Measurement of Soil Compaction in the Field

It is necessary in many instances to know the wet/dry densities of the soil in the field, especially before planning activities such as compaction. The same becomes particularly important in instances such as the construction of earth dams, embankments etc. to control the field compaction, that is, to know how close are the field values to the laboratory values of ϒ_{d max} and OMC. This information is vital since the design of the earth structure has been carried out based on properties such as shear strength of the soil as compacted in the laboratory.

Method-1 [The Core Cutter Method]

This is a simple and direct method for determining the field density.

The apparatus consists of a cutter, dolly and rammer (Fig-1). The cutter weight and dimensions must be noted prior to the test. The cutter with the collar fitted on top is driven down by a weight falling on a wooden cushion (dolly) placed on top of the collar.

When the driving is complete the side soils are removed by a spade. The cutter with the soil is cut at the base and brought out. The collar is removed and the soil protruding at the ends is trimmed off. The cutter with the soil is now weighed so that the difference with the empty weight gives the wet weight of the sample which when divided by the volume gives the wet density. The soil specimen is removed from the cutter and samples taken for the determination of w. From ϒ_wet and w we calculate the dry density.

The method is not suitable for dry sand as the same cannot be retained in the cutter while lifting, besides the compaction induced by the vibration generated by the weight falling on the dolly, which affects the natural density.

Also Read: Core Cutter Test of Soil [Step-by-Step Guide]

Method-2 [The Sand Replacement Method]

Keeping a rimmed pan with a central hole of dia. 100 mm on the ground a cylindrical hole is excavated from which the soil occupying the hole is carefully collected and kept in the pan (Fig-2a). The weight of the soil is determined. Samples from the soil so collected are used to determine w.

To obtain the field (wet) density we need to determine the volume of this hole which constitutes the rest of the work.

This makes use of an equipment in the form of a sand pouring cylinder with a calibrating can (Fig-2b). The volume of the cylindrical mould can be determined by measurement. The can is typically of dia. 100 mm and its depth is supposed to be the same as the depth of the hole excavated in the soil. The sand pouring cylinder has a conical (upright) funnel-like (inverted) part at the bottom with a stopper at the junction which allows/closes the flow of sand from the cylinder.

The sand pouring cylinder is nearly filled with dry uniform sand (of size within 600 – 300 μm range). In order to determine the bulk unit weight of the sand the can is placed at the bottom of the cylinder and sand is allowed to flow down filling the can and the conical part. The sand in excess of what is needed to fill the can is removed by running a scale or spatula horizontally across the top of the can. From the weight and volume of the sand filling the can its unit weight is obtained.

Our next effort is to determine the weight of sand filling the excavated hole so that dividing the same by the unit weight of sand gives us the volume of the hole.

For this we must first determine the weight of sand filling the conical part. For this we weigh the cylinder with sand and keeping it on a level surface (Fig-2c) we run sand to fill the cone, close and weigh the cylinder again. The difference gives the weight of sand filling the cone.

In the final phase of the work, we weigh the pouring cylinder nearly filled with sand and bring it on top of the cylindrical hole excavated in the ground and empty it until it fills the hole and the conical part (Fig-2d). Now close the stopper and take the weight. The difference in weight gives the weight of sand filling the cylindrical hole and the cone. From this the weight of sand filling the cone already determined is subtracted which gives us the weight of sand filling the hole alone. Dividing it by the unit weight of sand gives the volume of the excavated cylinder. Knowing the weight and volume of the excavated soil so obtained we can determine the field density, and from the water content, the dry density.

Also Read: Sand Replacement Test of Soil [Step-by-Step Guide]

Article written by

Dr. Nainan P. Kurian