A mat foundation is proposed to support a structure for a site where the bearing materials would be classified as fine sand with a blow count between 20 and 25 blows per foot. According to the Buffalo building code (see code attached) what is the maximum contact pressure that the mat should be allowed to develop? Give reasons for your answer in a few sentences. Hint: code bearing pressures are intended for rectangular or strip footings typically. (15 pts) Solution From the provisions mentioned in the attached Buffalo Buiding Code, we can categorize the fine sand under the mat as firm granular soil as its blow count lies between 20 and 25 (i.e. between 11 and 30 as per code). So, the fine sand beneath the mat foundation can be classified under Class-8 which have the maximum allowable presumptive bearing value as 2 tons/sq.ft. A structure with a spread footing can tolerate a differential settlement of about 0.75 times the maximum permissible settlements if the footing is on firm sand. Compared to spread/sallow footings, the differential settlement in a mat foundation is much more reduced as the mat bridges over the loose pockets and lenses of a soil deposit. It has been found that, for the same loading intensity and the soil condition, the allowable differential settlement is about 0.5 times that of a spread foundation. From the above fact, we can have: Maximum allowable contact pressure that the mat should be allowed to develop = 0.75x0.5x2 tons/sq.ft = 0.75 tons/sq.ft.

1. A mat foundation is proposed to support a structure for a site where the bearing materials would
be classified as fine sand with a blow count between 20 and 25 blows per foot. According to the
Buffalo building code (see code attached) what is the maximum contact pressure that the mat
should be allowed to develop? Give reasons for your answer in a few sentences. Hint: code
bearing pressures are intended for rectangular or strip footings typically. (15 pts)
Solution
From the provisions mentioned in the attached Buffalo Buiding Code, we can categorize the fine
sand under the mat as firm granular soil as its blow count lies between 20 and 25 (i.e. between 11
and 30 as per code).
So, the fine sand beneath the mat foundation can be classified under Class-8 which have the
maximum allowable presumptive bearing value as 2 tons/sq.ft.
A structure with a spread footing can tolerate a differential settlement of about 0.75 times the
maximum permissible settlements if the footing is on firm sand. Compared to spread/sallow
footings, the differential settlement in a mat foundation is much more reduced as the mat bridges
over the loose pockets and lenses of a soil deposit. It has been found that, for the same loading
intensity and the soil condition, the allowable differential settlement is about 0.5 times that of a
spread foundation.
From the above fact, we can have:
Maximum allowable contact pressure that the mat should be allowed to develop = 0.75x0.5x2
tons/sq.ft
= 0.75 tons/sq.ft