The liquefaction of sand is one of the primary factors leading to the damage of the structures during earthquakes. The liquefaction normally occurs due to the generation of excess pore pressure under undrained loading in sandy soil. Hence, it is imperative to take countermeasures against liquefaction and suggests the approach to combat it such that while the soil liquefies, the damage is minimum.
IntroductionOver the years, some of the most spectacular, and costly damage to the earth slopes and the foundation of structures has been due to liquefaction of sands during earthquake. When an earthquake shakes loose saturated sand, the grain structure of soil tends to consolidate into more compact packing. Since all these movements happen rapidly, there is no chance to reduce the volume through the dissipation of pore water pressure from within the soil mass. Therefore, the incompressible pore fluid takes up the entire applied stress and consequently, the effective stress approaches zero and ultimately the deposit "liquefies." Since a liquid has no shear strength, occurrences of disastrous consequences due to the failure of earth slopes and foundations are inevitable.
The devastating effects of liquefaction drew considerable attention of geotechnical earthquake engineers. In 1964, the Good Friday earthquake (Mw= 9.2) in Alaska and the Neegata earthquake (Ms = 7.5) in Japan occurred. Both earthquakes produced splendid examples of liquefaction leading to slope failures as well as foundation failures of bridges and buildings. Even the recent earthquakes of India such as Bihar Earthquake of 1988, Uttar Kashi Earthquake of 1991, Bhuj Earthquake of 2001, etc. have witnessed the liquefaction of soil leading to slope and foundation failures.
|Figure 1: Liquefaction during Bhuj Earthquake (26.01.2001)||Figure 2: Ground Rupture due to Liquefaction near the Epicentre of Bhuj Earthquake (26.01.2001)|
Some of the photographs of liquefaction of soil mass taken during Bhuj Earthquake is shown in Fig. 1. Ground rupture due to liquefaction and its effect near the epicenter of Bhuj Earthquake of 2001 are shown in Fig. 2 and Fig. 3 respectively.
|Figure 3: Effects of Ground Rupture near Epicentre of Bhuj Earthquake (26.01.2001)|
Mechanism of LiquefactionLiquefaction is a phenomenon experienced by soil when it loses its entire strength due to build up of excess pore water pressure. Under this condition, soil undergoes continuous deformation with low or no residual resistance. The term liquefaction is given in term of shear strength.
σ1 = γd Z1 + γsat Z2 - γw Z2
When water table is at ground surface, effective stress at any elevation 'Z' from ground level,
But under dynamic loading, an increase in the pore pressure (+Δu ) = γw Z' due to the vibration of the ground, the shear strength may be expressed as
where σ1dyn tan Φ1 is the effective dynamic stress and Z1 is the height of water rise in piezometer tube above ground level as shown in Fig. 4. Therefore, with the development of additional positive pore pressure, the shear strength of sand (c1=0) reduces. For complete loss of shear strength,
γ1Z = γw Z1
or Z1 / Z = γ1 / γw = (G – 1) / (1 + e) = icr
where G is the specific gravity of the soil solids, e is the void ratio and icr is the critical hydraulic gradient.
It has been widely understood that during earthquake, ground shakes due to the propagation of shear waves from the hypocenter, the location where the seismic energy is released and a series of cyclic stresses are induced for a very short duration on the soil. Therefore, the drainage of water cannot take place during the earthquake. But soil particles try to undergo volume change. In order to keep the loose sand at constant volume, some stresses build up as pore water pressure. When the excess pore water pressure becomes equal in magnitude to initial effective stress, soil loses its entire intergranular stress and the particles of soil literally starts to flow in water without any contact. Such a state is called liquefaction.
Quick Sand and LiquefactionWhen water flows through soils in upward direction, it exerts forces called seepage forces. As the seepage forces increase (due to increase in head), they gradually overcome the gravitational forces acting on the soil column and eventually a quick condition (quick = alive) will occur. This phenomenon is also known as quick sand. Another phenomenon related to quick sand is liquefaction. After the quick sand condition, the flow is reversed and the water level is allowed to drop just slightly below the sand surface. If a sharp blow, which eventually occurs during earthquake, is applied to the side of the tank, the entire soil mass liquefies and the sand loses all bearing capacity. And the phenomenon is termed as liquefaction. Normally, under static loading the permeability of the sand is sufficient for pore water pressure dissipation.
Hence there is a silver lining between the quick sand condition and the liquefaction as the former occurs due to seepage forces while the latter is outcome of dynamic forces.
Determination of Liquefaction SusceptibilityA number of approaches are available for determining the potential for initiation of liquefaction. The most common type of approach is cyclic stress approach.
Cyclic Stress ApproachIn this procedure, the following two cyclic stresses are determined:
- Cyclic stresses induced in the soil due to earthquake
- Cyclic resistance available in the soil measured either through laboratory tests or field tests.
