Vibro-Replacement extends the range of soils that can be improved by vibratory techniques to include cohesive soils. Reinforcement of the soil with compacted granular columns or “stone columns” is accomplished by the top-feed method. The important Vibro-replacement stone columns are Ground conditions, Relative density, Degree of saturation, Permeation. Principles of Vibro-Replacement Technique The stone columns and intervening soil form and integrated foundation support system having low compressibility and improved load bearing capacity. In cohesive soils, excess pore water pressure is readily dissipated by the stone columns and for this reason, reduced settlements occur at a faster rate than is normally the case with cohesive soils. • • • • • • • • • There are different types of installation methods which can be broadly classified in the following manner: Wet top feed method Dry bottom feed method Offshore bottom feed method Summary of Vibro Replacement Method Applications of Vibro-Replacement for Ground Improvement: Reduction of foundation settlement Improve bearing capacity/reduce footing size requirements Reduction of the risk of liquefaction due to seismic activity Slope stabilization Permit construction on fills Permit shallow footing construction Ground Type Relativ Sands Excellent Silty sands Excellent Silts Good Clays Marginal to good Principle • • Reinforcement Drainage Applicable soil(s) • • • • Mixed deposits of clay, silt and sandMinespoils Soft and ultra soft silts (slimes) Dumped fill Soft and ultra soft clays Garbage fills Garbage Effect(s) • • • Increased shear strength Increased stiffness Reduced liquefaction potential Common applications • • • • • • • Airport taxiways and runways Chemical plants Storage tanks & silos Pipelines Bridge abutments and approaches Offshore bridge abutments Road and railway embankments Maximum depth • 20-40 m Land / offshore application • Both Excellent (depending on grad Good Not applicable Mechanically Stabilized Earth Structures A segmental, precast facing mechanically stabilized earth wall employs metallic (strip or bar mat) or geosynthetic (geogrid or geotextile) reinforcement that is connected to a precast concrete or prefabricated metal facing panel to create a reinforced soil mass.
• • • • • • • • • Principles of Mechanically Stabilized Earth Structures: The reinforcement is placed in horizontal layers between successive layers of granular soil backfill. Each layer of backfill consists of one or more compacted lifts. A free draining, non plastic backfill soil is required to ensure adequate performance of the wall system. For walls reinforced with metallic strips, load is transferred from the backfill soil to the strip reinforcement by shear along the interface. For walls with ribbed strips, bar mats, or grid reinforcement, load is similarly transferred but an additional component of strength is obtained through the passive resistance on the transverse members of the reinforcement. Facing panels are typically square, rectangular, hexagonal or cruciform in shape and are up to 4.5m ^2 in area. MSEW- Mechanically Stabilized Earth Walls, when the face batter is generally steeper than 70 degrees. RSS- Reinforced Soil Slopes, when the face batter is shallower. Applications of Mechanically Stabilized Earth Structures: RSS structures are cost effective alternatives for new construction where the cost of embankment fill, right-of-way, and other consideration may make a steeper slope desirable. Another use of reinforcement in engineered slopes is to improve compaction at the edges of a slope to decrease the tendency for surface sloughing. Design: Current practice consists of determining the geometric reinforcement to prevent internal and external failure using limit equilibrium of analysis. • Soil Nailing Technique for Ground Improvement • • • • • • The fundamental concept of soil nailing consists of reinforcing the ground by passive inclusions, closely spaced, to create insitu soil and restrain its displacements. The basic design consists of transferring the resisting tensile forces generated in the inclusions into the ground through the friction mobilized at the interfaces. • Applications of Soil Nailing Technique: Stabilization of railroad and highway cut slopes • • Excavation retaining structures in urban areas for high-rise building and underground facilities Tunnel portals in steep and unstable stratified slopes Construction and retrofitting of bridge abutments with complex boundaries involving wall support under piled foundations Micropiles for Ground Improvement Micropiles are small diameter piles (up to 300 mm), with the capability of sustaining high loads (compressive loads of over 5000 KN).The drilling equipment and methods allows micropiles to be drilled through virtually every ground conditions, natural and artificial, with minimal vibration, disturbances and noise, at any angle below horizontal. The equipment can be further adapted to operate in locations with low headroom and severely restricted access. Applications of Micropiles for Ground Improvement For Structural Support and stability Foundation for new structures Repair / Replacement of existing foundations Arresting / Prevention of movement Embankment, slope and landslide stabilization Soil strengthening and protection Example of Micro Piles for Ground Improvement: In India, in some circumstances steel pipes, coated wooden piles are used as cost-effective Options in improving the bearing capacity of foundation or restrict Displacements to tolerable levels and similar uses in stabilization of slopes, strengthening of foundations are common. Sridharan and Murthy (1993) described a Case study in which a ten-storeyed building, originally in a precarious condition due To differential settlement, was restored to safety using micropiles. Galvanized steel Pipes of 100 mm diameter and 10 m long with bottom end closed with shoe, driven at An angle of 60o with the horizontal were used and the friction between the pile and the soil was used as the design basis in evolving the remedial measures. General Grouting for Ground Improvement Grouting is the injection of pumpable materials into a soil or rock formation to change the physical characteristics of the formation. Grouting selection considerations are Site specific requirement, Soil type, Soil groutability, Porosity. Grouting can be prevented by Collapse of granular soils, Settlement under adjacent foundations, Utilities damage,
• • • • • • • • • • • Day lighting. Grouting can provideIncreased soil strength and rigidity, reduced ground movement, Predictable degree of improvement. Steps for General Grouting Technique for Soil Stabilization Identify underground construction problem. Establish objectives of grouting program. Perform special geotechnical study. Develop initial grouting program. Develop performance prediction. Compare with other solutions. Refine design and prepare specifications. Grouting Techniques The various injection grouting techniques used by grouting contractors for ground improvement / ground modification can be summarized as follows: Permeation Compaction Grouting Claquage Jet Grouting Jet Grouting Technique for Ground Improvement Jet grouting is a general term used by grouting contractors to describe various construction techniques used for ground modification or ground improvement. Grouting contractors use ultra high-pressure fluids or binders that are injected into the soils at high velocities. These binders break up the soil structure completely and mix the soil particles in-situ to create a homogeneous mass, which in turn solidifies. This ground modification / ground improvement of the soil plays an important role in the fields of foundation stability, particularly in the treatment of load bearing soils under new and existing buildings; in the in-depth impermeabilization of water bearing soils; in tunnel construction; and to mitigate the movement of impacted soils and groundwater.