5. Consider a deep deposit of uniform sand with Noom- 25 and groundwater table a

Question

5. Consider a deep deposit of uniform sand with Noom- 25 and groundwater table at 5 m depth. The sand has unit weight 17 kN/m3. A building is to be constructed with foundation units located at 3 m depth. a) A certain shallow foundation plan calls for 1.5 m by 1.5 m square footings with minimum 0.3 m depth of cover (D) located on a grid pattern 3 m by 3 m, and supporting a superstructure over a rectangular area 12 m by 18 m. Using the Peck design approach, what would be the safe load capacity for a single foundation unit, and for the foundation system as a whole? (10 marks) b) Compare both the system load capacity as well as the allowable foundation pressure for the above foundation system with that of a raft foundation also 12 m by 18 m and founded at 3 m depth. (10 marks)

Explanation / Answer

For question A

Safe load capacity for a single unit of 1.5 m square shallow foundation at a depth of 3 m below natural ground level can be obtained by using peck equation for ultimate bearing capacity given as

Qult = 1.2 c Nc + YDf Nq + 0.4 Y B Ny

Where c is cohesion which is zero for cohesion less sand soil

Nc Nq and Ny are bearing capacity factors and can be determined by chart given by peck relating angle of internal friction , corrected N value or SPT value.

Assuming angle of internal friction as 36 degree and sand deposit to be dense sand deposit a fair approximation of bearing capacity factors from the chart can be obtained as :

Nc = 50 ; Nq= 40 ; Ny = 40

Given Y or unit weight of soil is 17 KN/m3 and depth of foundation Df = 3 m substituting in above equation we get

Qult = 2448 KN/ m2

Qnult = Qult - YDf

Qnult = 2397 KN/ m2

Assuming factor of safety of 3, Qnet safe = Qnult / 3 = 799KN/ m2

Safe bearing capacity = Q net safe + Y Df = 850 KN/ m2

Thus for 1.5 * 1.5 m square shallow foundation safe load capacity = 850*1.5*1.5 = 1912.5 KN

The foundation is 3m x 3m grid pattern in an area of 12m x 18 m number of square foundations would be 35 and hence the safe load capacity for system as a whole would be 1912.5 * 35 = 66937.5 KN

For part B of the question ,

Net allowable safe pressure for raft of 12m x 18 m as per peck design is given as Qs = 21 Ncorr KN/ m2

Qs= 21* 25 = 525 KN/ m2

Safe load capacity of raft = 525*12*18 = 113400 KN

Thus as compared to system load capacity of square foundations system the load capacity of raft is more.

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