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Ground Improvement Techniques

Bureau Built Expressions

Introduction

One of the major functions of Geotechnical Engineering is to design, implement and evaluate ground improvement schemes for construction projects. In the last two decades significantly new and advanced technologies and methods have been developed and implemented to assist the Geotechnical Engineer to provide cost-effective solutions for construction sites poor soil conditions.

Ground Improvement refers to a technique that enhances the engineering properties of the soil mass including shear strength, stiffness and permeability. Ground improvement has developed into a sophisticated tool to support foundations for a wide variety of structures. Properly applied, i.e. after giving due consideration to the nature of the ground being improved and the type and sensitivity of the structures being built, ground improvement often reduces direct costs and saves time.

Ground improvement construction methods are used to improve poor/unsuitable subsurface soils and/or to improve the performance of embankments or structures. These methods are used when replacement of the in-situ soils is impractical because of physical limitations, environmental concerns, or is too costly.

Though the term Ground Improvement has been familiar to Civil Engineers, the design approach is still empirical, mostly based on past experience. Well defined design procedure, construction procedures and codal provisions are yet to be developed. In the absence of above, execution of the same is difficult and sometimes lead to contract disputes. On the other hand, adequacy of Ground Improvement for supporting even large structures has been proved beyond doubts. Application of Ground Improvement is not only cheaper but reduces the construction time significantly.

Built Expressions catches up with experts in the field to know more on the methods, testing and evaluation systems

Prof. B. R. Srinivasa Murthy, formerly professor of Civil Engineering, Indian Institute of Science, Bangalore and Advisory Consulting Civil Engineer, says, "The geotechnical engineering practice has now reached a stage of scientific maturity where many innovative solutions can be provided to resolve field problems which had not been possible two decades earlier."  The recent increase in the rate of urbanization has thrust on the geotechnical engineers many new challenges such as deep excavations, poor and marginal foundation soils and underground constructions. The conventional solutions may be risky and cost prohibitive.

Primary Function

Ground Improvement Techniques involve several methods which result in changing soil characteristics by a physical action, such as vibration, or by the inclusion or mixing in the soil of a stronger material. There are three strategies available to accomplish the intent namely; Increase shear strength, density, and/or decrease compressibility of foundation soil, Use lightweight fills to significantly reduce the applied load on the foundation soil, and Transfer the load to a more competent (deeper) foundation soil.

Ground improvement methodologies have the primary functions to:

• Increase bearing capacity, shear, or frictional strength,
• Increase density,
• Control deformations,
• Accelerate consolidation,
• Decrease imposed loads,
• Provide/increase lateral stability,
• Form seepage cut-offs or fill voids,
• Increase resistance to liquefaction.

The Significance

Before starting any construction project, it is required to ensure the quality and sustainability of the ground. Ground improvement technique is used to improve the strength of soil from marginal levels to high levels based on the project requirement. Ground improvement techniques are generally adopted for infrastructure projects over large areas.  Mr. T. Rambabu, director, UR Ground Engineering Services Pvt. Ltd., says, "When it is concerned with building construction in urban scenario, structural loads are generally large and concentrated ground improvement technique is generally not adopted unless there is a specific requirement.  In building construction instead of ground improvement technique the loads are transferred to stronger strata using transfer elements such as piles of various kinds."

If the construction of building is not done on solid ground, then it may lead to repairs and instability in future. Therefore, modern techniques such as dynamic compaction and jet grouting are essential. For strong basement, ground improvement is necessary, because it reduces the risk of any damage later on.

However, for ground improvement, combination of binder and water is essential, because this can break up the soil and harden the ground. Thus, with the help of these techniques, you can turn any soil for construction. Each technique has its drawbacks and benefits thus; using it on the requirement of the project can really improve the stability of ground. Thus, improvement of ground is essential if you want your building to long last.

Ground Improvement Techniques

Rapid urban and industrial growth demands more land for further development. In order to meet this demand land reclamation and utilization of unsuitable and environmentally affected lands have been taken up. These, hitherto useless lands for construction have been converted to be useful ones by adopting one or more ground improvement techniques. The field of ground improvement techniques has been recognized as an important and rapidly expanding one.

There are various treatment techniques that are available to improve the condition of poor or unstable ground by altering the nature of the soil insitu. For example, where loose sands or man- made fill are encountered, deep densification methods can be used to increase the density of the material to a degree that allows for construction of shallow foundations or, in the case of loose sands, mitigates potential for liquefaction during a seismic event. Otherwise, costly over-excavation of the unsuitable material or deep foundations may be required.

