Recently there has been an increase in landslips due, it is believed, to high rainfall last winter. A hot summer is projected, and its effect could be to open up the clay shrinkage cracks deeper and wider than usual. Thus, during subsequent winter rain, especially taking into account our apparent new rainfall pattern, the enhanced hydrostatic pressure of water in these cracks, will again promote more landslip.
What is landslip?
In clay one can usually have three types of landslip failure: clay creep, deep-seated soil slip and shallow slip
- Clay creep: the surface of the clay gradually moves down the slope. In summertime the clay dries out, shrinks and cracks form. The weight of the clay above pushes down and the cracks fill up and become compacted. In wintertime, the clay absorbs water and expands again. But the new expansion tends to be downwards at right angles to the inclination of the slope rather than upwards along the line of the slope. The following summer the cycle repeats, and so the clay gradually moves down the hill. A problem with an insurance claim for this failure mode is that many insurers have never heard of clay creep or at least maintain they have not. As the clay expansion is lateral, one might regard it as a combination of diagonal heave and subsidence but most policies talk of vertical movement when dealing with subsidence and heave.
- A second type of landslip, the most usual, is like the third but is a shallow slip with the slip circle close to the sloping edge.
- The third form of landslip in clay is a more deep-seated type of movement in which the soil slips, often along the arc of a circle of similar failure plane. (See diagram). If the clay, as in one instance I dealt with, has a layer of more moist clay in it, then the failure surface will often be found to follow the plane of the higher moisture content. Sometimes a minor failure takes place, thus giving rise to a plane along which the moisture can travel. Thus, when boreholes are bored, a local zone of higher than usual moisture in the clay is indicative of the presence of a failure plane. If part of the house in question, or whatever structure is involved, is founded on the top part of the failure zone, then there will be a reduction in support of that part of the structure with consequential cracking and possibly eventual collapse.
Where does landslip occur?
Potential landslip areas can be identified by the topography, by example considering if the slope is steeper than usual for the type of soil involved. Railway cuttings in the Croydon area, are a potential risk area. Homes at the bottom of a railway embankment may eventually be at risk from the embankment, but whoever owns the railway, has a far greater vested interest in sorting that out. Clay creep seems to occur in London Clay in the South Norwood and Upper Norwood areas.
How can I deal with landslip?
Dealing with the problem of landslip under an insurance policy is more straightforward, as often the failure mode is visible and obvious, so the most awkward loss adjuster has difficulty in saying “no”. If one underpins the house down to a level of soil not being affected, one may save the house. This does not come cheap, and one has to detect the level of the failure plane and make sure that another one does not exist or will not form below the level of the first one. The failure zone is often indicated by more water in the soil near the plane of failure.
In material other than clay areas where there is for example rock, erosion of the rock can give rise to weakness and thus collapse. Scouring at the foot of cliffs by the action of water can cause sudden failure of the cliff. Rock anchors can be drilled into the rock to pin back potentially unstable rock by connecting it to the more stable part, as done in Gibralter. Similar can be done with soils, such as in Hong Kong, if access for the drilling equipment is feasible, both actually and economically.
Steel or carbon fibre anchors are placed in pre-drilled holes and concrete pumped in to form a connection between the soil and the anchor. Some anchors can be driven in with an explosive charge that drives the anchor into the embankment. The means of supporting the rigs whilst working on a slope forms an important part of the cost. If the house is to be underpinned as well, the pile location and the anchor location need to be well thought out, to avoid a clash. Thus, whilst not as prevalent as subsidence, if this problem does occur, then financial consequences are worse.