Note: This highly informative and elaborate paper was written by the Author many years back and holds valuable information. Readers should update themselves with the latest developments that might have occurred after the presentation of this paper.
Most of the special structures such as tall RCC Chimneys, natural draught cooling towers and high rise buildings require pile foundations to bear and transform various kinds of loads safely to the ground. Tall structures face all kinds of heavy dead loads, live loads, wind loads, seismic loads, temperature stresses. Due to the large height involved, wind and seismic loads are prominent and need special attention. Their safe transmission to the ground is possible only through pile foundations.
Type of piles: Depending upon the soil conditions met at the site of construction, engineers have devised many types of pile foundations to provide a safe base for the structures. Whatever be the soil conditions, the challenge lies in countering the adverse conditions instead of abandoning a site. Among various kinds of pile foundations, bored cast in situ piles and driven piles are the most commonly used ones.
Driven piles: are generally provided when a study of soil conditions reveals that it will be better to displace the soil in the ground instead of removing it. Another advantage of driven piles over bored piles is that these piles well compact the soil around them as only displacement of soil takes place, not its removal. Also, no muck or slushy conditions are created at site.
Bored cast-in-situ piles: are chosen as the right kind of foundations when a study of soil conditions reveals that it will be better to remove soil from the ground and fill the space so created with concrete instead of displacing the soil and driving in the piles. The type of soil plays a significant part in deciding the type of piles to be used. If the soil is cohesive, the bore made in it can stand itself without any support. Bored cast-in-situ piles are therefore suitable in clayey soils.
Soil suitability: The type of soil plays a significant part in deciding the type of piles to be used. If the soil is sandy or granular, it can be easily displaced and compacted. Driven piles are therefore more suitable in sandy soils. Sometimes, position of existing structures also plays a role in selection of type of piles. During the driving of driven piles, heavy stresses or vibrations get induced in the surrounding area. If the existing structures are in close proximity of the construction site, bored cast in situ piles gain preference over driven piles.
Site advantages: Driven piles help in early completion of foundation work as piles can be cast and stored in a separate yard. However, these piles need to be designed for handling and transportation stresses. Handling loads are important and shouldn’t be taken lightly to avoid additional stresses in piles. Bored Cast in situ piles have the advantage of avoiding all the job of creating a yard for pre casting the piles, their curing, stacking, handling and transportation. In their case, the bore hole is to be created, the reinforcement cage is to be lowered and concrete is to be filled in the pile hole. Choice has therefore to be made by keeping all plus and minus points as per site conditions.
Driven piles equipment: For driven piles, hammering method is used to drive piles inside the ground. If the piles are pre cast, the hammer strikes the head of the pile. If the driven pile is cast in situ, the hammer strikes the shoe provided at the bottom of the casing pipe. The free fall of the hammer is controlled through a guide provided for the purpose. The weight of the hammer is worked out by taking the weight of pile, soil conditions and free fall distance in to account. The hammer energy is regulated to avoid too hard or too light strokes of hammer. Hard strokes may damage the pile head while light strokes may not produce desired penetration.
The Hammers: Earlier, simple drop hammers were used to drive piles inside the ground. Then, either Single acting hammer or double acting hammer was used for driving the piles. In single acting hammer, a steam or hydraulic system was used to raise the hammer back to top position. The steam or hydraulic pressure was released on raising the hammer and it fell freely under gravity. In double acting hammer however, the fall was also forced by steam or hydraulic system. Therefore, double acting hammers provided fast piling work. Compressed air has also been used instead of steam or hydraulic oil for these hammers.
The strokes: In single acting hammers, the number of strokes is around one per second. In double acting hammers, the number of strokes can increase from 60 per minute to 100 per minute. As the energy required to drive the pile depends on the weight of hammer ram and the fall of hammer, these are to be adjusted to produce required amount of energy. A heavy ram-short fall combination proves better than a light ram-long fall combination.
Ideal ram weight: Only point to be kept in view while fixing hammer ram weight is that it should not damage pile head. Otherwise, heavier is the ram, less is the loss of energy due to rebound of hammer. A general method of deciding ram weight is to work out the weight of pile to be driven and to keep ram weight almost equal to it.
Differential acting hammers: Differential hammers have the piston in two different diameters. Large diameter piston is kept at the top and smaller diameter piston at the bottom. These hammers combine the advantages of both single acting and double acting hammers. Weight advantage of single acting and speed advantage of double acting hammers gets combined in them. Number of strokes by these hammers may be as high as 140 per minute. All differential acting hammers use hydraulic oil for operation. Today, hydraulic differential acting hammers have taken over the piling scene except certain circumstances where vibratory hammers prove useful.
