Phenomenon where strength and stiffness of a loose, saturated cohesionless soil is reduced by earthquake shaking is known as Liquefaction. During Liquefaction, pore pressure increases in undrained shearing process causing a reduction in effective stress which in turn reduces the shear strength. Often Liquefaction appears in the form of sand foundations.Even in Cohesive soils this phenomenon takes place. It is technically called as Clay Pumping or Mud Pumping. Here cohesive forces cannot withstand high pore water pressures during earthquake as whole clayey soil is pumped out.In partially saturated conditions also soils can liquefy. This phenomenon is technically called as Shear Fluidization. To ensure safety of all the structures built on the earth a study on liquefaction and various factors influencing it is essential. Based on the conditions, suitable mitigation technique can be designed.
Catastrophic damage is caused due to liquefaction because of the following factors.
- Differential settlements in the foundations of the structure
- Sand boils due to excess pore water pressure generated during earthquake
- Cyclic mobility or level ground liquefaction.
Liquefaction failure due to earthquake causes failure of important utility structures like dam, bridge, highways, underground pipelines and services and many more.. To counteract the damage caused by liquefaction during an earthquake, there is a need to mitigate it in the form of ground improvement.
Liquefaction Mitigation methods
Soil densification is generally considered highly reliable, and the standard remedial measure against liquefaction. It reduces the void space of the soil, thereby decreasing the potential for volumetric change that would lead to liquefaction. Resistance to shear deformation also increases with increased density.
a. Vibro-Compaction: The basic principle behind the vibro compaction process is that particles of non-cohesive soils can be rearranged into a denser state by means of vibration. Vibration is achieved by means of powerful vibrator at deeper depths. The vibrator is connected to a source of electric power and a high-pressure water pump. Extension tubes are added as necessary, depending on the treatment depth, and the whole assemblage is suspended from a crane.
b. Vibro-Replacement: The stabilization of weak deposits by displacing the soil radially with the help of a depth vibrator, refilling the resulting space with granular material and compacting the same with the vibrator is referred to as Vibro Replacement. The resulting matrix of compacted soil and stone columns has improved load bearing and settlement characteristic. Keeping the site conditions in view vibro stone columns can be installed either wet method (top feed) or dry method (bottom feed). Technically and functionally, vibro stone columns installed in both methods serve similar purpose.
Solidification is considered a highly reliable remedial measure against liquefaction. It prevents soil particle movement and provides cohesive strength. Various methods are grouting, lime pile, pre-mixing method and many more
Drainage method has been used for a number of liquefaction remediation projects. It is indicated that gravel drains can accelerate the dissipation of excess pore water pressures, thereby limiting the loss of shear strength and reducing the uplift pressures acting on buried structures. It is observed that quay walls having backfill treated by the gravel drain pile and sand compaction pile techniques suffered no damage, while quay walls having untreated backfill were severely damaged due to liquefaction.
Lowering the ground water level by dewatering reduces the degree of saturation, thereby preventing the development of excess pore water pressure which would lead to liquefaction. Dewatering is a difficult and very expensive task, since both upstream and downstream seepage cutoffs are usually required, and pumps must be maintained constantly.
5 New methods:
a. Passive site stabilization:
The passive site remediation/stabilization method causes minimal disruption to existing structures. Two methods involving nanomaterials belong to this category: grouting of colloidal silica and bentonite suspension.
- Colloidal Silica Grouting: Low-viscosity diluted colloidal silica is injected and transported from the site boundary to the target area through augmented or natural groundwater flow. After some time, the fluid gradually restores its viscosity as it transforms into the colloidal state, thus strengthening the bonding of the grains in the soil. The mechanism of liquefaction mitigation is mainly related to the cementation of individual sand grains by colloidal silica. In addition, the viscosity of pore fluid may be a beneficial factor that contributes to the reduction in the excess pore water pressure by reducing the hydraulic conductivity and this needs further study. Since the gelling time of colloidal silica can be adjusted by changing the pH and the ionic strength of the solution, colloidal silica can be injected into the proper location before it gels. It has been demonstrated that both the shear modulus and damping ratio increase with increasing colloidal silica content. The material cost of colloidal silica at 5 % concentration is approximately the same as that of microfine cement used to stabilize an equal volume of soil. In addition, since its viscosity with 5 % concentration is close to that of water, it can permeate the foundation at low pressure. Thus, the cost of this method is much lower than that of microfine cement grouting. Colloidal silica grouting creates minimal disruption to the foundation and can therefore be applied to existing structures that are susceptible to liquefaction. Moreover, colloidal silica is colorless, has stable biological and chemical properties, and is environmentally friendly.
