Abhishek, tarachand and satyanarayana reddy igc 2013 roorkee


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Case study on failure of Retaining wall

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Abhishek, tarachand and satyanarayana reddy igc 2013 roorkee

  1. 1. Proceedings of Indian Geotechnical Conference December 22-24,2013, Roorkee CASE STUDY OF FAILURE OF RETAINING WALL AT DWARAKANAGAR, VISAKHAPATNAM S.V. Abhishek, PG Student, A.U. College of Engineering, Visakhapatnam, svabhi.92@gmail.com V. Tarachand, PG Student, A.U. College of Engineering, Visakhapatnam, vtarachandg@gmail.com C.N.V. Satyanarayana Reddy, Professor, A.U. College of Engg., Visakhapatnam, cnvsnreddy@rediffmail.com ABSTRACT: A 6.1 m high cantilever basement retaining wall of a proposed multi-storeyed structure failed during heavy rains caused by tropical storm “Neelam” on November 3, 2012 at Dwarakanagar, Visakhapatnam. The retaining wall was designed by a structural engineer analogous to a framed structure using incorrect backfill properties and was constructed with inadequate weep holes. The walls on all four sides of the boundary eventually yielded in, with severe cracking at the corners. To investigate into the failure, samples of the backfill are collected and analysed in the laboratory for its properties. Based on the properties obtained, the retaining wall is redesigned for the expected lateral earth pressure and thereafter compared comprehensively with the design given by the structural engineer. The causes for failure of the retaining wall are determined and suitable measures are suggested to prevent the possible recurrence of such failures in the future. INTRODUCTION Most of the failures of retaining walls are due to adoption of incorrect design parameters, improper execution/construction or a combination of both. Although the design of retaining walls is considered to be the job of structural engineers, geotechnical engineers play a significant role with regard to selection of appropriate backfill, design of wall for surcharge loads and suggestion of measures for drainage of the backfill if suitable materials are unavailable. Increased land costs and lack of sufficient space has resulted in construction of many multi-storeyed structures with underground retaining walls to facilitate cellar and sub-cellar parking. The construction of these retaining walls needs proper attention if they had been initially designed akin to a framed structure, i.e., in conjunction with beams, columns and slab. The present paper deals with the failure of a basement retaining wall of a proposed multistoreyed building at Dwarakanagar, Visakhapatnam. The failure occurred on November 3, 2012 after the onset of tropical storm “Neelam”. The building consists of eight storeys accommodating two basement floors, one stilt floor with five upper floors and is proposed to be used partly for residential purpose and partly for commercial establishments. The retaining wall is 6.1 m high and is of cantilever type. The retaining wall was designed by a structural engineer of a private firm, similar to a framed structure using incorrect backfill properties. During the site visit, it is observed that insufficient weep holes are provided in the retaining wall and the walls on all four sides of the boundary yielded in with severe cracking at the corners (Fig. 1). Crack at corner Fig. 1 Failure of retaining wall with crack at corner To investigate into the failure, samples of the backfill are collected and analysed in the laboratory for its properties. The retaining wall has been redesigned based on these properties and thereafter compared comprehensively with the design given by the structural engineer. The causes for failure of the retaining wall are determined and Page 1 of 4
  2. 2. S.V. Abhishek, V. Tarachand & C.N.V. Satyanarayana Reddy specific measures are suggested to prevent the possible recurrence of such failures in the future. SUBSOIL PROFILE Prior to construction of the retaining wall, field investigation in the form of standard penetration test was conducted in five boreholes by a private soil exploration agency in Visakhapatnam. Core drilling using double core barrels was carried out on encountering rocky strata and rock cores were obtained. In general, the subsoil profile at the site consisted of yellowish brown clayey sand in the top 4.5 m with Standard Penetration Resistance (N) of 12, overlying a 2.0 m thick layer of soft disintegrated rock (SDR) with N>100. About 3.0 m of soft rock with Core Recovery (CR) of 53% lies below the SDR layer. This in turn is underlain by a thick layer of hard rock having Core Recovery of 62% and Rock Quality Designation (RQD) of 33%. The ground water table was not encountered within the depth of exploration. Properties of Backfill During investigation of failure of the retaining wall, samples of the backfill are collected and laboratory tests are conducted as per IS:2720 [1]. The properties of the backfill obtained are presented in Table 1. Table 1 Backfill Properties S. No. Property 1. Specific Gravity 2. Particle Size Distribution a) Gravel (%) b) Sand (%) c) Fines (%) 3. Plasticity Characteristics a) Liquid Limit (%) b) Plastic Limit (%) c) Plasticity Index (%) 4. IS Classification Symbol 5. Shrinkage Limit (%) 6. Natural Moisture Content (%) 7. In-Situ Density (g/cc) 8. Shear Parameters a) Cohesion (kN/m2) b) Angle of Internal Friction Based on the particle size distribution and plasticity characteristics, the backfill is classified as clayey sand (SC) as per Indian Standard Soil Classification System. For an in-situ density of 2.16 g/cc and natural moisture content of 18.2%, the in-situ dry density of the backfill works out to be 1.83 g/cc. The in-situ density is treated as saturated density since the natural moisture content is greater than liquid limit. The shear parameters reported in Table 1 correspond to the saturated state of the backfill. DESIGN OF RETAINING WALL Retaining Wall Designed as Framed Structure Figure 1 shows the cross section of the retaining wall and the detailing of reinforcement according to the structural engineer’s design. The retaining wall was designed considering the friction angle and unit weight of the backfill as 370 and 19 kN/m3 respectively. Bending moments in the retaining wall were calculated using STAAD software and the area of reinforcement was fixed accordingly. Maximum bending moment at the base of the stem was 40 kNm. Value 2.67 1 62 37 26.5 18.0 8.5 SC 16.4 18.2 2.16 10 260 (a) Cross section of retaining wall Page 2 of 4
