Well foundation

Introduction

Well foundations are one of the types of deep foundations that provide a solid and massive foundation typically for bridges and heavy structures. Well foundations are also useful for transmission line towers, where uplift loads are large. In earlier practice, well foundations were constructed with stone or brick, but today they are mostly of reinforced concrete. The advantages of well foundations are that they are monolithic and rigid, being a massive sub­structure. They have better lateral load resistance than pile foundations. Well foundations can also be conveniently installed in a boulder stratum as well.

Well foundations had their origin in India and have been used for hundreds of years for providing deep founda­tions for important buildings and bridges. Many Mughal monuments, including the famous Taj Mahal and several bridges, were supported on well foundations. The largest well used in the world in the early part of 20th century is the 73.8-m deep caisson provided for the San Francisco Oakland Bridge, in California. Well foundations have been used for most of the bridges in India. The main towers of the Howrah Bridge were provided with 31-m deep well foundations.

1. Types of well foundation 

Open Caissons:

Open caissons, also called well foundations, are caissons in which the top and bottom of the caisson are open during construction. An open caisson may be circular, rectangular, or oblong in plan. Figure 21.1 shows an open caisson with double rectangular cross section. It has a cutting edge at the bottom, which is fabricated at the site along with the first segment of the shaft. When the well sinks by self-weight, the soil inside the shaft is dredged by suitable means, which aids further sinking into the ground.

The next segment of the shaft is then added to it. The process of sinking by self-weight, as well as by dredging, is continued till it reaches the required depth. The bottom of the well is then sealed with concrete, which also forms the base of the well foundation. The hollow shaft is filled with sand and a concrete seal is provided at the top, known as a top plug. Open caissons can be constructed up to any depth and the cost of construction is relatively low.

The depth up to which the caisson is to be sunk depends on the loads on the caisson, the bearing capacity of the soil, skin friction resistance of the sides, and the minimum grip length to be used below the scour level.

Advan­tages

i. The caisson can be constructed to large depths.

ii. The cost of construction is relatively less than other types of caissons.

Disadvantages

i. Progress of construction in boulder deposits is very slow.

ii. The concrete sealed under water is not very effective.

iii. Inspection of the bottom of the well is not possible.

Box Caissons:

Box caissons are open at the top, but closed at the bottom, as shown in Fig. 21.2. It is first cast on land and then towed to the site, where it is sunk onto a previously leveled foundation base. Sinking of the caisson is facilitated by filling with sand, gravel, or concrete blocks inside the caisson. Box caissons are also called floating caissons, and are used where loads are not very heavy and a bearing stratum is available at shallow depth.

Advantages

i. The cost of construction of box caissons is low.

ii. It can be used where other types of caissons cannot be constructed.

Disadvantages

i. It is difficult to provide the foundation base below the water level, especially for deep excavations.

ii. Bearing capacity of the foundation base has to be properly assessed. Care has to be taken to protect the founda­tion base from scour.

Pneumatic Caissons:

In pneumatic caissons, the internal air pressure of the closed chamber is kept high to prevent water from entering the chamber (Fig. 21.3). The working chamber is thus kept dry to facilitate skilled persons to work in the chamber. Air locks are provided at the top. The caisson is sunk under complete controlled conditions by skilled persons and supervisory staff in the working chamber. The working chamber is filled with concrete after the final depth is reached and sinking of the caisson is completed.

Advantages 

i. There is a complete control over the sinking of the caisson, so that tilts and shifts can be detected immediately by the staff in the working chamber and corrective measures can be taken effectively.

ii. The bottom of the chamber can be sealed effectively as it is maintained under dry conditions.

iii. Obstructions to sinking, such as boulders, can be removed easily.

Disadvantages 

i. Pneumatic caissons are costlier than other types of caissons.

ii. The depth of the caisson below the groundwater table is limited to about 35 m during construction, as the staff in the working chamber cannot withstand a pressure more than 35 t/m2.

