In practical engineering the direct simple shear, DSS, tests are a valuable tool for parameter assessment, despite its known shortcomings. The shear stress mobilised at the end of the test is often used as a proxy for critical state strength. Several aspects complicate conducting reliable tests at large strain, in specific for clay samples. One of these effects is slippage between the sample and the platens. When slippage occurs, the measured resistance follows from the interface properties rather than soil behaviour. To better understand the critical state behaviour of clays during a DSS tests a test series is conducted on an anthropogenic stiff silty clay. The test series not only demonstrates that critical state conditions are reached before the end of the tests, but also that slippage tends to occur after reaching critical state conditions. Complimentary series of direct shear, DS, tests were conducted to further confirm critical state conditions are obtained. This confirms the applicability of DSS tests to obtain a soil specific proxy for critical state conditions without the influence of slippage. Hence, the presented method to derive critical state strengths from DSS tests, for anthropogenic clay, and thereby discarding the influence of slippage provides useful and reliable strength parameters for design.