Changes affecting construction arise from many sources, at random times and without coordination. It is necessary to re-evaluate construction practice to take account of these factors. Change may be driven by Government through legalisation, by equipment manufacturers, by client demand and many other forces. Foundation piling is in need of re-evaluation.
1. THE PROBLEM
The most commonly used material for foundation piles at the start of twenty-first century is concrete. Of these piles, the majority of 250 mm diameter upwards are formed by a process known as continuous flight auger (CFA). By this process a spiral fluted auger is screwed into the ground to a required depth. The auger is then pulled out of the ground either without rotation or with a small positive rotation (but definitely no reverse rotation) and at the same time pumped concrete is injected through the auger stem to replace the plug of earth being removed. Reinforcing steel comprised of a cage of bars or a single structural profile is usually inserted in the wet concrete after the auger has been removed. This system has evolved for several reasons
it is the lowest cost method of pile installation
the process has been actively promoted by drill rig manufacturers
concrete pumps and pumpable concrete mixes have become commonly available over the last 35 years of the twentieth century
the noise and vibration associated with driven pile installation does not occur with this process.
These apparent merits overlook an environmental catastrophe. Redundant concrete piles cannot be removed in such a way that the site is returned to its original condition. If they are removed by drilling the cost is far higher than installation, it produces contaminated low-grade aggregate of negative value and leaves a hole in the ground where the pile was. In the short term it is easier and cheaper to abandon piles of a previous structure. This process may be repeated two or three times before the site becomes permanently derelict with pile contamination. This situation already exists in many cities. The rate of change to dereliction will accelerate for two main reasons.
Many more structures are being piled where previously they were of strip or raft foundation.
Structures are designed and built for shorter working lives. The cycle of reconstruction comes round faster.
This form of construction needs to be changed as soon as possible in order to prevent further contamination or permanent dereliction. Property owners need to be made aware of the dangers of turning a potentially valuable city centre site into a ‘valueless space’ at the end of a structure's life. For minimal short-term price considerations during construction, massive removal costs will be incurred on redevelopment if that is practical.
It should also be politically and socially unacceptable for a property owner, who is in fact a temporary custodian, to generate permanent dereliction for future generations.
As the number of concrete piles of 250 mm diameter and upwards probably exceeds 200 000 each year in the UK alone, it is appropriate to re-evaluate the suitability of this method of construction relative to the permanent damage that it causes to the environment.
2. THE SOLUTION
There is a tried and proven alternative to the use of concrete piles: steel. Steel can accommodate both high compressive forces on installation and high tensile forces on removal. Steel occupies negligible space, creates negligible displacement of the ground on installation and permits complete or largely complete restoration of soil on removal. A construction site may be reused an indefinite number of times where steel has provided the transfer of structural loads from the superstructure into the substrata.
Steel will also last indefinitely in the oxygen-free environment beneath a structure.
Steel H-piles have been used for working loads of 50 to 400 t. Generally, steel pipe piles cover the range 50 to 1000 t or more. These profiles have traditionally been ‘driven’ using heavy, noisy equipment, which also causes large vibrations. Using engineering techniques and materials that have become generally available over the last 35 years it is possible to configure a steel foundation pile capable of carrying the full range of required loads.
The reasons why steel is not used more commonly as bearing piles are as follows.
Steel is more expensive in the Western hemisphere (Europe and the USA) than in the East. Where steel costs considerably less in the Eastern hemisphere it is more commonly used.
Noise and vibration are associated with the installation of most types of preformed piles, such as steel, precast concrete and timber.
The first issue of global pricing is outside the scope of this paper but needs to be addressed in the appropriate place. The second issue is discussed below.
3. OVERCOMING NOISE AND VIBRATION CONCERNS
The second issue of noise and vibration has already been the subject of 35 years of engineering development. Jacking piles into the ground using hydraulic cylinders is a well established and very satisfactory method. It is silent, vibrationless and pile resistance is measured easily. It is a commonly used practice when retrofitting piles to an existing structure where settlement is a risk or has occurred. It is also used in steel sheet-piling where a row of driven sheet piles provide the ‘reaction’ for driving further piles. This process is not reliable in all ground conditions.
In the range of higher loadings, a steel pipe pile is replaced by a ring of Larssen steel sheet-piles, connected together using Omega bars. There is a wide variety of sheet-pile profile widths, depths and thickness in regular production.
