SUBSIDENCE

Lime Barrier

how does Root Management and Soils Stabilisation work?

In the UK the shrinkage and swelling of clay soils, particularly when influenced by trees, is the single most common cause of foundation movements that damage low rise buildings.

Trees are known to cause clay soils to shrink by drawing water through their roots, predominantly during spring and summer. This shrinkage results in both vertical and horizontal ground movements, that when transmitted to a building's foundations, cause damage to the building structure.

The amount of shrinkage depends on the characteristics of clay soil, the type and size of vegetation, plus variations in climate. Trees growing under grass cover are forced to compete for their water and to extract water from greater depths than they might otherwise do, as is the case in this instance.

The water content of a shrinkable clay soil will vary with depth remote from and near to a large tree. Near the ground surface, there can be relatively large changes in soil water content between summer and winter as a result of evaporation from the ground surface and transpiration by the grass. Such variations are normally confined to the top 1m of the ground, possibly less adjacent to buildings. Where mature trees/vegetation grow at the same location, then the water-content profiles will vary and the seasonal fluctuations in soil water content are both larger and extend to greater depths. Soil volume changes and hence ground movements with attendant subsidence damage to low rise structures will thereby be greater.

Soil Stabilisation

Subsidence cracks arising in the summer months due to shrinkable clay subsoils will close up when ground moisture contents recover over subsequent winter periods. The intention of the Soil Stabilisation Method is to mitigate against this periodically damaging effect. The solution adopted in this case introduces a naturally occurring mineral at the surface of the clay particles which decreases their volume change potential. Thus decreasing water uptake by the trees and thereby lessening subsidence risk conserving soil moisture beneath the foundations and reducing clay shrink/swell effects. The benefits here are also seen here in severing the existing roots and then restricting their future growth potential by the creation of environments inhospitable to their re-development.

In addition, the shrink/swell properties of clay subsoils are limited by the introduction of lime additives thus reducing both plasticity and volume change potential. Consequent cation exchange/clay aggregation reduces the clay mineral effective surface area and affinity for water.

n addition, the shrink/swell properties of clay subsoils are limited by the introduction of lime additives thus reducing both plasticity and volume change potential. Consequent cation exchange/clay aggregation reduces the clay mineral effective surface area and affinity for water.

Cation exchange leads to soil property changes resulting in the modification of the clay structure mainly involving calcium ions and is regarded as a rapid cation exchange process occurring on the surface of clay particles. Clay particles typically exhibit surface charge imbalances and the negative charges are balanced by hydrated cations. Accordingly, individual clay particles are surrounded by adsorbed water in the diffuse double layer arrangement.

  • Lime is used as an excellent soil stabilising material for active clays which undergo frequent volumetric change through shrinkage and swelling

  • Lime acts immediately and improves various soil properties of the clay such as bearing capacity, resistance to shrink/swell during seasonal climatic changes, reduction in Plasticity Index provides an environment hostile to root growth

  • The reaction is very quick and stabilisation of soil starts within a few hours

  • Provides a valuable modification to the behaviour of tree roots whilst enabling the vegetation to be retained

    in place

  • Provides tangible sustainability benefits enabling trees to be retained rather than removed

  • The presence of lime does not constitute an eco-system burden or impact on groundwater

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