Question 4 A sample of soil has been taken 1m below an existing water table (a)
ID: 116397 • Letter: Q
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Question 4 A sample of soil has been taken 1m below an existing water table (a) Would you expect this sample to be saturated? (1 mark) When tested, the total density of the soil was shown to be 1800 kg/m3. Further testing showed that the specific gravity of the solid particles was 2.7 and the moisture content (on mass basis) of the sample was 25% b) Calculate the relative proportions (in terms of volume) of solids and water in the soil sample. (8 marks) Further testing showed the soil sample to be highly expansive. Excavation of the area, following a lowering of the water table, for residential buildings is planned for the site where the soil is located. (c) What recommendations would you make as to foundation pressures for the buildings or soil modification procedures to prevent possible damage to the buildings? (6 marks)Explanation / Answer
(a) Yes, since the sample is below the water table hence it is expected to be a saturated sample.
(b) Moisture content on the basis of mass is 25%
Mass of moisture = (25/100)1800
=450 kg
Moisture is soil refers to the water having denstity of 1000kg/m^3
Using formula, Density=Mass/Volume
volume = mass/density = 450/1000 =0.45
volume in percentage of water(moisture)= 0.45*100= 45%
volume in percentage of solids = 100% - 45% = 55%
(c) If the moisture content is below or near plastic limit, the soils have high potential to swell. It has been reported that expansive soils with liquidity index* in the range of 0.20 to 0.40 will tend to experience little additional swell.
There are a number of factors that can influence whether a soil might shrink or swell and the magnitude of this movement.
Antecedent Rainfall Ratio: This is a measure of the local climate and is defined as the total monthly rainfall for the month of and the month prior to laying the slab divided by twice the average monthly rate measured for the period. The intent of this ratio is to give a relative measure of ground moisture conditions at the time the slab is placed. Thus, if a slab is placed at the end of a wet period, the slab should be expected to experience some loss of support around the perimeter as the wet soil begins to dry out and shrink. The opposite effect could be anticipated if the slab is placed at the end of an extended dry period; as the wet season occurs, uplift around the perimeter may occur as the soil at the edge of the slab gains in moisture content.
Age of Slab: The length of time since the slab was cast provides an indication of the type of swelling of the soil profile that can be expected to be found beneath the slab.
Lot Drainage: This provides a measure of the slope of the ground surface with respect to available free surface water that may accumulate around the slab. Most builders are aware of the importance of sloping the final grade of the soil away from the structure so that rain water is not allowed to collect and pond against or adjacent to the foundations. If water were allowed to accumulate next to the foundation, it would provide an available source of free water to the expansive soil underlying the foundation. Similarly, surface water drainage patterns or swales must not be altered so that runoff is allowed to collect next to the foundation.
Topography: This provides a measure of the downhill movement that is associated with light foundations built on slopes in expansive soil areas. The designer should be aware that as the soil swells, it heaves perpendicularly to the ground surface or slope, but when it shrinks, it recedes in the direction of gravity and gradually moves downslope in a sawtooth fashion over a number of shrink-swell cycles. In addition to the shrink-swell influence, the soil will exhibit viscoelastic properties and creep downhill under the steady influence of the weight of the soil. Therefore, if the building constructed on this slope is not to move downhill with the soil, it must be designed to compensate for this lateral soil influence.
Pre-Construction Vegetation: Large amount of vegetation existing on a site before construction may have desiccated the site to some degree, especially where large trees grew before clearing. Constructing over a desiccated soil can produce some dramatic instances of heave and associated structural distress and damage as it wets up.
Post-Construction Vegetation: The type, amount, and location of vegetation that has been allowed to grow since construction can cause localized desiccation. Planting trees or large shrubs near a building can result in loss of foundation support as the tree or shrub removes water from the soil and dries it out. Conversely, the opposite effect can occur if flowerbeds or shrubs are planted next to the foundation and these beds are kept well-watered or flooded. This practice can result in swelling of the soil around the perimeter where the soil is kept wet.
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