Soil Investigation Techniques

Methods of Sub-Surface Investigation

• Various techniques available: test pits, boring and sampling, field tests, geophysical methods, and laboratory tests.

Test Pits

• Simplest and most cost-effective for shallow soil exploration (3-4 meters depth).
• Facilitates visual examination of in-situ soil conditions.
• Allows easy collection of disturbed or undisturbed soil samples.

Soil Boring and Sampling

• Most widely used method for obtaining soil samples at varying depths.
• Includes multiple methods: auger boring, wash boring, percussion drilling, and rotary drilling.

Auger Boring

• Can be operated manually or powered.
• Hand-operated augers (15-20 cm diameter) come in types like post-hole and helical.
• Provides highly disturbed samples; power augers can reach depths of 30m in many soil types above the water table.

Wash Boring

• Utilizes a power-operated system with a chopping bit and drilling fluid (usually water).
• Suitable for sandy and clayey soils but not for very hard materials like boulders.
• Penetration depth can exceed 60m; identifies soil strata changes through sound and slurry examination.

Percussion Drilling

• Power-operated, it progresses by delivering repeated blows from a heavy chisel.
• The resulting slurry at the borehole bottom is removed with a bailer or sand pump.

Dynamic Penetration Tests

• Standard Penetration Test (SPT):
  • Measures soil resistance to dynamic penetration using a split spoon sampler.
  • Driven by a 63.5 kg mass dropped from a height of 76 cm, initial and subsequent driving are recorded for standard penetration value (N).

Correlation of Penetration Data

• Relationship between N (number of blows) and various soil properties, such as relative density and unconfined compressive strength.
• Meyerhof's equation relates density and cone resistance, which can estimate compressibility for sand.

Vane Shear Test

• Measures undrained shear strength of soft clays using a steel rod with stainless steel bladed vanes.
• Torque applied to shear the soil, with maximum torque indicating shear strength.

Plate Loading Test

• Involves applying static load on a steel plate to assess soil settlement.
• Procedure includes proper plate positioning and controlled loading to measure bearing pressure against settlement.
• Limitations include short duration tests not capturing full consolidation behavior under certain soil conditions.

Pile Loading Test

• Most reliable for ascertaining pile load capacity.
• Testing closely resembles the plate-loading test, providing data on allowable bearing capacity and load-settlement relationships.

Geophysical Methods

• Effective for identifying strata locations and quick evaluations of sub-soil characteristics.
• Broadly categorized into seismic refraction and electrical resistivity methods.

Soil Exploration Report

• Typically includes: purpose of investigation, site description, geology, detailed findings from tests, lab results, discussion related to foundation design, and conclusions with recommendations for foundation types and allowable bearing pressures.