Question

Your boss asks you to design a commercial-industrial building with planned column loads of 2,000 kN. For each of the locations described below, list the type(s) of foundations you would consider in preliminary planning. Briefly explain your choice.

1) Urban fill extending to depths of ~5 m, followed by ~50-60 m of a soft lacustrine fat clay deposit.

2) Peat to a depth of 2 m underlain by a thick dense granular deposit, the groundwater table is near the surface.

3) A thin (<0.5 m) layer of topsoil over a thick (~10 m) marine sand deposit over competent bedrock.

4) Variable glacial deposits to a depth of ~20 m. These deposits consist of smaller layers of dense granular deposits and overconsolidated fine-grained deposits with cobbles and present at developed locations in the area. Groundwater is variable with several perched aquifers.

Answer 1

1) Since most of the soil in the case 1 is soft fat clay it is recommended to drive group of friction piles as clay has good cohesive strength. The friction piles may be concrete piles or more preferably prefabricated vertiical sand drains or stone columns to boost the consolidation if water table is present (or) a mat foundation is suitable but it may be more costlier since it is a large commercial industrial building. So friction piles are chosen.

2) Shallow foundation may be suitable for this case as there is thick dense granular deposit which have a large friction angle and less settlements and higher bearing capacity. The foundation may be laid below the peat layer and above the dense granular layer.

3) In this case end bearing friction piles are suitable as there is sand available to bear friction and hard bed rock is also at a suitable depth which have good bearing strength.

4) As the layers consist of dense granular deposits and overconsolidated fine grained soil shallow foundation may be preferred as granular deposits have good bearing strength. The settlements will be less due to the presence of the granular deposits as well as overconsolidated fine grained soil.