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Lec4_Piled Raft

20-Oct-15 Realistic

20-Oct-15 Realistic estimation of subgrade modulus: ‣ Estimate settlements considering all soil layers upto 1.5 times the actual raft width. ‣ Calculate the k value as ratio of actual bearing on the raft to the calculated settlement. ‣ Structural designer to use this value for structural design of the raft. Prof. V.S.Raju Consultants 15 Structural analysis of the Piled Raft: Pile Spring (Ps) upto 4 to 6 cm settlement, Ps = (P u / 4 to 6) in kg/cm. If the settlements exceeds 4 to 6 cm, replace springs with Pu. Prof. V.S.Raju Consultants 16 V.S.Raju Consultants 8

20-Oct-15 Drawbacks of Current Practice: ‣ In a plate load test, dimensions of the plate are much smaller than actual raft size. ‣ Hence k value obtained from plate load test does not take into account settlement contribution of all the soil layers upto 1.5 times width of the raft. ‣ This may lead to over estimation of subgrade modulus leading to under-design of the raft (unsafe). ‣ For eg. for a raft size 30 x 60 m, thickness = 2.5 m, analysis was done for 2 cases with k = 1 kg/cm 3 and k = 0.5 kg/cm 3 . In the 2 nd case, moments were higher by 20%, resulting in 20% higher reinforcement steel or 10% thicker raft. Prof. V.S.Raju Consultants 17 Predicting the Settlements on cohesionless soils S = Settlement S = H ∆σ z Es H = Thickness of the layer ∆σ z = Pressure increase @ middle of the layer (obtained Es = Soil Modulus from stress distribution curves) Es is determined by correlating (a) (b) (c) With SPT N value Static cone penetration value Footing / Plate load test (at shallow depth only) Prof. V.S.Raju Consultants 18 V.S.Raju Consultants 9

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