25-06-2013, 02:39 PM
PRACTICAL APPLICATIONS OF GROUND IMPROVEMENT
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ABSTRACT:
Ground Improvement techniques are often used to improve sub soil properties in terms of their
bearing capacity, shear strength, settlement characteristics, drainage, etc. These techniques have a wide range
of applicability from coarse grained soils to fine grained soils. Depending upon the loading conditions and nature
of soil, a suitable technique which is also economical needs to be adopted. This paper gives the concept and
theory of a few ground improvement techniques and describes the practical application of these techniques
along with a case history for each of the techniques.
Ground Improvement Overview
Ground improvement, is the modification of existing site foundation soils to provide better performance under
design and/or operational loading conditions. Ground improvement techniques are used increasingly for new
projects to allow utilization of sites with poor subsurface conditions. Previously, these poor soils were considered
as economically unjustifiable or technically not feasible and are often replaced with an engineered fill or location
of the project is changed. In short, Ground improvement is executed to increase the bearing capacity, reduce the
magnitude of settlements and the time in which it occurs, retard seepage, accelerate the rate at which drainage
occurs, increase the stability of slopes, mitigation of liquefaction potential, etc.
Ground Improvement Using Vibro compaction
Concept of Vibro Compaction
Particles of granular soil can be rearranged by vibration in such a way that they obtain a higher density. In non
cohesive soils (granular soils), the effective depth of surface compactor and vibratory roller is limited to a few
meters below ground level and the larger depths can be reached by deep compaction methods using depth
vibrators. The method is referred as Vibro compaction.
The depth vibrator is lowered into the ground under its own weight assisted by water flushing from jets
positioned near the tip of the vibrator (i.e. bottom jets). Experience has shown that penetration is most effective if
a high water flow rate is used, as opposed to high water pressure. On reaching the designated final depth, the
bottom jets are closed and flushing continued by water from jets positioned near the top of the vibrator. These
jets direct water radially outward, assisting the surrounding sand to fall into the space around the vibrator.
Quality Control & Quality Assurance
In general, quality management of Viro compaction works are divided into two categories, namely monitoring of
compaction parameters and post-compaction testing. The compaction parameters (depth and power
consumption) are monitored using real-time computerised system throughout the construction process. The
recorded data also printed simultaneously in real-time along with the probe reference number, date and time of
compaction. This ensures proper documentation of the work done in order to verify desired end product is
acheived.
Post- compaction testing is performed to ensure that the specifications are met. Typically, sounding methods are
used to assess the effectiveness of the Vibro compaction. Dynamic penetrometer tests (DPT), standard
penetration tests (SPT) and cone penetration tests (CPT) can be used. At present, CPT is the most popular
post-compaction test. It is suggested to perform post-compaction testing at least one week after the compaction
work such that excess pore water reduced to the initial level before compaction.
Concept
Vibro Compaction method reaches its technical limits where the fines content is high (i.e. in excess of 15 to
20%) as the fine particles cannot respond to the vibration and necessitates the need for externally introduced
reinforcement material such as gravels or stones. To overcome the limitations of the Vibro Compaction method,
Vibro Replacement method was first developed in 1956. In this method, a hole is created in the ground and is
filled with coarse aggregate such as stones, section by section. The coarse aggregate is then densified along
with the surrounding soil by repetitive use of the depth vibrator. This process produces vibro stone column that is
integral to the surrounding soil.
Conclusions
Ground Improvement techniques forms technically sound and cost effective solution where the sub soils are
weak and needs to be treated to enable the intended construction. Its applicability has been proven in the recent
past for a wide range of structures such as roads, runways, ports, power plants, railways, dams, slope
stabilisation, excavations, tunneling and other infrastructure facilities (Raju V.R. ,2004). These techniques have
been used all over the world for a wide range of soils starting from loose sands, silts, marine clays to weak
rocks. Based on the soil conditions, loading intensity and intended performance, an appropriate ground
improvement technique can be designed to attain the desired performance.