In order to improve the stability and strength of structures, soil investigation is necessary for construction projects. The qualities of the soil must be checked to find out its bearing capacity, composition and possible risks before the placement of any foundation. Poor soil analysis can result in excessive settlement, structural problems, or even building collapse.
With the help of well-planned geotechnical research, engineers can stay away from costly corrective treatments by building suitable foundations and choosing the right materials. In order to assess the subsurface conditions, a number of soil investigation methods are used, each suited to meet the needs of project and site conditions. The main methods of soil exploration that are mostly used in construction are discussed in this article.
Why is Soil Investigation Necessary?
Soil investigation is done before foundation design or construction site selection to:
- Check that the soil is able to carry the desired load by calculating its bearing capacity.
- Assess the permeability, shear strength, moisture content and particle size of the soil.
- Find out possible risks by observing the signs of settlement, expansive soils or liquefaction.
- Choose the most cost-effective and efficient type of foundation for the best foundation design.
- Reduce construction failures by avoiding problems like structural instability and uneven settling.
Neglecting soil investigation can result in costly modifications, delays and even catastrophic structural failures.
Methods of Soil Exploration
Soil investigation can be grouped into the following methods:
1. Direct Methods
2. Semi-Direct Methods
3. Geophysical Methods
Below is a detailed explanation of these methods of soil exploration.
1. Direct Methods
These methods include direct access to soil layers, providing a clear view of subsurface conditions. Direct methods are straightforward and often involve excavation to inspect the soil. They are:
Test Pits and Trenches (H4)
Process: Excavating pits to visually inspect the soil profile.
Best for: Depending on site conditions, the shallow foundation projects normally go up to the depth of the water table, which is usually between 1.2 and 3 meters.
Advantages: Simple, cost-effective and provides an accurate representation of soil strata.
Limitations: Labour-intensive, time-consuming and not suitable for deep investigations.
2. Semi-Direct Methods
These methods involve drilling and boring to collect soil samples without extensive excavation. Two commonly used techniques are:
(i) Auger and Shell Boring
Process: A hand or power-driven auger drills into the soil, and a shell collects samples.
Best for: Soft soil, clay and silt. Generally, hand augers work well up to 8-10 meters, however, depending on the situation, shell and auger drilling can go as deep as 60 meters.
Advantages: Economical, easy to operate and effective for cohesive soils. It allows for quick sample collection.
Limitations: Ineffective for hard or gravelly soils and cannot penetrate deeper layers. It needs manual or mechanical effort.
(ii) Wash Boring Method of Soil Exploration
Process: A hollow drill pipe is inserted into the ground, and water is forced through it to bring soil particles to the surface.
Best for: Loose sands, silts, and clayey deposits.
Advantages: It can go deeper than hand auger drilling and works well with various types of soils, except for those that have stones and gravel.
Limitations: It requires water and careful handling of samples. It may not provide undisturbed samples.
3. Geophysical Methods
Geophysical methods of soil exploration are advanced methods using scientific principles to assess subsurface conditions without extensive drilling. Common geophysical methods are:
(i) Seismic Refraction
Process: Seismic waves are generated and their travel times through soil layers are recorded to determine stiffness and rock depth.
Best for: Large-scale projects can include shallow to moderate-depth investigation. To find the bedrock depth, more advanced methods or tools might be needed.
Advantages: Non-invasive, quick, and effective in rocky terrain.
Limitations: It needs specific tools and knowledge.
(ii) Electrical Resistivity Method
Process: Passing an electrical current through the soil and measuring its resistance to detect water tables and subsurface materials.
Best for: Locating groundwater, clay layers, and voids. The depth of investigation depends on electrode spacing and soil conditions.
Advantages: Useful for large areas and provides continuous data.
Limitations: Affected by moisture content and needs interpretation.
Additional Key Soil Testing Methods
Apart from standard drilling and boring methods, in-situ tests help determine soil properties at the construction site.
1. Standard Penetration Test (SPT)
The standard penetration test is used in in-situ soil exploration methods. It involves driving a split-barrel sampler into the soil using a hammer of standardised weight and height. The number of hammer blows needed to drive the sampler a specific distance is recorded as the SPT N-value. This value is an indicator of soil strength and density.
Procedure:
1. The required depth is reached by drilling a borehole.
2. At the bottom of the borehole there is a split-barrel sampler.
3. The sampler is driven into the soil using a hammer with a specified energy.
4. The number of blows required to drive the sampler in three successive increments of 150mm is recorded. The sum of the blows for the last two increments is the SPT N-value.
Application: In foundation design for buildings and bridges. To estimate soil strength and density and to evaluate liquefaction potential in seismic zones.
2. Dynamic Cone Penetration Test (DCPT)
A quick and cheap way to test the strength and compaction of soil is using the dynamic cone penetration test. It involves driving a cone-tripped rod into the ground using a falling weight. Soil resistance can be measured by counting the number of blows needed to push the cone to a given depth.
Advantages: It is quick, cost-effective, and suitable for shallow investigations. It helps assess soil compaction and uniformity.
Limitations: It is unsuitable for deep exploration and is limited to granular soils.
Also Read: Soil Testing in Construction: Why it’s Required & What Are Its Methods
Purpose of Soil Investigation
Understanding the need for soil investigation is important before going deep into any specific methods. Geotechnical investigation helps engineers:
- Determine soil bearing capacity.
- Find any possible issues with the soil like liquefaction, expansive soil or settlement.
- Plan foundation designs tailored to site conditions.
- Stay away from costly construction errors.
Construction projects face a risk of serious damage due to structural instability if proper soil exploration is not done.
Factors Affecting the Selection of Soil Investigation Methods
Choosing the best soil investigation method depends on a few factors:
- Depth of Investigation: For small buildings, shallow methods like test pits are enough. For multi-story structures, deeper methods like wash boring are needed.
- Soil Type: Cohesive soils are good for auger boring. However, non-cohesive or gravelly soils might need wash boring or geophysical methods.
- Site accessibility: Because geophysical methods of soil exploration cause less disturbance, they might be more practical in urban or remote areas.
- Budget and time constraints: Direct methods are economical for small projects, while geophysical methods, though costlier, save time on larger sites.
Applications of Soil Investigation
Soil exploration is integral to various phases of construction, including:
- Foundation design: The correct foundation size and type are ensured by calculating the soil bearing capacity. In cases where soil conditions are challenging, implementing proper soil stabilization techniques can further enhance foundation performance and structural integrity.
- Earthworks and embankments: The properties of the soil affect the selection of materials and the level of compaction needed for roads, dams, and embankments.
- Site selection and feasibility studies: Geotechnical investigation helps in choosing the best locations for construction projects.
Case Study: Choosing the Right Method for a High-Rise Building
There are several soil investigation methods that can be used for an urban high-rise development project, such as:
1. Initial assessment: Seismic refraction and other geophysical methods provide complete information about the subsurface conditions of the soil.
2. Detailed investigation: Wash boring collects samples from deeper layers for laboratory testing to determine soil strength and compressibility.
3. Supplementary testing: Electrical resistivity identifies potential groundwater issues.
Engineers can design a safe and cost-effective foundation system for high-rise structures by combining different methods.
Soil investigation is the first step in ensuring the safety and success of building projects. Project requirements, soil type and site conditions all affect the method selection, ranging from simple methods like test pits to advanced geophysical methods. Whether using the wash boring method or auger and shell boring, a full geotechnical investigation reduces risks and creates the base for solid foundation designs.
Hemali Patel