Ground improvement

A simple definition of in situ improvement of a soil deposit is the increase in its shear strength along with a reduction in its compressibility.

14.5.1 Drainage or consolidation techniques Surcharge loading

Surcharge loading is probably the simplest method of ground improvement and can be applied to cohesive soils. The technique involves subjecting the surface of the soil to a temporary loading using some method such as the placing of temporary earth fill, water filled tanks or tension piles secured to some form of framework, etc. The soil experiences consolidation under this loading and both its stiffness and shear strength increase. The time taken for full consolidation depends upon the length of the drainage path which can be decreased by the insertion of vertical drainage wells.

Stage construction

This technique is also used for cohesive soils and involves determining the rate of construction that will allow the soil to consolidate and increase in strength sufficiently to maintain an adequate factor of safety against bearing capacity failure for the corresponding increment of construction loading. By proceeding in constructional steps the foundation soil eventually becomes sufficiently strong to support the full construction loading. Because the soil settles during the construction phase the method is usually applied to earth embankments rather than to rigid foundations. The stress path method evolved by Lambe (1964, 1967), which has been described in Chapter 9, can be used for this approach but it is usually also necessary to monitor the actual soil behaviour during construction using some form of instrumentation installed at the start of the work.

Electro-osmosis

This method causes water within a soil to drain away under the action of an electrical potential and can be very effective in fine grained soils, such as silts and clayey silts where well point systems (see Chapters 2 and 10) cannot be used because of the low permeability of the soil. The system was first used by the Germans in World War II during the construction of U-boat pens at Trondheim, in Norway, and its application has been described by Casagrande (1947).

Steel or aluminium rods, from 10 to 100 mm diameter are driven into the soil over the area to be treated. These rods act as anodes, their corresponding cathodes being conventional well points. An electrical potential of some 50 volts per metre is created by direct current and the water within the soil is gradually driven towards the well points, which are pumped out at intervals.

The method is only rarely used, possibly because of the high installation and running costs.

14.5.2 Compactive techniques

Possibly the most common method for improving ground is by compaction of the soil in a series of layers. Compaction is described in Chapter 11 and is particularly suitable for fill material. However, with existing soil deposits, modern compaction plant can only improve the soil for a depth of 1 or 2 m below its surface so that, for the improvement of a deep soil deposit where deep compaction is required, some other method must be used.

Vibro-compaction

This method cannot be used in cohesive soils and is most effective in granular soils, although soils with up to some 25 per cent silt can also be treated. A large vibrating probe, suspended from a crane, is lowered into the ground. The probe penetrates downwards under its own weight and compacts the surrounding soil, up to a distance of about 2.5 m from the probe, by virtue of the temporary reduction in effective stress caused by the vibration. Probes are normally spaced at 1.5 to 3.0 m and can compact suitable soils to a depth of about 12 m.

Vibro-flotation

In order to assist the penetration of the vibrating probe into the ground, water jets can be fitted at the top and bottom of the probe. In this case the process is referred to as vibro-flotation and the probe is called a vibrofloat.

Vibro-replacement

This technique can be used to improve the load-carrying capacity of soft silts and clays. Essentially the soil is reinforced by the insertion of stone columns. This is achieved with the use of a vibrating probe, similar to the technique used in vibro-compaction. The probe is allowed to penetrate the soil and does so by displacing the soil radially. Once the required depth has been reached the probe is withdrawn and the hole created by the probe backfilled with graded aggregate, up to 75 mm in size. The probe is then reintroduced to both compact and radially displace the aggregate. The process is repeated until the required stone column has been created. With soft clays soil is removed, not displaced, by means of water jets fitted to the probe. The method is really only suitable for light foundation loads as heavy loads can cause excessive settlement

Dynamic consolidation

This method involves the dropping of a large weight, 100 to 400 kN, from a height of 5 to 30 m, on to the surface of the soil. It is seen that the energy delivered to the soil per blow can be as high as 12 000 kNm although the energy values normally used lie between 1500 to 5000 kNm. The impact of the weight with the soil creates shock waves that can penetrate to a depth of 10 m. In cohesionless soils these shock waves create liquefaction, immediately followed by compaction of the soil, whereas in cohesive soils they create excessive pore water pressures, which are followed by the consolidation of the soil.

The work is generally carried out by a specialist contractor whose engineering judgement can be used to give a reasonable estimate of the energy requirements for a particular site.

Before the work is commenced the area to be treated is covered with a layer of granular material of thickness between 0.5 to 1.0 m. The layer acts as a working platform for the equipment and helps to prevent excessive penetration of the weight. It also provides a pre-load surcharge of some 10 to 20 kN/m2 and helps to drain away water as it is driven out of the soil.

