Vertical drains are constructed in order to improve the quality of soil specially cohesive soil. Vertical drains are mainly of 3 types: Sand Drains, Band Drains, Sand wicks. Sand Drain is the most commonly used vertical drain. Let us explain sand drains in detail as under –

What is Sand Drains?

Sand drain is a system in which radial consolidation rate of drainage is increased making vertical bore holes. These holes is back filled with suitable grade of sand. All the holes are connected at above with a layer of sand known as Sand Blanket. Further a surcharge load is placed over it to increase in consolidation rate or to accelerate the drainage. Surcharge is generally in the form of dumped soil. Due to surcharge load, the pore water pressure increases in the embankment. The drainage occurs in the vertical and horizontal directions. The horizontal drainage occurs because of dissipation of excess pore water created by surcharge.

Pattern of Sand Drains

The drains are laid either in a Square Pattern or Triangular Pattern. The spacing of drains is kept smaller than the thickness of the embankment in order to reduce the length of the radial drainage path. The zone of influence of each drain in a triangular pattern is hexagonal in plan. Similarly, the zone of influence of each drain in a square pattern is circular in plan.

The zone of influence is approximately equivalent to a circle of radius R, where R is given by :

For triangular pattern; R = 0.525 S

For square pattern; R = 0.564 S

The radius of sand drain is represented by rw

The spacing between sand drain is represented by S

Theory of Sand Drains

The theory of Sand Drains was given by Rendulic in 1935 and Barron in 1948. Later Richart in 1959 summarized the theories. Depending upon the type of strain there are 2 cases – (i) Free strain case (ii) Equal strain case.

(i) Free strain case

If the surcharge load placed over the sand blanket is flexible, free strain case occurs. In this case, there is uniform distribution of surface loads, but the settlements at the surface are uneven.

(ii) Equal strain case

This case occurs when the surcharge applied is rigid, such as heavy steel plates. In this case settlements are uniform, but the distribution of pressure is non uniform.

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The word coffer means a casket, chest or trunk. A coffer dam is a temporary structure built to enclose an area for excavation of foundation. Coffer dams are used when the size of excavations is very large and sheeting and bracing system becomes difficult, inconvenient or uneconomical. Coffer dams are generally required for foundations of structures, such as bridge piers, docks, locks, and dams, which are built in open water. These are also used for laying foundations on open land where there is a high ground water table. A coffer dam generally consists of a relatively impervious wall built around the periphery of the proposed excavation to prevent the flow of water into the excavation so that the foundation may be laid in dry condition.

Types of Coffer Dams –

The following are different types of coffer dams commonly used in practice.

1. Earth Coffer dams –

These are the simplest type of coffer dams well-adapted to depths of water upto 3 m. Earth embankments are constructed around the area to be dewatered. The earth coffer dams are built of local soils, preferably fine sand. These usually have a clay core or a vertically driven sheet piling in the middle. The upstream slope of the bank is covered with a rip rap. A successful coffer dam need not be completely watertight.

For reasons of economy, it is not possible to make it watertight and hence some seepage of water into the excavation is usually tolerated. The water collected is pumped out of the excavation. The embankment should be provided with a minimum free board of 1 m to prevent over-topping by waves.Sand-bag coffer dams are used in an emergency.

2. Rockfill Coffer dams –

Rockfill coffer dams made of rockfill are sometimes used to enclose the site to be dewatered. These are permeable and are usually provided with an impervious membrane of soil to reduce seepage. The crest and the upper part of the impervious membrane are provided with rip rap to provide protection against wave action. Overtopping does not cause serious damage in case of rockfill coffer dams. The slopes of a rockfill coffer dam can be made as steep as 1 horizontal to 1.5 vertical.

