage Construction and Factors of Safety/ Ground Improvement Design at Approach of (14/1) Stage No Estimated design cohesion C u ( t/m 2 ) Layer 1: 2.90 Layer 2: 1.90 Layer 1: 3.98 Layer 2: 2.98 Layer 1: 5.06 Layer 2: 4.06 Layer 1: 5.96 Layer 2: 4.96 Height of stage Cumulative height of fill at the end of stage F.O.S. in rotational stability F.O.S.in bearing capacity Details of PVD Depth of Band Drains = c/c Spacing = 890 mm in triangular pattern For more details of the ground improvement methodology and the formulae used for determining the spacing of PVDs, percentage radial, vertical and combined degrees of consolidation, choice of waiting period, progress of degree of consolidation with time and settlement with time i.e design methodology adopted in, reference may be made to: IRC- HRB Part A (1994) and Part B (1995), Holtz (1987) and Van Impe (1989). Details are not included due to lack of space. At each of the 12 approach locations, where high embankments were built, the waiting period for each stage of loading ranged from 115 days to 35 days. The spacing of band drains ranged from 890 to 1300 mm in a triangular pattern and the depth of band drains ranged from m to 5.00 m corresponding to the depth of soft clay layer. To achieve the height of maximum m for the embankments, 4 stages of loading and corresponding waiting periods were required. Also, at most of the locations, the soft clay subsoil deposit consisted of layers of differing shear strength (undrained), and was divided upto 4 nos. layers. Strength gain at the end of each stage for each layer was estimated for every site and factors of safety against failure in bearing capacity and rotational mode were estimated. Table 3 gives the details for one such location viz. Str. No. 14/1, Chandikhol Side Approach. 2.1 Installation of PVDs Properties of Prefabricated Vertical Drain (PVD) are given in Table 4 and Figure 2 (Photo) shows the rig actually used for installing PVDs, band drain spool and lance. Sand blanket and some of the PVD installed are also seen. Installation of PVDs is a very fast process, installation of each PVD takes about 1 minute. The PVD mounted on the spool is fed to the lance and the lance is pushed into the soil, the force for the same being provided by the rig. The lance with the band drain, base plate and shoe goes down into the soft soil. After the lance is pressed down to the desired depth, the band drain is cut of with scissors, above the level of the half sand blanket. The base plate and cutting shoe remain in the ground, serving as anchors for the PVD. The lance is withdrawn from the soft soil. The oozing out of water was observed during installation of PVD even without stage preloading. Table 4: Properties of Prefabricated Vertical Drain (PVD) Unit Mean Value Composite Weight g/m 70 Width m 0.1 Thickness mm 5 Tensile strength kn 2.1 Elongation at 2.0 kn % 25 Strength at 10% elongation kn 1.3 Discharge capacity qw at 250 kn/m 2 Index text using deformeable foam layers m 3 /s Straight embedded in bentonite m 3 /s Buckled embedded in bentonite m 3 /s Filter Tensile strength kn / m 11 Elongation at break % 25 A.O.S. (095) mm <75 Permittivity s Permeability (Kv) m / s

4 Referring to Fig. 1, it may be seen that additional 3 rows of PVDs extend beyond either toe of embankment. These are required to ensure lateral stability of the ground beyond toe. Figure 3 shows, the laying of non-woven geotextile fabric separator at the base, between the soft soil and the granular sand blanket. The total quantity of PVDs installed in the approach embankments amounted to 17,00,979 running meters. Colbond PVDs were selected for used in the project and they conformed to the project specification. The quantity of geotextile separator fabric used in the approach embankments was 1,05,123 sq.m. Fig. 2: Installation of Vertical Drain (PVDs) Fig. 3: Laying of Non-woven Geotextile Fabric Separator 3. CONSTRUCTION CONTROL AND PERFORMANCE MONITORING The construction of the different stages are heavily dependent on the dissipation of pore water pressure and the strength gain of the soft clay layer, due to the preceding stage load. Hence strict and elaborate construction control and monitoring was essential, which was specified and adopted. The details are summarized in the Table 5 below: Item monitored 1. Build up and dissipation of pore pressures 2. Gain in shear strength of subsoil Table 5: Monitoring Installations Equipment installed Casagrande Open Stand Pipe type Piezometer By recovery of undisturbed samples from bore holes at regular intervals 3. Settlements Platform type Settlement Gauges No. of installations 6 to 8 nos per site 4 to 6 boreholes per site, based on the number of stages 6 to 8 nos per site Piezometers and settlement gauges