The weight, or pounder, is usually dropped five to ten times at each selected point, the points being spaced on a square grid, 5 to 15 m in dimension. In the case of cohesive soils not all the blows are delivered at once as it is necessary to have pauses in order that full consolidation for a particular compaction of a treated area is first achieved. These pauses can extend to weeks in some cases.

14.5.3 Grouting techniques

The engineering properties of a soil can be improved by the injection of chemical fluids which solidify and hence strengthen the soil structure. Obviously the system is only effective if the voids of the soil can be penetrated by the grout and it therefore has little application for cohesive soils, except when fissures require to be sealed. Grouting is mainly restricted to granular soil and weathered rocks. The procedure is expensive and is only used when other methods of soil improvement are not applicable.

Cement grouts

Cement grouts are used to seal fissured rocks and to decrease permeability in sand and gravel deposits.

Bentonite and bitumen slurries

Suspensions of bentonite and emulsions of bitumen can be used to reduce the permeability of sands and gravels provided the grain size is not less than medium sand.

Chemical grouts

For fine sands chemical grouts, such as sodium silicate which comes in the form of a syrupy liquid, are made to react with a compound, such as calcium chloride, to form a stiff silica gel. The two agents can be mixed together along with a retarding agent so that gelification does not happen until the grouting process is completed or they can be injected separately so that they react together within the soil mass. The latter process has been used successfully for many years but has the disadvantage that a large number of grout holes, spread at not more than 700 mm, are required.

14.5.4 Geotextiles

The use of fabrics in ground improvement techniques is a recent development which has been highly successful and has taken place over the last 30 years.

The first use of fabrics was as a temporary expedient whereby the surfaces of soft soils were covered with fibre grids on to which temporary roads could then be constructed. Nowadays fabrics are used in the permanent construction of most forms of earthworks.

Fabrics range from natural products, such as cotton, jute and wool, to the polymer plastics produced from long chain hydrocarbon molecules which are now being increasingly used and are briefly described in Chapter 7. A generic term, geotextiles, is used to cover all the various different fabrics. In this chapter we will only concern ourselves with plastic materials, which now account for at least 75 per cent of the fabrics used in civil engineering.

Functions of geotextiles

Geotextiles are incorporated into a soil structure to satisfy at least one of the following functions:

(i) separation;

(ii) filtration;

(iii) drainage;

(iv) reinforcement.

Separation

The base of a pavement construction may be subjected to separation if it is placed directly on to the surface of a soft subgrade. Separation is the upward migration of particles of the fine subgrade soil accompanied by the downward movement of the denser base particles. Such intermixing of soil particles can create a weak zone at the interface between the two materials resulting in considerable reduction in bearing capacity strength.

The placing of a relatively weak strength geotextile fabric on the surface of a soft subgrade, prior to constructing the base, is all that is necessary to provide a permanent solution to separation between the two materials.

Filtration

Where a cohesive soil is subjected to seepage a suitable geotextile can be used to prevent the migration of the fine soil particles in exactly the same way as the granular filters described in Chapter 2. A geotextile filter, placed at the end of the seepage path, operates in a different manner to a granular filter. Soil particles tend to collect at the boundary between the soil and the geotextile and this appears to induce a self-filtration effect within the soil.

Drainage

Special types of permeable geotextile fabrics can be used to form drainage layers in basements and behind retaining walls in exactly the same manner as the layers of granular material illustrated in Figs 6.22c, d and e.

Reinforcement

The use of plastic reinforcement in reinforced soil retaining walls is now well established and is increasing. The technique is mentioned in Chapter 7.

In the construction of an earth embankment on top of a soft foundation soil a layer of geotextile fabric, placed on the surface of the soft soil, can give enough tensile strength to allow it to support an incremental layer of the embankment without spreading or edge failure during consolidation and thus permit stage construction to be carried out.

The sub-bases of roads supported by soft subgrades can be strengthened by the inclusion of layers of a geotextile fabric.

14.6 Environmental geotechnics

Environmental geotechnics brings together the principles of geotechnical engineering with the concerns for the protection of the environment and the subject is becoming increasingly important to the geotechnical engineer. Applications of environmental geotechnics include contaminated land (both its control and reclamation), containment of toxic wastes, design of landfill sites, and the management of mining wastes. These applications have a number of common features which epitomise environmental geotechnics problems: soil water flow problems, soil chemistry, and local and national government legislation. Many of the environmental geotechnics issues concern the leaching of toxins into the soil and groundwater supplies, and so the soil properties which are of greatest significance are permeability, void ratio and plasticity. The study of environmental geotechnics is a subject in its own right and is beyond the scope of this book. Readers interested in this aspect of geotechnics should refer to the texts of Attewell (1993), Cairney (1993), and Harris (1994) for a description of the subject.

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