3. Single-Sheet Pile Coffer dams –

Single-sheet piling coffer dams are generally used to enclose small foundation sites in water for bridges at a relatively shallow depth. In this type of coffer dams, there is a single row of cantilever sheet piles. The piles are sometimes heavily braced. Joints in the sheet piles are properly sealed. This type of coffer dams are suitable for moderate flow velocities of water and for depths upto 4 m. The depth of penetration below ground surface is about 0.25 h for coarse sand and gravels, 0.50 for fine sand and 0.85 h for silts, where h is the depth of water. Sometimes, single sheet pile coffer dams are provided with earth fills on one or both sides to increase the lateral stability.

4. Double-Wall Sheet Piling Coffer dams –

A double-wall sheet piling coffer dam consists of two straight, parallel vertical walls of sheet piling, tied to each other and the space between walls filled with soil. The width between the parallel piles is empirically set as (h/2 + 1.50 m), where h is height of water. Double-wall sheet piling coffer dams higher than 2.5 m should be strutted. Sometimes, an inside berm is provided to keep the phreatic line within the berm.

The fill material should have a high coefficient of friction and unit weight so that it performs as a massive body to give the coffer dam. stability against sliding and overturning. Suitable measures should be adopted to reduce the uplift on the coffer dam. This is generally done by driving the sheet piling on the upstream as deep as possible.

The double wall sheet piling coffer dam has the advantage of having less leakage than that in a single wall coffer dam. These coffer dams are suitable upto a height of 10 m.

5. Braced Coffer dams –

A braced coffer dam is formed by driving two rows of vertical sheeting and bracing with wales and struts.These are similar to sheeting and bracing system,with one basic difference that braced cuts are required for excavations in dry areas whereas braced coffer dams are used to isolate a working area surrounded by water. The braced coffer dams are susceptible

~ Land Coffer dams –

Braced coffer dams are sometimes used as land coffer dams to prevent ground water from entering the foundation pit on land and to support the soil so as to prevent cave in. After the pit is dewatered, the structure is then concreted. When concreting has been completed above the water level, the coffer dam is removed.

6. Cellular Coffer dams –

A cellular coffer dam is constructed by driving sheet piles of special shapes to form a series of cells. The cells are interconnected to form a watertight wall. These cells are filled with soil to provide stabilising force against lateral pressure. Basically, there are two types of cellular coffer dams that are commonly used.

(a) Diaphragm type –

This type of cellular coffer dam consists of circular arcs on the inner and outer sides which are connected by straight diaphragm walls. The connection between the curved and the diaphragms are made by means of a specially fabricated parts Y-element. The coffer dam is thus made from inter-connected steel sheet piles. The cells are filled with coarse-grained soils which increase the weight of the coffer dam and its stability. The leakage through the coffer dam is also reduced. To avoid rupture of diaphragms due to unequal pressure on the two sides; it is essential to fill all the cells at approximately the same rate. One advantage of the diaphragm type is that the effective length of the coffer dam thus may be increased easily by lengthening the diaphragm.

(b) Circular type –

It consists of a set of large diameter main circular cells interconnected by arcs of smaller cells. The walls of the connecting cells are thus perpendicular to to walls of the main circular cells of large diameter. The segmental arcs are joined by special T-piles to the main cells.

The circular-type cellular coffer dams are self-sustaining, and therefore independent of the adjacent circular cells. Each cell can be thus filled independently. The stability of such cells is much greater as compared with that of the diaphragm type. However, the circular cells are more expensive than the diaphragm type as these require more sheet piles and greater skill in setting and driving the piles. Because the diameter of circular cells is limited by interlock tension; their ability to resist large lateral pressure due to high heads is limited.

Read also about – Pile Foundation

Introduction

When the soil at or near the ground surface is not capable of supporting a structure, deep foundations are required to transfer the loads to deeper strata. Deep foundations are, therefore, used when soil surface is unsuitable for shallow foundation. The most common types of deep foundations are pile, pier and also caisson foundation. The mechanism of transfer of the soil is essentially the same in all types of deep foundations.

A deep foundation is generally much more expensive than a shallow foundation. It should be adopted only when a shallow foundation is not feasible. In certain situations, a fully compensated floating raft may be more economical than a deep foundation.

