Soil investigation
Purpose of Soil Investigation
The design of a structure’s foundation requires a thorough understanding of the soil’s chemical and physical properties, as well as its geotechnical parameters. This information is obtained through an investigation of both surface and subsurface soil strata. Boreholes are strategically selected to collect samples that represent various soil conditions relevant to the structure’s foundation.
Figure 1
The design of a structure’s foundation requires a thorough understanding of the soil’s chemical and physical properties, as well as its geotechnical parameters. This information is obtained through an investigation of both surface and subsurface soil strata. Boreholes are strategically selected to collect samples that represent various soil conditions relevant to the structure’s foundation.
Sampling
The choice of sampling method depends on soil conditions. For soft soils, the ASTM D1587-00 method is recommended, which involves using a thin-walled tube for sample collection. For gravel, large cemented soil particles, and hard soil, ASTM D1586-99 is the preferred method, utilizing a split-barrel sampler to obtain samples. All collected samples should be placed in sealed plastic bags to prevent moisture loss. Each sample must be properly labeled and identified.
The choice of sampling method depends on soil conditions. For soft soils, the ASTM D1587-00 method is recommended, which involves using a thin-walled tube for sample collection. For gravel, large cemented soil particles, and hard soil, ASTM D1586-99 is the preferred method, utilizing a split-barrel sampler to obtain samples. All collected samples should be placed in sealed plastic bags to prevent moisture loss. Each sample must be properly labeled and identified.
Soil Tests
1. Standard Penetration Test (SPT)
The SPT assesses the relative density and consistency of soil. The test apparatus includes a split-barrel sampler, drill rod, and drive weight assembly. The SPT value is determined by the number of blows required for the split-barrel sampler to penetrate 300 mm after an initial seating of 150 mm, using a 63.5 kg hammer falling from a height of 0.76 m. The test is conducted at various depths.
Figure 2
The SPT assesses the relative density and consistency of soil. The test apparatus includes a split-barrel sampler, drill rod, and drive weight assembly. The SPT value is determined by the number of blows required for the split-barrel sampler to penetrate 300 mm after an initial seating of 150 mm, using a 63.5 kg hammer falling from a height of 0.76 m. The test is conducted at various depths.
2. Particle Size Analysis
Particle size distribution is determined using ASTM D422-63. For particles larger than 75 μm (not passing through sieve No. 200), a sieve analysis is performed. For particles passing through sieve No. 200, the sedimentation process is used, as sedimentation velocity depends on particle size. A hydrometer is utilized to measure particle size in this case.
Particle size distribution is determined using ASTM D422-63. For particles larger than 75 μm (not passing through sieve No. 200), a sieve analysis is performed. For particles passing through sieve No. 200, the sedimentation process is used, as sedimentation velocity depends on particle size. A hydrometer is utilized to measure particle size in this case.
3. Unconfined Compression Test
This test determines the unconfined compressive strength of cohesive soils, following ASTM D2166. The specimen should be cylindrical with ends perpendicular to its longitudinal axis. It is subjected to an axial force, inducing an axial strain of 0.5% to 2%. Measurements of load, deformation, and time are recorded. The strain rate must be controlled so that failure occurs within 15 minutes. The loading continues until the load decreases with increasing strain or reaches 15% strain.
The unconfined compressive strength (qu) is the stress at failure, while the shear strength (su) is half of the unconfined compressive strength.
Figure 3
This test determines the unconfined compressive strength of cohesive soils, following ASTM D2166. The specimen should be cylindrical with ends perpendicular to its longitudinal axis. It is subjected to an axial force, inducing an axial strain of 0.5% to 2%. Measurements of load, deformation, and time are recorded. The strain rate must be controlled so that failure occurs within 15 minutes. The loading continues until the load decreases with increasing strain or reaches 15% strain.
The unconfined compressive strength (qu) is the stress at failure, while the shear strength (su) is half of the unconfined compressive strength.
4. Atterberg Limits
Developed by Albert Atterberg, these limits define soil consistency. Originally six, they are now primarily the plastic limit, liquid limit, and shrinkage limit.
Plastic Limit: The water content at which soil transitions from a semi-solid state to plastic state. It is determined using ASTM D4318 by rolling a soil sample into a 3.2 mm diameter thread until it crumbles. The water content at this point is the plastic limit.
Liquid Limit: Determined using a liquid limit device. The soil sample is spread inside the cup and split with a groove tool. The cup is then repeatedly dropped until the groove closes at the bottom. The number of blows and corresponding water content are recorded for multiple trials. The liquid limit is calculated as:
Where:
- LLn = Liquid limit for a given trial
- Wn = Water content for the trial
- N = Number of blows required to close the groove
Figure 4
Figure 5
Developed by Albert Atterberg, these limits define soil consistency. Originally six, they are now primarily the plastic limit, liquid limit, and shrinkage limit.
Plastic Limit: The water content at which soil transitions from a semi-solid state to plastic state. It is determined using ASTM D4318 by rolling a soil sample into a 3.2 mm diameter thread until it crumbles. The water content at this point is the plastic limit.
Liquid Limit: Determined using a liquid limit device. The soil sample is spread inside the cup and split with a groove tool. The cup is then repeatedly dropped until the groove closes at the bottom. The number of blows and corresponding water content are recorded for multiple trials. The liquid limit is calculated as:
Where:
- LLn = Liquid limit for a given trial
- Wn = Water content for the trial
- N = Number of blows required to close the groove
5. Water Content
Water content is measured according to ASTM D2216. A soil sample is dried in an oven at 105°C for 18 to 24 hours. The difference in mass before and after drying represents the water mass. Water content is calculated as the ratio of water mass to the mass of solid particles.
6. Point Load Strength Index of Rock
This test determines the point load strength index of rock. A specimen is subjected to increasing point load using coaxial truncated conical platens until failure occurs, typically through splitting.
Figure 6
7. California Bearing Ratio (CBR)
The CBR test evaluates the mechanical strength of soil. A soil sample is compacted in a cylindrical mold, and a 50 mm diameter plunger is forced into it at a controlled rate. The CBR is calculated as the ratio of the load required to penetrate 2.5 mm and 5 mm to the load required for standard well-graded crushed stone.
7. Chemical and Groundwater Analysis
Soil and groundwater samples are collected for chemical analysis to determine their composition. This is crucial for selecting appropriate construction materials. For instance, if sulfate concentration is high, sulfate-resistant concrete must be used to prevent sulfate-induced expansion, cracking, and spalling of concrete structures.
8. Soil Investigation Report
The soil investigation report should include:
- Results of all conducted tests
- A profile of soil strata
- Recommendations for bearing capacity of shallow and deep foundations
- Estimated settlement predictions for foundations
- The level of underground water
- Results of all conducted tests
- A profile of soil strata
- Recommendations for bearing capacity of shallow and deep foundations
- Estimated settlement predictions for foundations
- The level of underground water
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