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

Chapter 8 GROUND IMPROVEMENT
Removal and Replacement (or re-compaction) technique is one of the most common and relatively
less expensive methods used in infrastructures development such as road and earthworks
construction. However, its usage is limited or constraint by:-
a. Thickness of unsuitable soft soil
Often, this technique is only applicable to soft soil layers with thickness less than 3 meter.
Thick removal may require massive temporary shoring to be in place and end up being
more costly.
b. Availability of replacement material
Availability of replacement material is an important factor as it will govern the overall
construction cost. Sometimes, light weight material such as Expanded Polystyrene System
(EPS) is used as an alternative replacement material to minimize excessive consolidation
settlement and bearing failure of thick fill area.
8.2.2 Surcharging
Covering poor soils with a temporary surcharge fill, as shown in Figure 8.3, causes them to
consolidate more rapidly. When the temporary fill is removed, some or all of the soil is now
overconsolidated, and thus stronger and less compressible. Often, preloading (by surcharging) has
been used to improve saturated silts and clays because these soils are most conducive to
consolidation under static loads. Sandy and gravelly soils respond better to vibratory loads.
If the soil is saturated, the time required for it to consolidate depends on the ability of the excess
pore water to move out of the soil voids (see the discussion of consolidation theory in Chapter 4).
This depends on the thickness of the soil deposit, its coefficient of permeability, and other factors,
and can be estimated using the principles of soil mechanics. The time required could range from only
a few weeks to thirty years or more. Allowable construction period is an important factor to
determine the height of surcharge. Lesser surcharge height will require longer surcharge time.
For condition where high embankment or surcharge load is required, stage construction can be
introduced to avoid bearing failure during construction. Consolidation process during stage
construction will increase soil strength in order to allow higher load at the next stages.
The consolidation process can be accelerated by an order of magnitude or more by installing vertical
drains in the natural soil, as discussed in Item 8.2.3. These drains provide a pathway for the excess
water to escape more easily. Preloading is less expensive than some other soil improvement
techniques, especially when the surcharge soils can be moved from place to place, thus preloading
the site in sections. Vertical drains, if needed will increase the cost substantially.
8.2.3 Sub Surface Drainage Improvement System
In general sub-drainage system, either horizontal or vertical (or both), can be used to accelerate
consolidation process by reducing drainage path. These drainage systems provide a pathway for the
excess water to escape more easily. Vertical drainage system is the most commonly used system for
embankment constructed on soft soil (provided there are no sand layers or lenses exist in the
ground) and the directional flows of these drains are as shown in Figure 8.2. The length of the
drainage path is determined by the thickness of the soft soil or by the existence of any drainage
layers such as sand layers or lenses. The longer the drainage path, the longer the time required to
achieve the desired degree of consolidation.
March 2009 8-3