5. CHOICE OF PILE TYPE AND DESIGN RESPONSIBILITY
5.2 FACTORS TO BE CONSIDERED IN CHOICE OF PILE TYPE
The determination of the need to use piles and the identification of the range of feasible pile types for a project form part of the design process. In choosing the most appropriate pile type, the factors to be considered include ground conditions, nature of loading, effects on surrounding structures and environs, site constraints, plant availability, safety, cost and programme, taking into account the design life of the piles.
Normally, more than one pile type will be technically feasible for a given project.
The selection process is in essence a balancing exercise between various, and sometimes conflicting, requirements. The choice of the most suitable type of pile is usually reached by first eliminating any technically unsuitable pile types followed by careful consideration of the advantages and disadvantages of the feasible options identified. Due regard has to be paid to technical, economical, operational, environmental and safety aspects. A flow chart showing the various factors to be considered in the selection of piles is given in Figure 5.1.
It should be noted that possible installation problems associated with the different pile types should not be the sole reason for rejection as these can generally be overcome by adherence to good piling practice and adoption of precautionary measures, albeit at a cost.
However, from a technical viewpoint, the choice of piles should be such as to minimise potential construction problems in the given site and ground conditions, and limit the risk of possible delays. Delays are especially undesirable where the project owner is paying financing cost.
5.2.1 Ground Conditions
The choice of pile type is, in most instances, affected by the prevailing ground conditions. The presence of obstructions, existing piles, soft ground, depth of founding stratum, cavities, faults, dykes and aggressive ground can have a significant influence on the suitability of each pile type.
Problems caused by obstructions are common in old reclamations, public dump sites, and ground with bouldery colluvium or corestones in saprolites. Driven piles are at risk of being deflected or damaged during driving. Measures that can be adopted to overcome obstructions are described in Sections 8.2.5.4 and 8.3.4.4.
Assess types of structures and foundation loads
Assess ground conditions
Choose shallow foundation types Are piles
necessary?
Technical Considerations for Different Pile Types Ground
conditions (Section 5.2.1
& 5.2.2)
Loading conditions (Section 5.2.3)
Environmental constraints (Section 5.2.4)
Site and plant constraints (Section 5.2.5)
Safety (Section 5.2.6)
Feasibility of reusing existing
piles, if present (Section 5.3)
List all technically feasible pile types and rank them in order of suitability based on technical consideration
Assess cost of each suitable pile type and rank them based on cost consideration
Make overall ranking of each pile type based on technical, cost and programme consideration
Submit individual and overall rankings of each pile type to client and make recommendations on the most suitable pile type
Figure 5.1 – Suggested Procedures for the Choice of Foundation Type for a Site No
Yes
In soft ground, such as marine mud or organic soils, cast-in-place piles can suffer necking unless care is taken when extracting the temporary casing. Construction of hand-dug caissons can be particularly hazardous because of possible piping or heaving at the base.
Machine-dug piles with permanent casings can be used to alleviate problems of squeezing. In these ground conditions, driven piles offer benefits as their performance is relatively independent of the presence of soft ground. However, soft ground conditions may exhibit consolidation settlement which will induce negative skin friction along the shafts of the driven piles. In case the settling strata are of substantial thickness, a large proportion of the structural capacity of the driven piles will be taken up by negative skin friction.
The depth of the founding stratum can dictate the feasibility of certain pile types.
Advance estimates of the depth at which a driven pile is likely to reach a satisfactory 'set' are usually made from a rule-of-thumb which relies on SPT results. The SPT N value at which large-displacement piles are expected to reach 'set' is quoted by different practitioners in Hong Kong in the range of 50 to 100, whilst the corresponding N value for steel H-piles to reach 'set' is quoted as two to three times greater.
Barrettes and large-diameter machine-dug piles are generally limited to depths of 60 m to 80 m although equipment capable of drilling to depths in excess of 90 m is readily available.
5.2.2 Complex Ground Conditions
Parts of Ma On Shan and the Northwest New Territories areas are underlain by marble and marble-bearing rocks. The upper surface of marble can be karstic and deep cavities may also be present. The assessment of piling options requires a careful consideration of the karst morphology.