- Bentonite Suspension grouting: The principle behind the bentonite suspension grouting method is similar to that of colloidal silica grouting. Bentonite suspension is permeated into the soil and becomes bentonite gel. Bentonite gel can take up a certain amount of strain, reducing the excess pore pressure under seismic vibration, thus increasing the resistance to liquefaction. After some time, the treated soil cures and is restored to its original status. Sodium pyrophosphate (SPP) may be added to bentonite suspension to reduce the initial viscosity, so that the bentonite suspension can permeate into the soil faster.
Nutrients and microorganisms are injected into the ground foundation at the site under low pressure, and then, a series of chemical reactions take place and products which can gel the sand grains are generated increasing the shear strength of the soil. In a biocementation method called microbial-induced carbonate precipitation (MICP) where injected nutrients are CaCl2 and Urea. The main factors influencing the generation rate of the calcium carbonate are, the initial bacterial concentration, the activity of the microbial enzymes in situ, nutrient solution concentration, PH, temperature, and sand particle size distribution.
In biocementation, nutrients and microorganisms are injected into the foundation. Therefore, it can be classified as a grouting method. Microorganisms such as S. pasteurii are widely available and are inexpensive since they can be cultivated specifically and optimized for this process. In addition, the chemical reactions are slow, so the treatment fluid can be injected into the foundation effectively before gelling.
c. Induced Partial Saturation:
Induced partial saturation means transferring the soil from a saturated to an unsaturated state by different measures. Transferring the soil from a saturated to an unsaturated state can be an economical liquefaction mitigation method. A small decrease in saturation leads to a great increase in liquefaction resistance. Silt does not liquefy when the saturation is below 60 %. There are basically 2 methods such as air injection and biogas.
- Air Injection: Air injection uses special equipment to inject air into the ground foundation and thus reduce the soil saturation. As air injection technology is widely used in ground water pollution treatment, it can serve as a technical basis for liquefaction mitigation technology.
- Biogas: This method uses microbial denitrification. Denitrifying bacteria can lead to nitrate reduction and release molecular nitrogen, which can occupy part of the pore water space and de-saturate the soil. Rate of nitrogen production is stable and can be controlled by the concentration of the reactants.
Compared to air injection, the merits of biogas are as follows:
(1) As the viscosity of the microbes and nutrients is low, the mixture can be diffused into the soil and thus the gas bubbles can be distributed evenly
(2) The gas bubbles in the pore water are small, making escape from the foundation difficult
(3) The method is energy efficient since no high-power device is used in the process
(4) It can achieve good liquefaction mitigation without re-compacting the foundation. Biogas creates minimal disruption to the site and can be applied to existing and vulnerable structures.
d. Mitigation using tire chips
Tire chips have low density and strong flexibility. As the rigidity modulus of the sand-tire chips mixture is lower than that of sand, volume contraction of the mixture can occur during the dynamic loading, which helps to decrease the excess pore pressure. Sand mixed with tire chips can be used as a drain which can dissipate the excess pore pressure, thus improving the liquefaction resistance. The arrangement of the tire chips can be selected according to the engineering requirements and can make full use of the material’s plasticity and its ability to dissipate excess pore pressure. Moreover, applying tire chips to geotechnical engineering helps to solve the problem of recycling used tires. Thus, besides providing an effective engineering solution, the method is economical and environmentally friendly.
- The presence of liquefaction susceptible soil does not mean that one has to abandon the site or install deep foundations.
- In seismic vulnerable zones, ground improvement solutions provide technically sound and cost effective solutions.
- Vibro-replacement and Vibro-compaction are the basic methods of increasing the density. These methods can be adopted and cross checked by conducting SPT, CPT and plate load tests.
- New methods like passive site remediation using low viscosity colloidal silica and bentonite suspensions can be used to treat the liquefaction susceptible soil beneath existing structure
H C Sumanth