How to Determine the In-Situ Density of Soils ?

It is often required to determine the density of soil in the natural state (in-situ density). There are several methods available for the determination of in-situ density. Two commonly used methods are: 1. Core Cutter Method 2. Sand Replacement Method.

1. Core Cutter Method:

Core cutter method is used to determine the in-situ density of fine-grained natural or compacted soils free from aggregates using a core cutter.

Principle:

In this method, a core cutter is driven into the undisturbed soil at the site and the in-situ density is determined by taking the weight and volume of the soil along with the core cutter. This method is suitable only for fine-grained natural or compacted soils free from aggregates.

Equipment:

The apparatus used in core cutter method, consists of the following:

i. Cylindrical core cutter of seamless steel tube, 13-cm long and 10-cm internal diameter, with a wall thickness of 0.3 cm, beveled at one end. The cutter should be kept properly greased or oiled.

ii. Steel dolly, 2.5 cm high and 10-cm internal diameter, with a wall thickness of 7.5 mm with a lip to enable it to be fitted on top of the core cutter.

iii. Steel rammer with solid mild steel foot, 140-mm diameter and 75-mm height, with a concentrically screwed 25-mm diameter solid mild steel staff. The overall length of the rammer including the foot as well as the staff should be approximately 900 mm. The rammer (foot and staff together) should weigh approximately 9 kg.

iv. Balance accurate to 1 g.

v. Palette knife of a convenient size; one having a blade approximately 20-cm long and 3-cm wide.

vi. Steel rule.

vii. Grafting tool or spade or pick axe.

viii. Straight edge, a steel strip about 30-cm long, 2.5-cm wide and 3- to 5-mm thick, with one bevelled edge will be suitable.

ix. Apparatus for the determination of water content.

Procedure:

The in-situ density of soil in core-cutter method is determined in the following steps:

i. The internal volume (V_c) of the core cutter in cubic centimeters is calculated from its dimensions which shall be measured to the nearest 0.25 mm.

ii. The cutter is weighed to the nearest gram (W_c).

iii. A small area, approximately 30 cm² of the soil layer to be tested, is exposed and levelled.

iv. The steel dolly is placed on the top of the cutter and the cutter is rammed down vertically into the soil layer until only about 15 mm of the dolly protrudes above the surface, care being taken not to rock the cutter.

v. The cutter is then dug out of the surrounding soil, care being taken to allow some soil to project from the lower end of the cutter. The ends of the soil core are then trimmed flat to the ends of the cutter by means of the straight edge.

vi. The cutter containing the soil core is weighed to the nearest gram (W_s).

vii. The soil core is removed from the cutter and a representative sample shall be placed in an air-tight container and its water content (ω) determined.

viii. It is necessary to make a number of repeat determinations (at least three) and to average results, since the dry density of the soil varies appreciably from point to point. The number of determinations should be such that an additional one would not alter the average significantly.

Calculations:

i. In-Situ Bulk Density of Soil:

In-situ bulk density is determined using Eqs. (4.72) and (4.73)

γ = (W_s – W_c)/V_c …(4.72)

V_c = d²h/4 …(4.73)

where W_c is the weight of empty core cutter, W_s is the weight of core cutter plus wet soil, V_c is the volume of core cutter, d is the inner diameter of core cutter, and h is the inner height of the core cutter without dolly.

ii. Natural Moisture Content:

Natural moisture content is calculated from the usual relation –

ω = [(W₂ – W₃)/(W₃ – W₁)] x 100

where W₁ is the weight of empty cup, W₂ is the weight of cup + wet soil, and W₃ is the weight of cup + dry soil. The average of three trials is taken as the natural moisture content

iii. In-Situ Dry Density of Soil:

In-situ bulk density is determined using the following usual relation:

γ_d = γ/(1 + w)

where ω is the natural moisture content of the soil.

2. Sand Replacement Method:

Sand replacement method is used to determine in-place dry density of natural or compacted fine-and medium-grained soils. This method is applicable to layers not exceeding 15 cm in thickness.

Principle:

In this method, the volume of excavated soil or site hole is determined indirectly by calibrating the density of a freely draining sand. The density of the calibrating sand is first determined in the laboratory. The sand is then filled in the site hole from where the soil is excavated. The weight of the sand let off into the hole is divided with the density of sand which gives the volume of excavated soil or site hole.

The method is useful for cohesionless soils for which undisturbed sample cannot be collected using the core cutter. The method is also useful where core cutter cannot be driven due to the presence of hard soil or stones in the soil mass.

Equipment:

The apparatus used in sand replacement method, consists of the following:

i. Sand-pouring cylinder.

ii. Tools for excavating holes – a scraper tool to make a level surface, bent spoon, and a dibber.

iii. Cylindrical calibrating container with an internal diameter of 100 mm and an internal depth of 150 mm fitted with a flange, approximately 50-mm wide and about 5-mm thick, surrounding the open end. The volume of the container should be given to an accuracy of 0.25%.

iv. Balance – accurate to 1 g.

v. Plane surface such as glass or perspex plate or other plane surface – about 450-mm square and 9-mm thick or larger.

vi. Metal containers to collect excavated soil. A convenient size is one about 150-mm diameter and 200-mm deep with a removable cover.

vii. Cylindrical steel core cutter of steel, 127.4 ± 0.1-mm long and 100 ± 0.1-mm internal diameter with a wall thick­ness of 3 mm and bevelled at one end. The cutter shall be kept adequately greased.

viii. Metal tray, 300 mm square and 40-mm deep, with a 100-mm hole in the center.

ix. Clean, uniformly graded natural sand passing the 1.0 mm IS sieve and retained on 600-micron IS sieve is used as the calibrating sand. It should be free from organic matter and shall have been oven dried and stored for a suitable period to allow its water content to reach equilibrium with atmospheric humidity.

