Trial pits also known as test pits are commonly used to investigate shallow ground conditions to develop an understanding of the profile of soils within the ground. Trial pits are holes dug prior to construction to help ensure the ground conditions are suitable for building and construction projects.

Trial pits can be more cost-effective than boreholes but they cannot reach the same depth. Trial pits can also be excavated relatively quickly. The biggest benefit of a test pit is cost savings.

They can be excavated by hand or using an excavator, generally to a depth of up to 3.5-4.5m. If a trial pit is deeper than 1.2 m and is intended to be entered by people, it should be made secure against the possibility of collapse.

Example of a trial pit

Trial pits are dug to determine:

  • Soil classification
  • Soil contamination
  • Determine the location of buried structures and underground utilities
  • Unsuitable soil conditions
  • Water table location
  • Sidewall stability
  • Groundwater seepage
  • Determine whether the project site can be trimmed, rammed, and leveled easily or not.
  • Determine whether fill material is present at the site or not and specify the depth it exists.
  • Potential challenges in project structure

Digging of Trial Pits

The location of trial pits should be near from foundation of the structure. Trial pits should be distributed properly to be able to obtain cross-sections along the major axis of the project site.

A trial pit should be at least deeper than the upcoming construction’s foundation.

Trial pitting enables many different samples (both disturbed and undisturbed) to be taken, and various sampling methods to be facilitated these may include, contamination tests, Vane Shear tests, Infiltration/soakaway tests, CBR testing, and in-situ strength testing.

Once samples are gathered backfill the area and document the conditions.

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