Table of Contents for Soil Tests for Construction-
- Introduction to soil tests for construction
- Why to test soil before construction
- Test 1: Particle Size Analysis Tests
- Test 2: Moisture Content Tests
- Test 3: Dry Density Tests
- Final Words
Introduction to Soil Tests for Construction
The foundation of the structure transfers loads of the building to the soil beneath it. The soil properties majorly govern the foundation design. Some of the soil properties that affect the foundation design are density, moisture content, soil type, soil particle distribution etc.
In this article, we will go through the top 3 tests that you can conduct at the site as per your requirement and convenience. The mentioned tests are for particle size distribution (sieve analysis), moisture content of the soil, and dry density. All the test methods are explained as per Indian Standards (BIS).
Before moving to the tests, let us start with the ‘why’ part first and clear our doubts about the need for these tests.
Why to test soil before construction?
The fundamental reasons for testing the soil are-
- Better stability and safety of structure
- Design optimisation
- To decide the foundation type
- To ascertain the soil settlement under the loads
The soil properties change with location, depth, geology, water table depth, vegetation, hydrology and local conditions. It is important to ascertain the soil properties before starting construction else the structure may collapse. Other than collapse, the structure may tilt or develop cracks due to uneven settlement. Thus, degrading the structure’s aesthetics.
The correct assessment of soil properties will optimise the foundation design for stability, durability and cost. Without the understanding of soil properties for that location, the foundation may be made bulkier to provide stability and safety but that will be uneconomical.
Also, the cost of soil testing is way lesser than the cost of extra concrete and the risk involved.
Test 1: Particle Size Analysis Test
The sieve test determines the soil particle size. The soil particle may vary from 0.002mm to 300mm depending upon the soil type. The Indian Standard Soil Classification System (ISSCS) categorises the soil into very fine, coarse and very coarse groups.
The ISSCS classification is as follows-
Relevant Code- IS-2720 (Part 4):1985 (Reaffirmed- May 2015), AASHTO T-88.
The soil particles retained on the 4.75mm IS sieve are tested separately from the particles passing through the sieve. The retained particles are dried in an oven at 105-110 degrees centigrade and weighed. Sieve the dried soil on 4.75 mm and higher sizes. Record the weight retained on each sieve.
The particles passing through the 4.75mm IS sieve are dried in the oven and weighed. Wash the soil on a 0.075mm sieve until the clean water starts coming out of the sieve. The remaining soil is again oven-dried and sieved through 2mm, 0.425 mm and 0.075 mm sieves.
The % cumulative weight retained on each sieve is deducted from 100 to get the % finer. This data is plotted against the size of the particle to get the particle size distribution curve.
- Curve A: Uniform Sand
- Curve B: Well-graded Soil
- Curve C: Poorly Graded Soil
- Curve D: Sandy Silt
- Curve E: Silty Clay
Test 2: Moisture Content Test
The soil has voids filled with air or water. The water present in these voids affects the soil properties. The soil density mainly depends upon the moisture content. Hence, it is necessary to determine the water content before assessing the density. Also, soil compaction is also directly linked to its moisture content.
The moisture content is the ratio of the weight of water to the weight of solids. This ratio is multiplied by 100 and represented in terms of percentage.
Relevant Code: IS-2720 (Part 2):1973 (Reaffirmed- May 2015).
The following in-situ methods are generally used to check the moisture content of soil-
- Pycnometer Method: It uses a bottle shaped container called pycnometer of capacity approx. 900 ml. The empty pycnometer is weighted (W1). It is then weighed with some soil. (W2). The pycnometer is now filled with water keeping the soil in it (W3). It is weighed fourth time with only water filled to its’ full capacity (W4). The formula for calculating the moisture content by pycnometer is below-
- Rapid Moisture Meter Method: Ituses the chemical reaction to determine the moisture content. The calcium carbide is put into the moisture meter that reacts with the soil moisture to produce acetylene. The meter attached gives the reading in % of water content of soil. This water content is on wet soil basis and should be converted to dry basis as follows-
- Sand Bath Method: Itis a quick in-situ method. Thesoil sample is weighed and heated on the sand bath for uniform heating. The weight loss (W) after heating is recorded and divided by the weight of dry soil (Ws) to get the moisture content.
Test 3: Dry Density Test
The density of soil is one of the most important properties of soil. It helps in assessing the compaction and voids. The field density (field dry density, FDD) is represented as a percentage of Standard Proctor density (maximum dry density, MDD).
The density is assessed on a dry basis as the bulk density of soil varies with the change in volume of soil due to varying moisture content.
Relevant Codes- IS 2720 (Part 28 & 29), AASHTO T99-86, ASTM D446-82
The dry density is tested by the following methods-
- Core Cutter Method: The core cutter method uses a 130mm long and 100 mm diameter core that is rammed in to the soil. The weight of the soil inside the core is calculated and divided by the core volume to get the bulk density of the soil.
The moisture content of the soil is also determined to get the dry density by the formula-
- Sand Replacement Method: It is a very common method of dry density determination. It uses a cylindrical apparatus and a sand filled cone. The conical part is filled with dry and graded sand. The calibration of cone apparatus is done first.
A hole is dug into the ground with a diameter equal to the density plate hole. The excavated soil is weighed. The cone is placed over the excavated hole and the shutter is opened to allow the sand to fill the excavated hole. The weight of the sand-filled in the hole is calculated by taking before and after readings of the cone.
Determine the moisture content of the soil.
The volume of the hole is calculated by dividing the sand weight in the hole by the density of soil. This volume is then used to divide the weight of soil to get the bulk density. The formula mentioned for the core cutter method is also used here to get the dry density.
Soil testing is of utmost importance in the design process of any structure as well as in the construction process. The tests mentioned above are the most basic and must be conducted to check the quality of work. However, only these tests are not sufficient to fully satisfy the design or quality needs and must be clubbed with other tests.
You can download the IS code test manuals here.
Please comment below and let me know what tests do you conduct at your site.