Soil liquefaction is a phenomenon primarily associated with saturated loose granular soils such as sands, some gravels, and non-plastic silts located close to the ground surface where in situ confining stresses are relatively low. During earthquake shaking, loose, saturated granular soils tend to contract which can cause an increase in the pore water pressure of the soil particles. If the shaking is strong enough to increase the pore water pressure beyond the effective confining stress of the soil, the pressure may force the soil particles apart and the soil will then behave similar to a liquid – hence the term “liquefaction”. Liquefaction can result in a significant reduction in the soil’s shear strength, as well as other ground distress such as sand boils, excessive settlement, and lateral movements.
Confidential Project, Abu Dhabi, UAE
Soil liquefaction can be a risk in the Middle East as groundwater levels are typically high within one to two kilometers of the coast. The phenomenon is potentially problematic in man-made land reclamation zones, as well as in areas where natural coastal loose sand deposits are encountered.
The liquefaction risk is often exaggerated, as engineers tend to overlook several risk mitigating circumstances, like the high content of fines (silt and clay) within the soil material; the additional surcharge (and hence increased confining stress) due to the subsequent construction of embankments; and also the existence of a surficial crust of dense sand that prevents the liquefaction induced settlement from reaching the surface. In some cases the relatively small thickness of the liquefiable sand layers means that even under a design earthquake event, the actual induced settlements are manageable from a serviceability perspective. Reclaimed areas may or may not be susceptible to liquefaction, depending on the quality of the sand compaction during reclamation and the content of the sand in fines. Typically, the reclamation can be performed in wet conditions by use of the hydraulic fill method, where the material is deposited by a flowing stream of water, or in dry conditions by compacting the imported fill material in layers. In the first case, the compaction of the reclaimed material typically takes place by use of the vibro-compaction method, while in the second case by the use of impact or vibration rollers. Hydraulic fills tend to be more susceptible to liquefaction, as the material usually lacks fines and as the compaction is performed following fill placement, so is therefore more difficult to achieve. Conversely, reclamation fill placed in dry conditions tend to be less susceptible to liquefaction as the material is compacted in layers and the compaction quality control is performed during fill construction.
In a recent case in Abu Dhabi, authorities accepted Langan’s view that the above mentioned mitigating circumstances eradicated the soil liquefaction risk, which resulted in cost savings of millions of dollars in ground improvement related construction costs.
About Alexandros Yiagos, Ph.D
U.S. educated (Princeton University) and native of Athens, Greece, Alexandros has 25 years of experience in the design and construction of earth dams, highways (embankments, slopes, and bridges), buildings, thermal power plants, refineries, hydraulic structures, marine structures, airports, wind farms, mines and environmental projects. As a senior project manager for Langan International in Dubai during the past three years, he has been involved in geotechnical engineering consulting for the design of high-rise and low-rise residential, office, hotel, hospital, and education buildings in the United Arab Emirates, Saudi Arabia, Oman, Qatar and India.