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The principle is to load the ground with a temporary embankment before the full structural load is applied, in order to cause advance compression and settlement of the ground. When the final load is acting, the ongoing settlement is therefore much reduced or negligible.

Pre-loading is applied up to the value of the structural load and then removed before construction takes place.

Surcharging is a pre-load which is in excess of the final structural load, applied with the intention of producing a greater rate of compression and less rebound on removal.

Both methods are designed to reduce the settlement that will occur after the final load is applied.

Tests have shown that surcharge of loose fill can produce greater pre-construction settlement and lower long-term settlement than dynamic compaction (Burford and Charles, 1992). Time for settlement of loose fill with large air voids and including clay lumps is rapid; hence the surcharge need not be applied for an extended period. A surcharge load is frequently placed on the drainage blanket over vertical drains (6.1) in compressible clay to increase the rate of consolidation.

Suitable for loose granular fills and soils where, given good drainage (k>10⁻⁴m/s), settlement can be rapid. In soft compressible clays, unless supplementary vertical drains are used to relieve the excess pore pressure induced by surcharging, settlement time will not be significantly reduced. Similarly, where a pre-loading embankment may cause shearing of weak soil, the addition of vertical drains is advisable.

Adequate low-cost material should be available locally for the embankment;

alternatively, loading with water tanks/bags may be feasible using natural sources.

Not recommended where time is limited for construction, unless a staged operation is possible (Tomlinson and Wilson, 1973).

Short-term pre-loading of peaty soils will not be effective if the secondary settlement is likely to dominate.

In cases where buildings or machinery or embankments are to be built on compressible soil or fill but cannot tolerate some ongoing settlement, the use of piles or a piled raft is likely to be preferred by designers (Card and Carter, 1995).

Applications are mainly to improve the shear strength and control total and differential settlement of fills, but some landfills are difficult to treat. The method is useful where

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embankments join viaducts or other more rigid structures.

Ground investigation must determine the strength characteristics of the soil (and of fill if this is to be treated and remain as a foundation material) so that detailed settlement calculations can be made for each layer of soil. Permeability and consolidation parameters are required, together with elastic modulus.

The settlement of foundations on fill will occur due to:

• its own weight and degradation of landfill

• applied load

• consolidation of the underlying ground.

Large-plate loading tests using skips or the construction of test embankments are recommended as part of the investigation to check whether the rate of settlement under pre-load will be effective.

Design requires assessment of the three phases of settlement—immediate, consolidation and creep (Tomlinson, 2001)—with and without the effects of the pre-load.

In weak soil the temporary loading embankment will have to be built up in short lifts so that shear failure does not occur. Vertical drains may improve the subsoil, allowing greater lifts and final height.

As a guide, the ratio of maximum effective vertical stress during surcharge to that due to the structure will be around 1.5. However, to minimise rebound of the ground on removal of the pre-load, surcharge greater than 3 times the imposed load may be needed.

Where a layer of permeable sand exists below the compressible clay the drainage under surcharge will be downwards. The water squeezed out can then be extracted from the sand with pumped wells—this can be accelerated by vertical drains into the sand layer.

Where the drainage is mainly upwards a drainage blanket, preferably with additional applied vacuum under a membrane cover, and collector drains are required around the pre-load fill.

Construction area of the surcharge must be larger than that loaded by the permanent structure. The amount of pre-load depends on the consolidation period and the rate of placement.

The pre-load/surcharge is removed when the degree of consolidation is at least 75 per cent of the total estimated; but if time is available it is worth waiting until 95 per cent is reached.

Monitoring will check the rate of settlement and pore pressure reduction. On weak foundations measurement of lateral movement at the toe of embankments may be appropriate.

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CPTs and triaxial tests before and after loading are routinely performed.

Cost comparisons of ground improvement methods ranging from pre-loading to piling:

• drainage and preloading are the cheapest per square metre to apply, but the time period required to produce improvement can be considerable

• dynamic and vibro-compaction are next in cost and results are virtually immediate

• jet grouting and deep soil mixing are 2–2.5 times the cost of drainage/pre-load, but provide considerably greater strength improvement

• piling and vibro-concrete columns for areal treatment are 4–5 times the cost of drainage/pre-load.

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