2. Grip Length:

The depth of the bottom of the well foundation, below the lowest scour level is known as the grip length. Well foun­dations should be provided with adequate grip length such that the required passive resistance of the soil on the rear side of the well is generated to resist lateral loads. The grip length may be taken as one-third of the maximum scour depth. The depth of the well foundation should not be less than 1.33 times the deepest scour depth below HFL. If a non-erodible stratum, such as rock is available at shallow depth, this depth may be reduced.:

Further, as per IRC – 45-1972, the foundation should be taken at least 2 m below the maximum scour depth for piers and abut­ments with arches, that is, the minimum grip length in such cases is 2 m. The minimum grip length is 1.2 m for piers and abutments supporting other types of superstructures.

3. Construction of well foundation 

It consists of the following stages: 

1. Laying the Cutting Edge 

2. Alignment Control 

3. Construction of Well Curb 

4. Construction of Well Steining 

5. Well Sinking.

Stage # 1. Laying the Cutting Edge:

Well foundations are constructed in stages by sinking under self-weight as well as dredging the soil inside the dredge hole and on the sides outside. The first step in the construction of a well foundation is to lay the cutting edge and well curb. If the river bed is dry, the cutting edge is placed in position after removing the top loose layer of sand.

Otherwise, a temporary structure, known as a sand island, is constructed using cofferdams for the purpose of excluding water, and soil is sufficiently constructed using continuous sheet piles all around the well foundation and filling the space with sand, which serves as a working platform for the work force and equipment. If the water is deeper than about 5 m, the cutting edge and the well curb are fabricated on the shore and towed to the sand island for installation.

Stage # 2. Alignment Control:

The centreline of the wells should coincide with the centreline of the abutments and piers and of the bridge. Masonry pillars are constructed on the centreline of the bridge to serve as station points for checking the alignment of the abutments and piers.

Stage # 3. Construction of Well Curb:

The well curb is assembled on wooden blocks or sand bags placed at suitable intervals so that it does not sink, while assembling the curb. Concreting is done after placing the reinforcement. M15 or richer grade concrete is used for the well curb. The well curb is then allowed to set for a week before sinking is started. The well curb is allowed to sink alone before raising the steining above it.

Stage # 4. Construction of Well Steining:

The grip length of the well is very small at the beginning of the sinking operation and the chances for tilting are more. The steining should not be raised too high in the initial stages of well sinking, which would otherwise lead to increasing the tilting further. Hence, the well curb is first allowed to sink alone and the steining is then raised in small heights of 1.5 m at a time, allowing minimum 24 h before adding the next height of steining.

The steining can be raised in installments of about 3 m, once the well has sunk to a sufficient depth to get a min­imum grip length of 6 m. The steining masonry should be constructed perfectly vertical to ensure vertical sinking of the Well. Straight edges of about 2 m length should be used for this purpose along the outer periphery of the steining at suitable intervals. When the steining is raised by this height, the straight edges are removed and fixed at a higher level and the entire height of the steining should be raised using this procedure.

Stage # 5. Well Sinking:

The soil in the dredge hole is excavated to facilitate sinking of the well. A large-size spade, known as jham, is used for excavation under water. Jham consists of a sector-shaped steel pan with edges connected by a rope and wooden bullies. When Jham is used, excavation is done manually and hence well sinking is slow. Alternatively, automatic grabs or dredgers can be used for excavation operated by a winch and crane. Bell’s dredger is commonly used for sandy soils.

As the sinking proceeds, more depth of the well foundation will be below the scour level, increasing the frictional resistance between the steining surface and the surrounding soil. The steining is loaded with kentledge through a suitable platform and with sand bags piled on it to aid well sinking, overcoming this frictional resistance. The platform for the kentledge is constructed in such a way that it does not obstruct the dredging process. Air and water jets are also used in addition to the kentledge to further overcome the frictional resistance. These jets consist of G.I. pipes of 2.5-5 cm diameter with a nozzle at one end.

