Liquefiable Materials

Liquefaction is a rare condition which is simple to understand. Vulnerable ground is easily identified and fixing it is not difficult.

Liquefaction can only affect a granular material which is fully saturated and where the grains form a very loose unbonded skeleton. All other materials are safe.

What happens is that when this weak and brittle skeleton receives a strong enough jolt, some joints are dislocated, and the result is like the collapse of a house of cards. The grains come tumbling down, through water.

This jolt can come from an earthquake. Rayleigh waves travelling along the surface are the most likely vehicle for disjointing the skeleton. The common assumption that the damage is done by shear stresses coming up from the bedrock is computationally convenient, but we believe, wrong.

Hodge showed in the Water in the Soil Part 2 GN Part2that as grains fall they gradually transfer their weight to the water. As the only strength the vulnerable soil types have comes from friction, the loss of solid weight is reflected in loss of strength. So, if the grains have room to fall far enough all strength is lost into temporarily pressurizing the groundwater during their fall. The ground turns to a heavy liquid, hence the term "liquefaction". That's all there is to it.

As the table shows, there are immense differences between grains of silt, sand or gravel, and this is reflected in how they behave. Fine sands can liquefy, gravels cannot. Neither can well graded materials of any kind simply because there's just not the room between grains for them to fall far enough. Silts are a different matter.

The preventative treatment is to shake the ground before an earthquake does. But do it a little bit at a time, so that at any one moment only a small part of the overall volume is being affected, and undergoing the temporarily destabilizing changes necessary to improve the soil-structure. There are many tools out there to do this; the Phoenix™ Machine is one of them.

Saturated silts are special/strange in how they respond to agitation: For although silts particles are smaller than sand sizes, there is a hydrodynamic reason why their loss of strength is not accompanied by an increase in pore water pressure. This is explained in detail in the Water in the Soil Part 4. GN Part4