LIQUIFICATION / SEISMIC HAZARDS AND GROUND IMPROVEMENTS

Liquefaction/Seismic Hazards and Ground Improvement:

Liquefaction is defined as the phenomenon in a soil mass, because of the development of excess pore pressures that suffers a substantial reduction in its shear strength to a constant value, and deforms continuously until the imposed shear stresses become equal to steady-state shear strength. It is commonly observed in quicksand, quick clay, turbidity currents, and as a result of earthquake shock in unconsolidated sediments. During earthquakes, excess pore pressures in saturated soil deposits may develop as a result of induced cyclic shear stresses resulting in liquefaction.

Soil liquefaction occurs in submerged granular soils during or after strong ground shaking. There are several requirements for liquefaction to occur. They are as follows:

• Soils must be submerged
• Soils must be primarily granular
• Soils must be contractive, that is, loose to medium-dense
• Ground motion must be intense
• Duration of shaking must be sufficient for the soils to lose shear resistance

Lateral spreading involves primarily lateral movement of earth materials due to ground shaking. It differs from slope failure in that complete ground failure involving large movement does not occur due to the relatively smaller gradient of the initial ground surface. Lateral spreading is demonstrated by near-vertical cracks with predominantly horizontal movement of the soil mass involved.

Seismic deformation analysis anticipates the results of a given seismic event. When an earthquake fault ruptures, it causes two types of deformation: static and dynamic. Static deformation is the permanent ground displacement, while dynamic deformation causes the waves we experience as ground shaking.

Landslides or slope failures are common occurrences during or soon after large earthquakes or heavy rainfalls.

Earthquake-Induced Flooding was caused by failure of dams or other water-retaining structures as a result of earthquakes.

Tsunamis are tidal waves generated by fault displacement or major ground movement.

Seiches are large waves generated in enclosed bodies of water in response to ground shaking.

Ground Improvement (Geo-textile, stone columns, dynamic compaction, soil cement, soil nailing, tie-back, compaction grouting, etc.) refers to any procedure undertaken to increase the strength, decrease the permeability or compressibility or otherwise render the physical properties of soil more suitable for engineering use. Procedures that may be used for ground improvement may include:

Geotextile is a reinforcing fabric that is used to construct walls and slopes by alternating layers of soil and the fabric as the wall is built from the bottom up.

Soil Nailing is used to stabilize an existing slope by excavating the unwanted soil in front of the face while proceeding from the top down. After each incremental excavation is complete a layer of steel rods, called soil nails, are driven or installed in holes drilled into the face of the slope and bonded to the soil by grouting. Shotcrete can be used to support and protect the face of the slope.

Stone columns (or vibro-displacement stone columns) are used to improve the soil consist of sand, silt and clay. Soils that has been displaced laterally by the vibroflot is replaced with crushed stone to form stone columns. Stone columns reduce the potential of liquefaction by both densification and drainage. Due to the drainage, the excess pore pressure generated during the earthquake will reduce the extent of liquefaction.

The borehole can be filled with stone from either the bottom of the hole or ground surface. Then, the stone is compacted and forced to surrounding ground. This process in repeated until the borehole is completely filled. With both methods, the penetration can be achieved with either air only or by a combination of compressed air and water.

Stone columns are very effective in sands and can be effective in silts and silty sands. The actual stone columns design and installation should be performed by experienced subcontractors.

Compaction grouting improves the ground condition by soil displacement. First, a grout pipe casing is driven to the ground to reach the liquefied area. Then, a very viscous (low Ðmobility), aggregate cement grout is pumped though the pipe and the pipe is withdrawn in stages. At each stage, compaction grout will displace the adjoining soils and densify the immediately surrounding soil. The grout densify the loosest soil and treat the most susceptible material. The grout injection rate should be slow enough to allow pore pressure dissipation. Soils that lose strength during remolding (saturated, fine-grained soils, sensitive clay) should be avoided.