were monitored daily, in stretches where fill placement was in progress. Weekly readings were taken after the desired fill height was reached. Shear strength of subsoil was checked at the end of waiting period of each stage, for each approach, two to three boreholes were drilled for this purpose. Undisturbed soil samples were collected and tested immediately. Actual strength gain at most approaches and for most stages, was as predicted by the relationship Δ Su = 0.20 Δ p' o., Where ΔS u = increase in undrained shear strength caused by an increase in effective stress Δ p' o. At one or two locations where the shear gain was less, one of the two alternatives was adopted: (a) Where adequate factor of safety was available for placing the complete height of next stage, the same was allowed. A minimum factor of safety of 1.20, for the full stage height immediately at the end of placement was specified. (b) Where the estimated factor of safety for the contemplated stage, based on the available strength, was less than 1.20, the height of the stage was reduced to keep the factor of safety at In practice, it was found that only a marginal decrease of stage height was needed. Following Table 6 shows typical strength increase with respect to the stage loading, at Chandikhol Side Approach of Str. No. 14/1. Table 6: Actual Increase in Shear Strength of Soft Clay Subsoil under Stage Loading at Approach of Str. No. (14/1) Stage Height of stage (m) Δp' o 1 st nd rd Layer No. C u Initial C u Final ΔC u /Δp' o

5 From the above table it is seen that the average ratio of (ΔCu /Δp' o ) with respect to actual strength gain, was around The observed settlement at most of the approaches was less than estimated value. Preloading (M) Water Head (RL in M) Loading Curve Pore Water Pressure & Settlement Curve 2/15/2006 2/25/2006 3/7/2006 3/17/2006 3/27/2006 4/6/2006 4/16/2006 4/26/2006 5/6/2006 5/16/2006 5/26/2006 6/5/2006 6/15/2006 6/25/2006 7/5/2006 7/15/2006 7/25/2006 8/4/2006 8/14/2006 8/24/2006 9/3/2006 9/13/2006 9/23/ /3/ /13/ /23/ /2/ /12/ /22/ /2/ /12/ /22/2006 1/1/2007 1/11/2007 1/21/2007 1/31/2007 2/10/2007 2/20/2007 3/2/2007 3/12/2007 3/22/2007 4/1/2007 4/11/2007 4/21/2007 5/1/2007 5/11/2007 5/21/2007 5/31/2007 6/10/2007 6/20/2007 6/30/2007 7/10/2007 7/20/2007 7/30/2007 8/9/2007 8/19/2007 8/29/2007 9/8/2007 Date Main Peizometer Dummy Piezometer-1 Dummy Piezometer-2 Settlement Fig. 4: Typical Loading, Pore Water Pressure and Settlement Curve at Approach of Str. No. (14/1) Figure 4 shows the typical loading, pore water pressure builtup curve and dissipation and progress of settlement with time at Chandikhol Side Approach of Structure No. 14/ Chronology of the Project and Performance of the Highway The design of the project, including the highway and bridges was completed in Construction was taken up in 2004 and was completed in The pavement condition remains even and smooth at present (Fig. 5). The condition of the pavement is an indication of the fact that the embankments are stable without any distress and anticipated strength increases have taken place in the subsoil clay layer, by adopting stage construction with PVDs Settlement (mm) 4. CONCLUSION High embankments of upto m height were successfully built on soft clay subsoil deposits whose depth was as much as 22.0 m. Ground improvement measure with PVDs in conjunction with stage construction was successfully adopted. Sand core embankment concept was successfully implemented. The success of the ground improvement project is based on detailed and elaborate monitoring procedures adopted during construction. Among the various ground improvement techniques for building embankment on soft ground, use of PVDs together with stage construction is far cheaper than such techniques as stone columns. However, due to the waiting periods needed, adequate time has to be available. This can be ensured by proper planning of the project construction. ACKNOWLEDGEMENTS The authors thank design and supervision team members of M/s. LEA Associates South Asia Pvt. Ltd. (LASA) for support and encouragement in the preparation of the paper and M/s. HCC, the project contractors for monitoring and data collection. REFERENCES IRC - HRB (1994). State of the Art: High Embankment on Soft Ground Part A Stage Construction. IRC - HRB (1995). State of the Art: High Embankment on Soft Ground Part B Ground Improvement. Holtz R.D. (1987). Preloading with Prefabricated Vertical Strip Drains, Geotextiles and Geomembranes, 6: Stabilenka Manual (1987). Design Guidelines for Reinforced Embankments on Soft Soils. Van Impe W, F. (1989). Soil Improvement Techniques and their Evaluation, Balkema A.A. Publishers, Rotterdam: Fig. 5: Present Condition of Highway and Embankment at Approach of Structure No. 14/1 802