Piling Equipment : On Strong Ground
The importance of pile foundations cannot be over-emphasised. Tall structures face all kinds of heavy dead loads, live loads, wind loads, seismic loads and temperature stresses. Their safe transmission to the ground is possible only through pile foundations. Er. Jagvir Goyal shares details on the kinds of piling equipment.
The importance of pile foundations cannot be over-emphasised. Most specialised 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 the soil condition, 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 compact the soil around them well, as only displacement of soil takes place not its removal. Also, no muck or slushy conditions are created at the site.
Bored cast-in-situ piles are chosen when a study of the soil conditions reveals that it will be better to remove soil from the ground and fill the space 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 by itself without any support. Bored cast-in-situ piles are therefore, suitable in clay soils.
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, the 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.
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 doing away with the work of creating a yard for pre-casting the piles, as well as their curing, stacking, handling and transportation. In this case, the bore hole is created, the reinforcement cage is lowered and concrete is filled in the pile hole. The choice has to be made by keeping all plus and minus points as per site conditions.
For driven piles, the 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 all into account. The hammer energy is regulated to avoid too hard or too light strokes; hard strokes may damage the pile head while light strokes may not produce the desired penetration.
Earlier, simple drop hammers were used to drive piles inside the ground. Then, either a single-acting hammer or a double-acting hammer was used for driving the piles. In the single- acting hammer, a steam or hydraulic system is used to raise the hammer back to top position. The steam or hydraulic pressure is released on raising the hammer and it falls freely under gravity. In the double-acting hammer, however, the fall is forced by the steam or hydraulic system. Therefore, double-acting hammers provide fast piling work. Compressed air is also used instead of steam or hydraulic oil for these hammers.
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 the hammer ram and the fall of hammer, these are adjusted to produce the required amount of energy. A heavy ram-shortfall combination proves better than a light ram-longfall combination.
The only point to be kept in view while fixing the hammer ram weight is that it should not damage the pile head. Otherwise, the heavier the ram, the less is the loss of energy, due to the rebound of hammer. A general method of deciding ram weight is to work out the weight of the pile to be driven and to keep the ram weight almost equal to it.
Differential hammers have the piston in two different diameters. The large diameter piston is kept at the top and the smaller diameter piston at the bottom. These hammers combine the advantages of both single-acting and double-acting hammers. The weight advantage of single-acting and speed advantage of double-acting hammers become combined in them. The 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 in certain circumstances where vibratory hammers prove useful.
More countries are becoming aware of smoke and noise prevention concerns and are passing legislation on them. Piling equipment has, therefore, it needs to be environment-friendly and should produce minimum smoke, noise and dust. In addition, it should cause minimum vibrations, and should have a low centre of gravity to have good stability. Of course, its efficiency, economy in running, lowest maintenance problems and low consumption of oil are all factor that score high with the user or hirer.
Pile-driving rigs move on crawlers and their centre of gravity is kept low. Therefore, they possess good stability. They have the ability to revolve fully in any direction and thus, can operate in a large area around them. Taking the 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 m and width of 4 m, 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 is generally capable of driving piles of 20 m length. A rig should be self-erecting and ready for work in a few minutes. The driver’s cabin is made completely safe with a good view, to allow him to concentrate fully on his work.
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 a 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 cushion blocks are made of wood. Anvil blocks transmit the hammer blow to the pile without damaging it.
The 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 can be done with it. The 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 piles 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
It is very important to stick to the designed layout of piles. Shifting of piles from their designed locations results in different load distribution from the pile cap than the designed distribution. If such a thing happens, it altogether alters the design calculations and some piles may face more loads than its load-carrying capacity (LCC) while full the LCC of another pile may not get utilised. Precautions, therefore, should be taken so that piles are driven exactly at designed locations.
Bored cast in situ piles may or may not need casing pipes. Wherever such strata is encountered during boring which cannot support itself, casing has to be provided. Generally, the presence of a layer of boulder or gravel demands the provision of casing. Also, casing is required for initial depths near ground surface because of the influence of outer loads, moving vehicles or ongoing construction work on the ground near the bore hole. Generally, the casing is provided up to the top of an underlying clay layer so that the borehole is fully secured.
It is not possible to provide temporary casing for the full depth of piles; such provision proves uneconomical. Not only that, it becomes very difficult to pull out temporary casing from greater depths without harming the borehole for the pile. Leaving the temporary casing below the ground also proves uneconomical and causes losses. To avoid such losses, an alternative is to use bentonite slurry in the borehole to hold its walls in position. Bentonite is a type of powder, cream in color and supplied in a 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 acts as a sort of casing for the piles. Bentonite is mixed with water to form a bentonite slurry which is circulated through the boreholes. Bentonite slurry- forming equipment is installed at the site and it keeps drawing water and bentonite from different compartments, mixes them well to desired consistency and supplies them to the pile boreholes.
