Subsidence & Heave
Subsidence may cause parts of a house to drop
Subsidence is a natural downwards movement in the ground relative to a constant such as sea level. The opposite of subsidence is heave, which increases the elevation of the ground.
How is Subsidence caused?
Many soils contain significant proportions of clay which because of the very small particle size are affected by changes in soil moisture content. Seasonal drying of the soil results in a reduction in soil volume and a lowering of the soil surface. If building foundations are above the level to which the seasonal drying reaches they will move and this can result in damage to the building in the form of tapering cracks. Trees and other vegetation can have a significant local effect on seasonal drying of soils. Over a number of years a cumulative drying occurs as the tree grows, this can lead to the opposite of subsidence, known as heave or swelling of the soil, when the tree declines or is felled. As the cumulative moisture deficit is reversed, over a period which can last as many as 25 years, the surface level around the tree will rise and expand laterally. This is often more damaging to buildings unless the foundations have been strengthened or designed to cope with the effect.
The habitation of lowlands, such as coastal or delta plains, requires drainage. The resulting aeration of the soil leads to the oxidation of its organic components, such as peat, and this decomposition process may cause significant land subsidence. This applies especially when ground water levels are periodically adapted to subsidence, in order to maintain desired unsaturated zone depths, exposing more and more peat to oxygen. In addition to this, drained soils consolidate as a result of increased effective stress. In this way, land subsidence has the potential of becoming self-perpetuating, having rates up to 5 cm/yr. Water management used to be tuned primarily to factors such as crop optimisation but, to varying extents, avoiding subsidence has come to be taken into account as well.
How does Target solve subsidence-related problems?
Heli Pile Installation
Target's Heli Pile system is a versatile, quick and simple solution to many foundation-related problems, including those caused by subsidence and heave.
Made from Grade LM25 (AlSi7-Mg0.5) cast aluminium alloy the Heli Pile has been tested by the University of Bath for its tensile, compressive and ductile strength. The corrosion resistance of aluminium alloy is well known, and this particular grade is well suited for its use under ground.
The threaded connector between each 1 metre (approx) length of alloy pile is made from Grade 304 stainless steel.
Used in combination with the Bar Flex Beaming system the Heli Pile is used as a standard pile and beam repair method. The system is very quick and easy to install giving an efficient and cost-effective solution.
The lightweight equipment ensures that there is little disruption, even on the wettest sites.
A number of Standard Details, which show the various uses of the Heli Pile, are available. Each Standard Detail includes a full method statement for the particular case outlined. Full specifications can also be supplied.
As the sections of Heli Piles are only 1 metre long, they are easily installed internally through a solid floor without the need for major disruption. If a longer pile is required, it is a simple matter to screw on additional lengths as each metre length is driven home.
Heli Pile Fully Installed
A Heli Spike is driven into the ground to form a pilot hole of the required size. No material is removed; it is simply compressed by squeezing, ready to accept the Heli Pile. The Heli Pile is driven into the pre-formed pilot hole compressing the surrounding substrate and increasing its effective diameter. Additional lengths are added as required to suit the ground conditions and loading requirement.
Because of the ease of testing, a proof test is recommended on each installation. Site testing is achieved by means of a small hydraulic testing kit and, to suit all site conditions, may be performed either in tension, for soil stabilisation, or compression where it is utilised as a pile, it is also possible to use a correlation between a tensile test and a compressive load.
Instead of working to characteristic loadings, which have been attained in laboratory conditions, the capability of in-situ proof testing allows the Heli Pile to be tested in the actual site conditions in which it is installed. Proof testing gives the specifier confidence in the Heli Pile's ability to cope with the imposed loads and allows a much lower factor of safety to be used.
Heli Pile Summary