Site Investigation and Subsurface Exploration

Why it matters

Site investigation and subsurface exploration are critical in geotechnical engineering as they provide essential information about the soil and rock conditions at a construction site. This information is vital for designing safe and cost-effective foundations and other structures.

Key ideas

• Purpose of Site Investigation: To gather data on soil properties, groundwater conditions, and other subsurface features that influence construction.
• Stages of Site Investigation: Includes desk study, site reconnaissance, and detailed exploration.
• Methods of Exploration:
  • Boring: Drilling holes to extract soil samples.
  • Test Pits: Excavating shallow pits to examine soil layers.
  • Geophysical Methods: Using seismic waves or electrical resistivity to infer subsurface conditions.
• Sampling Techniques:
  • Disturbed Samples: Used for classification tests.
  • Undisturbed Samples: Used for strength and compressibility tests.
• In-situ Testing:
  • Standard Penetration Test (SPT): Measures soil resistance to penetration.
  • Cone Penetration Test (CPT): Provides continuous soil profile.

Formulas

For a full 450 mm standard penetration drive, N = N_2 + N_3. The first 150 mm is the seating increment and is excluded. N is reported as blows per 300 mm of penetration. Corrected values such as N_60 are distinct and require equipment/energy information.

Worked example

Three successive 150 mm increments require 12, 15 and 18 blows.

Exclude the first 12 seating blows. Add the next two increments: N = 15 + 18 = 33 blows/300 mm.

Answer: Uncorrected SPT N = 33. Do not average the three counts. For incomplete penetration or refusal, record blows and achieved penetration according to the test procedure rather than inventing a full-penetration N value.

Common mistakes

• Misinterpreting the results of in-situ tests like SPT and CPT.
• Failing to account for groundwater conditions in site investigations.
• Incorrectly classifying soil samples due to improper sampling techniques.

For GATE CE

Questions often focus on interpreting data from SPT and CPT tests, understanding the purpose and methods of site investigation, and applying knowledge of soil sampling techniques. Practice solving problems related to these tests and understanding their implications on design.

Quick check

1. What is the purpose of a site investigation?
2. Name two in-situ testing methods used in subsurface exploration.
3. How is the uncorrected SPT N value calculated for a complete test?

Answers: 1. To gather data on soil properties and conditions. 2. Standard Penetration Test (SPT) and Cone Penetration Test (CPT). 3. By summing the second and third 150 mm blow counts, excluding the seating increment.

Reference

FHWA: Standard Penetration Test procedure.