Page 22 - Building Regulation and Design Guidelines - Structural (Grey Code)

SECTION: 1 GEOTECHNICAL GUIDELINES & REGULATIONS SECTION: 1 GEOTECHNICAL GUIDELINES & REGULATIONS

1.4.1.c Soil Replacement
Vibro-Replacement Stone Columns: Vibro-replacement stone
columns, Fig. (1.25), improve the resistance of cohesionless
soils to liquefaction by several mechanisms. The primary
mechanism of treatment is the densification of the native soil.
Secondary benefits may also come from the reinforcing effects
of the stone columns (e.g.,. they are usually stiffer than the
surrounding soil), an increase in the in-situ horizontal stress
(e.g., due to the packing of stone in the column), and the
drainage of earthquakeinduced pore water pressures through
the stone columns.
Vibro-displacement method uses compressed air to displace
the soil laterally as a probe is advanced through the weak
strata. Backfill is placed in to the hole in stages as the probe is
incrementally withdrawn and lowered again to compact the fill.
This process, also known as the ‘dry method’, forms a stone
column. The columns are typically smaller in diameter than the Fig. (1.26): Deep Mixing (Soil Mixing) Technique
‘wet’ method and are used in the stiffer soils.
Typically, the reagent is delivered in a slurry form (i.e. combined
with water), although dry delivery is also possible. Depending
on the soil to be mixed, the volume of slurry necessary ranges
from 20 to 30 percent by volume. Can be a variety of materials
including: Cement (Type I through V), Fly ash, Ground Blast
Furnace Slag, Lime, Additives.
No single tool will be the best for all soil types and, for this
reason, mix tools are often developed for individual projects.
Considerations include: soil type and available turning
equipment, often designed for particular site conditions, size
ranges from 1.6 to 11.5-ft diameter, can be a combination of
partial flighting, mix blades, injection ports and nozzles, and
Fig. (1.25): Vibro-replacement Stone Columns Technique shear blades. The in situ injection and mixing of cement into
weak soils is becoming more common. Recent applications
include liquefactionmitigation and the strengthening of weak
1.4.1.d Soil Mixing
cohesive soils adjacent to embankments, levees and bridge
Soil Mixing, also known as the Deep Mixing Method, Fig.(1.26), abutments.
is the mechanical blending of the in situ soil with cementitious
materials (reagent binder) using a hollow stem auger and 1.4.1.e Grouting
paddle arrangement. The intent of the soil mixing program is
to achieve improved character, generally a design compressive Grouting can stiffen and strengthen the soil layer by increasing
strength or shear strength and/or permeability. Soil mixing its density, increasing the lateral stresses, and acting as
can also be used to immobilize and/or fixate contaminants as reinforcement. Grouting may also be used to produce controlled
well as a treatment system for chemical reduction to a more heaving of the ground surface to re-level a structure that has
‘friendly’ substrate. been damaged by differential settlements.

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