Page 24 - Building Regulation and Design Guidelines - Structural (Grey Code)

SECTION: 1 GEOTECHNICAL GUIDELINES & REGULATIONS SECTION: 1 GEOTECHNICAL GUIDELINES & REGULATIONS
1.4.1.h Jet-Grout
Jet-grout is the form of jet-grout column “soil-crete pile” by
drilling a hole specified with its length in the relevant design
and then by jetting with proper mixing and pumping equipment
with the jetting parameters to achieve designed diameter.
Using a drilling rig holes between 400 to 700 mm diameters
will be drilled down to required column depth. The drilling
can be carried out by traditional rotary or rotary percussive
methods. And then jetting will be done while dragging the drill
set at a specified drag and revolution speed.
The rig must be equipped with automatically adjustable drag
and revolution speed controls. The jetting takes place at the
bottom of the drilling set at the special tool named “monitor”
with one or two nozzles the diameter of which is from 1.5 mm
to 3.0 mm depending on the design parameters. The cement-
water mix ejects from these nozzles at minimum pressure of
300 bars with 250 m/s jet speed.
Dragging the drilling set upwards with a pre-set dragging
rate while jetting is continued causes to destroy the natural
structure of the soil and then mix the soil with cement-water
mix at very high pressure therefore forms a jet-grout column
as per Fig. (1.31).
Fig. (1.31): Jet Grouting Technique
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The parameters to be used will be designed and checked with
trial columns prior to start of the works. With jet grouting, it
is possible to treat a broad range of grounds, consisting of
different type clays, loose sands and to overcome the drawbacks
of the other injection systems. It is a valid alternative to other
consolidation systems such as dewatering, micro-piles, stone
columns etc.
1.4.2 SAFETY AGAINST LIQUEFACTION
The hydraulic fill, loose, fine and saturated sands may subject to
liquefaction (significant loss of strength due to buildup pore water
pressure and subsequent deformation in some locations under the cyclic
loading of earthquakes).
To mitigate liquefaction hazards, soil improvement is required and its
efficiency shall be ensured from CPT readings pre and post-tests, (ASTM D
5778 or BS 1377: Part 9: Test 3.1 Amd 8264-95 and SSMFE test Procedure
for Cone Penetration Test (IRTP) - 1989 and updated 1997). The pre CPT
shall be carried out every 900 m2 maximum or as per project specs and
the results to be compared with the post compaction CPT results for the
same area as per Fig. (1.31). The locations of post CPTs shall be selected
at the central points and/or at one third the maximum distance between
the improved points. Proper weighted average for near and far tested
points should be considered.
For deep foundations, the achievement of 6.0 MPa (Seismic Zone: 2A) and
8 MPa (Seismic Zone 2B) weighted average of the tip resistance profile
for the post compaction CPT is an accepted criterion of the compaction
efficiency.
For shallow foundations, one plate load test/structure to be carried
out (as per ASTM D1194 – 94). The criteria of acceptance shall be the
achieving of targeted bearing pressure of 150 kPa corresponds to
maximum settlement of 25 mm and maximum distortion of 1:500.
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