Ground Improvement

The main goal of most soil improvement techniques used for reducing liquefaction hazards is to avoid large increases in pore water pressure during earthquake shaking. This can be achieved by densification of the soil and/or improvement of its drainage capacity.

1. Gravel Piles/ Rammed Stone Columns (0.2 to 1.2 m diameter, up to 25-30 m deep)

Ground improvement is an important requirement in today’s construction industry as land reclamation is becoming increasingly popular. Ground treatment by rammed granular piles can offer effective solution amongst various other improvement methods. Densification of the surrounding ground by rammed granular piles modifies the soil properties. The stone column technique is a very efficient method of improving the strength parameters of soil like bearing capacity and reducing consolidation settlement. It offers a much economical and sustainable alternative to piling and deep foundation solutions. Ground improvement when implemented through stone column technique aids in a much stable solution to construction in weak cohesive soils.

2. Sand Drains/Wick Drains

Sand drain is a process of radial consolidation which increase rate of drainage in the rate of drainage in the embankment by driving a casing into the embankment and making vertical bore holes. These holes are back filled with suitable grade of sand.

Wick drains, perhaps more accurately called strip or prefabricated vertical drains (PVDs), accelerate preconstruction soil consolidation.

The drains are composed of a plastic strip with drainage channels, wrapped in a non woven geotextile filter fabric (see photo in the box above). The geotextile filter prevents soil particles from entering the channels and clogging the drain.

The installation of the drains-often, but not always, vertically-into soft soils is performed using vibratory hammers or static methods, and the wick drain layout is typically a triangular or square pattern. Once installed, the PVD field looks like a series of wicks.

3. Foundation Strengthening

Foundations need strengthening in the case of applying additional loads. Widening and strengthening of existing foundations may be carried out by constructing a concrete jacket to the existing footings. The new jacket should be properly anchored to the existing footing and column neck in order to guarantee proper transfer of loads. Another possibility is to provide full anchorage length for longitudinal reinforcement by extending the column jacket at the top of footing.

When the bearing area of footing is not sufficient, the size of the footing should be increased. If the column is also being jacketed, the transfer of load from column to footing becomes easy. Attention shall be given during construction in order that the excavations for the new “jackets” do not affect the existing adjacent foundations.

4. Evaluation and Mitigation of Geotechnical failures

Natural hazard is unexpected or uncontrollable natural event of unusual magnitude that threatens the activities of people or people themselves. The natural hazards directly affect the ground or cause ground movements are called geotechnical hazards. Some geotechnical hazards are: earthquakes and earthquake related hazards like soil liquefaction, lateral spreading and tsunami; and landslides or sloping failures. Human activities can increase the occurrence and severity of a geotechnical hazard like building on top of unstable slope will increase the possibility of slope collapsing, steepened slope due to cutting into a hillside or embankment and too much logging operations may initiate landslides. Although natural geotechnical hazards cannot be prevented, there is greater possibility that we can control human activities that can cause disasters.

By effective mitigation techniques, we can reduce the damage, reduce the severity of its effects and reduce human sufferings that result from disasters. There is a critical need for increase efforts in understanding the causes of disasters, evaluating their risk, and developing procedures for mitigating their effects.

To prevent problems, failures and even disasters. There are several causes for geotechnical failure:

• There can be an exceptional large load, an exceptional low strength, or a combination of these two, in a foreseen failing mechanism.
• There can be an unknown or unforeseen failing mechanism or other lack of scientifical knowledge.
• There can be a calculation error from a well-qualified engineer.
• There can also be a lack of available knowledge or willingness (incompetence) at the designing part of the construction management, for example when a lack of time, money, qualified people or qualified material tempt or lead managers to take unacceptable known or unknown risks.

5. Remedial measures to support foundations of unsafe/distressed buildings/structures:

Causes of failure of foundations:

• Uneven loading of the structure.
• Poor selection of the foundation due to wrong or lack of soil analysis
• Poor drainage system within the site.

Remedies:

• For uneven loading, during design state, live and dead loads should be carefully distributed by introduction of columns, beams.
• Proper soil analysis should be carried out, this will give the inside of the type of foundation to be adapted for a particular structure.
• For poor drainage, this can be avoided by proper leveling of the site prior to construction, use of land drains or use of water repellant cement for the foundation concrete

6. Grouting of Soils & Rock – using cement, cement-bentonite, chemicals, etc.

Grouting technology has become a common ground improvement method used frequently for underground and foundation constructions. The process of grouting consists of filling pores or cavities in soil or rock with a liquid form material to decrease the permeability and improve the shear strength by increasing the cohesion when it is set. Cement base grout mixes are commonly used for gravely layers or fissure rock treatment. But the suspension grain size may be too big to penetrate sand or silty-sand layers. In this case, chemical or organic grout mixes are also used. In recent years, the availability of ultra fine grout mixes has extended the performance of hydraulic base grout for soil treatment.