Use of equipment
A tripod, piling winch, wire rope, the chisel and the bailer make one set of piling equipment. The winch is positioned at a distance while the tripod is centred exactly above the centre point of the pile to be cast in situ. Usually, 22 mm diameter wire ropes are used to avoid its snapping during the pulling out of the chisel or bailer. First, the heavy cylindrical chisel is used to strike repeatedly at the point where the pile is to be provided. It softens the soil. Next, the chisel is replaced by the bailer. It takes out the softened soil and borehole progresses. After achieving a certain depth only, can the bailer be used to take out soil through repetitive strokes. On completion of boring work, the nature of foundation stratum is examined by taking out a sample of soil. After cleaning of loose material from the bore, N values of the foundation stratum are taken. If the N value is more than the desired figure, the reinforcement cage and trimie pipe are lowered in to the borehole and concreting work is started.
Before lowering the trimie pipe into the borehole, the reinforcement cage is lowered into it. Care is taken that the cage is centrally placed, and that the walls of the borehole are not damaged during its lowering, and space for concrete cover is available all around it. The trimie pipe is generally of 200 to 250 mm diameter and is in small, one-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, there is a sliding plug to stop the flow of concrete when 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 activity (II)
The bored cast in situ piling work calls for the following sequence of activities in order to complete a job:
1. Marking the layout of the pile.
2. Driving in the temporary casing pipe.
3. Boring the hole in ground to required depth.
4. Cleaning the bottom of the hole on reaching required depth.
5. Taking N value of the foundation stratum by use of SPT apparatus.
6. Lowering of steel reinforcement cage inside the borehole.
7. Lowering of tremie pipe.
8. Production of concrete.
9. Laying of concrete in the borehole and extraction of tremie pipe.
10. Extraction of casing pipe on completion of concrete work.
11. Trimming of pile head to cut-off level.
It is very important to stick to the designed layout of piles. Shifting of piles from their designed locations results in the possibility of different load distribution from the pile cap, rather than the designed distribution. If such a thing happens, it altogether alters the design calculations and some piles may face more loads than their load- carrying capacity (LCC), while the full LCC of another pile may not get utilised. Precaution should therefore, be taken that piles are bored exactly at the designed locations.
All the muck generated during piling work should be cleared side- by- side, otherwise it poses a major problem and hinders the progress of the piling work. If too much muck is lying at the site, it not only creates problems in marking exact layout of piles but also causes extra concreting above cut-off level, 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 accidents may happen at the site because of the toppling of a tripod.
The area and sequence of moving of tripods should be well designed. The moving path of a tripod should be so kept that it moves away from the other tripod and no interference is caused by one in the other’s work. Also, boring of two adjacent piles should never be done simultaneously.
It is very important to clean the bottom of a borehole 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, the settlement of a pile may jeopardise its integrated action with the pile cap.
Precaution should be taken that during concreting work, the lower end of the trimie pipe always remains embedded in the concrete below. Otherwise, the whole system of concreting underwater will get disrupted.
Piling work companies
Deep-bore piling work is specialised work. Certain companies in India have gained proficiency in it; Simplex, Gammon India and L&T are some major concerns that have their own piling units. Many companies are now coming up exclusively do only piling jobs, with a number of pile- driving rigs in their equipment fleet.
(*Author, technical books; columnist, technical journals; Deputy Chief Engineer Civil, PSEB, Punjab.)
For successful installation
Bored cast in situ piles require the following equipment for successful installation of piles:
• Tripods with pulley arrangement.
• Piling winches.
• Temporary steel casing.
• Driving heads for casing pipes.
• Bentonite slurry equipment.
• Bailers with cast steel flaps.
• Heavy cylindrical chisels/shells.
• 22- mm diameter wire ropes.
• Trimie pipes.
• Funnels for trimie pipe.
• Concreting equipment.
• Concrete carrying hand trolleys.
• Jack hammers for trimming pile heads.
• Air compressors to run jack hammers.
• D-shackles, U clamps, 4-sheave and 2-sheave pulleys.
• Standard penetration test equipment.
• Stability of equipment.
• High stroke pressure.
• Maximum driving efficiency.
• Least noise production.
• Least environmental pollution.
• Least vibration levels.
• Automatically controllable.
• Least maintenance requirements.
• Easily shiftable.
• Operator’s safety.