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GROUND COMPACTION - URETEK Deep-Injection

CAUSES OF SUBSIDENCE

The causes of subsidence are multiple. Many are suited to correction by URETEK Deep-Injection.

Variation in ground moisture content
Long periods of drought result in the evaporation of water normally present in the ground, causing a reduction in its volume, particularly in some clays. This contraction is never uniform, either because of lack of homogeneity of the foundation soils or because the structure itself prevents the heat of the sun's rays from directly and uniformly reaching the ground. Differential settlement occurs and cracks begin to appear, perhaps opening and closing with the seasons.

Conversely a building built slab-on-ground during a very dry period can experience "ground heave". That is, the centre of the building rises causing confusion, as the appearance is that the perimeter has settled. Heave occurs because over a long period the slab traps what little moisture there is (somewhat like condensation) and the clay expands.

Trees or plants close to a building or with a very extended root systems, can remove water from under the building and cause similar consequences. (If a tree needs to be removed, the ground moisture must be allowed to stabilise over many months, before correction.)

In many cases, the effects of these factors occur over very long periods of time, making it difficult to establish the exact cause.

Leaking water pipes, sewers and drains, etc.
When certain soils become saturated, they lose their strength. Cohesive soils (clays in particular) can become very soft (plastic) and are then no longer able to uniformly support the load above - and subsidence occurs.

Common causes of subsidence are infiltration of uncontrolled surface water and water leakage from defective underground water services. A burst water supply may result in massive erosion of the foundation soils, whilst even a slow leak from any type of pipe can have the most detrimental affects.

Excavations adjacent to building.
Excavation adjacent to buildings, especially in non-cohesive ground (sand and gravel) can cause subsidence unless proper precautions are taken to support the building in advance, restrain the sides of the excavation and/or prevent a lowering of the water table (see below).

Certain soil types can be pre-stabilised by URETEK Deep-Injection to insure against such problems.

Inadequacy of the foundation structure.
Where the mass of a building exceeds the bearing capacity of the foundation soils, settlement is almost inevitable. Where conditions vary, the result may be differential settlement, with consequential cracks occurring.

Cohesive ground can react quite slowly to imposed pressure, sometimes taking a long time to reach a state of equilibrium. When considering the cause of a subsidence, one should be careful not to underestimate slow ground movements nor overestimate factors which produce effects in a short time.

Disturbed soil and backfill.
When ground is disturbed prior to the construction of a building, it loses its original supporting capacity and cannot be relied upon as an adequate foundation. Similarly with unconsolidated filled ground, which commonly occurs where ground level has been raised or in the vicinity of a retaining wall.

Unless such soils are re-consolidated effectively, subsidence will occur. Differential settlement may take years to become apparent - but can become very serious!

Additional causes of subsidence and movement

Lowering of water table
Dewatering of abandoned mine shafts, tunnelling, open-cut mining, building excavations and the like, can all lead to lowering of the water table. As pore pressure is reduced, so settlement begins.

Poor design

Footings too small to adequately spread load into foundation soils.

The absence of an organized footing system - common in very old buildings or in buildings with sections added-on at various times.

Differences in the dimensions and depths of the footings in different parts of the building, relative to load and soil conditions.

Differential movement due to buildings being founded on varying strata. (e.g. rock and clay - and this is often a problem with partial underpinning down to rock)

Vibration caused by traffic, machinery or construction activities.
Vibration due to machinery or passing traffic can cause ground densification. That is, the ground compacts and shrinks, leaving the structure above unsupported. Such problems also occur on properties adjacent to building excavations where there is rock excavation, or in the vicinity of underground tunnelling.

Buildings built on "Cut-and-Fill"
Where sloping sites are levelled to create a building platform, one of two problems can occur due to differences in consolidation or geological strata. A concrete slab resting across both the cut and inadequately consolidated fill can fail at the transition. Footings that are not in the same stratum will function differently. That is, footings on the "Fill" side need to be excavated through the fill, to just below the same type of stratum exposed in the "Cut".

Organic material in ground
Decomposition of organic materials (rubbish, peat, old tree roots, etc.) can eventually lead to voids and subsidence

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