Site investigation is an essential step in the construction and geotechnical engineering process, providing critical information about the subsurface conditions. The data collected during site investigations helps engineers design safe and stable foundations, roads, and other infrastructure projects. The primary techniques for site investigation include drilling, sampling, and in-situ testing. Each method has its specific purpose and application in determining the physical and mechanical properties of soil and rock at a construction site.
1. Importance of Site Investigation
Before any construction project begins, it is necessary to understand the subsurface conditions to avoid costly delays, unexpected failures, or other unforeseen issues. Site investigations aim to:
Identify soil and rock types and their properties.
Determine groundwater levels.
Assess the risk of natural hazards, such as landslides, liquefaction, or subsidence.
Provide data for designing foundations, retaining walls, tunnels, and other geotechnical structures.
Ensure compliance with building codes and standards.
The key steps in a site investigation include desk studies, fieldwork, and laboratory testing. The fieldwork often involves drilling boreholes, collecting samples, and performing in-situ tests.
2. Drilling Techniques
Drilling is a common method used to investigate the subsurface conditions of a site. It allows engineers to extract soil and rock samples from various depths for further analysis. Several drilling techniques can be used, depending on the site's soil and rock conditions.
2.1. Auger Drilling
Method: Auger drilling involves the use of a rotating helical screw, or auger, to penetrate the soil. The soil is lifted to the surface as the auger rotates.
Application: This method is ideal for shallow, unconsolidated soils. It is often used in the early stages of site investigation to obtain soil samples.
Advantages: Simple and cost-effective for shallow investigations. It allows continuous sampling.
Limitations: Auger drilling is unsuitable for hard rock and deep investigations.
2.2. Rotary Drilling
Method: Rotary drilling involves using a rotating drill bit that cuts through soil and rock. It is coupled with drilling fluid (often water or mud) to cool the bit and transport cuttings to the surface.
Application: Suitable for deeper investigations and can penetrate hard rock. Rotary drilling is often used in large infrastructure projects or when geotechnical conditions are complex.
Advantages: Can penetrate deep and hard materials, providing continuous cores of soil and rock for analysis.
Limitations: Requires specialized equipment and is more expensive than other methods.
2.3. Percussion Drilling
Method: Percussion drilling involves using a heavy tool to repeatedly strike the ground, breaking it up, and allowing material to be brought to the surface. The drill moves downward through the hammering action.
Application: This technique is useful for drilling through hard soils and rock layers.
Advantages: Effective for drilling in hard ground conditions, including rock layers.
Limitations: It can be slow and may disturb the soil samples, making them less suitable for detailed testing.
3. Sampling Techniques
Collecting samples from different depths is critical for laboratory analysis of soil properties, such as strength, compressibility, permeability, and composition. There are two main types of sampling methods: disturbed and undisturbed sampling.
3.1. Disturbed Sampling
In disturbed sampling, the soil structure is altered, and the natural layering is often lost. Disturbed samples are generally used for tests that do not require intact samples, such as grain size distribution or moisture content testing.
Method: Samples are obtained using tools such as augers or shovels. In some cases, split-barrel samplers are used during drilling.
Application: Suitable for determining physical characteristics like grain size and moisture content but not for testing properties that require an undisturbed sample (e.g., shear strength).
3.2. Undisturbed Sampling
Undisturbed samples preserve the natural structure and layering of the soil, making them suitable for laboratory testing that evaluates the mechanical behavior of the soil.
Method: Specialized sampling tubes, such as Shelby tubes or piston samplers, are driven into the soil to extract undisturbed samples.
Application: Used for laboratory tests requiring intact samples, such as triaxial tests, consolidation tests, or direct shear tests.
Advantages: Provides more accurate data on soil behavior under stress, making it essential for designing foundations and earthworks.
Limitations: Collecting undisturbed samples can be difficult and expensive, particularly in challenging ground conditions.
4. In-Situ Testing Techniques
In-situ tests are performed directly at the site, providing real-time information about the soil's properties without the need to transport samples to a laboratory. These tests are often used to assess soil strength, stiffness, and permeability at different depths. In-situ tests complement sampling and provide a better understanding of subsurface conditions.
4.1. Standard Penetration Test (SPT)
Method: The Standard Penetration Test involves driving a split-barrel sampler into the ground using a hammer. The number of blows required to penetrate the soil 30 cm (12 inches) is recorded as the "N-value," which provides an indication of soil density and strength.
Application: Widely used to estimate soil properties like relative density for sands and gravels or consistency for clays.
Advantages: Simple and widely used. Provides a rough estimate of soil strength.
Limitations: Results can be variable depending on operator technique. It provides less accurate information in cohesive soils.
4.2. Cone Penetration Test (CPT)
Method: The CPT involves pushing a cone-shaped probe into the soil at a constant rate and measuring the resistance to penetration. The test provides continuous data on soil strength, density, and stratigraphy.
Application: Used to determine the bearing capacity of soils, identify soil layers, and assess soil compaction.
Advantages: Provides more detailed and continuous data compared to the SPT. It is particularly effective in identifying stratigraphy and changes in soil type.
Limitations: Not suitable for very hard soils or soils with large particles, such as gravel.
4.3. Plate Load Test
Method: In the Plate Load Test, a steel plate is placed on the ground, and a load is applied to it incrementally. The settlement of the plate is measured at each load increment.
Application: Used to determine the bearing capacity of soils and estimate settlement under foundations.
Advantages: Provides direct data on bearing capacity.
Limitations: Can be time-consuming and limited to the size of the test plate.
4.4. Permeability Tests (Falling Head and Constant Head Tests)
Method: Permeability tests measure the rate at which water flows through the soil. In the falling head test, water is allowed to flow through the soil, and the drop in water level is measured over time. In the constant head test, water flows through the soil at a steady rate, and the flow rate is measured.
Application: Used to assess the permeability of soils, which is critical for drainage and groundwater control.
Advantages: Provides important information for foundation design and groundwater management.
Limitations: Results may vary depending on soil type and site conditions.
5. Site Investigation Report
After drilling, sampling, and in-situ testing, the data collected is compiled into a site investigation report. This report includes detailed information on soil types, subsurface conditions, groundwater levels, and laboratory test results. The report is used by engineers to design foundations, retaining walls, earthworks, and other structural components.
The site investigation report typically includes:
Borehole Logs: Detailed records of soil and rock layers encountered during drilling.
Test Results: Data from in-situ tests, such as SPT or CPT, and laboratory analysis.
Recommendations: Suggestions for foundation types, soil improvement techniques, and drainage systems.
Conclusion
Drilling, sampling, and in-situ testing are essential techniques in geotechnical site investigations. Each method provides unique insights into the subsurface conditions of a site, enabling engineers to make informed decisions about foundation design, construction materials, and soil treatment. By combining these techniques, construction projects can be designed with greater confidence, ensuring safety, stability, and longevity.
