Dr. Shashi Ranjan Kumar, Principal, Jagat Guru Dattaray College of Technology (JDCT), Indore, Dr. Achintya, Principal, Bhagalpur College of Engineering, Bhagalpur.

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.

Introduction

Over 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)
Liquefaction being one of the most important, interesting, complex and controversial topics in the geotechnical earthquake engineering, has been studied extensively by number of researchers around the world. A brief account of the phenomenon of liquefaction along with its countermeasures has been presented in this paper.

Mechanism of Liquefaction

Liquefaction 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.

Figure 4
When water table is at any depth Z1 below the ground surface as shown in Fig. 4, then effective stress at any depth Z2 (below the water table).

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 Liquefaction

When 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 Susceptibility

A 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 Approach

In 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 Soil

The 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 Resistance

To 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

where

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 Liquefaction

This 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
The average equivalent uniform shear stress induced by earthquake and cyclic shear stress required to cause liquefaction are shown in Fig.5 for different depths. If equivalent cyclic stress induced by earthquake exceeds the cyclic resistance available in the soil, liquefaction occurs.

Countermeasures Against Liquefaction

After 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.

Densification

Soil 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

Vibrofloatation

Figure 6: Schematic illustration of a typical vibrofloat (after Bell, 1993)
This method is most commonly used to densify cohesionless deposits of sand and gravel having not more than 20% silt or 10% clay. Vibration utilizes a cylindrical penetration. It is an equipment of about 4m in length and 400mm in diameter as shown in Fig. 6. The lower half is vibrator and the upper half is stationary part. The device has water jets at the top and bottom. Vibrofloat is itself lower under its own weight with bottom jet on which it induces the quick sand condition as soon as it reaches the desired depth. The flow of water is diverted to upper jet and is pulled out slowly. As the vibrofloat is pulled out, a crater is formed. Sand or gravel is added to the crater so formed and thus it results in compacting the sand.

Blasting

In 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.

Grouting

Grouting 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.

Compaction Grouting

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)
It is a thick, low mobility grout that remains in a homogeneous mass without entering the soil pores. It is shown in Fig. 7. As the grout mass expands, the installation of casing to the required depth either into a pre-drilled hole or through driving the casing. A stiff grout such as a soil-cement-water mix, is then pumped through the casing until typically one of the three criteria, namely target volume, maximum pressure or surface heave is reached.

Jet Grouting

In this, high-pressure fluid jets are used to erode and mix or replace soil with grout. The general installation procedure begins with the drilling of small nozzles, 90mm to 150mm in diameter, to the final depth. Grout is jetted into soil through small nozzles as the drill is rotated and withdrawn.

Filtration (Drainage)

Use of coarse material blanket and artificial drains reduce the length of drainage path and increases coefficient of permeability, thereby speeding up the drainage process. These are considered fully effective if the permeability of material of drains is about 200 times the permeability of sand in which they are installed. The rate of generation of pore pressure depends on number of cycles of stresses and in turn depends on frequency. Rate of pore pressure built up depends on the rate of dissipation of pore pressure, which is based on drainage. As the number of drains installed is increased, the non-liquefied zone increases. As the acceleration increases, the zone reduces gradually but the increase in time does not reduce the non-liquefied zone. Surface drains effectively prevent the foundation settlement. In order to obtain good results, it must be ensured that adequate depth and width of drains be designed and installed during installation of shallow drains and outside drains. Liquefaction occurs at points too far from the drains. A flexible vertical drain formed by organic fibers like jute can also be used.

Lowering of Ground Water Table

It is well-known that water is the worst enemy to the soil. Presence of water can reduce the strength, increase the compressibility characteristics and affect the durability behavior of soils. The problem of liquefaction is more pronounced due to water. Liquefaction can be completely eliminated if the ground is not fully saturated. It may be recalled that ground liquefies when it loses its effective stress that is due to the excess of built up pore water pressure. This is possible when the loading is faster than dissipation. Though it is almost impossible to lower ground water, it is possible to bail out water close to the surface either by pumping or by using gravity to advantage.

Application of Dead Weight

It is known that the increase in effective over-burden pressure increases the effective confining stress of soil. However, the increase in effective confining stress requires much higher level of shear to liquefy the ground. At a particular site, maximum design acceleration of shaking will be known. Hence, the amount of excess over-burden pressure to be placed on potentially liquefiable ground can be estimated.

Conclusion

Liquefaction of sandy soil is one of the primary causes of damages to the structures during earthquakes. The damages due to liquefaction during earthquakes include slope failures, foundation failures and floatation of buried structures. Liquefaction occurs in saturated sand deposits either because of formation of sand bolts and mud spots at the surface of the earth or because of the seepage of water through ground cracks. In some cases, it occurs on account of the progressive quick sand like conditions over a considerable large area. Liquefaction resistance may be influenced by the fine content of soil. The plasticity of the fines can also influence liquefaction resistance. The adhesion of plastic fines tends to resist the relative movement of individual soil particles and thereby reduce the generation of excess pore pressure during earthquakes. Zone of liquefaction has been evaluated and various methods to control the liquefaction have been suggested. Countermeasures against liquefaction must be applied on north Bihar soils as the soils are comparatively less plastic. Such areas need planning, investigation, provisions and manning of special facilities for timely mitigation of damages, their nature, degree of severity and exact location. "Learning From Nature" is process and not result. This process benefits to the geotechnical engineers to the maximum extent.
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