Liquefaction

The term ' I iquefaction' describes the phenomenon whereby perfectly stable soil underlying a flat site and possibly having provided a

Compression

Ground acceleration

▲ 7.1 Forces acting on a foundation from the superstructure and the ground.

Compression

Ground acceleration

▲ 7.1 Forces acting on a foundation from the superstructure and the ground.

Brick - - Container Sand and water

▲ 7.2 A simple demonstration of liquefaction.

▲ 7.2 A simple demonstration of liquefaction.

▲ 7.3 A building after liquefaction. Dagupan City, 1990 Philippines earthquake.

(Reproduced with permission from David C. Hopkins).

▲ 7.3 A building after liquefaction. Dagupan City, 1990 Philippines earthquake.

(Reproduced with permission from David C. Hopkins).

solid founding stratum for hundreds of years suddenly liquefies during a quake. Fortunately, most soils are not in danger of such a rapid phase-change with its subsequent destructive effects on the built environment. The three main prerequisites for liquefaction are: a layer of relatively loose sand or silt, a water table high enough to submerge a layer of loose soil, and finally, an intensity of ground shaking sufficient to increase the water pressure between soil particles to cause the soil-water mixture to liquefy. Even if the ground shaking does not cause'liquefaction', the increase in water pressure can lead to a reduction in the ability of foundation soils to withstand loads from buildings.

Liquefaction can be easily demonstrated by filling a bucket with dry or moist sand, and pouring in water. A brick placed upright in the sand tilts and sinks when the side of the bucket is tapped with enough force (Fig. 7.2). A sandy beach provides another opportunity to experience liquefaction. By rapidly vibrating your feet at the water's edge the sand-water mixture liquefies. The sand beneath your feet settles as it consolidates and water flows to the surface causing the sand to glisten.

During some quakes a liquefied sand-water mixture is naturally ejected under pressure through cracks in the ground and sand is deposited on the surface. Due to their appearance, these sandy deposits are referred to as 'sand boils'.

When a soil liquefies it completely loses its bearing capacity. Unless a building above it is designed to float like a boat it tilts and overturns (Fig. 7.3). It is usually impractical to design a building to float. The centre of gravity must be lower than the centre of flotation or the centre of gravity of the displaced fluid which, without a heavy ' keel ' , is unlikely.

If a site is susceptible to liquefaction, and relocation of the proposed building to a less vulnerable site is not an option, what are the alternatives? First, a building can be founded on piles. The objective of piling is to extend the foundations through the liquefiable deposit and found a building on firm soil or a rock stratum unaffected by seismic waves. A limitation of this approach is that the piles will not prevent the ground around and under the building from settling. Unless specifically designed for relative movement, possibly in the order of several hundreds of millimetres, power, water, gas and sewage services are at risk of rupture. The piles themselves are also vulnerable to damage especially due to relative horizontal movements between the ground strata above and below the layer of liquefiable soil. Structural engineers provide these piles with additional ductility.

As an alternative to piling that founds the building on a firm stratum, ground can be strengthened by one of several techniques known as 'ground improvement' (Fig. 7.4).2 The approaches to ground improvement consist of densification, drainage and reinforcement. Soil can be densified by several different methods. Vibroflotation consists of penetrating the soil with a large vibrating probe, possibly squirting water from its tip. It vibrates and densifies the soil as it sinks into it. In a variation of this method, stones are introduced as the vibrating probe is withdrawn to form a stone column that both reinforces the soil and allows any quake-induced water pressure to dissipate quickly. Stone columns are positioned typically on a one and a half metre grid. A far less sophisticated dynamic compaction method to densify loose soils shallower than approximately 12 m may be an option if there are no

(c) Closely spaced piles

▲ 7.4 Common methods of ground improvement to prevent liquefaction.

(c) Closely spaced piles

▲ 7.4 Common methods of ground improvement to prevent liquefaction.

adjacent buildings or facilities. A crane pounds the ground by dropping a weight of up to 30 tonnes from a height of between 10 m to 30 m. Once the resulting 'craters ' are filled with compacted material the building is founded on shallow foundations. At times, contractors dynamically compact soil utilizing an even heavier-handed approach involving blasting. Obviously this technique is limited to sites distant from existing buildings!

The formation of stone columns is one method of improving soils prone to liquefaction and providing a means of drainage. Another soil improvement technique includes driving many closely-spaced piles. This densifies the soil by compressing it and reduces the chance of liquefaction. In permeable soil deposits like sands and gravels, cement or chemical grout can be injected underground at close enough centres in plan to bind the soil particles together and prevent liquefaction.

One of the side affects of liquefaction is lateral spreading - the lateral or horizontal movement of flat or gently sloping reclamations and ground adjoining foreshores and rivers. This has been observed in many earthquakes; it caused extensive damage in the recent 1994 Northridge, California and 1995 Kobe, Japan earthquakes (Fig. 7.5). Depending on various engineering soil parameters, and the intensity of ground accelerations, horizontal displacements can vary from tens of millimetres to several metres.3 Although lateral spreading may occur in soft soils that do not liquefy, the range of mitigation techniques includes most of those that prevent liquefaction. The construction of sea walls to confine areas likely to flow, and location of buildings well away from free edges likely to undergo lateral spreading, are also recommended.

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