Procedure:

The procedure is done in two steps which are discussed below:

Step 1 – Calibration of Apparatus:

Calibration of the apparatus consists of the determination of the bulk density of sand and is as follows:

i. The pouring cylinder is filled with sand so that the level of the sand in the cylinder is within about 10 mm of the top. Its total initial weight (W₁) is found and shall be maintained constant throughout the tests for which the calibration is used.

ii. A volume of sand equivalent to that of the excavated hole in the soil (or equal to that of the calibrating container) is allowed to run out of the cylinder under gravity. The shutter on the pouring cylinder is then closed and the cylinder is placed on a plane surface such as a glass plate.

iii. The shutter on the pouring cylinder is opened and sand is allowed to run out. When no further movement of sand takes place in the cylinder, the shutter is closed and the cylinder is removed carefully.

iv. The sand that has filled the cone of the pouring cylinder (that is, the sand that is left on the plane surface) is collected and weighed to the nearest gram.

v. These measurements are repeated at least three times and the mean weight is taken.

vi. The internal volume (V) in ml of the calibrating container is determined from the weight of water contained in the container when filled to the brim. The volume may also be calculated from the measured internal dimensions of the container.

vii. The pouring cylinder is placed concentrically on the top of the calibrating container after being filled to the constant weight (W₁) as in step 1. The shutter on the pouring cylinder is closed during this operation. The shutter is opened and the sand is allowed to run out. When no further movement of sand takes place in the cylinder, the shutter is closed. The pouring cylinder is removed and weighed to the nearest gram. These measurements are repeated at least three times and the mean weight is taken.

Step 2 – Measurement of Soil Density:

The following method shall be followed for the measurement of soil density:

i. A flat area, approximately 450 mm², of the soil to be tested is exposed and trimmed down to a level surface pref­erably with the aid of a scraper tool.

ii. The metal tray with a central hole is laid on the prepared surface of the soil with the hole over the portion of the soil to be tested. The hole in the soil shall then be excavated using the hole in the tray as a pattern, to the depth of the layer to be tested up to a maximum of 150 mm.

iii. The excavated soil is carefully collected, leaving no loose material in the hole and weighed to the nearest gram (W_m). The metal tray is removed before the pouring cylinder is placed in position over the excavated hole.

iv. The water content (ω) of the excavated soil is determined by the method specified in IS – 2720 (Part 2) – 1973.

v. The pouring cylinder filled to the constant weight (W₁) as in step 3 is placed so that the base of the cylinder covers the hole concentrically. The shutter on the pouring cylinder is closed during this operation. The shutter is then opened and the sand is allowed to run out into the hole.

vi. When no further movement of sand takes place, the shutter is closed. The cylinder is removed and weighed to the nearest gram (W).

Precautions:

Following precautions should be taken while using the sand replacement method:

i. Since variations in atmospheric humidity affect the water content of the sand, and hence its bulk density, the cali­bration should be made (or at least checked) during each day’s work. To overcome the effects of slight variations in grading and particle shape between batches of sand, each batch should be sampled and calibrated.

ii. If for any reason, it is necessary to excavate the holes to depths other than 150 mm, the calibrating container should be replaced by one whose depth is the same as the hole excavated or its effective depth should be reduced to that of the hole excavated.

iii. Care should be taken in excavating the hole to see that the hole is not enlarged by levering the dibber against the side of the hole, as this will result in lower densities being recorded.

iv. It is necessary to make a number of repeated determinations (at least three) and to average the results because the dry density of the soil varies appreciably from point to point. The number of determinations should be such that an additional one would make no significant difference to the average.

v. Generally a storage period, after oven drying, of about 7 days is sufficient for the water content of the sand to reach equilibrium with the atmospheric humidity. The sand should not be stored in air-tight containers and should be thoroughly mixed before use. If sand is salvaged from holes in compacted soils after carrying out the test, it is advisable to sieve, dry, and store this and again before it is used in further sand replacement tests.

Calculations:

The in-situ density is calculated as explained below:

i. Bulk Density of Sand:

Bulk density of sand is calculated from –

γ_sand = W_s/ V₁ …(4.74)

W_s = W₁ – W₂ – W₃ …(4.75)

V₁ = d²h/4

where W_s is the weight of sand in the calibrating container, V₁ is the volume of calibrating container, W₁ is the weight of sand pouring cylinder plus sand, W₂ is the weight of sand pouring cylinder plus sand after filling the calibrating container, W₃ is the mean weight of sand in cone (of pouring cylinder), d is the inside diameter of the calibrating container, and h is the inside height of the calibrating container.

ii. Volume of Site Hole:

Volume of the excavated soil is calculated from –

V = W₇/γ_sand …(4.77)

W₇ = W₁ – W₃ – W₆ …(4.78)

where W₇ is the weight of sand filling the site hole and W₆ is the weight of sand pouring cylinder plus sand after filling the site hole.

iii. In-Situ Bulk Density of Soil:

It is given by –

γ = W₈/V …(4.79)

where W₈ is the weight of excavated soil and V is the volume of excavated soil.

iv. Natural Moisture Content:

Natural moisture content is calculated from the usual relation –

ω = [(W₁₀ – W₁₁)/(W₁₁ – W₉)] x 100

where W₉ is the weight of empty cup, W₁₀ is the weight of cup + wet soil, and W₁₁ is the weight of cup + dry soil. The average of three trials is taken as the natural moisture content.

v. In-Situ Dry Density of Soil:

It is given by the usual relation –

γd = γ/(1 + w)

where γ is the in-situ bulk density of soil and w is the natural moisture content of soil.