GEOTECHNICAL

Geotechnical Site Investigation

Boring, sampling + testing programme that characterises the ground for design

Also calledsoil investigationgeotechnical investigationsite investigationsoil explorationgeotechnical report
Related on InfraLens
CODES
IS 1892
Definition

Geotechnical site investigation is the planned programme of boreholes, in-situ tests (SPT, CPT, plate load), sampling and laboratory testing that establishes the soil/rock profile, ground-water regime and engineering parameters needed to design foundations safely and economically. IS 1892 is the code of practice for subsurface investigation for foundations — it specifies the number, spacing and depth of boreholes (depth typically 1.5-2× the foundation width or until firm strata), sampling intervals and the tests required.

Its deliverable, the geotechnical report, gives the recommended foundation type, safe bearing capacity / pile capacity, settlement estimates, ground-water level, and construction cautions (dewatering, shoring, expansive/liquefiable soils). Skipping or under-scoping investigation — designing on presumptive IS 1904 values for an important structure — is one of the costliest mistakes in construction.

Where used
  • Mandatory precursor to all foundation design
  • Bidding + risk assessment for civil projects
  • Selecting shallow vs deep foundation strategy
  • Dewatering, shoring + slope-stability planning
  • Forensic assessment of foundation distress
Acceptance / threshold
Borehole number/spacing/depth + tests per IS 1892 for the structure category. Final foundation design must use investigation-derived parameters, not presumptive IS 1904 Table 1 values (which are preliminary only).
Frequently asked
How deep should boreholes go?
Per IS 1892, generally to a depth where the stress increase from the foundation is small — commonly 1.5-2 times the foundation width below founding level, or to refusal in firm rock, whichever is less, deeper for pile groups.
Can presumptive bearing values replace a soil investigation?
Only for preliminary estimating. IS 1904 Table 1 presumptive values are not permitted for final design of any significant structure — a site-specific investigation per IS 1892 is mandatory.
Related terms
Borehole Log (Bore Log)
Depth-wise record of strata, SPT N-values, samples + water table in a borehole
Ground Improvement
Techniques that strengthen/stiffen weak ground to avoid deep foundations
Soil Stabilisation
Improving weak soil's strength/durability by additives or mechanical means
Modulus of Subgrade Reaction
Soil 'spring stiffness' (pressure per unit settlement) for raft + pavement design
Soil Liquefaction
Loss of soil shear strength under cyclic loading (earthquake) — saturated loose sand behaves like liquid.
Geotextile / Geomembrane
Synthetic fabrics for drainage, filtration, soil reinforcement
Soil Classification
Per IS 1498 / Unified Soil Classification System
Standard Penetration Test (SPT)
Soil density test in borehole per IS 2131
Lateral Earth Pressure
Horizontal soil pressure on retaining structures. Active (Ka), at-rest (K0), passive (Kp). Computed by Rankine or Coulomb theory.
Safe Bearing Capacity (SBC)
Max safe pressure soil can take. 100-500 kN/m² typical.