Pile is a slender structural member made of steel, concrete or wood. A pile is either driven into the soil or formed in-situ by excavating a hole and filling it with concrete. A pier is a vertical column of relatively larger cross section than a pile.

Necessity of Pile Foundation

Pile foundations are used in the following conditions:

1. When the strata is at / just below the ground surface is highly compressible and very weak to support the load transmitted by the structure.
2. When the plan of the structure is irregular relative to its outline and load distribution. It would cause non-uniform settlement if a shallow foundation is therefore constructed. A pile foundation is required to reduce differential settlement.
3. Pile foundation are required for the transmission of structural loads thus through deep water to a firm stratum.
4. Pile foundations are used to resists horizontal forces in addition to support the vertical loads in earth-retaining structures and tall structures that are subjected to horizontal forces due to wind and earthquake.
5. Piles are required when the soil conditions are such that a wash out, erosion or scour of soil may occur from underneath a shallow foundation.
6. Piles are used for the foundations of some structures, such as transmission towers, off-shore platforms; which are subjected to uplift.
7. In case of expansive soils, such as black cotton soil (swells or shrink as the water content changes), piles are used to transfer the load below the active zone.

Classification of Piles

Piles can be classified according to followings –

1. the material used
2. mode of transfer of load
3. method of construction
4. the use
5. displacement of soil

1. Classification according to material used

There are four types of piles according to material used :

1. Steel Piles
2. Concrete Piles
3. Timber Piles
4. Composite Piles

2. Classification based on mode of transfer of loads

Based on the mode of transfer of loads, the piles can be thus classified into 3 categories :

1. End-bearing Piles
2. Friction Piles
3. Combined end-bearing and friction Piles

3. Classification based on method of installation

Based on method of installation / construction the piles may be classified into the following 5 categories :

1. Driven Piles
2. Driven and Cast in-situ Piles
3. Bored and Cast in-situ Piles
4. Screw Piles
5. Jacked Piles

4. Classification based on use

Piles can be classified into the following 6 categories, depending upon their use :

1. Load bearing Piles
2. Compaction Piles
3. Tension Piles
4. Sheet Piles
5. Fender Piles
6. Anchor Piles

5. Classification based on displacement of soil

Based on the volume of the soil displaced during installation, the piles can be classified into 2 categories :

1. Displacement Piles
2. Non-displacement Piles

Pile Driving

Piles are driven into the ground by means of hammers or similarly by using vibratory driver. Such piles are thus called driven piles. In some special cases, piles are installed by jetting or partial augering.

Following methods are commonly used –

1. Hammer Driving

• Drop Hammer
• Single acting Hammer
• Double-acting Hammer
• Diesel Hammer

2. Vibratory Pile Driver

3. Jetting Techniques

4. Partial Augering Method

Construction of bored piles

1. Drilling Holes

Bored piles are constructed after making a hole in the ground and however filling it with concrete. Following methods are used for drilling the hole.

• Hang Auger
• Mechanical Auger
• Boring Rig
• Belling Bucket

2. Concreting

Before placing of concrete, the bored hole is bailed dry of water. Any loose or softened soil is cleaned out and the bottom of the hole is rammed. A layer of dry concrete is placed and rammed if the bottom of the hole is wet. Then the concrete with a readily workable mix, not leaner than 300 kg cement/m3 of concrete, is poured into a hopper placed at the mouth of the hole. A retarder is added to the concrete if there there is a risk of the concrete setting before the casting is lifted out. However, the quality of concreting done under water is not good. This method should be avoided as far as possible.

Read also about – Coffer Dams

Soil Stabilization is the process of improving the engineering properties of the soil and thus making it more stable. It is require when the soil available for construction is not suitable for the intend purpose. In its broadest senses, stabilization includes compaction, pre consolidation, drainage and many other such processes. However, the term stabilization is generally restrict to the processes which alter the soil material itself for improvement of its properties. A cementing material or a chemical is add to a natural soil for the purpose of stabilization.

