Pile Heave
This is the phenomenon in which a previously installed pile is lifted by upward movement of the soil surrounding it caused by the driving of an adjacent pile. It occurs in saturated clayey and silty soils and does not occur in clean sand. It only occurs with driven displacement type piles and not with bored piles.
Where pile heave lifts the whole pile it is generally thought that bearing capacity is not affected materially. In some cases, however, pile heave has been found to be detrimental to the bearing capacity of the pile. This is believed to be due to the separation of the shaft from the base due to insufficient tension transfer mainly caused by low bond figures in the green concrete. By test loading a pile that has heaved one can establish whether the heave has been detrimental to the pile's bearing capacity. If this proves negative then the problem can be ignored although any simple measures to reduce the heave are nevertheless advisable.
Predrilling or coring a pile as discussed in the above VARIATION TO INSTALLATION TECHNIQUE section can largely reduce the amount of heave because displacement takes place over a reduced depth. If predrilling or coring is carried out for the full pile depth then the risk of shaft-base separation is largely eliminated. Heave may still take place but this will lift the base of the pile and, from experience, is not usually detrimental to the pile's bearing capacity.
Another simple measure that can reduce or eliminate pile heave is to leave the pile for three to four days before driving the pile immediately alongside. This time delay enables the bond between the reinforcing bars and the concrete to build up. Welding a shear key to the steel to improve the bond is also a possibility.
When considering the use of a driven displacement pile in saturated cohesive soils the number and spacing of the piles in the group is an important consideration. A single tank base with a large number of piles in one group is a real potential problem and more suited to a bored pile solution. An open warehouse type structure with groups of two and three piles should not present a major problem. The spacing of the piles in the pile cap should be increased if pile heave is a potential problem.
Vibration During Driving and Forming of the Enlarged Base
The act of driving a pile causes a certain amount of vibration. In general the smaller the pile the smaller the energy applied and the less the vibration. Exceptions to this rule have occurred and are thought to occur when the frequency of driving is resonant with the natural frequency of the ground. Normally, however, the vibrations experienced with a Light pile would be much lower than those experienced with a Heavy.
The vibration levels generated by the Franki system are generally not that severe. They can and have resulted in minor cracks forming in buildings and the extension of existing cracks. The discomfort of feeling the vibration is not normally a problem but the longer the contract the more sensitive people become.
Measures such as predrilling, jetting and coring can be used to reduce the levels of vibration. The vibration experienced when forming the base is however always there but generally of a lower level than when driving because the energy levels are lower.
Contracts close to residential buildings should be avoided unless the piles are predrilled, jetted or cored. The more sensitive parts of city centres should also be avoided.
Noise Pollution
Noise levels are not much above that of the main engine noise. There is the thump from the hammer impact but this takes place inside the tube so is fairly muffled. The odd clang of a wire rope hitting the side of the mast does not seem to worry people. Noise is generally not a problem with the Franki system.
Artesian Conditions
The risk of artesian conditions is very low but they are known to occur. When a cast-in-situ pile is formed through an artesian layer the ground water which is under pressure tends to travel up the side of the pile and in so doing washes out the cement in the concrete. The amount of defective shaft resulting from this action will depend on how strong the water source is. In the one recorded case of artesian conditions the effect on the pile shafts was serious. Artesian conditions should be identified and reported by the drilling operator during the geotechnical investigation. This condition should then be fully investigated. A Franki pile with a precast shaft or a permanent casing or one of the preformed piling systems could provide a better choice of pile type if artesian conditions are present.
7.2 DRIVEN TUBE PILES
This system is not used extensively mainly due to the high cost of the steel tubes. There are, however, situations where the positive features of the system outweigh the costs. Small pile sizes are commonly used for underpinning houses and light buildings with limited headroom and poor access. Medium pile sizes are commonly used for piling new column foundations within existing buildings or in difficult access areas. The larger sizes are used mostly for river bridge foundations and in marine construction.
Positive features
• There is an extensive range of pile sizes.
• The system can achieve considerable depths ( >60 metres in a suitable profile ). • The pile is permanently cased and thus ideal for river and marine work.
• It can be installed in limited headroom.
• It can be installed in areas with very difficult access. • The shaft is cast in the dry so quality control is good. • Noise levels are not high.
Negative Features
• It is a relatively expensive system.
• There is vibration associated with the driving of the tube.
Steel piling tubes can be installed either open ended or closed ended. With the closed ended technique the toe end of the piling tube is sealed off with a steel plate so that there is full displacement during driving. It is this more common technique which is covered in this section. The use of open ended steel tube piles is associated with temporary staging structures as well as marine work and is not covered in this text due to its specialised nature.
PILE DETAILS
The working load shaft stress is generally in the range 8 to 10 MPa. Shaft stresses of up to 16
MPa have been used on deep piles where there is a significant friction component and the piles are driven onto a competent founding stratum. The strength of the shaft concrete has to be raised in line with the higher shaft stress.
Pile Diameter 150 250 400 500 600
Typical working load (kN) 125 400 1000 1750 2500 Typical max. depth (m) 12 20 40 50 50 Pile spacing c/c (mm) 500 750 1200 1500 1750 Maximum rake 1:4 1:4 1:4 1:4 1:4