Soil Pressure in Foundation Wall
Dear reader we know foundation wall is an essential part of building and we have discussed many post about this term. Foundation wall should be designed considering lateral loads exerted by soil as per international building code.
The main aim of design and construction of foundation wall is to resist soil loads exerted on it; we usually call this lateral soil loads. Dear reader we are providing a table below which depicts minimum value of design lateral loads as per IBC.
Here 1 lb/ft2/ft depth=1 psf/ft depth=0.157 Kpa/m
1. The lateral soil loads used for design as in table should be of moist condition at optimum densities for specified soil. In this regard, field condition should govern-saturated or submerged soil pressure and should include hydrostatic loads plus weight of buoyant soil.
2. Not accepted as backfill
Wide range of backfill materials are used to classify and defining these materials. The classification and definition also are based on ASTM D2487 [Standard Practice for Classification of Soils for Engineering Purpose (Unified soil classification system)].
We know about active earth pressure and earth pressure at-rest. We think, it is not required to explain-why passive earth pressure is not included here?
Now think about application of table-is this versatile table for designing foundation wall? Obviously not-the values provided are minimum value and we should not design based on minimum values; for economic design the actual value or more rigorous investigation should be done to have satisfactory design values.
When foundation walls are restrained by any means to have horizontal movement both at top and bottom we should use at-rest pressure from table. When foundation walls are restrained at bottom by any means may be by mat foundation or pile cap or piles and free at top (that is free to rotate or move) active earth pressure values should use.
In case of questionable soil like expansive soil, the design vales of earth pressure should be increased. When surcharge included in building site, the design value should include design lateral earth pressure for surcharge.
When drainage system is not defined or designed around foundation wall, full pressure from hydrostatic action in backfill should consider in design lateral earth pressure. Dear reader we have discussed about foundation drain in our previous posts.
When a foundation wall extends less than 8 feet below grade and flexible diaphragm at the top provides lateral support to it, active pressure values from the table should be used to design it.