The various methods of soil stabilization and their effects on the engineering properties of the soils.

• Mechanical Stabilization.
• Cement Stabilization.
• Lime Stabilization.
• Bituminous Stabilization.
• Thermal Stabilization.
• Chemical Stabilization.
• Electrical Stabilization.
• Stabilization by Grouting.
• Stabilization by Geo textile and Fabrics.
• Reinforce Earth.

Mechanical Stabilization

It is the process of improving the properties of the soil by changing its gradation. Two or more types of natural soils are mix to obtain a composite material which is superior to any of its components. To achieve the desire grading, sometimes the soils with coarse particles are add or the soils with fine particles are remove. It is also refer as granular stabilization.

For the purpose of mechanical stabilization, the soils are subdivide into two categories :

• Aggregates ( particles larger than 75 micron )
• Binder ( particles smaller than 75 micron )

Cement Stabilization

Cement Stabilization is do by mixing pulverize soil and Portland cement with water and compacting the mix to attain a strong material. The material obtain by mixing soil and cement is refer as soil cement. The soil cement becomes a hard and durable structure material as the cement hydrates and develops strength.

Types of Soil cement

• Normal Soil Cement
• Plastic Soil Cement
• Cement Modified Cement

Normal Soil Cement

It consists of 5 to 14 % of cement by volume. The quantity of cement mix with soil is sufficient to produce a hard and durable construction material. The quantity of water use should be just sufficient to satisfy hydration requirements of cement and to make the mixture workable.

Plastic Soil Cement

This type of soil cement also contains cement 5 to 14 % by volume, but it has more quantity of water to have wet consistency similar to that of plastering mortar at the time of placement. Plastic soil cement can be place on steep or irregular slopes where it is difficult to use normal road making equipment. It has also be successfully use for water proof lining of canals and reservoirs.

Cement Modified soil

It is a type of soil cement that contains less than 5 % of cement by volume. Cement Modified soil is a semi hardened product of soil and cement. It is quite inferior to the other two types.

Lime Stabilization

Lime stabilization is do by adding lime to a soil. It is useful for stabilization of clayey soils. When lime reacts with soil, there is exchange of cations in the absorb water layer and a decrease in plasticity of the soil occurs. The resulting material is more friable than the original clay, and is, therefore, more suitable as sub grade

Lime is produce by burning of lime stone in kilns. The quality of lime obtain depends upon the parent material and the production process. There are basically 5 types of limes.

• High calcium, quick lime ( Cao )
• Hydrate, high calcium lime ( Ca (OH)2 )
• Dolomitic lime ( CaO + MgO )
• Normal, Hyrate dolomitic lime ( Ca (OH)2 + MgO )
• Pressure hydrate dolomitic lime ( Ca (OH)2 + MgO2 )

The Quick lime is more effective as stabilizer than the hydrate lime; but the latter is more safe and convenient to handle. Generally, the hydrate lime is use. It is also refer as slaked lime.

Bituminous Stabilization

Bitumen are non aqueous systems of hydrocarbons that are soluble in carbon di-sulfide. Tars are obtain by the destructive distillation of organic materials such as coal. Asphalts are materials in which the primary components are natural or refine petroleum bitumen.

The amount of bitumen require generally varies between 4 to 7 % by weight. The actual amount is determine by trail.

Types of soil bitumen

According to the Highway Research Board of USA, there are four types of soil bitumen.

• Soil bitumen
• Sand bitumen
• Water Proof Clay concrete
• Oil earth

Soil bitumen

This is water proof, cohesive soil system. The best results are obtain if the soil satisfies the following criteria.

• Passing No. 4 ( 4.76 mm ) Sieve 50%.
• Passing No. 40 ( 0.425 mm ) Sieve 35 to 100 %.
• Passing No. 200 ( 0.074 mm ) Sieve 10 to 50 %.
• Plastic limit less than 18 %.
• Liquid limit less than 40 %.
• The maximum size of the particle should not be greater than one-third the compact thickness of the soil bitumen.

