Question

Q2 A concrete strip footing with a width of B is founded at the depth of D. below the surface of a level site. The water table is at a depth of d below the base of footing. The soil at the site consists of clay having the following properties The unit weight = y, the undrained cohesion=, the undrained friction angle = 1, the drained cohesion=c' and the drained friction angle = Use Hansen's theory of bearing capacity to determine the allowable bearing pressure, which could be placed on the footing assuming a factor of safety of F for the following cases: a) Immediately after loading the footing b) Long time after loading the footing Notes: The load is vertical. The ground and the base of footing are horizontal. D should be less than or equal to 2.5 B. The variable d can be positive (below the base), negative (above the base) or zero (at the base). Assume water level cannot be above the ground surface.

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Answer 1

According to Hansen, the ultimate bearing capacity is given by Va= CN Sedric + q Ng Lg dq iq +0.58B Ny Sy dog in where, Ne, Nq and Ny are Hansen's bearing capacity factors, and q is the effective Surcharge at the base levd (= N'Df), and Sc, Sq, sy → shape frietors . dova, dy depth factory Ichigolo inclination factors. The bearing capacity factors are given by No = ten(45+')Ttand') : Ne - (Nq-1) for $! NAF 1.80 (Nq-1) tong! For strip footing , Sc esq = Sy = 1:0 des dq = 1+0.25(%) , dp =1.00. where Op = depth of footing ; B = width of footing. As the load is vertical, Icalq = 1y = 1.0. (a) Immediately after loading :- qu = CNC do + q Nq dq to stong- (b) Long term after loading : V y = C'Nc de + q Nq dq +0.5 TBNN

Water table is at a depth 'd' from the base of the footing Then, q q = CN cd ct q ng da +0.5VON If the water table depth is >es from the base of footing then the effect of water table on bearing capacity is negligible la: B = 7.d + $'(0-) Pa = YA+qCP-d). Net allowable bearing pressure (Yna) = Vas Yu-TD All units are in S'I System