Foundations in seismic zones are subjected to additional forces due to seismic waves. If the structure is not able to withstand these forces, it may collapse leading to serious catastrophes.
A foundation is the lowest part of a structure, which is normally below the ground level. Foundations are provided to transmit the load of the superstructure to the underlying soil.
It is necessary that the subsoil is able to withstand these loads. For this, the superimposed load should be lower than the safe bearing capacity of the soil. In case of foundations in seismic zones, additional loads are created due to the seismic vibrations. The design should take into account the additional forces.
General Considerations in the Seismic Design of Foundations
- Site investigations and determination of soil properties
- Details of geological and geotechnical environment
- Identification of loads - static and dynamic
- Type of foundation
- Safety verification as per building codes
Soil Investigations for Seismic Designs
For the design of foundations, the soil properties are to be ascertained. For this, both static and dynamic tests are to be conducted in the laboratory as well as in the field.
The important soil parameters to be analyzed are:
- Particle size distribution
- Relative density
- Shear modulus
- Damping factors
The important field tests to be conducted are:
- SPT Hammer Energy
- Pressure Meter Testing
- Shear wave velocity Measurement
- Cone Penetrometer Test
- Seismic Piezo cone Penetrometer
The purpose of these laboratory and field tests are to define soil deposit details, hydraulic conditions, soil index properties, static and dynamic stress-strain soil behavior.
Main Factors That Influence Site Effects
- Intensity and frequency characteristics of bed rocks in seismological environment
- Duration of bed rock motions
- Nonlinear behavior of soils
- Elastic vibration characteristics of soils
- Topography of underlying bedrock
- Non-horizontal soil deposit layering
- Soil deposit thickness
- Type of under lying rock
The main effect of an earthquake is the horizontal forces that are generated in the structure. The horizontal ground accelerations give a measure of this force. The granular soils get compacted due to the vibrations. This in turn causes the settlement of the ground surface.
Another effect of the vibrations is liquefaction. The degree of liquefaction depends on the relative density of the soil, percentage of fines, depth of water table and the ground acceleration.
Effects of Earthquake
- The dynamic stress and induced pore water pressure may reduce the bearing capacity of the soil
- Loose granular soils are compacted by ground motion. This causes large subsidence of the ground surface.
- Compaction of loose granular soil may induce excess pore water pressure, which causes liquefaction of soil
- The vibration due to earthquake may cause structural damage
Measures to prevent Liquefaction
1. Compaction of loose soil
- with vibratory rollers
- compaction piles
- vibro floatation
2. Grouting and chemical stabilization
3. Application of surcharge
For the design of foundations in seismic zones, knowledge in a wide spectrum of disciplines such as soil dynamics, geology, seismology and structural dynamics is required. The design should be safe and at the same time economical.
1.Theory and Practice of Foundation of Foundation Design, N N Som & S C Das, Prentice Hall of India, New Delhi,2003
2 Soil Mechanics and Foundation Engineering V N S Muthy, UBS Publishers and Distributors, New Delhi, 2002
3.Building Construction and Construction Materials, Dhanpat Rai Publishing Company, New Delhi, 2007
Links: Bright Hub, How Soil Liquefaction Occurs