The various techniques that may be employed include:

• Jet Grouting
• Chemical and Permeation Grouting
• Compaction Grouting
• Soil Mixing
•  Deep Densification (Vibro compaction, deep dynamic compaction)
• Stone Columns
• Blast-induced Compactions
• Preloading/Surcharging
•  Geosynthetics and Soil Reinforcement
• Cement/Lime Stabilization

The ground can be improved by adapting any of the ground improvement techniques mentioned above individually or in combination. Vibro-compaction increases the density of the soil by using powerful depth vibrators. Vacuum consolidation is used for improving soft soils by using a vacuum pump. Preloading method is used to remove pore water over time. Heating is used to form a crystalline or glass product by electric current. Ground freezing converts pore water to ice to increase their combined strength and make them impervious. Vibro replacement stone columns improve the bearing capacity of soil whereas Vibro displacement method displaces the soil. Electro osmosis makes water flow through fine grained soils. Electro kinetic stabilization is the application of electro osmosis. Soil nailing increases the shear strength of the in-situ soil and restrains its displacement. Micro pile gives the structural support and used for repair/replacement of existing foundations. Grouting is injection of pumpable materials to increase its rigidity.

Tests and evaluation systems

Before starting a construction process, an experienced Geotechnical engineer, experienced in Ground Improvement Techniques, need to assess the requirements of the project and available information.  Based on the assessment one can advise the required scheme for further field investigations, depth of investigations and specific type of tests to be carried at laboratory.  Basically one needs to find out by geotechnical investigations the existing strength of soil and expected settlement with proposed imposed loads from the development. 

Certain testing and evaluating methods help in determining the quality of the soil before commencing the construction process. Dr. C R Parthasarathy, Group Managing Director, Sarathy Geotech & Engineering Services Pvt. Ltd., says, "Primarily, classification test is conducted on soil to determine the type of the soil. To understand the strength of the soil, a test to determine the engineering properties of the soil is conducted that are used for the designs. Thorough geotechnical/geophysical investigation supplemented with laboratory tests before the treatment will help to identify judiciously the methods of Ground improvement techniques to be adopted. Concurrent with the improvement techniques, site investigation (in-situ and laboratory testing) after treatment will assist in the estimating the intended improvement."

Scope of Application

The scope of application of the various techniques depends mainly on the type and grading of the soils that require improving and the basis for choosing a particular technique or in combination depends on;

• Case studies
• Numerical & statistical analyses
• Laboratory tests on soil specimens (QA/QC)
• Benefit vs. cost analysis
• Constructability issues + engineering judgment

The graph shows Ground Improvement techniques based on soil characterization.

Ground Improvement techniques based on soil characterization

Current trends

"As said earlier, one needs to assess the existing strength and estimated settlement of soil with respect to required for the project.  Based on the gap of existing to expected, a suitable method either by replacement or in-situ improvement is selected," adds Mr. Rambabu.

"Experienced Geotechnical engineers need to assess the requirements of the project and available information. Based on the assessment he will devise the required scheme for further field investigations, depth of investigations and type of tests to be carried at laboratory.  Basically one needs to find out by geotechnical investigations the existing strength of soil and expected settlement with proposed imposed loads from the development," adds Dr. Parthasarathy.

Thorough evaluation of soil properties prior to the construction is significant to get the bearing capacity of the soil, analysis for preventing side collapse due to excavation. Stabilization of soil by mechanical, cementing and chemical if needed. Adopting proper side protection system is necessary to prevent failure thus not jeopardizing safety. Appropriate methods of side protection (vis., soldier piles, sheet piles, Soil nailing, grouting, diaphragm walls, grouted nails etc) shall be determined based on soil characteristics. Economical solutions can be arrived by proper site investigation designs by analytical methods verified by numerical approach and model tests. During construction, it is strongly recommended to monitor the construction progress by instrumentation to measure the stress and deformation, thus verifying the designs (observational approach).

Compaction grouting is the injection of a thick, low mobility grout that remains in a homogeneous mass without entering soil pores. As the grout mass expands, the surrounding soil is displaced and densified. Before zeroing in on a specific type of soil improvement, certain factors should be considered including, noting the type soil type is essential, whether it's purely a soil, clay, organic, etc. The area and depth of treatment required should be analysed depend on the geometric characteristics of the soil deposit and the nature of facilities proposed for construction. It is imperative to consider the type of structure and load distribution of the structure. In addition, understanding the soil properties such as strength, compressibility and permeability is needed.

Certain material availability including stone, sand, water, admixture, stabilizers and availability of skills and equipment would also determine which type of improvement technique a soil should be subjected to. Environmental considerations-waste disposal, erosion, water pollution, local experience and preferences and economics aspect can never be ignored.

Conclusion

Engineers must be aware of the capabilities and limitations of available liquefaction remediation Ground improvement techniques and how better understanding of the physical phenomena associated with each technique will help improve the current state of practice regarding quality control and construction.

Rapid urbanisation and industrial growth are demanding more land for further development. In order to meet this demand, improvement and utilization of unsuitable lands is the need of the hour. In such a scenario, Ground improvement techniques will play a major role. Ground Improvement Techniques will play increasing role with time as the developers are left with no choice but to use lands which were earlier considered as unusable for projects due to various factors but are now forced use same.  Ground Improvement Technique will play a major role in this scenario as it offers cheaper alternative to improve existing soil and utilizing the same with modern techniques and required men and machinery is available in the country today compared to few years ago.