Well sinking may stop and tilting may also occur when the cutting edge encounters an obstruction below it. The obstruction can be removed by dewatering the well using pulsometer pumps and blowing of sand into it. The dewatering process should be continuously watched and should be suspended if there is a tendency for tilt­ing of the well. Dewatering also should not be used unless the well has sunk to a sufficient length of a minimum of 9 m.

CAUSES OF FAILURES OF FOUNDATIONS AND REMEDIAL MEASURES.

The foundations may fail due to the following reasons:

1. Unequal settlement of sub-soil. Unequal settlement of the sub-soil may lead to cracks in the structural components and rotation thereof. Unequal settlement of sub-soil may be due to (i) non-uniform nature of sub-soil throughout the foundation, (ii) unequal load distribution of the soil strata, and (iii) eccentric loading. The failures of foundation due to unequal settlement can be checked by : (i) resting the foundation on rigid strata, such as rock or hard moorum, (ii) proper design of the base of footing, so that it can resist cracking, (iii) limiting the pressure in the soil, and (iv)avoiding eccentric loading.

2. Unequal settlement of masonry. As stated earlier, foundation includes the portion of the structure which is below ground level. This portion of masonry, situated between the ground level and concrete footing(base) has mortar joints which may either shrink or compress, leading to unequal settlement of masoray. Due to this, the superstructure will also have cracks. This could be checked by (i) using mortar of proper strength, (ii) using thin mortar joints, (iii) restricting the height of masonry to 1 m per day if lime mortar is used and 1.5 m per day if cement mortar is used, and (iv) properly watering the masonry.

3. Sub-soil moisture movement. This is one of the major causes of failures of footings on cohesive soil, where the sub-soil water level fluctuates. When water table drops down, shrinkage of sub-soil takes place. Due to this, there is lack of sub-soil support to the footings which crack, resulting in the cracks in the building.

During upward movement of moisture, the soil (specially if it is expansive) swells resulting in high swelling pressure. If the foundation and superstructure is unable to resist the swelling pressure, cracks are induced.

4. Lateral pressure on the walls. The walls transmitling the load to the foundation may be subjected to lateral pressure or thrust from a pitched roof or an arch or wind action. Due to this, the foundation will be subjected to a moment (or resultant eccentric load). If the foundation has not been designed for such a situation, it may fail by either overturning or by generation of tensile stresses on one side and high compressive stresses on the other side of the footing.

5. Lateral Movement of sub-soil This is applicable to very soft soil which are liable to move out or squeeze out laterally under vertical loads, specially at locations where the ground is sloping. Such a situation may also arise in granular soils where a big pit is excavated in the near vicinity of the foundation. Due to such movement, excessive settlements take place, or the structure may even collapse. If such a situation exists, sheet piles should be driven to prevent the lateral movement or escape of the soil.

6. Weathering of sub-soil due to trees and shrubs. Sometimes, small trees, shrubs or hedge is grown very near to the wall. The roots of these shrubs absorb moisture from the foundation soil, resulting in reduction of their voids and even weathering. Due to this the ground near the wall depresses down. If the roots penetrates below the level of footing, settlements may increase, resulting in foundation cracks.

7. Atmospheric action. The behaviour of foundation may be adversely affected due to atmospheric agents such as sun, wind, and rains. If the depth of foundaion is shallow, moisture movements due to rains or drought may cause trouble. If  the building lies in a low lying area, foundation may even be scoured. If the water remains stagnant near the foundation, it will remain constantly damp, resulting in the decrease in the strength of footing or foundation wall. Hence it is always recommended to provide suitable plinth protection along the external walls by (i) filling back the foundation trenches with good soil and compacting it, (ii) providing gentle ground slope away from the wall and (iii) providing a narrow, sloping strip of impervious material (such as of lime or lean cement concrete) along the exterior walls.