Factors to Consider in Foundation Design Chapter # 02 Lec. # 04

key Factors Foundations in Sand and Silt deposits Foundations on clays and clayey silts Foundation on Loess and other collapsible soils Foundation on Residual Soils Foundation on unsaturated soils subject to volume change with change in water contents Environmental Considerations

7-Foundations in Sand and Silt deposits Foundation in sand and silt will require consideration of the following Bearing capacity Densification of loose deposits to control settlement. Placing the footing at a sufficient depth that the soil beneath the footing is confined. If silt or sand is not confined, it will roll out from the footing perimeter with a loss of density and bearing capacity. Wind and water may erode sand or silt from beneath a footing that is too near the ground surface.

Uncontaminated glacial silt Uncontaminated glacial silt deposits can have a large capillary rise because of the small particles sizes. Sometime these deposits can be stabilized by excavation to a depth of 0.6 to 1 m, followed by placement of a geotextile water barrier. The silt is then backfilled and compacted to provide a suitable foundation.

Foundations on clays and clayey silts Clay and clayey silts may range from very soft, normally consolidated, to very stiff, highly over consolidated deposits. Major problems are often associated with the very soft to soft, deposits from both bearing-capacity considerations and consolidation settlements. Silt with a large IP (PL) and /or wL (LL)may be called plastic silts.

Remarks Consolidation test should be made to determine the expected settlement if the structure has a relatively high cost per unit area. For smaller or less important structures, some type of settlement estimate based on the index properties might be justified. The net ultimate bearing pressure for vertical loads on clay soils is normally computed as a simplification of either the Meyerhof or Hansan equations; qult = cNc Sc dc+ q Nq Sq dq-q

9-Foundation on Loess Loess having particle size range 0.01 to 0.1mm, specific gravity 2.6 to 2.8, in situe dry densities ranges from 10 to 16.5 kN/m3 and Atterberg limits 25-55 & 15-30 percents is predominant collapsible soil.

Foundation on Loess and other collapsible soils Collapsible soils are generally wind-blown (Aeolian) deposits of silts, dune sands and volcanic ash. Typically they are loose but stable, Certain conditions of load + wetting produce a collapse of the soil structure, resulting large settlement.

Improvement of site conditions Compact (excavate and replace) the soil to γdry > 15.5 kN/m3 Use an admixture during compaction. Admixtures may lime, lime/fly ash, or Portland cement. Use of piles through the collapsing soils to a more competent underlying stratum.

10-Foundation on Residual Soils A residual soil is produced from physical and chemical weathering of rock. i.e sedimentary, metamorhic or igneous. Soils produced in this manner tend to be sandy silts or silty sands often with some mica particles and clay contamination. In many cases, to make a reliable design, some soil exploration is necessary in all residual soils.

11-Foundation on unsaturated soils subject to volume change with change in water contents Expansive soils undergo volume changes upon wetting and drying. For a volume changes to occur these soils must be initially unsaturated at some water content w0. when water content changes to a new value w1,the volume increase if w1> w0 or decreases if w1< w0 unless w0 is the shrinkage limit where w0 = ws.

Remarks These soils occur in an active zone, which starts at the ground surface and goes down to the saturated part of the zone of capillary rise above the ground water table.

Other approaches related with volume changes Volume-changes related to consolidation Volume change related to the expansion index E1 Volume change based on soil suction Volume change correlations using soil index properties

Designing structure on soils susceptible to volume change Alter the soil (using lime cement, or other admixtures) Compact the soil well on the wet side of the OMC. Control the direction of expansion. By allowing the soil to expand into activities built in the foundation, the foundation movements may be reduced to tolerable amounts. A common practice is to build “waffles” slabs so that the ribs support the structure while the waffle voids allow soil expansion.

Control the soil water. The soil may be excavated to a depth such that the excavated overburden mass of soil will control heave, lay a plastic fabric within the excavation, and then backfill. The rising water vapor is trapped by the geo-textile and any subsequent volume change is controlled by the weight of overlying material and construction. The surface moisture will also have to be controlled by paving, grading, etc.

Check whether a granular blanket of 0 Check whether a granular blanket of 0.3 to 1m or more depth will control capillary water and maintain nearly constant water content in the clay. Ignore the heave. By placing the footings at a sufficient depth and leaving an adequate expansion zone between the ground surface and the building, swell can take place without causing detrimental movement. A common procedure is to use belled piers with the bell at sufficient depth in the ground that the soil swell produces pull-out tension on the shaft of the whole system heaves.

Load the soil to sufficient pressure intensity to balance swell pressure. Using spread footing or replacing granular material with soil may treat the swell.

Environmental Considerations Foundation engineers have the responsibility to ensure that their portion of the total design does not have a detrimental effect on the environment. Examples; Soil boring through sanitary landfills can pollute the ground water via seepage through the boreholes. Soil boring logs should be checked for indication of effect of site of excavation on the environment in terms of runoff, pollution in runoff, odor problems, dust and noise.

Environmental Considerations One should investigate means to salvage topsoil for landscaping. Pile driver noise and vibration can be objectionable. It should be determined whether soil borings near streams cause piping problems during high water periods. These may be avoided by careful plugging of the boreholes. The effect of river and marine structures on aquatic life must be minimized.

Improving site soils for foundations use Mechanical stabilization Compaction Pre-loading Drainage Densification using vibratory equipments Use of in-situ reinforcement Use of geo-textile Chemical stabilization

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