Introduction:
Liquefaction or quick sand condition of soils is a common term in earthquake prone soils. it is one of the natural hazards associated with earthquakes. Other hazards may include: ground shaking, surface rupture, landslides, lateral spreading, shear failure of foundations, settlement of structures, failure of retaining walls and Tsunami/Seiches.
Liquefaction commonly occurs in saturated cohesionless soils and involves the reduction of the shear strength of the soil to zero due to pore water pressure caused by vibration during the earthquake such the soil starts behaving like a liquid.
Effects of liquefaction of soils
- Bearing capacity failure
- Sinking or tilting of buildings
- Landslides
- Lateral spreads
- Floatation of underground structures
Conditions for liquefaction to occur in soils
- The soil is cohesionless e.g sand and sometimes quick clays
- The soil is loose
- The soil is saturated
- There is shaking of ground of the required intensity and duration
- The undrained conditions develop in the soil due to its limited permeability
Factors affecting liquefaction of soils
- Soil type
- Particle size and gradation
- Initial relative density
- Length of drainage path
- Surcharge loads
- Characteristics of vibration
- Age of soil deposit
- Trapped air
- Soil structure
- Method of soil deformation
How to assess soil susceptibility of soil to liquefaction using the common cyclic stress approach
The cyclic stress ratio is defined as the ratio of the cyclic shear stress (τ) to the initial effective stress (σo1). The cyclic stress produced by the earthquake (τ/ σo1)d is compared with that required to induce liquefaction in the soil (τ/ σo1)l. the factor of safety (F) against liquefaction is estimated as;
F = (τ/ σo1)l/ (τ/ σo1)d
Liquefaction would occur if F is less than 1. A minimum value of F should be between 1.25 and 1.50 inclusive.
Cyclic stress ration produced by earthquake
The cyclic stress ratio produced by the earthquake can be determined by the expression below;
(τ/ σo1)d = 0.65 (αmax/g) (σo/ σo1)rd
Where,
αmax = maximum horizontal acceleration (MHA) at the ground surface. This value decreases as the epicentral distance increases and may be determined from suitable charts.
g = acceleration due to gravity (= 9.81 m/s2)
σo = total verticals stress at the point of interest
σo1 = effective vertical stress at the same point
(τ/ σo1)d = cyclic stress ratio produced by the design earthquake
rd = stress reduction factor = 1 – (0.008 x depth (m))
Cyclic stress ratio to induce liquefaction
The cyclic stress ratio to induce liquefaction in the soil (τ/ σo1)l can be determined from a correlation between the cyclic stress ratio against the corrected standard penetration test (SPT) number usually indicated as (N1)60, the magnitude of the earthquake and the particle size distribution where the percentage of fines (quantity passing No 200 sieve (0.075 mm)) should be known. The knowledge of the percentage of fines in important because as the percentage of fines decreases, liquefaction potential increases.
Measures to prevent liquefaction in soils
- Provision of deep foundations such as piles to bypass the susceptible zone
- Compaction of soils to increase its shear resistance
- Replacing of the liquefiable soil with better soil
- Grouting the soil with cement slurry or other suitable substances
- Drainage of the soils to reduce pore water pressure
- Provision of stone columns
- Application of surcharge loads
Example
Estimate the cyclic stress ratio produced by an earthquake at a site from the following data:
Stress reduction factor, rd = 0.95
Maximum horizontal ground acceleration, αmax = 0.1 g
Total stress at the given depth, σo = 120 kN/m2
Effective stress at the same depth, σo1 = 66 kN/m2
Solution
rd = 0.95
αmax = 0.1 g
σo = 120 kN/m2
σo1 = 66 kN/m2
Cyclic stress ratio, (τ/ σo1)d = 0.65 (αmax/g) (σo/σo1)rd = 0.65 (0.1/9.81) (120/66) x 0.95 0.01144