Essential features of Piling Equipment: Good pile driving equipment should have the following essential features-
1. Stability of equipment.
2. High stroke pressure.
3. Maximum driving efficiency.
4. Least noise production.
5. Least environmental pollution.
6. Least vibration levels.
7. Automatically controllable.
8. Least maintenance requirements.
9. Easily shift able.
10. Operator’s safety.
More and more countries are becoming aware of smoke and noise prevention concerns and are passing legislations to avoid them at site. Piling equipment has therefore to be environment friendly and should produce minimum smoke, noise and dust. In addition, it should cause minimum vibrations, should have a low centre of gravity to have good stability. Of course, its efficiency, economy in running it, least maintenance problems and low consumption of oils come above all for the user or hirer.
Pile driving rigs: Pile driving rigs move on crawlers and their centre of gravity is kept low. Therefore these possess good stability. These have the ability to revolve fully in any direction and thus can operate in a large area around them. Taking example of a medium weight piling rig, it is able to drive both steel and concrete piles in to the ground, has a crawler length of 5 metres and width of 4 metres, ram weight of about 5 to 10 tonnes, engine power of 240 KW, hydraulic tank volume of 650 to 700 litres, fuel tank volume of 400 to 450 litres and generally capable of driving piles of 20 metres length. A rig should be self erecting and ready to work in a few minutes. Driver’s cabin is made fully safe with complete view to allow him to concentrate fully on his work.
Rig accessories: Pile driving rigs need some accessories such as Pile driving heads, Anvil blocks and cushion blocks to carry out the job effectively. Made of cast steel, pile driving heads transfer the hammer blow to the pile head in an uniform manner. Cushion blocks provided over pile heads soften the impact of hammer on the head though full energy is transferred to the pile for penetration. Mostly these are made of wood. Anvil blocks transmit the hammer blow to the pile without damaging it.
A Heavy duty piling rig: To have an idea of the parameters, here are the major features of a heavy duty pile driving rig designed to drive 36 metre long concrete or steel piles into the ground-
Ram weight: 10 tonnes.
Maximum capacity ( pile + hammer): 35 tonnes
Engine power: 400 hp
Fuel tank volume: 850 litres
Hydraulic tank volume: 1100 litres
Counterweight: 12.8 tonnes
Crawler length: 5.7 m
Crawler width: 5.0 m
Total weight (without hammer, counterweight): 83 tonnes
A piling rig should have the versatility of replacing its pile driving hammer with a rotary drill head so that both, the boring as well as driving work could be done with it.
Sequence of activities: The piling work shall require the following sequence of activities to complete a job—
1. Marking the layout of the pile.
2. Positioning the piling rig above the pile point.
3. Driving in the casing pipe (for cast in situ piles) to required depth.
4. Lowering of reinforcement cage (for cast in situ piles)
5. Production of concrete (for cast in situ piles)
6. Laying of concrete in the bore-hole and extraction of casing pipe.
7. Trimming of pile head to cut off level.
8. Transportation of pre cast pile to site. (for pre cast piles)
9. Positioning of the pile above pile point.
10. Driving in the pile.
11. Checking the alignment of pile.
Checklist for piling work: Most important factor to be kept in view while providing driven piles is that the piles are provided exactly as per designed alignment. Mostly, vertical plies are used. It has therefore to be seen that the piles don’t get inclined to vertical. Wherever batter piles are to be provided, these are to be driven exactly at the designed inclination to the vertical. Maximum tolerance limits should never be exceeded.
Precautions during piling work: It is very important to stick to the designed layout of piles. Shifting of piles from their designed locations results in their facing different load distribution from the pile cap than the designed distribution. If such a thing happens, it altogether alters the design calculations and some pile may be facing more load than its load carrying capacity (LCC) while full LCC of another pile may not get utilized. Precaution should therefore be taken that piles are driven exactly at designed locations.
Bored cast in situ piling equipment: Bored cast in situ piles may or may not need casing pipes. Wherever such a strata is encountered during boring which can not support itself, casing is required to be provided. Generally, presence of a layer of boulder or gravel demands provision of casing. Also, casing is required for initial depth near ground surface because of influence of outer loads, moving vehicles or ongoing construction work on the ground on the bore hole. Generally, the casing is provided up to the top of an underlying clay layer so that the borehole is fully secured.
Bentonite Slurry: It is not possible to provide temporary casing for the full depth of piles. Such a provision proves uneconomical. Not only that, it becomes very difficult to pull out temporary casing from greater depths without harming the bore hole for the pile. Leaving the temporary casing below the ground itself proves uneconomical and causes losses. To avoid such losses, an alternative is to use Bentonite slurry in the bore hole to hold its walls in position. Bentonite is a type of powder, cream in color and supplied in dry form in jute or plastic bags. It has great cohesive properties and sticks to the walls of the borehole thus saving it from collapse. It therefore acts as a sort of casing for the piles. Bentonite is mixed with water to form a ‘Bentonite slurry’ which is circulated through the bore holes. Bentonite slurry forming equipment is installed at site and it keeps drawing water and bentonite from different compartments, mixes them well to desired consistency and supplies to the pile bore holes.
