Analysis of undrained shear strength from drop-cone tests

Abstract: The Swedish Geotechnical Institute, SGI, has been given the assignment of mapping the risk of landslides in the area by the river Göta älv. Tests were made to determine the shear strength of soil on land and in the river. It has been shown that shear strength values evaluated with drop cone tests from points under the river are higher compared to other test methods although experience shows that the shear strength values from fall cone tests usually is slightly lower compared with those from other test methods. The soil under the river is more over consolidated then the surrounding soil because of years of erosion from the river. When evaluating undrained shear strength from CPT- and field vane shear tests, consideration to over consolidation is taken. SGI is now interested to look into if this needs to be done when evaluating undrained shear strength from fall cone tests as well. This thesis compares values from fall cone tests without correction for the over consolidation with values which is corrected to see if a correction is necessary. All data concerning the soils effective stress is necessary for calculating the over consolidation ratio and is evaluated manually. There is no set method to correct fall cone tests for the over consolidation so the existing formula used in the correction of field vane shear tests in over consolidated soils were used, both in its original form and with the exponent -0,15 adjusted. Own evaluations were made on the CPT- and field vane shear tests for comparisons with the results from the fall con tests. All processed data comes from SGI's own measurements. Results show that shear strength values from fall cone test and CPT field tests match better after corrections due to the over consolidation with the presented formula.