Types & Purpose of In-situ Rock Tests for Foundation Design

Introduction on In-situ Rock Tests

In-situ Rock Tests are done to determine in-situ stresses & deformation characteristics and shear strength of jointed rock mass or critically weak seams within the rock mass, residual stresses within the rock mass, anchor capacities, and rock mass permeability. In-situ tests are often the best means for determining the engineering properties of subsurface materials and, in some cases, may be the only way to obtain meaningful results. List of in-situ tests and their purposes are given below.

Large-scaled in-situ tests tend to average out the effect of complex interactions. In-situ tests in rock are frequently expensive and should be reserved for projects with large, concentrated loads. Well-conducted tests may be useful in reducing overly conservative assumptions. Such tests should be located in the same general area as a proposed structure and test loading should be applied in the same direction as the proposed structural loading.

Laboratory tests are usually performed in addition to and after field observations and tests. These tests serve to determine index values for identification and correlation, further refining the geologic model of the site and they provide values for engineering properties of the rock used in the analysis and design of foundations and cut slopes.

Summary of Purpose and Type of In-Situ Rock Tests

Purpose of Test : Strength

1. Field Vane Shear test – Primarily for clay shales, badly decomposed, or moderately soft rocks, and rock with soft seams.
2. Direct Shear test
3. Pressuremeter test – Less frequently used.
4. Uniaxial Compressive test – Less frequently used.
5. Borehole Jacking – Less frequently used.

Purpose of Test : Bearing Capacity

1. Plate Bearing Test – Primarily for clay shales, badly decomposed, or moderately soft rocks, and rock with soft seams.
2. Standard Penetration Test – Primarily for clay shales, badly decomposed, or moderately soft rocks, and rock with soft seams.

Purpose of Test : Stress Conditions

1. Hydraulic Fracturing Test
2. Pressuremeter Test
3. Overcoring Test
4. Flat Jack Test
5. Uniaxial (Tunnel) Jacking Test – Less frequently used.
6. Chamber (Gallery) Pressure Test – Less frequently used.

Purpose of Test : Mass Deformability

1. Geophysical (Refraction) Test – Dynamic deformability.
2. Pressuremeter or Dilatometer Test
3. Plate Bearing Test
4. Uniaxial (Tunnel) Jacking Test – Less frequently used.
5. Borehole Jacking Test – Less frequently used.
6. Chamber (Gallery) Pressure Test – Less frequently used.

Purpose of Test : Anchor Capacity

1. Anchor/Rockbolt Loading

Purpose of Test : Rock Mass Permeability

1. Constant Head
2. Rising or Falling Head
3. Well Slug Pumping
4. Pressure Injection