ABSTRACT
Two broad slope conditions have been the focus of previous studies on the role of subsurface soil pipes and stability: the bank or free face failure condition and the intra slope landslip condition. The presence of pressures, piping and consequent sapping on banks has received the most attention and the link between mass movement and piping has been firmly established for this condition by a number of empirical and theoretical studies. On the other hand, the link between hill slope landslips and piping remains less clear. Four propositions are examined in this paper: piper preferentially direct water into susceptible slope locations, creating critical porewater pressures; pipes act as effective drainage systems, thereby retarding the development of critical porewater pressures; pipes lead to critical conditions only if they become blocked; the creation of subsurface voids through conduit development reduces structural support and decreases stability. The first three of these propositions are examined by using combined hydrology/slope stability computer simulation model. Three scenarios are examined: piped slopes, un-piped slopes, and slopes with blocked pipes. In each case conditions representing a typical New Zealand hill country slope are examined during a simulated rainstorm to determine changes in the factor of safety. Parameters used in the model have been obtained from a field site where slope hydrology, pipeflow and climate are continuously monitored. The results are constrained by the degree of simplification required by the model but support the experimental findings of Pierson (1983). All cases showed a reduction of stability resulting from the application of rain. Compared to the other two conditions the presence of pipes appears to retard the development of positive porewater pressure. Blocked pipes and un-piped slopes reached their lowest factor of safety much sooner than piped slopes and overall resulted in lower factors of safety.
