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Landfill CQA Questions: Triaxial Compression Testing or Direct Shear Testing?

A question for landfill construction quality assurance engineers which arises regularly is whether they should use Triaxial Compression Testing or Direct Shear Testing.

Well although triaxial compression testing is the most frequently used small scale direct shear testing does have certain advanatges and can be appropriate under certian circumstances.  

There is an interesting dicussion of this issue at the Engineering-Tips Forum. Where there is an excerpt from Holtz and Kovacs (section 10.5) that addresses this issue, as follows:

The advantages of the triaxial test over the direct shear test are:

  • Progressive effects are less in the triaxial
  • The measurement of specimen volume changes are more accurate in the triaxial
  • The complete state of stress is assumed to be known at all stages during the triaxial test, whereas only the stresses at failure are known in the direct shear test

The triaxial machine is more adaptable to special requirements.

The advantages of the direct shear test are:

  • Direct shear machine is simpler and faster to operate
  • A thinner soil sample is used in the direct shear test, thus facilitating drainage of the pore water from a saturated specimen. 

Landfill Construction Quality Assurance places a dual requirement on engineers and technicians which is seldom the case for general earthworks projects. In landfill construction lining and capping soils data must demonstrate both structural strength capability, and provide a seal against the passage of water (leachate) and landfill gas, so reliable testing for the low permeability of these materials is also necessary.

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