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SOLIFLUCTED CLAY

Understanding the geology is critical for successful preventative or remedial stabilisation works.

Why, you might ask, should engineers in the UK be concerned with solifluction processes that operate under periglacial climatic conditions, when no such conditions are found in the UK today?

The answer is simple. Past solifluction processes have left many of the slopes in south and central England in a meta-stable (marginally unstable) condition. This means alterations to the slope profile during development or civil engineering works may cause a slope failure unless preventative measures are built.

The British Isles experienced an extended period of periglacial conditions during the Pleistocene Ice Age, which resulted in significant changes to the engineering properties of some near-surface soils and rocks.

These periglacial conditions had a profound effect on our overconsolidated clays. This was most severe in south England, to the south of the maximum advance of the ice sheets where the surface strata were exposed to continuous periglacial climatic conditions for a prolonged period.

Periglacial processes
Periglacial conditions are characterised by two diagnostic criteria (French, 1996) that are usually both present although they do occur in isolation in some areas. These are intense freezing and thawing oscillations and the presence of permafrost.

There are several periglacial processes that operate under periglacial climatic conditions, two of which, cryoturbation and solifluction, are significant as far as the stability of clay slopes is concerned. Both occur primarily in the active layer, which is the surface layer that thaws each summer in regions affected by permafrost, although they can also occur in seasonally frozen ground where there is no underlying permafrost.

Cryoturbation is a collective term for all soil movements caused by cyclical freezing and thawing of the upper soil layers. It is responsible for "patterned ground" phenomena, so called because the freeze-thaw processes create a variety of geometric shapes and may also sort the soils by grain size. The precise processes that created many of the cryoturbation phenomena are still the subject of debate, but it is the resulting effects on the engineering properties of the soils that are of current interest.
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Cryoturbation effects caused physical disruption of the soil fabric in both weathered and otherwise unweathered clays, and have been recorded in the overconsolidated clays of south England to depths going down to 5m or 6m below natural ground level. However, care must be taken to separate cryoturbation effects from depositional brecciation fabrics present in some over-consolidated clays.

Cryoturbation of overconsolidated clays has also been shown to have induced undulose shear surfaces aligned broadly parallel with the ground surface at the base of patterned ground structures (Weeks, 1970).

The solifluction process involves the movement downslope of saturated soil. It is particularly effective when the whole active layer slides over frozen ground at depth, creating extensive, broadly slope-parallel slip surfaces towards the base of each solifluction lobe or sheet. The solifluction process also typically led to intense fissuring within the soliflucted clays.

Solifluction movements were able to take place on very shallow slopes owing to the development of high pore water pressures when the ground surface froze either diurnally or each autumn as the active layer re-froze downwards.

Solifluction in England
Fluctuating climatic conditions in the Pleistocene Ice Age allowed several phases of solifluction to occur so more than one solifluction sheet may still be identified in some areas of south England. The total depth of solifluction deposits is typically between 1m and 3m, however, it does occasionally exceed this. Conversely, some clay slopes are completely devoid of solifluction deposits.

The clays affected include the Lias, Oxford, Wadhurst, Weald, Atherfield, Gault and London Clays.

Since the end of the Pleistocene Ice Age, approximately 10,000 years ago, the soliflucted slopes have generally re-graded to a condition that is stable in the summer months but, when pore water pressures rise in winter, become marginally unstable. This can cause small downslope creep movements to occur on the slip surfaces.

Artificial instability
Any artificial disturbance of these soliflucted slopes during civil engineering or building works, such as the addition of fill material on the upper parts of the slope, excavations into the lower parts, or even the removal of trees, can lead to instability. This instability may occur immediately or, more usually, during subsequent wet winters.

Obtaining a detailed understanding of the geology and hydrogeology of sloping soliflucted clay sites is therefore essential for civil engineering and building developments.

Determining the lowest level and geometry of the solifluction slip surfaces is critical because the shear strengths along the surfaces will have been reduced to residual values. These can be as low as half or less. The geology can be complicated further by the possible presence of relict landslips beneath the solifluction mantle and the presence of other bedding-parallel shear surfaces, all of which need to be distinguished from the solifluction sheets / lobes.

Once the geology and groundwater regime is understood, appropriate preventative measures can be designed to ensure the slope remains stable. These measures do not necessarily require the use of bored pile or other cantilevered retaining walls, the cost of which can make a development scheme unviable. Preventative stabilisation solutions can be employed using a combination of:

- Land drainage
- Granular fill and lightweight fill tailored to the geometry of the solifluction slip surfaces
- Careful design of the proposed slope profiles to keep loadings compatible with the geometry of both existing and potential slip surfaces.

There are, of course, occasions when ground engineering expertise is only sought after soliflucted slopes have started failing. As with all landslide investigations, identifying the geometry of the failure surface and establishing the pore water pressures acting within the slip surface are the first priorities.

As soliflucted slopes usually fail during particularly wet periods, a two-phase strategy for remedial works is often appropriate. The first phase, usually urgent, involves installing temporary land drainage and either adding weight at the toe or removing load from the head of the sliding mass.

Once movement has been arrested, permanent works can be built. If these can be delayed until the summer then requirements for temporary support of excavations are likely to be reduced, simplifying construction of a drainage and granular fill type scheme. An example of one such failure and a granular fill remedial works scheme is illustrated above.

The slopes had been created by a combination of placing up to 2.5m of clay fill at the crest and cutting into the natural slope by up to 1.5m at the toe. The presence of solifluction slip surfaces resulted in a series of failures with backscarps of up to 3m in height appearing overnight; an extremely worrying prospect for nearby residents.

The granular fill remedial works were constructed the following summer after detailed investigations and stability analyses of both the solifluction mantle and the underlying bedding parallel shear surfaces.

Conclusions
Solifluction and cryoturbation processes during the Pleistocene Ice Age have left many slopes in the over-consolidated clays of south and central England in a marginally unstable condition. Disturbance of these slopes during civil engineering and building works can result in failures either immediately or in subsequent wet winters.

To design preventative or remedial stabilisation works, a detailed understanding of the site's geology and the geometry of the slip surfaces is essential.

Stabilisation works do not necessarily require cantilevered retaining walls. The use of a combination of drainage, granular fill and lightweight fill can provide a cost-effective solution for stabilising soliflucted slopes.

Keith Gabriel is managing director of Gabriel GeoConsulting.

References:
French, H.M., 1996. The Periglacial Environment, 2nd edition. London: Longman.
Weeks, A.G., 1970. Stability of the Lower Greensand escarpment in Kent. PhD Thesis, University of Surrey. 107-109.

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