  3. 3. Case study of failure of retaining wall at Dwarakanagar, Visakhapatnam shear failure, the safe bearing capacity estimated from Teng’s equation [2] is 90 t/m2. But for an allowable settlement of 25 mm, the safe settlement pressure obtained from the equation specified by IS:8009 (Part 1) [3] is 25 t/m2. As a result, allowable bearing capacity of 25 t/m2 is adopted for design of retaining wall. (b) Detailing of reinforcement in wall and column (c) Reinforcement detailing in beam and base slab Fig. 1 Design of retaining wall as framed structure The wall was founded in the SDR layer and was constructed using M 25 grade concrete and Fe 415 grade steel with a clear cover of 40 mm and 25 mm to the reinforcement on earth side and other faces, respectively. Columns of size 450 mm x 300 mm were proposed to be constructed at intervals of 3.2 m for proper bearing of floor beams onto the retaining wall. The bottom beam of 300 mm width and 600 mm depth was aimed at providing stiffness to the columns and ensuring uniform distribution of load onto the base slab. The retaining wall was proposed to be connected to the main building at the cellar roof slab level and again at the ground floor level. Unfortunately, it failed soon after construction, before the columns and beams could be built. Redesign of Retaining Wall To verify the design given by the structural engineer, the retaining wall is redesigned as a reinforced cement concrete (R.C.C.) cantilever wall (Fig. 2) based on limit state by incorporating the shear parameters and density of backfill given in Table 1. Since the retaining wall is founded in SDR, a corrected standard penetration resistance of 50 is considered. Considering possible rise of ground water table upto ground surface and adopting a factor of safety of 3.0 against risk of Fig. 2 Retaining wall redesigned as an R.C.C. cantilever wall The computed maximum bending moment and shear force in the stem, toe slab and heel slab are 138 kNm, 67 kNm, 65 kNm and 89 kN, 94 kN, 100 kN respectively. The area of reinforcement and development length are calculated as per IS:456 [4]. The clear cover provided to all reinforcement in the stem and base slab are 40 mm and 50 mm respectively [4]. To satisfy the development length criterion, the main and distribution reinforcement of the stem are to be anchored into the base slab over a distance of 840 mm and 340 mm respectively. DISCUSSION Table 2 compares the two retaining wall designs illustrated earlier. It can be observed that by designing the retaining wall as a conventional R.C.C. cantilever wall, the section and percentage Page 3 of 4
  4. 4. S.V. Abhishek, V. Tarachand & C.N.V. Satyanarayana Reddy of reinforcement required are much higher when compared to the integrated frame design. Due to unforeseen delay in construction of beams and columns caused by various reasons and due to saturation of backfill owing to heavy rains of storm “Neelam”, the retaining wall yielded in. This is reflected by the very low factor of safety (F.S.) of 0.37 with respect to overturning. Table 2 Comparison of retaining wall design Description Design of Redesign Retaining as R.C.C. Wall as Cantilever Base Pressure at Heel -593.9 kPa 19.0 kPa F.S. (Overturning) 0.37 2.21 F.S. (Sliding) 0.71 1.84 Main Reinforcement (a) Stem 754 mm2 2244 mm2 (b) Toe Slab 754 mm2 1436 mm2 (c) Heel Slab 524 mm2 1745 mm2 Distribution Steel (a) Stem 524 mm2 457 mm2 2 (b) Base Slab 393 mm 457 mm2 Although the factor of safety with respect to sliding is also quite low, the mobilization of passive resistance of soil possibly averted sliding of the retaining wall. The formation of cracks at the corners of the boundary is attributed to deficient wall section, underprovided reinforcement and separation of heel slab from foundation soil due to overturning. Absence of proper weep holes also resulted in additional lateral thrust being exerted on the wall by the saturated backfill. Lack of provision of a temporary supporting system during setback in progress of work is furthermore considered to be one of the reasons behind the failure of the retaining wall. Foundation Depth Base Slab (a) Toe Slab Width (b) Heel Slab Width (c) Total Width (b) Thickness Stem (a) Height (b) Thickness Resultant Eccentricity Base Pressure at Toe Framed Structure 0.60 m Retaining Wall 1.00 m 0.67 m 0.53 m 1.20 m 230 mm 1.00 m 1.50 m 2.50 m 350 mm 5.87 m 230 mm 2.12 m 717.5 kPa 6.15 m 350 mm 0.33 m 161.4 kPa grouping it with the design of beams and columns (unlike a framed structure). Otherwise, suitable temporary supporting systems must be assembled to support the wall in the eventuality of any unanticipated delay in construction of the cellar and sub-cellar structural components. REFERENCES 1. IS:2720, Methods of tests for soils – relevant parts, BIS, New Delhi. 2. Teng, W.C. (1962), Foundation Design, Wiley, New York. 3. IS:8009 (Part 1)-1976, Code of practice for calculation of settlement of foundations (shallow foundations subjected to symmetrical static vertical loads), BIS, New Delhi. 4. IS:456-2000, Code of practice for plain and reinforced concrete, BIS, New Delhi. CONCLUSIONS A combination of various factors such as improper interpretation of backfill properties, absence of proper weep holes and alteration in the behaviour of the wall due to delay in progress of work, are considered to be responsible for failure of the retaining wall. It is desirable to design and construct a basement retaining wall as a conventional, distinct retaining wall rather than Page 4 of 4