EARTHQUAKE GEOTECHNICAL ENGINEERING AND ITS PRACTICES-IV-GROUND IMPROVEMENT FOR LIQUEFACTION HAZARD MITIGATION

Ground Improvement for Liquefaction Hazard Mitigation

Ground Improvement in IS Code

        “In poor and weak subsoils, the design of conventional shallow foundation for structures and equipment may present problems with respect to both sizing of foundations as well as control of foundation settlements. Traditionally, pile foundations have been employed often at enormous costs. A more viable alternative in certain solutions, developed over the recent years, is to improve the subsoil itself to an extent such that the subsoil improvement would have resultant settlements within acceptable limits. The techniques for ground improvement has developed rapidly and has found large scale application in industrial projects.”

IS 13094 : 1992 (Reaffirmed 1997)

          Ground improvement is indicated if

• Net loading intensity of the foundation exceeds the allowable bearing pressure as per IS 6403:1981
• Resultant settlement or differential settlement (per IS 8009 Part 1 or 2) exceeds acceptable limits for the structure
• The subsoil is prone to liquefaction in seismic event

Types of Ground Improvement by Function

1. Excavation, fill placement, groundwater table lowering
2. Densification through vibration or compaction
3. Drainage through dissipation of excess pore water pressure
4. Resistant through inclusions
5.  Stiffening through cement or chemical addition

Densification through vibration and compaction

Vibrating probe/vibroflotation

·         Vibrations of probe cause grain structure to collapse densifying soil; raised and lowered in grid pattern

Most Suitable Soil Type	Saturated or dry clean sand
Max effective treatment depth	20 m, ineffective in upper 3-4 m.
Special materials required	None
Special equipment required	Vibratory pile driver or vibroflot equipment
Properties of treated material	Can obtain up to Dr = 80%
Special advantages and limitations	+ Rapid, simple, cheaper than VR stone columns, compaction piles – less effective than methods that employ compaction as well as vibration, difficult to penetrate stiff overlayers, may be ineffective for layered systems
Relative Cost	Moderate

Vibro-compaction/replacement stone/sand columns

• Steel casing is driven in to the soil, gravel or sand is filled from the top and tamped with a drop hammer as the steel casing is successfully withdrawn, displacing the soil

Most Suitable Soil Type	Cohesionless soil with less than 20% fines
Max effective treatment depth	30 m
Special materials required	Granular Backfill
Special equipment required	Vibrofolt equipment, steel casing, hopper for backfill
Properties of treated material	Can obtain high relative density
Special advantages and limitations	+ Rapid, useful for a wide range of soil types – May require a large volume of backfill, noisy
Relative Cost	Moderate

Dynamic Densification (heavy tamping)

•         A heavy weight is dropped in a grid pattern, for several passes

Most Suitable Soil Type	Cohesionless soil, waste fills, partly saturated soils, soils with fines
Max effective treatment depth	30 m, less at the surface, degree of improvement usually decreases with depth
Special materials required	None
Special equipment required	Tamper and crane
Properties of treated material	Good improvement and reasonable uniformity
Special advantages and limitations	+ Rapid, simple, may be suitable for soils with fines – lack of uniformity with depth, not possible near existing structures, may granular backfill surface layer
Relative Cost	low

          Other methods

• Displacement piles: densification by displacement of pile volume, usually precast concrete or timber piles
• Compaction grouting: densification by displacement of grout volume

Stiffening through cement or chemical addition

Permeation or penetrating grouting: High permeability grout is injected into the ground at numerous points, results in solidified soil mass

Most Suitable Soil Type	Saturated medium to coarse sand
Max effective treatment depth	> 30m
Special materials required	Grout
Special equipment required	Mixers, tanks, pumps, hoses, monitoring equipment
Properties of treated material	Impervious, high strength where completely mixed
Special advantages and limitations	+ Produces a hard, stiff  mass of soil, useful for existing structures as it causes little or no settlement or disturbance, low noise – Area of permeation can vary, can be blocked by pockets of soil with fines, difficult to determine the improved area, requires curing time
Relative Cost	Least expensive of grout systems, but moderately expensive compared to vibro methods

Earthquake resistant design of geotechnical structures

Geotechnical structures like,

          Retaining wall/Sheet pile

        Slope

        Shallow foundations

        Deep foundations

Must be designed to withstand the earthquake loading

Seismic Design of Retaining Wall

Mononobe-Okabe (1926, 1929) Method

Seismic Slope Stability

Wedge Method of Analysis by Terzaghi (1950)

Seismic Bearing Capacity of Shallow Foundations

Seismic Bearing Capacity of Shallow Strip Footings

Guideline as per Indian Code

•         According to IS 1893, isolated RCC footing without tie beams, or unreinforced strip foundation shall not be permitted in soft soils

•         Shallow foundation elements should be tied together so that they move uniformly, bridge over areas of local settlements, resist soil movements which ultimately reduces the level of shear forces induced in the elements resting on the foundation

•         Buried utilities, such as sewage and water pipes, should have ductile connections to the structure to accommodate the large movements and settlements that can occur under seismic loading