Use of Standard Laboratory Tests in the Assessment of Slope Stability of Peat

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In December 2006 the Scottish Executive (SE) published a best practice guide for proposed electricity generation developments entitled “Peat landslide hazard and risk assessments”. The SE document includes a requirement for slope stability analysis using the infinite slope equation. While there is nothing wrong with the equation, it pre-supposes that the conventional effective stress parameters are appropriate for peat and furthermore, by implication, that they can be obtained from standard laboratory testing.

In his major work on peat, Hobbs specifically excluded any discussion on shear strength noting that it is clear that the strength depends not only on effective stress but also on time as the void ratio continuously decreases under maintained load. Other authors have reported exceptionally high drained strengths and regard the effective stress tests as problematic.

Notwithstanding the foregoing paragraph, there has been a noticeable increase in demand for effective stress triaxial and direct shear tests on peat to satisfy the requirements of the SE document. Inevitably these tests create major difficulties for the test laboratory and then provide strengths which are not credible to the engineer doing the assessment.

The following comments need careful consideration prior to scheduling what can be time consuming and expensive laboratory tests :

  • Assessment based on compressive or shear stresses may be insufficient for peat in which the main strength may well be tensile due to fibrous structure.
  • For effective stress triaxial and direct shear tests the rate of strain in shearing is derived from the preceding consolidation stages. For peat there are well documented difficulties in identifying primary and secondary compression, and their associated parameters. Standard test procedures rely on data for primary consolidation only in order to calculate compression rates. In peat, secondary compression may be more dominant.
  • In undrained compression, pore pressures increase rapidly and are sustained very close to the applied total stress, thus producing very low effective stresses, often within the uncertainty of measurement of standard testing. This is particularly true where the stresses are initially very low as would be the case for superficial peats. Such low effective stresses are inherently inaccurate and lead to derivation of very high effective angle of friction, 50o to 80o have been reported.
  • In drained compression or direct shear the rapid flow of water from specimens often produces an untypically concave upward trend in the stress/strain curve without achieving failure even at very high deviatoric stress, again producing a very high angle of friction.

 

This cautionary note should not detract from the main requirements and recommendations of the guide, but needs to be given serious consideration when assessing the method of analysis to be used for slope stability in peat. If such laboratory testing is to be used, it is prudent to discuss individual requirements with the laboratory prior to scheduling, but still expect the test results to be problematic.

Peter Keeton, Environmental Services Group Ltd