Soil Sample


The tests are carried out in the soil in order to determine its physical and chemical properties by doing a number of an experiment in the laboratory . The soil is being tested first before performing the number of construction works.

Types of soil sample

i. Disturbed sample

In these types of sample, the natural structure of the soil gets disturbed during sampling. However, this sample represents the composition and mineral content of the soil. Disturbed sample can be used to determine the index properties of soil like grain size, plasticity index, specific gravity etc.

ii. Undisturbed sample

These are the sample in which the natural structure of soil and water content are not-disturbed (retained). However, it is not possible to get truly undisturbed sample even when great care is taken during sampling. The undisturbed sample is used for determining the engineering properties of soil such as compressibility, bearing capacity, shear strength, permeability etc.

Criteria for deciding the location and number of tests pits and bores:

i. The location extent (depth) of exploration and spacing of boreholes depends mainly on the variation of the strata in the horizontal direction. The exploration should be extensive so as to indicate the major changes in the properties of the sub-surface strata or subsoil.

ii. For small and less important building, single borehole or tests pits at the centre is sufficient.

iii. For compact building covering an area more than 0.4 hectares, there should be at least 5 boreholes i.e. 1 at the centre and other four near the corners.

iv. For large multi-storage buildings, the boreholes should be drilled at all the corners and at an important location. The spacing between boreholes is generally kept between 10 to 30 meters.

v. For highway sub-surface exploration is usually carried out along the purposed centre line at the spacing between 150-300 m, but if sub-surface is erratic, spacing may be reduced to 30 m.

Field identification of soil

A. Dilatancy Test

Select enough material from the specimen to mould into a ball of about 12 mm in diameter. Mould the material adding water if necessary until it becomes soft but not sticky inconsistency. The soil ball is smoothened in the palm of one hand with the blade of a knife. Shake horizontally the soil ball striking the side of hand vigorously against the other hand several time. Note the reaction of the water appearing on the surface of the soil ball. Squeeze the sample by closing the hand or pinching the soil between the finger and note the reaction as none, slow, rapid according to the following criteria:

i. None: No visible changes in the specimen.

ii. Slow: Water slowly appears on the surface of the specimen during shaking and does not disappear or disappear slowly upon squeezing.

iii. Water quickly appears on the surface of the specimen during shaking and disappear upon squeezing.

B. Dry Strength Test

Select enough material from specimen sample to mould into a ball of about 25 m in diameter. The material is moulded until it has the consistency of plastic state adding water if necessary. From the mould sample, make at least 3 test specimen of the ball of diameter 12 mm.Allow the test specimen to dry in sun or air or by artificial means not exceeding 60 degree Celsius. Test the strength of dry ball or lumps of soil by crushing them between the fingers and note the strength as none, low, medium, high and very high.

a. None: The dry specimen crumbles with unnoticeable pressure (mere pressure) of handling.

b. Low: The dry specimen crumbles with some finger pressure.

c. medium: The dry specimen crumbles into pieces or crumbles with considerable pressure.

d. High: The dry specimen cannot be broken with finger pressure. The specimen will brake into pieces between thumb and hard surface.

e. Very high: The dry specimen cannot be broken with thumb and hard surface.

C. Toughness Test

After completing the dilatancy test, The specimen is shaped into an elongated part and rolled by hand on a smooth surface or between the palm into a thread of about 3 mm in diameter. If the sample is too wet to roll, allow some water to evaporate. Fold the sample thread and re-roll repeatedly until the thread crumbles at a diameter of about 3 mm when the soil is near the plastic limit. Note the time required to roll the thread near the plastic limit and also note the strength of the thread. After the thread crumbles the pieces should be lumped together and knead until the lumped crumbled. Note the toughness of the material during kneading.

Describe the toughness of the thread lumped as low, medium or high according to the criteria.

1. Low: Only slight pressure is required to roll the thread and both thread and lump are weak and soft.

2. Medium: Medium pressure is required to roll the thread and the lump and thread have medium stiffness.

3. High: Considerable pressure is required to roll the thread near the plastic limit. The thread has very high stiffness

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