It is often necessary to improve the properties of the soil whether as a foundation material or as a construction material because it is not suitable for its intended purpose. The fundamental techniques for improving the properties of natural materials are compaction, modification, stabilization, drainage, precompression, vibrocompaction, soil reinforcement, which includes soil nailing, and the use of geotextiles. The principles and methods of compaction are discussed: compaction parameters like maximum dry density and the optimum moisture content, zero-air-voids curve for different degrees of saturation, factors affecting compaction. The chapter further discusses field compaction, compaction control, and compaction equipment. The chapter also considers the other fundamental techniques for improving the properties of soils. Finally, the chapter briefly considers the types and requirements of a pavement and the two essential methods of design namely semi-empirical method and rational method of design.
Top10.0 Improvement Techniques
10.0.1 Purpose and Type
In many engineering problems which involve soils, it is often necessary to improve the properties of the soil whether as a foundation material or as a construction material because it is not suitable for its intended purpose. The soil may be weak, highly compressible or it may have a higher permeability than desirable from an engineering and economic point of view. When such unsuitable soils are encountered on an engineering project, the engineer may have to move the structure from that site or redesign the structure taking into consideration the soil conditions. These alternatives often lead to escalation of the cost of the project.
Another alternative is to improve, modify or stabilize the in-situ soil to increase its capacity to sustain the applied loads without serious deformations and to retain its strength and stability at the same time. Some of the objectives of soil improvement are to:
- 1.
Reduce the settlement of the structure on the soil;
- 2.
Improve the shear strength of the soil and thus increase the bearing capacity;
- 3.
Reduce the shrinkage and swelling characteristics of soils;
- 4.
Minimize the possibility of slope failure of embankments and dams;
- 5.
Reduce liquefaction potential
- 6.
Decrease the permeability in some cases; and
- 7.
Reduce the water absorption capacity of the soil.
The fundamental techniques for improving the properties of natural materials are compaction, modification, stabilization, drainage, precompression, vibrocompaction, soil reinforcement which includes soil nailing and the use of geotextiles.
10.1 Compaction
Compaction can be defined as the artificial densification of soils. It is a simple ground improvement technique, where the soil is densified through external compactive effort. It involves the application of mechanical energy to a soil to rearrange the particles and reduce the void ratio. It is a process of increasing the density of soil in a fill by reducing its void space by the expulsion of air. Compaction is different from consolidation which is the gradual compression of a saturated soil layer under a steady load by expulsion of water from its pores. Compaction is usually achieved by subjecting the soil to a changing state of stress such as few repetitive cycles of ramming, rolling or vibrations. This results in a decrease in the volume of the air voids and an increase in the density of the soil.
R.R. Proctor (1933) was the first to define and apply in a systematic way the principles of soil compaction. The standard laboratory method of compaction is therefore named after him.
10.1.1 Benefits of Compaction
Compaction of soil is probably one of the largest contributors to the site work economy; in other words-it is important. Large sums of money are spent on it every day. Soil is a very flexible and inexpensive construction material. It can be manipulated to produce a material with a wide range of properties. Control of compaction in the field permits civil engineers to engineer a soil to produce a material with properties that are optimized for a project.
Densification of the soil by compaction has many beneficial effects: The principal reason for compacting soil is to reduce subsequent settlement under working loads. It increases the strength of the soil and by reducing the proportion of air void or water space, it restricts the passage of water from the surface, and thereby reduces both the softening effect and the dimensional changes associated with changes in water content. Compaction can also prevent the buildup of large water pressures that cause soil to liquefy during earthquakes. If the moisture content of the soil can be controlled during compaction, there will be an increase of the cohesion owing to capillary tensions in the soil moisture.