Testing of soil for building construction is the first step in the building planning process to determine the suitability of the soil for the planned construction activities. The foundation design is based on the construction site’s soil test report.
To provide outstanding performance, the soil that withstands the loads of the structure should be properly tested. If the soil is not properly tested, the entire building or structure may get damaged or even collapsed. Through this article, we bring you information on different types of soil tests carried out before construction.
Importance of Soil Testing
Soil testing is done primarily to determine bearing capability. During this process, the chemical and physical content of the soil is examined. The soil needs to be able to stand the weight of the construction.
Soil quality is affected by a variety of elements, including weather, climate change, and how the land has been used in recent years for what purpose and what was previously present.
The size of the pillars is determined based on the soil condition. Soil testing is the only means to determine the water level in the soil. If soil testing is not performed, the building will be exposed to unforeseen threats, which could prove fatal.
Types of Soil Test
Soil inspection or testing is the first step in the construction process. Various soil tests are carried out to determine soil quality for building construction. Some tests are performed in a laboratory, while others are performed in the field. The following are the popular types of soil tests:
- Moisture content test
- Specific gravity of soil
- Dry density of soil
- Atterberg limits tests
- Proctor’s Compaction test
1. Moisture Content Test
Moisture is critical in determining the soil’s weight-bearing capacity. Moisture content, or the amount of water in the soil, is a significant factor in building construction.
It is computed as the weight of a moist soil sample divided by the weight of the sample after drying. Low moisture equals greater soil strength. It is estimated by different methods, which are as follows:
- Method of drying in the oven
- Method of calcium carbide
- Torsion balance Method
- Pycnometer Method
- Sand bath Method
- Radiation Method
- The alcohol Method
The most popular and accurate drying method is oven drying. In this process, a soil sample is collected, weighed, and dried in an oven at 110o ± 5oC. After 24 hours, the soil is removed and weighed. The difference in weight is referred to as the weight of water or moisture content in the soil.
2. Specific Gravity Test
The specific gravity of soil is the ratio of the unit weight of soil solids to that of water. It is useful to know how permeable the soil is.
The optimal range of soil specific gravity for construction purposes is 2.65-2.85. There are numerous methods for determining it.
- The density bottle method
- Pycnometer method
- The procedure of the gas jar
- Method of shrinkage limitation
- Method for measuring flasks
The density bottle and Pycnometer methods are basic and widely used approaches. Pycnometers are weighed at room temperature in four different cases:
empty weight (M1), empty + dry soil (M2), empty + water + dry soil (M3), and Pycnometer filled with water (M4). The formula below is used to calculate specific gravity from these four masses.
3. Dry Density Test
The weight of soil particles in a given volume of sample is defined as the dry density of soil. The void ratio and specific gravity of the soil determine the dry density of the soil.
Based on its dry density, the soil is classified into three types: dense, moderately dense, and loose. The dry density is calculated using the core cutter method, sand replacement method, and water displacement method.
Core Cutter Method
A cylindrical core cutter of conventional dimensions is used in this approach to cut the dirt in the ground and pull the cutter up with a soil sample.
The sample is weighed and recorded. Finally, the water content of the sample is determined, and the dry density is calculated using the equation below.
Sand Replacement Method
In this approach, a hole is dug in the earth by digging the soil to determine its dry density. The pit is filled with known dry density homogeneous sand.
So the volume of the hole is calculated by dividing the amount of sand thrown into the hole by the dry density of sand. So, using the formula mentioned above, we can compute the soil dry density.
4. Atterberg Limits Test
To evaluate the critical water content of fine-grained soil, Atterberg proposed three limits that highlight the properties of fine-grained soil under different conditions. There are three limits: liquid, plastic, and shrinkage. Individual tests calculate these limits as follows.
Liquid Limit Test
Casagrande’s liquid limit gadget, which consists of a cup with a sliding up and down mechanism, is employed in this test. The cup is filled with a soil sample, and a groove is made in the center of the cup with the appropriate equipment.
When the cup is moved up and down with the handle, the groove eventually closes. Keep track of how many blows it takes to seal the groove. The soil’s water content is then determined.
Repeat this technique three times and create a graph showing the relationship between log N and soil water content. The liquid limit of soil is the water quantity equivalent to N = 25.
Plastic Limit Test
Add enough water to the soil sample to make it pliable to mold into a small ball. After some time, place the ball in the glass plate and roll it into 3mm diameter threads. If the threads do not break when we roll it to less than 3mm diameter, the water content is greater than the plastic limit.
Reduce the amount of water and continue the method until crumbling occurs at 3mm diameter. Finally, determine the water content of the resulting soil, whose value is nothing more than a plastic limit.
Shrinkage Limit Test
In the case of a shrinkage limit, the water content in the soil is just enough to fill the soil gaps. The saturation level is set to 100%. As a result, lowering the shrinkage limit does not affect the volume of soil. For the given soil sample, it is determined by the formula below.
- M1 = initial mass,
- V1 = initial volume
- M2 = dry mass
- V2 = Volume after drying
- Pw = density of water
5. Proctor’s Compaction Test
Proctor’s test is used to measure soil compaction characteristics. Soil compaction is simply the densification of air spaces in the soil. Compaction is assessed in terms of the dry density of the soil.
In Proctor’s Compaction Test, a soil sample is sieved through 20mm and 4.75mm sieves. The percentage passing 4.75mm and the percentage detained at 4.75mm are blended in specific quantities.
Fill it with water and store it in an airtight container for 20 hours. Mix the soil and divide it into 6 – 8 equal parts. Position the mold and pour one part soil into it in three layers, using 25 ramming strokes for each layer.
Remove the base plate and weigh the soil and mold together. Remove the dirt from the mold and collect a small quantity of soil from different layers to perform a water content test.
Determine the dry density of soil and water content from the values and prepare a graph between them, noting the maximum dry density and optimum water content of the compacted soil sample at the highest point on the curve.
Though soil testing before construction is required, the standards vary depending on the soil and the project requirements. The significance of soil tests may be liked through the reality that most governments of the world have made it a mandatory process by law and will only approve a building’s construction plan after receiving the soil test report.
Nonetheless, many people skip this process to save money or simply because they are clueless. The outcomes of such occurrences are almost always fatal. The soil test is the first step towards a long and safe future. Hope this article has deepened your understanding of the different types of soil tests required for building construction.