Earthwork and Ground Technology
Site Improvement
Methods of site
improvement
•
Removal
and replacement
•
Preloading
•
Vertical
drains
• In-situ densification
Removal and
replacement
•
One
of oldest and simplest methods is simply
to remove and replace the soil
•
Soils
that will have to be replaced include
contaminated soils or organic soils
•
Method
is usually practical only above the
groundwater table
Preloading
•
Simply
place a surcharge fill on top of the soil that
requires consolidation
•
Once
sufficient consolidation has taken place, the
fill can be removed and construction takes place
•
Surcharge
fills are typically 10-25 feet thick and
generally produces settlement of 1 to 3 feet.
•
Most
effective in clay soil
Advantages of
preloading
•
Requires
only conventional earthmoving equipment
•
Any
grading contractor can perform the work
•
Long
track record of success
Disadvantages
of preloading
•
Surcharge
fill must extend horizontally at least
10 m beyond the perimeter of the planned
construction, which may not be possible
at confined sites
•
Transport
of large quantities of soil required
•
Surcharge
must remain in place for months or
years, thus delaying construction
Vertical Drains
•
Vertical
drains are installed under a surcharge load
to accelerate the drainage of impervious soils and thus speed up consolidation
•
These
drains provide a shorter path for the water
to flow through to get away from the soil
•
Time
to drain clay layers can be reduced from
years to a couple of months
PVD
(Prefabricated Vertical Drain)
•
Geosynthetics
used as a substitute to sand columns
•
Installed
by being pushed or vibrated into the ground
•
Most
are about 100 mm wide and 5 mm thick
Typical installation of PVD
•
Typically
spaced 3 m on centers
Prefabricated
Drains Available
•Nylex
In-situ
densification
·
Most
effective in sands
·
Methods
used in conventional earthwork are only
effective to about 2 m below the surface
·
In-situ
methods like dynamic deep compaction are
for soils deeper than soil can be
compacted from the surface
Vibratory probe
compaction
•
Long
probe mounted onto a vibratory pile
driver compacts the soil around the probe; penetrations spaced in a grid pattern
similar to vertical drains
Vibroflotation
•
Probe
includes the vibrator mechanism and water
jets
•
Probe
is lowered into the ground using a crane
•
Vibratory
eccentric force induces densification
and water jets assist in insertion and extraction
•
Vibratory
probe compaction is effective if silt
content is less than 12-15% and clay is less than 3%
•
Probes
inserted in grid pattern at a spacing of 1.5
to 3 m
