The stress on any plane surface can be resolved into the normal stress, σn, which acts perpendicular to the surface and the shearing stress, τ, which act along the surface, the magnitude of the resistance being given by Coulomb's equation:
τ = c + σn tan Ø
Where,
τ = shear strength of soil
c = cohesion
σn = normal stress
Ø = angle of friction
Shear Strength of Clays
The shear strength of clays depends not only on the soil type and composition, but also on factors related to the mineralogy, grain, size and shape, adsorbed water and water chemistry of the clay minerals present. Shear strength also depends to a great extent upon the initial moisture content of the clay and the rate at which the soil structure can expel or take in water during a test.
In a shear test on a particular type of clay the three factors which are of the greatest significant here are:
- the water content (expressed in terms of liquidity index);
- the degree of saturation - whether fully saturated or partially saturated;
- the rate of shear displacement in relation to the permeability of the soil - whether "quick" (allowing no drainage), or "slow" (allowing full drainage, i.e. the dissipation of any excess pore water pressure set up during shear).
The undrained shear strength of saturated plastic clays is usually determined from compression tests. The shearbox test being less satisfactory for these soils. However their shear strength can be measured directly by the vane apparatus [11]. Clay soils are cohesive in that they possess some strength at zero normal pressure. Consequently, their strength envelope looks like that shown in Figure 2.3. The slope of the strength envelope depends greatly on drainage conditions as the clay is being loaded. For most short-term loading conditions, the usual situation during construction, the slope of the strength envelope is near zero if the clay is saturated. The strength of the clay is, therefore, expressed by the cohesion term, c [9]. For a given clay, unconfined strength or cohesive strength will depend strongly on the water content. Table 7.2 indicates typical ranges of strength corresponding to standard nomenclature for soil consistency. Soils of stiff to hard consistency seldom present problems when encountered in construction [9].
Moisture Content
Naturally occurring soils nearly always contain water as part of their structure. The moisture content of a soil is assumed to be the amount of water within the pore space between the soil grains which is removable by oven drying at 105-110°C, expressed as a percentage of the mass of dry soil. By "dry" is meant the result of oven drying at that temperature to constant mass, usually for a period of about 12-24 hours. In non-cohesive granular soils this procedure removes all water present.
There are several ways in which water is held in cohesive soils, which contain clay minerals existing as plate-like particles of less than 2 μm across. The shape and very small size of these particles, and their chemical composition, enable them to combine with or hold on to water by several complex means [12]. For the purpose of routine soil testing, moisture content relates only to the water which is removable by oven drying at 105-110°C. Moisture content is usually expressed as a percentage, always on the basis of the oven-dry mass of soil.
Methodology
Research methodology is used to described the overall work plans of the research for the whole semester. It is normally written in flow chart that can be easily to read and understand.
Research flow chart is a guideline that will be followed throughout the research to achieve the best result as shown below.
- Research Title
- Literature Review
- Selection of Research Area
- Sample Preparation for Laboratory Test
- Laboratory Test
- Undisturbed sample
- Direct Shear Test
- Vane Shear Test
- Moisture Content Test
- Free Swell Test
- Disturbed Sample
- Direct Shear Test
- Vane Shear Test
- Moisture Content Test
- Data Collection
- Data Analysis
- Conclusion and Recommendation
Moisture Content Test (BS 1377: Part 2: 1990: Clause 3)
The standard method that will be used for this test is the oven-drying method and this is the procedure recommended for a soils laboratory.
This moisture content test includes of disturbed and undisturbed sample. Undisturbed sample is needed to determine the actual moisture content of sample taken from the research area. To change the percentage of water for the purpose of this study, disturbed sample will be used. The percentage of water will be increased from 15% to 65% with gap of 10%. To determine the mass of water needed to produce 15% until 65% of water content, the following equation can be used:
w (%) = mw x100
mp
where;
w = moisture content
mw = loss of moisture
mp = dry mass
Free Swell Test