The quantity of bitumen varies from 4 to 7 % of the dry weight.

Sand bitumen

This is a bitumen stabilize cohesion less soil system. The sand should be free from vegetal matter or lumps of clay. The sand may require filler for its mechanical stability. However, it should not contain more than 25 % minus No. 200 sieve material ( i.e. the material finer than No. 200 sieve ) for dune sands and not more than 12 % in case in other types of sand.

Water Proof Clay Concrete

A soil possessing a good gradation is water proof by a uniform distribution of 1 to 3 % of bitumen in this system. Soils of three different gradations have be recommend. For the three gradations, the percentage passing No. 200 sieve varies between (i) 8 to 12 ; (ii) 10 to 16 and (iii) 13 to 30.

Oil earth

In this system, a soil surface consisting of silt clay material is made water proof by spraying bitumen in two or three applications. Slow or medium curing bitumen or emulsions are use. The bitumen penetrates only a short depth into the soil. The amount of bitumen require is about 5 litres per square meter of the soil surface.

Chemical Stabilization

In chemical stabilization, soils are stabilize by adding different chemicals. The main advantage of chemical stabilization is that setting time and curing time can be control. Chemical stabilization is however generally more expensive than other types of stabilization.

The following chemicals have be successfully use.

• Calcium chloride
• Sodium chloride
• Sodium silicate
• Polymers
• Chrome Lignin
• Other Chemicals

Thermal Stabilization

It change causes a mark improvement in the properties of the soil. Thermal stabilization is do either by heating the soil or by cooling it.

Heating :- As the soil is heat, its water content decreases. Electric repulsion between clay particles is decrease and the strength of the soil is increase. When the temperature is increase to more than 100 degree celsius, the absorb water is driven off and the strength is further increase.

Freezing :- Cooling causes a small loss of strength of clayey soils due to an increase in inter particle repulsion. However, if the temperature is reduce to the freezing point, the pore water freezes and the soil is stabilize. Ice so form acts as a cementing agent.

Stabilization by grouting

In this method of stabilization, stabilizers are introduce by injection into the soil. As the grouting is always do under pressure, the stabilizers with high viscosity are suitable only for soils with high permeability. This method is not suitable for stabilizing clay because of their very low permeability.

The grouting method is costlier as compare with direct blending methods. The method is suitable for stabilizing buried zones of relatively limit extent, such as pervious stratum below a dam. The method is use to improve the soil that cannot be disturb. An area close to an existing building can be stabilize by this method.

Types of Grouting

• Cement grouting
• Clay grouting
• Chemical grouting
• Chrome lignin grouting
• Polymer grouting
• Bituminous grouting

Stabilization by Geo textile and Fabrics

The soil can be stabilize by introducing Geo textiles and fabrics which are make of synthetic materials, such as polyethylene, polyester, nylon. The Geo textile sheets are manufacture in different thickness ranging from 10 to 300 mils ( 1 mil = 0.0254 mm = 25.4 micron ). The width of the sheet can be up to 10 m. These are available in rolls of length up to about 600 m. Geo textiles are manufacture in different patterns, such as woven, non woven, grid, and hybrid. The woven Geo textiles are make from continuous mono filament or silt film fibers. The non woven Geo textiles are make by use of thermal or chemical bonding of continuous fibers and then press through rollers into relatively thin sheets.

Reinforce Earth

The soil can be stabilize by introducing thin strips in it. In reinforce earth, thin metal strips or strips of wire or Geo synthetics are use as reinforcement to reinforce the soil. The essential feature of the reinforce earth is that friction develops between the reinforcement and the soil. By means of friction, the soil transfer the forces built up in the earth mass to the reinforcement. Thus tension develops in the reinforcements when the soil mass is subject to share stresses under loads.