Cyclic Stress Induced in the SoilThe level of excess pore pressure required to initiate liquefaction is related to the amplitude and duration of the earthquake induced cyclic loading. The loading can be predicted either through site amplification or by a simplified approach. In our country, the simplified approach is preferred. Actual earthquake ground motions recorded have quick irregular time histories. However, laboratory data from which liquefaction resistance is estimated are typically obtained from the uniform cyclic shear stress amplitudes. Hence, the recorded strong motion data are converted to an equivalent series of uniform cyclic stress. A weighted procedure was used by Seed to determine the number of uniform cyclic stress (Neq) (at amplitude of 65% of the peak cyclic shear stress) that would produce an increase in pore pressure equivalent to earthquakes of different magnitudes. The uniform cyclic shear stress or average shear stress (amplitude) can also be estimated from a simplified procedure (Seed and Idriss, 1971) which is as follows:
z = Height of the soil column considered
γ = Unit weight of soil
g = Acceleration due to gravity;
amax = Maximum surface ground acceleration
rd = Stress reduction factor
Seed and Idriss recommended the use of charts for obtaining the values of rd at various depths. The following equation may be used to estimate average values of rd:
rd = 1.0 – 0.00765z for z < 9.15m
rd = 1.174 – 0.0267z for 9.15m < z < 23m
The critical depth for development of liquefaction is usually less than 12m.
Evaluation of Liquefaction ResistanceTo evaluate the susceptibility of liquefaction, after determining the average shear stress due to the earthquake loading, the next important step is to estimate the resistance offered by the soil (shear stress of soil causing liquefaction). In addition to laboratory tests, several field tests (SPT, CPT, etc.) have been used for the evaluation of liquefaction resistance.
The resistance available in the soil is determined in the cyclic stress ratio (CSR).
For the cyclic triaxial test, (CSR)tx = σdc / 2σ31
σdc = Deviator stress = σ11 - σ31
σ11 = Additional axial stress, and
σ31 = Initial effective confining pressure
and Maximum cyclic shear stress = σ11 - σ31 / 2
Here (CSR)SS = cr (CSR)tx
The correction factor cr is estimated from Table 3.1 as shown below.
In contrast to the laboratory tests, earthquakes produce shear stresses in different directions. It was suggested that the CSR required to initiate liquefaction in the field was about 10% less than that required in unidirectional cyclic simple shear tests. Therefore the liquefaction resistance of an element of soil in the field is given by the cyclic stress ratio (CSR) which is as follows:
(CSR)field = 0.9 (CSR)SS = 0.9cr (CSR)tx
Evaluation of Zone of LiquefactionThis is determined by comparing the average shear stress (tav) due to earthquake loading and the shear stress (t) causing liquefaction, as expressed earlier, by the following relations:
where symbols have their usual meanings.
|Figure 5: Zone of liquefaction in the field|
Countermeasures Against LiquefactionAfter determining zone of liquefaction, we control liquefaction by various countermeasures. It is appropriate to expel out water from the liquefaction zone as far as possible in order to avoid building up excess pore water pressure during the countermeasures. The common methods of ground improvement techniques that can be applied to counter liquefaction are discussed below.
DensificationSoil densification termed as compaction is one of the most common methods of soil stabilization. It may be natural or artificial process by which soil is made stronger and more resistant to deformation under applied loads. Sand is densified by means of rollers, rammers or vibrators. Compaction involves spreading of sand in thin layers called 'lifts' by means of the hauling equipments supplemented by blade graders and bulldozers. Each lift is generally 150mm to 300mm thick. On the other hand, in the case of a rather wet, rebounding type of sand, the thickness may be reduced. The objective is to obtain the desired soil density throughout
- the rolling with rubber tyre rollers
- the compaction with vibratory plates and vibratory rollers
- the driving of piles
|Figure 6: Schematic illustration of a typical vibrofloat (after Bell, 1993)|
BlastingIn blasting, explosion of buried charges is responsible for escaping of pore water from the soil mass to facilitate rearrangement of the soil particles and thus leading the sand to more compacted state.
GroutingGrouting is a technique of inserting some kind of stabilizing agent into the soil voids in a limited space around the injection tube. The agent reacts with the soil to form a stable mass. The most common grout is a mixture of cement and water with or without sand. Grouting is particularly useful where it is desired to increase the soil strength without disturbance to the soil or to any other structure. Permeation through soil by a cementitious grout increases soil cohesion and angle of internal friction.There are various methods of grouting which are described below.
|Figure 7: Compacting grouting, Low-slump gout is pumped under high pressure to form a bulb that displaces and densifies the surrounding soils. By raising the grout tube while pumping, a column of grout can be created in the soil (after Hausmann, 1990)|