Piling Equipment: Bored cast in situ piles require following equipment for successful installation of piles:
1. Tripods with pulley arrangement.
2. Piling Winches.
3. Temporary steel casing.
4. Driving heads for casing pipes.
5. Bentonite slurry equipment.
6. Bailers with cast steel flaps.
7. Heavy cylindrical chisels/shells.
8. 22 mm diameter wire ropes.
9. Trimie pipes.
10. Funnels for trimie pipe.
11. Concreting equipment.
12. Concrete carrying hand-trolleys.
13. Jack hammers for trimming pile heads.
14. Air compressors to run jack hammers.
15. D-shackles, U clamps, 4-sheave and 2-sheave pulleys.
16. Standard penetration test equipment.
Use of piling equipment: A tripod, piling winch, wire rope, the chisel and the bailer make one set. The winch is positioned at a distance while the tripod is centred exactly above the centre point of pile to be cast-in-situ. Usually 22 mm diameter wire ropes are used to avoid its snapping during the pulling out of chisel or bailer. First, the heavy cylindrical chisel is used to strike repeatedly at the point where pile is to be provided. It softens the soil. Now, the chisel is replaced by the bailer. It takes out the softened soil and bore hole progresses. After achieving certain depth, only the bailer can be used to take out soil through repetitive strokes. On completion of boring work, the nature of founding stratum is examined by taking out a sample of soil. After cleaning of bore of loose material, N Values of the founding stratum are taken. If N value is more than the desired figure, the reinforcement cage and trimie pipe are lowered in to the bore hole and concreting work is started.
Before lowering the trimie pipe in the bore hole, the reinforcement cage is lowered in it. Care is taken that the cage is centrally placed, walls of bore hole are not damaged during its lowering and space for concrete cover is available all around it. Trimie pipe is generally of 200 to 250 mm diameter and is in small 1 metre pieces that are screwed to one another. On the top of trimie pipe, a funnel is provided to receive the concrete. Inside the funnel, a sliding plug is provided to stop flow of concrete till the funnel is full. When the funnel is full of concrete, the plug is pulled out and the whole mass of concrete flows down the trimie pipe as one mass. Reaching the bottom, it displaces water and settles there. Water can be seen flowing out at the top around the trimie pipe. The process continues till concrete starts coming out instead of water. It is a signal that the pile has been fully concreted.
Sequence of activities: The bored cast in situ piling work shall require the following sequence of activities to complete a job—
12. Marking the layout of the pile.
13. Driving in the temporary casing pipe.
14. Boring the hole in ground to required depth.
15. Cleaning the bottom of the hole on reaching required depth.
16. Taking N value of the founding stratum by use of SPT apparatus.
17. Lowering of steel reinforcement cage inside the bore hole.
18. Lowering of tremie pipe.
19. Production of concrete.
20. Laying of concrete in the bore-hole and extraction of tremie pipe.
21. Extraction of casing pipe on completion of concrete work.
22. Trimming of pile head to cut off level.
Designed layout: It is very important to stick to the designed layout of piles. Shifting of piles from their designed locations results in their facing different load distribution from the pile cap than the designed distribution. If such a thing happens, it altogether alters the design calculations and some pile may be facing more load than its load carrying capacity (LCC) while full LCC of another pile may not get utilized. Precaution should therefore be taken that piles are bored exactly at designed locations.
Removal of muck: All the muck generated during piling work should be cleared side by side otherwise it poses a major problem with the progress of piling work. If too much muck is lying at site, it not only creates problems in marking exact layout of piles but also causes extra concreting above cut off level of piles thereby resulting in wastage of concrete. Not only this, sometimes, the slush so generated makes it difficult to keep the tripods in a stable position and accident may happen at site because of toppling of a tripod.
Sequence planning: Area and sequence of moving of tripods should be well designed. The moving path of each tripod should be so kept that it moves away from the other tripod and no interference is caused by one in other’s work. Boring of two adjacent piles should never be done simultaneously.
Pile base: It is very important to clean the bottom of a bore hole of all the muck and loose material before start of concreting to avoid false settlement of piles. A pile resting on loose soil will show immediate settlement till it compacts the loose material below on application of load on pile. This settlements may create confusion and doubt about the load carrying capacity of piles. Moreover, settlement of a pile may jeopardize its integrated action with pile cap.
Trimie pipe end: Precaution should be taken that during concreting work, lower end of trimie pipe always remains embedded in concrete below. Otherwise the whole system of concreting under water will get disrupted.
Piling work companies: Deep bore piling work is a specialized work. Certain companies in India have gained proficiency and specialization in it. Simplex, Gammon India ltd and L&T are some major concerns that have their own piling units. Many companies now coming up exclusively do the piling jobs only and have many pile driving rigs in their equipment fleet.