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Subsidence

Subsidence Explained

Subsidence-Shrink-swell National Potential MapSoils generally fall into two categories; Cohesive (clays) and non-cohesive (gravels, sands, silts). The soil type may have some bearing on the type of foundation but more importantly, it is the range of factors and external influences that dictate foundation type. Cohesive soils have a very small molecular structure and hold water as part of the clay molecule. In a non-cohesive soil, water is held within the gaps between the particles and therefore, water can pass freely through. This unique feature of cohesive soils is what makes them impermeable and used in things such as landfill liners and lining natural ponds and lakes, as they will not allow water to pass through them.

Trees and other vegetation demand moisture from the ground in order to survive and flourish. This water is normally provided by rainfall collecting in watercourses and in the natural water table. The water is then given up by the ground into a capillary root system of the tree, which conveys this back to the central tree system. In cohesive soils however, the replacement of that water is a very slow process due to its impermeability and over a period, this will lead to a reduced moisture content (mc). Moisture content is worked out by taking a sample of clay, weighing it, drying it in a kiln and weighing it again.

Mc =     wet weight - dry weight .          x 100%

                      Wet weight

As the moisture content reduces in a cohesive soil, we reach a critical point where the soil moves from an elastic state to a plastic state and this is known as desiccation. In other words, the soil starts to shrink as more water is extracted. This situation is often experienced at ground level and will be noticed by gardeners in the summer months when the flowerbeds become hard to dig and the grass in a cohesive soil area starts to go brown and the ground begins to crack. It is for this reason that foundations in cohesive based soils, need to be located at a depth beyond the zone of seasonal influence or in the case of large trees, beyond the influence of the root zone.

By using Shire Stabilizers to solve a subsidence issue, we can quickly and simply drive our piles to depth beyond the root influence of trees and install the helical section of the pile into soils, which, by virtue of the depth within the ground, exhibit uniform characteristics throughout the year and are not subject to this seasonal fluctuation, experienced at much shallower depths.

In non-cohesive soils, vegetation is not critical at all. It is perhaps more critical to know whether the ground is natural or indeed a fill material. This is to say, has the ground been placed geologically over a large period of time or is the ground the result of some artificial working, such as cut and fill to create terracing on a hill side. The greatest threat in non-cohesive soils is drains or water coursing. When water passes through a granular soil, it hydraulically compacts it, drawing the finer particles deeper into the ground and allowing a secondary consolidation of the larger particles.

Trees:

As previously stated, trees and vegetation are only an issue in cohesive soils so if your property is in sand, the presence of vegetation is not an issue. In cohesive soils, trees extract moisture from the ground causing a reduction in the volume of the soil and hence a downward movement occurs. If this was to occur uniformly across a site, it is unlikely that the occupant would be aware of it, as this would simply mean the property moves up and down as one unit. The problem arises when the degree of movement is more pronounced in one area of the structure and this is where trees have their greatest effect. This “differential movement” is what most people frequently refer to as subsidence and is the cause of the majority of domestic subsidence claims in the south of England.

The species and maturity of a tree will also have a bearing on the threat it poses to a building and how deep a foundation will have to be. At Thor Helical Remedial our design principles are based upon going 1.5 m beyond the last detected root in a site investigation or using NHBC guidance plus 1.5m, whichever is deeper. This ensures that the helical section of the pile (the part of the pile, which interacts with the ground and derives bearing capacity) is beyond the influence of tree roots and prevents the structure from subsiding.

Shire Piles and Shire Stablizers.

Shire Stabilizers are used to repair subsidence issues and are designed to work with the existing structure and can provide full or partial support. Typical uses would be the stabilisation of foundations resting on soft ground or ground subject to clay shrinkage movement (subsidence). If you require a repair to solve subsidence using Shire Stabilizers please complete our Find an Approved Installer page and you will be contacted by return.

Shire Piles are used where the support cannot rely on assistance from the existing structure. Typical uses would be new build garage, conservatory, garden rooms, oak frmaed buildings and orangeries. A specialist foundation system based on Shire Piles is available from our sister Company Quickbase Foundations Ltd.

 

Map reproduced by permission of the ‘British Geological Survey © NERC.  All rights reserved. IPR/130-87CY

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