What every resident should know about geology, groundwater and basement excavation
Summary of a talk to the Society on 23 May 2013 by Dr Michael de Freitas, Chartered Geologist and Director of First Steps Ltd, also Emeritus Reader of Engineering Geology at Imperial College London.
The key facts that residents should know are as follows.
Engineering is perfectly capable of creating basements that do not cause problems to neighbours but this requires advanced knowledge of the ground based on good facts, sophisticated design and meticulous supervision; that is expensive. Insufficient investment in understanding the ground is invariably the cause of developments that create trouble for neighbours. This means that damage to neighbouring properties arising from basement excavation is often a price the neighbours pay for owners not investing sufficiently in the work they commission to increase and realise the value of their estate.
An argument is made that it is unreasonable to invest such sums of money as are required without first obtaining permission to build. This is not a fair description of the situation. The problem is two-fold; design and construction. An argument can be made that it is reasonable to withhold permission to build until design demonstrates the work will not cause the ground beneath neighbouring properties to change in a damaging way.
It is not possible to excavate a hole without the ground responding and the key issue for neighbours is the extent of that response and its magnitude. The ground in Hampstead, especially near the Heath, is not as simple as most proposals suggest. The upper layer of the ground is formed from a mantle of gravel, sand and clay which has flowed down the hill sides in glacial times, and now forms an apron of permeable material; this is the shallow aquifer that causes so much problem with wet basements and cellars – it is often misidentified as “made ground”. This can often be separated from ground in its “virgin” state by clay that has been softened and possibly deformed. The balance of horizontal and vertical stresses in the “virgin” ground can be difficult to know and this affects the prediction of ground response to unloading and reloading during basement excavation and construction.
Dr Michael de Freitas
The movement of ground water in this zone can be very variable as in this zone are located the old drains and sewers, many of them leaky, the trenches in which they were laid, and the receptors for soak-aways. When it rains, much water is shed rapidly from roof tops, covered ground and roadways, and pulses of water pass through the drainage system which, if it is leaking, allows the water to discharge to the “made ground”. That is why the occasional measure of water levels in a ground investigation is by no means an adequate assessment of ground water conditions that could be met over longer periods.
“Insufficient investment in understanding the ground is invariably the cause of developments that create trouble for neighbours.”
Many of the responses of the silts and clays in the ground are time-dependent because they occur in response to the decay of pore water pressure in the clay horizons of the Claygate Beds and in the London Clay itself. This means that the ground will continue to respond to basement development long after the basement work is complete. The response may be small in absolute terms but it may not be uniform, meaning that neighbouring properties can be gradually distorted as the ground nearest the excavation moves more than that furthest from it.
What should every resident do? Every resident wishing either to develop a basement or cope with a basement development next door should insist on a good quality ground investigation of the site involved. Adequate design which avoids damage to existing and neighbouring buildings requires a verifiable model of the ground, good representative samples of the ground, tested for their mechanical properties, together with long term monitoring of water levels and their response to rainfall. If the Basement Impact Assessment reveals with Screening any issues that need Scoping then those issues should be scoped (i.e. quantified) before a request for permission to build is given. It is not appropriate for matters to be left to “good industrial practice” once planning permission is given – predicting ground response and responding so as to mitigate its effects is, in many parts of Hampstead, beyond what “good industrial practice” is able to guarantee.
Residents neighbouring a proposed basement development should therefore expect a Basement Impact Assessment that is supported by a good quality, site-specific ground investigation accompanied by long-term monitoring of water levels. They should also expect to receive a prediction of the ground movement that is likely to accompany the excavation and an assessment of the time over which those movements are likely to occur. All this can be undone by poor construction methods and a clear statement of construction sequence should accompany the submission.
Engineering in the ground is substantially more difficult than engineering above ground and there is a case to be made for councils to ensure, by appropriate training, that those councillors and council staff involved with decisions in these areas are aware of the technical issues involved before recommending and granting planning permission.