Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 8 GROUND IMPROVEMENT
Fortunately, engineers and contractors have developed methods of coping with these problematic
soils and have successfully built many large structures on very poor sites. Among the methods used
(either individually or in combination) include:-
a) Support the structures on deep foundations that penetrate through the weak soils
b) Support the structure on shallow foundations and design them to accommodate the weak
soils
c) Use a floating foundation, either deep or shallow
d) Remove the poor material and replace with good materials. This approach is only effective
if the poor soil material is relatively thin and good replacement soil materials can be easily
found on site.
e) Improve the engineering properties of the soils. Various methods of ground improvement
techniques are available which basically aim to reduce the pore water pressure, reduce
the volume of voids in the soil, add stronger materials and additives (such as lime or
cementitious grout) to enhance its soil properties
f) Avoid the poor ground either by re-alignment or shifting the location of the structures (if
availability of land is not a constraint)
The main objectives of ground improvements are to:-
• Reduce settlement of structures
• Improve shear strength and bearing capacity of shallow foundations
• Increase factor of safety against possible slope failure of embankments and dams.
• Reduce shrinkage and swelling of soils
The most common techniques often used in our country for solving and stabilizing soft ground
problems are listed below:-
a) Structure support system using the shallow foundation or deep foundation and incorporating
either partially or fully floating foundation principle. Readers are advised to refer to Chapter 5
and Chapter 9 for shallow foundation and deep foundation respectively.
b) Soil improvement and stabilization works include
i) Removal and replacement
ii) Surcharging
iii) Sub-surface drainage improvement system
iv) Vibro floatation
v) Deep mixing – Lime column
vi) Dynamic compaction
8.2 SOIL IMPROVEMENT TECHNIQUES
8.2.1 Removal and Replacement
Sometimes poor soils can simply be removed and replaced with good quality compacted fill. This
alternative is especially attractive if the thickness of the deposit is small, the groundwater table is
deep and good quality fill material is readily available. If the soil is inorganic and not too wet, then it
probably is not necessary to haul it away. Such soils can be improved by simply compacting them. In
this case, the contractor excavates the soil until firm ground is exposed and then places the
excavated soil back in its original location, compacting it in lifts. This technique is often called
removed and re-compaction. If necessary, the soil can be reinforced with geosynthetics to spreads
the applied load over a larger area, thus reducing the change in effective stress and reducing the
consolidation settlement as well as increasing the bearing capacity.
8-2 March 2009