8.3.1 Effect of soil liquefaction on structures
Liquefaction is a process by which non-cohesive or granular sediments below the water table temporarily lose strength and behave as a viscous liquid rather than a solid hen subjected to strong ground shaking during an earthquake. ypically, saturated, poorly graded, loose, granular deposits ith a lo fines content are ost susceptible to liquefaction. Liquefaction does not occur at random, but is restricted to certain geological and hydrological environments, primarily recently deposited sands and silts in areas ith high ground ater levels. ense and ore clayey soils, including well compacted fills, and older deposits (Pleistocene deposits; Youd and Perkins, 1978) have low susceptibility to liquefaction.
he liquefaction process itself ay not necessarily be particularly damaging or hazardous. For engineering purposes, it is not the occurrence of liquefaction that is of importance, but the capability of the process and associated hazards to cause damage to structures. The adverse effects of liquefaction can be summarised as follows:
• Flow failures - completely liquefied soil or blocks of intact material ride on a layer of liquefied soil. Flows can be large and develop on moderate to steep slopes.
• Lateral spreads - involve lateral displacement of superficial blocks of soil as a result of liquefaction of a subsurface layer. Spreads generally develop on gentle slopes and move toward a free face such as an incised river channel or coastline.
• Ground oscillation - where the ground is flat or the slope too gentle to allow lateral displacement, liquefaction at depth may disconnect overlying soils from the underlying ground, allowing the upper soil to oscillate back and forth in the form of ground waves. These oscillations are usually accompanied by ground fissures and fracture of rigid structures such as pavements and pipelines.
• Loss or reduction in bearing capacity - liquefaction is induced hen earthquake shaking increases pore water pressures, which in turn causes the soil to lose its strength and hence bearing capacity.
• Settlement - soil settlement may occur as the pore-water pressures dissipate and the soil densifies after liquefaction. Settlement of structures ay occur due to the reduction in bearing capacity or due to the ground displacements noted above.
• Increased lateral pressure on retaining walls - occurs when the soil behind a wall liquefies and so behaves as a 'heavy' fluid with no internal friction.
• Flotation of buried structures - occurs when buried structures such as tanks and pipes become buoyant in the liquefied soil.
ther anifestations of liquefaction, such as sand boils, can also occur and ay pose a risk to structures, particularly through loss or reduction in bearing capacity and settlement.
Section 4.1.4 of EN 1998-5 describes the requirements for assessing liquefaction potential. Furthermore it provides a normative methodology in Annex B. It should, however, be noted that there have been numerous developments in liquefaction assessment methodologies in recent years (e.g. Seed et al, 2003; Boulanger and Idriss, 2004 etc.) and the methods described in the code ay be potentially unconservative, especially for aterials ith high fines content. It is therefore recommended that an expert should be employed to carry out liquefaction assessment.
A liquefaction susceptibility evaluation should be made when the soil includes extended layers of thick lenses of loose sand (with or without
Figure 8.4 Liquefaction assessment using corrected SPT values
Figure 8.4 Liquefaction assessment using corrected SPT values silt/clay fines), beneath the water table and when the water table level is close to the ground surface. EN 1998-5 recommends that the shear stress approach is applied. In this method, the horizontal shear stresses generated by the earthquake are compared with the resistance available to prevent liquefaction. In Annex B of EN 1998-5 a set of liquefaction potential charts can be found for a magnitude M = 7.5 earthquake. The shear stresses 'demand' are expressed in terms of a cyclic stress ratio (CSR), and the 'capacity' in terms of a cyclic resistance ratio (CRR).
The CRR is assessed based on corrected SPT blow count using the empirically derived liquefaction charts, which are shown schematically for silty sand in Figure 8.4. These charts compare CRR (x/o'v0), with corrected SPT blow count (N^)). In Figure 8.4 the dependence of liquefaction potential on the percentage fines content in the silty soil is also seen by comparing the three lines. For a given corrected SPT blow count, clean sands with fines content of <5 per cent liquefy more easily compared to silty sands ith a greater percentage of fines content. he procedure for correcting the field N values to obtain the corrected N1(g0) is explained later in Section 8.3.3.
The CSR is assessed by first calculating the cyclic shear stress (t) using Equation (8.6).
where a is the ratio of the design ground acceleration on type A ground, a, to the acceleration of gravity, g, S is the soil factor and sv0 is the overburden pressure.
It must be pointed out that Equation (8.6) is conservative because it neglects the stress reduction factor with depth (rd).
This expression may not be applied for depths larger than 20 m. A soil shall be considered susceptible to liquefaction whenever CRR > l X CSR, where l is recommended to be 0.8, which corresponds to a factor of safety of 1.25.
If soils are found to be susceptible to liquefaction, itigation easures such as ground improvement and piling (to transfer loads to layers not susceptible to liquefaction), should be considered to ensure foundation stability.
he use of pile foundations alone should be considered ith caution due to the large forces induced in the piles by the loss of soil support in the liquefiable layers, and to the inevitable uncertainties in determining the location and thickness of such layers.
For buildings on shallow foundations, liquefaction evaluation may be o itted hen the saturated sandy soils are found at depths greater than 15 .
8.3.3 Design example on determination of liquefaction potential
In this section we shall outline the liquefaction assessment for Site A, as described in Chapter 4. The foundations for the hotel building can take the for of shallo foundation provided that the chosen site does not pose a major risk of liquefaction. In other words, liquefaction potential of the chosen site should be lo . he design of shallo foundations ill be considered in this chapter. However, in certain sites where there is significant liquefaction risk, pile foundation ay be preferred. he design of pile foundation ill be considered in Chapter 9. In either case, it is important to carry out an assessment of liquefaction potential for any building site. The method for carrying out such an assessment on Site A' is shown in this section. As explained in Chapter 4, Site A' has loose sand layers below the water table.
he borehole data obtained fro site investigation is presented in Figure 8.5 along with the strength parameters and the water table.
Water table + 12.53m v
Was this article helpful?