By using multiple sheet-piles in a box or circular configuration, it is possible to drive one element using the others for reaction. Each sheet pile would have a hydraulically operated clamp attached to its top. The clamp would be connected to a double acting hydraulic cylinder of, for example, 200 t capacity. This enables the piles to be pushed, as well as pulled, along its axis. The 200 t capacity cylinders are ganged together so that they transfer the reaction forces to the driving force and vice versa.
Further variations are achievable with a push/pull driving machine. In place of interlocked sheet piles, individual H-piles may be gripped and driven by each cylinder in a close group.
During driving, each pile, clamp and cylinder is driven or pressed downwards forcing the pile into the ground by whatever stroke length the 200 t cylinder has, e.g. 750 mm. On completion of this movement this pile becomes a static reaction pile and another pile becomes the driven pile. When every pile in the ring has been installed by 750 mm, the body of the driving machine then lowers on all cylinders and the driving cycle begins again.
Additional pile driving reaction may be derived from the mass of the piling rig, which could weigh 80 t and provide up to 30 t of down crowd. This is particularly useful on start-up when the pile and driving head may only weigh 15 t, which may be insufficient for initial penetration.
The push/pull concept of driving piles works very well in cohesive soils where high skin friction develops good reaction forces. It is not reliable in granular material where high end bearing resistance and low skin friction result in reaction piles pulling out against a static driven pile.
This limitation may be overcome using vibration of an appropriate frequency. Vibrations currently used in the construction industry are suitable for driving piles in granular material up to SPT of 50. Construction equipment vibrators use contrarotating eccentric weights. These need to be mounted in ball or roller bearings. As bearing life reduces as an exponential function of the increase in speed of revolution, there is a practical maximum frequency between 2500 and 3000 cycles per minute (41–50 Hz). These and lower frequencies travel through the ground and can cause disturbance up to 100 m away.
This damaging side effect can be avoided by the use of a different form of vibrator known as an ‘actuator’. This is commonly used in the materials testing industry. By this means it is possible to generate higher frequencies than those achievable by the current industry method of rotating eccentric weights. Pile driving at this frequency is very effective in a wider variety of ground conditions. Also, the vibrations of high frequency and low amplitude decay rapidly, avoiding collateral damage or disturbance.
Other engineering tools are available to assist the push/pull driving process. Electro-osmosis may be used to lubricate a pile being driven through cohesive soils of low water content. This process uses a low voltage (110 V maximum) and a light direct current of, for example, 5 A. The pile being driven is wired as the cathode and a previously driven pile becomes the anode. The cell created produces water at the cathode, thereby lubricating the pile during driving. On completion, the polarity is reversed in order to revert to normal. The soil will also revert to normal by natural means.
4. EXTRACTION
Withdrawal of steel sheet box piles or individual H-piles can be done with the same machine that installed them. Bonding of steel to soil over a period of years may make extraction more difficult. This is counteracted by the absence of end bearing, or resistance, on the pile being extracted. End bearing also benefits the reaction piles during extraction.
5. SUMMARY OF ADVANTAGES/DISADVANTAGES
Piles or pile groups having working load capacities of 1500 kN to 15 000 kN can be installed with the following benefits
very low noise emissions
very low ground vibration
no soil removal to landfill
no ground heave
piles are removable, returning the site to as near ‘as found’ as possible
extraction at the end of the structure life is relatively low cost and simple
the material, once extracted, can be reused or recycled
the load carrying capacity of each pile can be measured as a product of installation by hydraulic jacking
materials to site are lower mass, lower volume than comparable materials
immediate load carrying is possible.
The disadvantages may be
possibly, though not necessarily, a higher supply and installation price may arise
material is supplied in predetermined lengths and some wastage may arise (waste can be recycled)
some site ground conditions will not be suitable for this type of installation e.g. boulders, obstructions, etc.–these can also be dealt with but at some loss of system benefits (e.g. noise and vibration).
6. CONCLUSION
Society needs to decide whether the issue of dereliction and removal of a structure needs to be considered and valued at the design stage. If the cost of removal of piled foundations is valued from the outset, either due to the commercial cost of permanent dereliction or due to environmental sustainability considerations, the whole life cost of a piled foundation will almost certainly be lower using steel than any other material.
