Lec4_Piled Raft

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