Soil Exploration and Subsurface Investigations

Soil Exploration

• Soil exploration involves identifying soil layers and their characteristics beneath a proposed structure.
• Field and laboratory investigations to gather subsoil information are called soil exploration or soil investigation.
• Subsurface explorations are crucial for structure design and construction planning.
• The success of a foundation depends on reliable soil parameters obtained from field investigations and laboratory testing.

Stages in Subsurface Explorations

• 1. Site Reconnaissance: The initial stage, involving site visits to study maps and relevant records. This stage helps determine future site investigations' scope of work, exploration methods, sample types, and laboratory testing procedures.

  • Features assessed include general topography, settlement cracks, evidence of landslides and slope creep/shrinkage, high flood marks, depth of groundwater (wells), existence of springs/swamps, drainage patterns, and vegetation.
  • Information collected during reconnaissance will aid in choosing appropriate techniques for future investigations.

• 2. Preliminary Exploration: Aims to determine the depth, thickness, extent, and composition of each soil stratum at the site. Involves limited borings or test pits to assess soil properties like strength and compressibility.

  • Cone penetrometers and sounding rods may be used to gather data during preliminary explorations.

• 3. Detailed Explorations: This stage involves extensive boring programs, sample collection, and laboratory testing of samples to determine soil properties in different strata.

  • Common field tests include vane shear tests, plate load tests, and permeability tests.
  • Detailed investigations used for complex projects such as bridges, dams, and high-rise buildings but not always required for small projects with uniform strata.
  • Reconnaissance and preliminary exploration data are usually sufficient for small projects.

Sampling in Soil

• Classification: Soils are heterogeneous mixtures.

  • Coarse-grained soil: particles larger than 0.075mm
  • Fine-grained soil: particles smaller than 0.075mm

• Types of Sampling: Soil sampling can be either disturbed or undisturbed.

    - Disturbed samples reflect all constituents proportionally but with altered structure
        - Undisturbed samples represent the in-situ conditions
  

• Methods for disturbed sampling include: backhoes, hand augers, or drill rigs. Tools like split spoon samplers, shelby tubes, or macrocore push samplers may also be used. Disturbed samples collected with these methods require careful inspection for signs of disturbance, and affected portions should be discarded in subsequent analysis.

• Methods for undisturbed sampling involve using piston samplers, pitcher barrel samplers, or specialized equipment. This method aims to capture soil's structure as it exists.

• The choice of sampling method is determined by factors such as time constraints, topography, cost, and the reason for soil analysis.

Exploratory Borings in the Field

• Hand-operated augers: Suitable for a considerable depth in various soil types above the water table (particularly clayey soils).

• Power-driven augers: Suitable for soils including sandy soils below the water table but not suitable for soils mixed with gravel/cobbles. This method can be used to drill holes quickly up to a 60m depth.

• Shell and Auger methods: Useful for dense sandy, stiff clay soils, or even those mixed with gravel.

• Wash boring techniques: Effective for sand, silt, and clay soils. Useful for soil studies but not for rocks.

• Rotary drilling: Used in various soils (sand, clay, rocks) and generates boreholes with diameters of 50-203mm. The drilling mud is a slurry of water and bentonite, which is useful for preventing soil caving.

• Percussion drilling: This method creates holes in hard strata like rocks, boulders, using a chisel repeatedly lifted and dropped into a vertical hole. Not ideal for collecting undisturbed samples, especially those containing boulders in the subsoil.

• Core drilling: Commonly used to drill holes and gather rock cores. Diamond-studded bits are most superior but costly, compared to cutting-edge bits that are of simpler types.

Soil Exploration Report

• A report is produced at the end of the soil exploration program.
• Reports feature information on the scope of the investigation, structure under consideration, descriptions of the soil, water drainage conditions, analysis of subsoil conditions based on soil and rock samples collected, and the groundwater table observed from the borehole.
• Other aspects the report should provide include detailed foundation recommendations, anticipated construction problems, limitations of the investigation, site location maps, borehole locations, lab test results, and other supplementary documentation.

In-Situ Tests

• Tests often used to determine soil properties at the site without altering the soil's natural state include:
  • Standard Penetration Test (SPT)
  • Cone Penetration Test (CPT)
  • Plate Load Test
  • Vane Shear Test
  • Pressuremeter Test (PMT)
  • Field Permeability test
  • Dynamic Cone Penetrometer (DCP) Test
  • Electrical Resistivity Test
• Each test has specific procedures and is used to determine distinct properties, such as soil density, strength, bearing capacity, and settlement under load. Also used for liquefaction potential, stratigraphy, and determining soil types.
• In-situ tests are beneficial because they reflect real soil behavior under natural conditions and minimize soil disturbance.