Field tests for operational shear strength assessment in peat at Uitdam, The Netherlands

Stability assessment of existing dikes build on soft soil generates questions on the available stability calculation techniques and material models. Especially peat behaviour is not easily captured by standard design methods. This has led to disapproval, based on calculations, of dikes that have been in operation for several hundreds of years. To establish the weak points in the stability assessment prescribed by the available handbooks on dike engineering, a series of 5 field tests is started. These field tests give the option to compare the different design methods and different parameter assessment techniques to the test results. The series includes single stage and multi stage loading. This paper focuses on the single stage loading tests. The tests result in a failure mode that differs from the circular type failure planes that are used in engineering practice. Horizontal and vertical fractures dominate the active side of the failure while at the passive side the peat is compressed. The tests indicate that tensile strength of peat, or peat fibres, is important in understanding stability problems in which thick peat deposits are involved. Furthermore, the tests show that over-consolidated behaviour is important to understand the measurements, while in engineering practice peat is usually modelled as a normally consolidated material.