Civil and Building Engineering.

Between the 1950s and 1970s, geotechnical engineers recognized that the undrained shear strength of many soils followed a characteristic pattern. Initially, many engineers utilized this knowledge through their local experience but did not formalize their thinking in any particular manner. Eventually, several well-known geotechnical engineers began to present graphs of the shear strength that became popular. Foremost among these engineers were Professor Alan W. Bishop of the Imperial College of Science and Technology
in London and Professor Charles C. Ladd of the Massachusetts Institute of Technology. In 1974, Professor Ladd formalized a system to present and characterize the undrained shear strength of soils. This system is known as the SHANSEP (Stress History And Normalized Soil Engineering Properties) system. This system is probably the most widely used system for characterizing the shear strength of soils for engineering in practice in the Englishspeaking world. The following is a brief presentation of the SHANSEP
system.

The SHANSEP system is based on the observation that the shear strength of many soils can be normalized with respect to the vertical consolidation pressure. One example of normalized behavior is illustrated in Figure 3.62. Here, two stress-strain curves measured in consolidated-undrained (CU) triaxial tests are plotted. In the upper half of the figure, the unnormalized stressstrain curves are plotted. In the lower half of the figure, the normalized stress-strain curve is plotted. In the normalized plot, the strain axis is unchanged and the stress axis is normalized by dividing the axial stress difference by the vertical consolidation pressure.


When normalized plots are made using CU triaxial data, unique curves are obtained for each value of the OCR. As the overconsolidation ratio increases from 1 to higher values, the strain at peak stress decreases and the normalized stress-strain curves plots higher on the figure.

If a soil exhibits normalized behavior, a figure of strength ratio versus log OCR can be plotted. The strength ratio is defined as the undrained shear strength c divided by the vertical effective stress. Traditionally, the most commonly used symbol for strength ratio is c/p . Other symbols are also used. The SHANSEP procedure is as follows:

1. A soil investigation is conducted, and a sufficient number of samples are obtained from the soil strata of interest.
2. A series of one-dimensional consolidation tests are run on samples from various depths to define the overconsolidation ratio versus depth in the soil profile.
3. A series of CU test series are performed at confining pressures corresponding to OCR values of 1, 1.5, 2, 4. and 6. Often, this testing program requires that the test specimens be consolidated to stresses well above those found in the field and then rebounded to lower levels of effective stress to obtain the desired OCRs.
4. Figures similar to those shown above are plotted using the test data.
5. The SHANSEP figures can be used to estimate undrained shear strengths in many situations. The general procedure used to determine undrained shear strength is presented in Steps 6 to 9.
6. The effective stress at the depth of interest is computed.
7. The OCR is computed using the known value of maximum past consolidation pressure. (If the new effective stress exceeds the maximum past consolidation pressure, then the soil becomes normally consoli-
dated and OCR 1.0.)
8. The OCR is then used to obtain the strength ratio from the chart of strength ratio versus OCR logarithm.
9. The undrained shear strength is computed by multiplying by the vertical effective stress by the strength ratio.

When soil is to be placed on or excavated from the site, it is often necessary to estimate the changes in undrained shear strength resulting from consolidation or rebound due to changes in levels of effective stress. If this is the case, the effective stress computed for Step 1 may be computed both prior to and at the end of consolidation or rebound. The change in undrained shear strength is then the difference in the two values of computed strength.

If the stress-strain behavior is of interest, the OCR can be used to estimate the stress-strain curve.

Noted that if one is interested in the undrained shear strength under a foundation, one must compute the vertical distribution of stresses due to the foundation. A similar situation exists for stresses under an embankment.