Analysis Report 1

Deep Foundation System

A foundation transfers the Structural load reliably from a building into the ground. A large building requires a deeper foundation considering the weight of the concrete and the heavy structure above it. The foundation system reaches through the layers of soil to the rock to counter the massive load. Foundation design is a highly specialized field of geotechnical engineering. The requirements of a good deep foundation system are noted below:

1. The foundation including the underlying rock must be safe to prevent any structural failure and collapse.

2. The foundation must not or show any sign of settlement during its life to prevent any damage to the structure or impair its function.

3. The foundation should not have adverse effects to the surroundings.

Socketed Caissons and End Bearing piles are two of the best examples of deep foundation system utilized for high-rise buildings.

Socketed CaissonsCaissons are concrete cylinders poured into drilled into the hard stratum and transfers the load primarily by friction by friction between soil and the caisson itself. Sometimes temporary steel casing is provided to prevent flooding from ground water during the construction the caisson hole.

Socketed Caisson

1. Materials and equipment required to construct

Socketed caissons are concrete cylinders which have to be drilled into holes which can be hand dug to achieve a required bearing area. Large Auger Drills are required for drilling as shown the picture on the right. Steel casings are also provided to prevent ground water flooding into the hole. The hole is filled with concrete. Reinforcing is seldom used with the concrete filling with steel rods.

2. Suitability

Socketed caissons are mostly used in areas where the shallow foundation is not an option.The structure is a big building with heavy weight where the caissons are the most suitable deep foundation system employed.The Soil in the area has high bearing capacity and a rock bed is present to support the structure.There is little or no run off water or seepage. The caissons are made of concrete and continuous exposure could lead to corrosion and collapse.The socketed caissons mostly rely on the friction between soil and rock and the sides of the caisson for strength and support. Hence, these systems are

suitable only at those places where the soil’s adhesive forces comes into play to promote friction.

3. Sequence of Installation

1. A Geotechnical investigation and analysis of the data obtained is essential to decide the type of foundation that has to be used.

2. The accuracy of setting out of piles is a critical initial step which must be carried out to the accuracy stipulated in the specifications.

3. Temporary casings if any are provided.4. Holes are drilled or done manually depending on soil characteristics.5. Large Augers are used with sharp teeth edges to cut through the soil

and even rock if required.6. The Augers experience cracking when a complimentary depth is

reached. This marks the end of drilling.7. The Concrete is poured into the hole and allowed to settle preventing

any runoff or water inflow.

4. Potential problem areas

The potential problems related to socketed caissons are encountered mostly during the construction phase.

Sometimes, before the concrete hardens a vertical subsidence of 3 – 4 inches is witnessed in the middle of the caisson leading to anchor bolts getting misplaced. This happens due to unconsolidated concrete getting consolidated due to the weight of the structure and mass of concrete above it forces air from lower portion of the caisson. This is an immediate problem faced mostly.

Methods of correction and repair suggested include grouting, addition of steel in core holes, partial removal and replacement, complete removal and replacement and additional caissons with transfer girders. The importance of adequate inspection and safe construction procedures has been emphasized.

End Bearing Piles

End bearing (steel or precast) piles are distinguished forms of caisson. They are driven into the soil rather than a distinct hole filled with concrete. They are used where non-cohesive soil, subsurface water condition or excessive depth of bearing strata make caissons impractical. The piles are ex-situ manufactured or retrieved and driven into the earth with repeated blows by a heavy mechanical hammer. The end bearing pile when driven experiences a certain resistance from the layer of rock beneath.

1. Materials and Equipment required to construct

Pile hammer and a hydraulic pile driver are the basic essential equipment required for end bearing piles. Most of the friction is developed at the base of the pile bearing on hard layer. Different types of piles can be used which are ex-situ manufactured and drilled straight into the ground until the “point of refusal”.The piles are driven with the help of these large machines which work on hydraulic power for extra strength with a heavy piston mechanism and hence inserted deep down the hard soil through the soft soil above.

2. Suitability

The end bearing piles are mostly suited for that that have soft compressive layer above and a hard incompressible layer below. They are used where non-cohesive soil, subsurface water condition or excessive depth of bearing strata make caissons impracticalAn analysis of the soil layers confirms what kind of pile work is to be done. If the toe of the pile is supposed to face a hard strata of soil such as rock, end bearing piles give the best results.

3. Sequence of Installation

1. A geotechnical analysis of the soil is done and the enginer determines the type of foundation system required.

2. Since the piles are pre cast, the strength of the piles is determined by a structural engineer.

3. With the help of a pile driver hammer, the end bearing piles are driven to the ground in to a depth of 150 feet (variable) to a pint of refusal. It is the point where little or no penetration is made with the hammer signifying that the pile is embedded firmly into the ground.

4. Potential Problems

1. Piling problems in a typical limestone bedrock.

The potential problem in a typical limestone bed rock is the biggest problem end bearing pile face. The presence of sinkholes, cavity and residual soils makes the limestone bedrock corrugated. The rock being the base of the pile finally results inr settlement or collapse.

2. Damage to pile due to poor workmanship

The Piles are driven deep into the ground with the help of strong hydraulic powered psitons which sometimes lead excessive stress in the pile resulting in cracks due to tention.Poor workmanship of the pile can also leave small cracks which while hammering can lead to failure of the pile.

3. Other Problems that remain undetected

The pile heads sometimes get tilted and due the soft compressive soil on the upper layers. This leads to crushng of the head.The hard strata below gets crushed due to the immense force applied because of the pile hammer.

Seepage, runoff or other nearby water sources oxidise the stone which turns out to be reactive to water and may corrode. This leads to collapse of the pile due to absence of a strong base.The pressure from pile driver leads to bending under stress leading to unequal setllement.

Conclusion

Overall, Socketed Caissons are mostly used where the soil is uniform throughout and the cohesive forces of the soil are not predominant whereas the End bearing piles are used when the top layer of soil is soft and compressive and the botton layer is hard and incompressive. The toe of the end bearing has enough strength to drive through the layers of such incompressive soil.

The socketed caissons require a hole to be drilled which can be done with manually which is not the case in piles. The holes of caisson is filled with concrete and allowed to settle. In case of piles the ex-site or in-situ manufactured piles are driven straight into the ground to a point of refusal. This saves time and cost for piles.

The piles have reported immediate potential problems during construction like problems with limestone beds, corrosion due to reaction with water, cracking due to bending stress. The socketed caissons are more reliable considered to piles.

The piles need to be manufactured and need a lot of skilled workmanship. Any minute error might lead to breakage and collapse. The socketed caissons system do not require skilled workmanship. The high powered machines make work easier and less complicated.

Submitted By:Sourav SinhaID: 916830