Site Improvement

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