LIQUEFACTION OF SOIL- A BIG CHALLENGE & CONCERN FOR STRUCTURAL ENGINEER
“Even a sound and structurally stable structure is susceptible to failure if soil supporting structure undergoes liquefaction during earthquake. This is one of the biggest challenge for structural engineer. Reading carefully and attentively the geo-technical report and providing robust foundation or soil improvement is the only solution for this challenge. For solution, it is essential to know what is liquefaction and how does it happens. Without knowing the complete anatomy behind liquefaction, a suitable solution can never be found.
Feel the structure-MSA
As a structural engineer, we design foundation on the basis of data provided by geo-technical consultant. Whatever is the number of bore hole and SPT test done at the site, water table is mentioned in the report for each borehole. Also, soil type is clearly mentioned in the report. If the soil type is fully sandy or sandy and silty type(SM Type) and water table is high, then in this case, soil has a tendency to liquefy when earthquake occurs.
Even a sound and structurally stable structure is susceptible to failure if soil supporting structure undergoes liquefaction during earthquake. This is one of the biggest challenge for structural engineer. Reading carefully and attentively the geo-technical report and providing suitable foundation or soil improvement is the only solution for this challenge. For solution, it is essential to know what is liquefaction and how does it happens. Without knowing the complete anatomy behind liquefaction, a suitable solution can never be found.
Generally shallow foundation is provided at a depth of 1.5 m to 2.5 m below natural ground level(NGL).If water table is very high, say water table level is 1.5 to 2 m below NGL, then soil in this region is fully submerged condition. And if soil in this region is cohesion less i.e fully Sandy or Sandy -Silty type(SM Type) then whole soil in this region will undergo in liquefaction i.e soil will behave like a fluid and will have shearing strength zero. Liquefaction state of soil is analogue to fluid in terms of shearing strength. As fluid has zero shearing strength, hence soil in the state of liquefaction may have some shearing strength or has zero shearing strength depending upon the liquefaction potential of the soil. You can relate this with an example, when you try to put your hand into the bucket full of water, you will feel negligible resistance as a shear by water. In the same manner, when soil is liquefied, then it does not respond any shearing resistance to the force exerted by the footings supported on it. Hence, in this case structure starts settling/sinking during liquefaction. As liquefaction is not uniform below each footing, hence differential settlement occurs and due to this building may partially or as a whole may collapse depending on the liquefaction potential of the soil. Although structural elements of building may or may not have damaged during earthquake but excessive sinking may lead to building non functional. Not only building, even a light weight car can sink when it is standing over liquefied soil because soil is not any potion to support any loads in liquefaction state.
How Liquefaction Occurs:
To take precautionary action before construction on a site having liquefied soil or doing retrofitting of a damaged building over liquefied soil after earthquake, we need to understand the complete anatomy behind the occurrence of liquefaction of soils during earthquake.
Saturated cohesion less soil(Sandy or Sandy + Silty) is susceptible to liquefaction during earthquake. Soil under foundation and it’s vicinity is under extremely stressed condition. If soil is saturated, there will be pore water pressure which keep soil particles together. But when earthquake occurs, it produces extreme vibration/turbulence/shaking to the soil particles. This extreme vibrations increases the pore water pressure and also soil gets loosely packed giving a path to water to flow upwards. As we know, there is always a tendency to flow fluid from high pressure to low pressure. As there is low pressure at the ground, hence water will start flowing from below to ground floor. In this case, voids will be created at the displaced water particle position. In the vicinity of these created voids, loosened/loosely packed soil particles start moving to fill these voids. Due to this, sinking of soil starts taking place. As the soil supporting structure starts sinking, hence structure also starts sinking. This phenomenon happens in such way, it seems structure is supported over fluid which does not have any shearing strength. This sinking can be from cm to meter. As the turbulence/vibration occurs differently in different foundation system, hence it leads differential settlement. Building may tilt excessively or may collapsed depending upon the liquefaction potential of the soil.
Serious notes to keep in mind for Soil prone to liquefaction:
This blog will motivate structural engineer to study thoroughly key areas of the Geo-technical report such as the soils types, N-values & water table before designing any foundation. Geo-technical consultant may or may not provide you a statement about whether soil is susceptible to liquefy or not. Knowing the soil types and water table, you have to understand whether soil will liquefy or not. I have noticed one thing, most of the people are only concerned about the safe bearing capacity(SBC).Sometimes, even structural engineer focuses on SBC only without knowing the soil characteristics like type of soil, N-values, soil compactness like loose, medium dense or highly dense. It is surprising to note, a soil having high potential to liquefy may have satisfactory safe bearing capacity such as 100 KN/m2 or 120 KN/m2. If somebody design isolated foundation with this data then foundation system may collapse during earthquake.
In recent past, I have gone through the foundation design of an industrial shed proposed at Nepal for Steel Melting Shop(SMS) & Rolling Mill. In Geo-technical report, there was SBC mentioned as 120KN/m2 having average N-values as 10 at a depth of 2.0m. But there was a warning in this report highlighted as soil upto a depth of 4.0 m is prone to liquefy. I talked to Geo-technical consultant regarding the same and drew his attention that isolated foundation will not be safe. He re-analysed the soil characteristics and recommended pile foundation for the same. Raft foundation is also an alternate solution for soil prone to liquefy as it some how stops water to come out above the foundation as it blocks all possible path of water to come from below during excessive shaking in earthquake.
Mohammad Sohel Akhtar(MSA)