Behaviour of discrete piles used to stabilise a tree-covered railway embankment

Many clay railway embankments in the UK are well over 100 years old, and now suffer from a range of stability and serviceability problems. A common means of remediating earthworks that have either suffered, or are assessed as being at risk of, deep-seated slope failure is to install a row of discrete piles along the mid-slope. This paper presents field data from a 9 m high, pile-stabilised embankment at Mill Hill, north London. Initial pore water pressure measurements had shown that the trees present on the earthwork maintained low pore water pressures during winter; thus the trees were left in place after piling to aid stability. The piles initially bent upslope, as a result of inward shrinkage of the embankment over a period of dry weather, before then being loaded by shallower downslope movements of the clay. Later cycles of seasonal movement caused a small but gradual ratcheting upward of pile bending moments. The largest bending moments measured over 6 years of monitoring were those resulting from the initial inward shrinkage of embankment, which reached about 25% of the design capacity of the piles. At the end of the monitoring period, the measured bending moments resulting from shallower downslope ground movements were about 20% of the pile design capacity.