Soil liquefaction is a hazard where saturated, loose soil temporarily loses its strength and stiffness, causing it to behave like a viscous liquid. This transformation occurs during an earthquake when intense seismic shaking triggers the phenomenon. Liquefaction can lead to catastrophic failure, causing buildings to tilt, sink, or collapse, and damaging underground infrastructure. Preventing this requires a multi-faceted approach, focusing first on identifying the risk and then implementing engineering solutions to alter the soil’s structure or the building’s interaction with the ground.
Understanding Liquefaction Susceptibility
Liquefaction occurs only when three specific conditions are present simultaneously. The first is loose, granular material, typically fine sands or silts, which are cohesionless. These soils tend to compress when loaded, but water prevents immediate compaction.
The second condition is saturation, meaning the pores between the soil particles are completely filled. This is common in areas with a high water table, such as coastal zones or near rivers and lakes. When saturated, the water pressure in the pores, known as pore water pressure, dictates the effective stress holding the particles together.
The final condition is a trigger, usually intense, cyclic seismic shaking from an earthquake. The vibration causes particles to temporarily lose contact. This rearrangement leads to a rapid increase in pore water pressure, reducing the effective stress between the grains to near zero. When the soil loses effective stress, it loses its ability to support loads and behaves like a fluid. Geotechnical engineers must assess susceptibility before construction using tools like the Standard Penetration Test (SPT) or Cone Penetration Test (CPT).
Ground Improvement Through Densification
One of the most direct prevention strategies is to eliminate the “loose soil” condition by physically increasing the ground’s density. Densification rearranges the soil particles into a tighter, more stable configuration, which prevents the pore water pressure from building up during shaking. This process enhances the soil’s strength and reduces its compressibility. Several mechanical methods are used for densification:
- Vibro-compaction, or vibro-flotation, uses large vibrating probes inserted deep into the ground. The powerful vibrations, often assisted by water jets, allow the soil particles to settle into a denser state as the probe is slowly withdrawn. This method is highly effective for improving large volumes of clean, granular soils.
- Dynamic compaction involves repeatedly dropping a heavy weight, or tamper, from a crane onto the ground surface. The impact energy creates shock waves that penetrate the subsurface, forcing the loose soil layers to compact. This technique is suitable for granular soils and achieves densification across a large area.
- Compaction grouting offers a low-vibration alternative, valuable near existing structures. This technique involves injecting a stiff, low-mobility mortar grout into the soil under high pressure. The grout remains in a bulb shape, physically displacing and compressing the surrounding soil mass.
Controlling Pore Water Pressure and Soil Stabilization
Liquefaction can be prevented by addressing the soil’s saturation or by chemically binding the particles together, referred to as stabilization. These methods either manage the water that causes pressure build-up or change the soil’s composition to make it cohesive.
Gravel Drains and Stone Columns
Gravel drains or stone columns manage pore water pressure by providing vertical pathways for water to escape. These columns are constructed by drilling or vibrating a hole and then backfilling it with highly permeable material, typically crushed stone or gravel. During an earthquake, these drains rapidly dissipate the excess pore water pressure, preventing the soil’s effective stress from dropping to zero.
Deep Soil Mixing (DSM)
Deep Soil Mixing (DSM) transforms liquefiable soil into a rigid, non-liquefiable mass. This process uses large, rotating augers to mechanically mix the in-situ soil with cementitious binding agents, such as cement or lime. The resulting soil-cement columns have increased shear strength and stiffness, preventing the movement and pore pressure generation that leads to liquefaction.
Chemical Grouting
Chemical grouting, specifically permeation grouting, involves injecting a low-viscosity chemical agent into the soil pores without displacing the soil. The grout flows through the void spaces of granular soils and then hardens, cementing the individual particles together. This creates a strengthened, less permeable matrix resistant to cyclic loading, and is well-suited for fine-grained sands that are difficult to densify mechanically.
Designing Liquefaction-Resistant Foundations
Even when ground improvement techniques are used, the final layer of protection involves designing the structure’s foundation to withstand ground movement. This structural mitigation ensures the building remains stable even if the soil experiences some degree of liquefaction or lateral spreading. The design goal is to either bypass the unstable layer or allow the structure to move uniformly with the ground.
Deep foundations, such as piles or drilled shafts, transfer the structure’s weight through the liquefiable soil layer. These foundations extend down to a stable, non-liquefiable stratum, such as bedrock or dense soil, well below the zone of influence. Piles must be designed with sufficient diameter and reinforcement to resist the vertical load and the large lateral forces that occur if the surrounding liquefied soil flows horizontally.
Alternatively, structures can be placed on a stiff mat foundation, often referred to as a raft foundation. This involves creating a single, highly reinforced concrete slab beneath the entire building footprint. The mat foundation distributes the loads widely and acts as a rigid unit, allowing the structure to settle or tilt uniformly if the underlying soil liquefies. This uniform movement minimizes differential settlement and shearing forces that cause structural damage.