Advance Foundation Engineering

Black Cotton Soils&Under Reamed Piles

BySyed Jeelani Basha

14481D8720

INTRODUCTION

In India, expansive soils are called as Black Cotton soil.The name Back Cotton as an agricultural origin. Most of these soils are black in colour and are good for growing Cotton.  All the black soils are not expansive soils and all the expansive soils are not black in colour.  These soils passed high strength in summer and decreased rapidly in winter.The soil has a swelling property due to the presence of MONTMORILLONITE mineral.

 The swelling soils of India have their origin in subaQueous decomposition of basalt rocks or weathering In-Situ. It is very necessary to discuss about the Engineering behaviour of soils. From this information civil structures can be prevented from the damages causing Expansive soils.

CHEMICAL COMPOSITION OF BLACK COTTON SOILS

Black Cotton Soils are made of varying proportions of clay minerals like Montmorllionite, Illite and Kaolinite.Chemicals like Iron oxide and Calcium Carbonate and organic matter like humus.Montmorllionite is a Predominant mineral of black cotton soils. The swelling and shrinkage behaviour of black cotton soils originate mainly from this mineral.Clay minerals are hydra silicates of aluminium and magnesium. They are made of sheets of silica and alumina stacked one above the other forming sheet like structure with expanding lattice. The structure of some aluminium is by magnesium ions and the mineral becomes chemically active. The mineral has high activity, and has the Base Exchange capacity. They attract water molecules (dipoles) and various types of hydrated cations to the surface causing the soil to increase the volume.

n·H2O+cations

Montmorllionite Illite Kaolinite

potassium

            PH Value 8.9            Organic Content   0.4 to 2.4 %            CaCo3 5-15%            Sio2 50-55%            Al203 3.5 %            Montmorllionite minerals 30-50 %

CHEMICAL COMPOSITION OF BLACK COTTON SOILS

DAMAGES When the structure is built on the black cotton soil in the dry seasons, there may be no damage in the structure in that season.  When the rainy seasons is starts due to the swelling property the soil get expands and the strength in the foundation structure decreases, which causes uneven settlement leads to crack in the walls.

Due to uneven settlement the beam gets deflected which affects the plastering to the walls.

Small fissure cracks are seen on the floors and on the walls.

Due to high degree of expansive soils, the buildings may fails which will be very dangerous.

Cracks of soil Cracks due to expansive soil

Tests

SIEVE ANALYSIS

Wet Sieve Dry Sieve

75 micron sieve used for both analysis

Tests

ATTERBERG’S LIMITSLIQUID LIMIT

50 – 100%.

PLASTIC LIMIT20-65%

SHRINKAGE LIMIT9-14%

Casagrande apparatus

Parameters

Differential Free SwellDFS= settled soil vol in water – settled soil vol in kerosenesettled soil vol in kerosene

Swelling PotentialSP = 3.6 x 10-

5

A2

.

4

4

C3

.

4

4

C = % of clayA = PI /C

EXPANSIVE SOIL

REMEDIAL TREATMENTS

Replacement of the expansive soil

With Sand, Gravel and both

Design of foundations to width

stand swellingRigid, Flexible

structure and Isolating foundations

Replacement of the expansive soil

With Sand, Gravel and mix of both

Vibro Compaction Vibro Replacement

Water Pressure Injection Wet Soil Mixing

Dry Soil Mixing

Under-Reamed PilesThe under-reamed piles developed by the CBRI, Roorke,

are commonly used in India for foundation for expansive soils.The under-reamed piles anchor the structure to a stable soil mass well below the unstable soil which is subjected to ground movement because of seasonal changes.

The unstable zone is known as Active zone and its depth is normally 3 to 4 m at most place in India.

The under-reamed piles are bored, cast-in situ concrete piles with bulb or under-ream at its bottom. In a double under-reamed pile, there are two bulbs IS:2911 – part 3 (1980) recommends a maximum of two bulbs as shown in fig.

