Soil Pressuremeter Test
Figure 1: BST Shear Head (left) and Shear Dynamometer (right)
Benefits of the Pressuremeter Test:
- Immediate results save the owner and engineer time and money, particularly with failed slopes
- Measures the drained shear strength of soils in place
- A new development allows for residual effective shear strength measurements
- Used for designing earth slopes, retaining walls and determining the bearing capacity of foundations
Iowa Borehole Shear Test (BST): While the shear strength of soils can be critical for the design of earth slopes, the calculation of earth pressure against retaining walls, and the determination of foundation bearing capacity, it can be a time consuming and expensive to measure with laboratory shear tests. The BST (Figure 1), developed by Dr. R. L. Handy at Iowa State University, provides a convenient method to accurately measure the drained shear strength of soils in-situ. Tests typically require between 30 and 60 minutes to perform, and the results are immediately available. It is similar to a laboratory direct shear test with the sides of the borehole being sheared.
To perform the BST, the operator inserts the shear head into a 3‑inch diameter borehole to the chosen test depth. A normal stress is then applied to push apart two serrated stainless steel plates (total area 10 in2), pressing them laterally against the sidewalls of a borehole (Figure 2). After allowing the soil to consolidate at the applied normal stress, usually between 5 minutes for cohesionless soil and about 10 to 20 minutes for cohesive soil, the operator pulls the shear head upward to measure the shear strength of the soil in contact with the plates (Figure 3). This shear test is typically repeated four to five times at progressively higher normal stresses to prepare a plot of normal stress versus shear stress. In sands, silts, and stiff clays, the BST provides a drained test, while results for softer cohesive soils may be partially drained. An available pore pressure sensor located in the shear head can provide an indication of drainage. Because the same soil is tested, the data can usually be fitted linearly with a coefficient of correlation of 0.99 or better.
Figure 2: Applying the Normal Stress
Figure 3: Shearing the Soil
For soils with an N60 value of 15 or more blows per foot, the smaller set of plates (total area 1.6 in2) should be used to ensure that the plates are fully embedded into the soil. Because the pressure gauges are calibrated to measure the stress of the larger (standard size) plates, for the smaller plates the recorded pressures must be multiplied by 6.25 to account for the differences in the plate areas.
An oversize borehole can adversely affect the accuracy of the test results, as can loosening or softening of the borehole sidewalls. A borehole prepared with a 76-mm (3 inch) diameter Shelby tube usually tends to minimize disturbance. Hand augers are also a good choice for more remote locations. Boreholes prepared using mud‑rotary drilling methods will reduce the shear strength until the normal stress causes the shear heads to penetrate through any mud‑caking.
Research is being performed to evaluate the residual shear strength in over-consolidated clays. After measuring the peak shear strength value, the BST plates are collapsed and lowered back to the starting depth for the data point. A normal stress equal to about 90% of the peak normal stress is then reapplied to the clay and the plates are pulled upward to the ending depth of the peak value. The resulting shear stress is recorded. This procedure is repeated until the shear stress becomes a constant value. An example set of residual borehole shear test data is shown as Figure 4.
Figure 4: Peak and Residual BST