In order to determine the suitability of a particular soil it is essential to know the proportion of each material in the soil and also whether the particular material possesses the properties required of it.
For instance, a road soil that is 95 percent sand will not cohere, and clay that does not possess the property of cohesion is useless as a road material and will do nothing but make dust.
Therefore it is required to perform some field test prior to use soil in road work. The following 4 filed test can be made to check its suitability for road work.
1. TO DETERMINE PROPORTION OF SAND IN THE SOIL
Take a sample of the dry soil and weigh it. Put it in a glass and fill with water. Agitate it and pour off the clay. Do this several times until nothing but sand remains in the glass. Dry the sand and weigh it. The result will give the percentage of sand in the soil. The remainder is clay and silt.
2. TO DETERMINE THE PROPORTION OF CLAY & SILT
Silt is generally darker in colour than clay and a sample that contains too high a percentage of silt will not have the characteristic of brown colored clay. Silt settles more rapidly than clay.
If the sample is put into a glass and mixed with water and allowed to settle the clay will remain muddy while the silt will settle within a few minutes. A sample that clears very quickly has too much silt. In this type of soil it is required to add some clay before using it as a road material.
3. TO DETERMINE QUALITY OF CLAY
Pure clay is very retentive of moisture and becomes plastic and unstable when wet, and as it abrades easily, produces all dust when dry. The extent to which these objections occur depends on whether the clay is of slaking or non-slaking variety. The slaking variety is undesirable as it is muddier in wet weather and dustier in dry weather. To determine the qualities of various clays perform slaking test as described below.
Make several balls of the same size of the different clays and dry them out. Place them in water so that they are covered entirely. The balls which hold their shape longest after being placed in water have the highest resistance to slaking, and that clay is to be preferred for use in the road construction.
It is important in this test that if various clays are being compared, the proportion of sand in each sample should be the same and should not exceed 25 percent. If sand is in excess, it should be removed before doing the slaking test.
If the clay is of the slaking variety, the balls will disintegrate almost as soon as they are put in water, such a clay is not suitable for road work. Samples that contain too much silt will not show good non-slaking qualities and will break up at once in water. Clay requires to be added in such soils before using it.
4. TO DETERMINE THE SUITABILITY OF SAND
Place a sample of the sand in a vessel containing water and agitate the water until the sand is thoroughly in suspension. Then when the sand has been allowed to settle pour off the water slowly. Good quality sand will not be carried off with the water but will remain in the vessel until practically all the water has been drained off. Bad quality sand will not meet this test and is not suitable of use on road works.