The bulbs provide adequate bearing and anchorage to the pile.

Single under-reamed pile and Double under-reamed pile

Design of under-reamed pile

1) Minimum length of the pile in the deposit of expansive soil should be 3 to 5m below the ground surface. The top 1.2 m (or 2 times of shaft diameter ) is usually neglected while calculating the skin friction.

2) The diameter of the bulb is taken 2 to 3 times of shaft diameter. The shaft diameter is 20 to 30 cm.

3) The vertical spacing b/w the two bulbs should not be less than 1.5 times the bulb diameter for the piles up to 30 cm diameter. For larger diameter piles, the minimum spacing is 1.25 times the bulb diameter. The spacing is usually kept 1.5 to3 m.

4) The minimum horizontal spacing of piles should be 2 times the bulb diameter under normal loading. The maximum spacing should not exceed 2.5 m to avoid pile cap problems.

5) The depth of the most bulb from the ground surface should be at least 2 times the bulb diameter. This depth should not be less than 1.75 m .

6) The piles are connected at their top by a RCC plinth beam which supports the super-structure.

Load Carrying CapacityThe ultimate load for single bulb, under-reamed pile is given by

Qu = Qp + Qs

Qu = [qsh Ash +qb (Ab -Ash)] + f As

Qu = [π/4 D2 (9c) + π/4 (Du2- D2)(9c`)] + α ca As

The ultimate load for double bulbs, under-reamed pile is given by

Qu = [qsh Ash + qb (Ab - Ash)] + f As + f `As`

Qu = [π/4 D2 (9c) + π/4 (Du2 - D2)(9c`)] + α ca As + ca` As`

Where qb bearing capacity of the soil at the under-reamed section(9c) qsh bearing capacity of the soil at the bulbAb cross –sectional area of the bulb (π/4 Du

2) Ash cross –sectional area of the shaft (π/4 D2)D and Du are diameter of shaft and bulbf unit skin friction (adhesion) on the shaft above and below the bulbAs surface area of the shaft of the pile above and below the bulbc unit cohesion at the basec` unit cohesion at the bulbca average cohesion on the shaftα adhesion factor =(1, 0.7, 0.4 and 0.3 for soft, medium, stiff and very stiff soils)As` surface area of the cylindrical circumscribing the bulbsf ` unit skin friction (adhesion) b/w soil along the cylindrical surface

Construction

Construction of under-reamed piles is quite easy and convenient in most cases.The equipment can be easily handled manually.In soft soils for the construction of an under-reamed pile up to 3.5 m dept, a bore hole is made using a spiral auger. The auger works through a boring guide frame fixed on the ground. When the desired depth of the bulb is reached, an under reaming tool is inserted into the bore hole. When the under reamer is pressed down and rotated, it gradually widens out and cuts the soils from the sides of the bore hole. The excavated soil enters the under reamer. When the under reamer is full of soil, it is taken out and emptied at the ground. This process is repeated till the complete bulb is formed.

For bore holes deeper than 3.5 m and also for bore holes larger than 30 cm diameters, a portable hoist with manually operated which used to handle the equipment.

After the completion of bore hole, the reinforcement cage is lowered into it. The cement concrete poured into the bore hole, compaction by rodding

Advantages:1. Enlarged base (under reamed), also known as bell, which increases the base bearing capacity and load carrying capacity.2. Decreases the vertical settlement and differential settlement.3. It act as foundation for unstable and cohesive expansive soil such as black cotton soil.4. The cost advantages of under-reamed piles are due to the reduced pile shaft diameter, resulting in less concrete needed to replace the excavated material.5. Provision of under reamed bulbs has the advantage of increasing the bearing and uplift capacities.6. It also provides better anchorage at greater depths7. These piles are efficiently used in machine foundations, over bridges, electrical transmission tower foundation sand water tanks

REFERENCEIS 2911-1980 (Part 3)

Soil mechanics and foundation Engineering – AroraGeotechnical Engineer – C. Venkata Ramaiah

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