Table 7.12.

Trial pits and trenches

Trial pits/trenchesMethodApplicationsLimitations
Graphic
  • Machine or hand excavated;

  • Trial pits typically 3 m long × 0.5–1 m wide by up to about 5 m deep. Trenches may extend to tens of metres;

  • 5–10 machine excavated trial pits to 5 m depth in a day, depending on sampling requirements.

  • Examination of soils in situ and obtain large bulk samples for testing;

  • Assessment of ease of excavation, stability and groundwater issues;

  • Disturbed geotechnical and geoenvironmental sampling;

  • Block samples.

  • Relatively level site with vehicle access;

  • Benefits of depth >5 m questionable due to instability/H&S and low light levels;

  • Potential adverse influence on existing/proposed infrastructure;

  • Excavation limited in water-bearing or loose granular soils;

  • Breakers required where obstruction/pavement is present;

  • Pits should be treated as ‘confined spaces’.

Cost Factor: low, but shoring/reinstatement costs may be high
Relevance to glacial/periglacial depositsAllowing soils to be viewed in situ provides the opportunity for detailed logging and identification of small- and large-scale structural and textural changes in the deposits that may not be identifiable in boreholes. Machine-excavated pits may result in smearing of soils; however, a wide range of disturbed and undisturbed samples can be taken. Pitting can be undertaken in the sides of landforms and slopes and provide the necessary size of exposure for investigating geomorphological processes in detail.
  • H&S are the requirements to satisfy national legislation for Health and Safety.