There are three marble-bearing geological units in the Northwest New Territories areas, including Ma Tin Member and Long Ping Member of the Yuen Long Formation and the Tin Shui Wai Member of the Tuen Mun Formation (Sewell et al, 2000; Frost, 1992). The Ma Tin Member is a massively bedded, white to light grey, medium- to coarse-grained crystalline marble, comprising more than 90% of carbonate rock. Karst features are most strongly developed in this pure marble rock.
The Long Ping Member dominantly comprises grey to dark grey, fine- to medium- grained crystalline marble with intercalated bands of calcareous meta-sedimentary rock.
Karst features in the Long Ping Member are poorly developed. The impure marble contains up to one third of insoluble residues. These residues have the potential to accumulate and restrict the water flow paths that are opened up by dissolution, thus limiting the development of karst features.
Marble in the Tin Shui Wai Member of the Tuen Mun Formation exists as clasts in volcaniclastic rocks (Frost, 1992; Lai et al, 2004). The marble clasts in the volcaniclastic rocks are generally not interconnected. Dissolution of the marble clasts is localised, typically leading to a honeycomb structure of the rock. This structure does not usually develop into the karst features that are common in marble of the Yuen Long Formation. While large cavities are rare in the volcaniclastic rocks, there are in a few occasions where relatively large
cavities were encountered, which could have geotechnical significance to the design of foundation (Darigo, 1990).
Marble in the Ma On Shan area consists of bluish grey to white, fine- to medium- grained crystalline marble. The marble has been assigned to the Ma On Shan Formation (Frost, 1991; Sewell, 1996). Cavities in the Ma On Shan Formation indicate the development of karst features similar to those of the Ma Tin Member of the Yuen Long Formation in Northwest New Territories. The karstic top of the marble has caused significant engineering problems.
In sites traversed by faults, shear zones or dykes, the geology and the weathering profile can be highly variable and complex. Dykes are especially common in the Lantau Granite, Tai Lam Granite and Sha Tin Granite Formations in the western part of Hong Kong (Sewell et al, 2000).
Complex geological ground conditions may also be encountered in the Northshore Lantau. Weathering of granite and rhyolite dykes associated with faulting may lead to a very deep rockhead profile. In some locations, the rockhead is encountered at depths in excess of 160 m below ground level. In addition, large blocks of meta-sedimentary rock embedded within the intrusive rocks, may contain carbonate and carbonate-bearing rock, including marble. Cavities or infilled cavities can be found in these marble blocks. There have been cases where planned developments were abandoned because of the complex geological ground conditions in the Northshore Lantau area (GEO, 2004; ETWB, 2004).
The choice of piles will be affected by the need to cope with variable ground conditions and the feasibility of the different pile types will be dependent on the capability of the drilling equipment or driveability considerations.
Experience in Hong Kong indicates that heavy steel H-pile sections (e.g. 305 mm x 305 mm x 186 kg/m or 223 kg/m) with reinforced tips can generally be driven to seat on marble surface under hard driving. However, pre-boring may have to be adopted for sites with unfavourable karst features such as large overhangs. Large-diameter bored piles have also been constructed through cavernous marble (e.g. Li, 1992; Lee et al, 2000; Domanski et al, 2002).
Precast concrete piles are prone to being deflected where the rock surface is steeply inclined or highly irregular and may suffer damage under hard driving. Most types of driven cast-in-place piles are unsuitable because of difficulty in seating the piles in sound marble.
The use of hand-dug caissons should be avoided because of the risk of sinkholes induced by dewatering and potential inrush of soft cavity infill. Barrettes may be difficult to construct because of the possibility of sudden loss of bentonite slurry through open cavities.
Corrosion of piles should be a particular design consideration in situations such as those involving acidic soils, industrial contaminants, the splash zone of marine structures and in ground where there is a fluctuating groundwater level (Section 6.14). In general, precast prestressed spun concrete piles, which allow stringent quality control and the use of high strength material, are preferred in aggressive or contaminated ground.
5.2.3 Nature of Loading
Pile selection should take into account the nature and magnitude of the imposed loads.
In circumstances where individual spacing between driven piles could result in the problem of 'pile saturation', i.e. piles are arranged in minimum spacing, the use of large-diameter replacement piles may need to be considered.
For structures subject to cyclic and/or impact lateral loading such as in jetties and quay structures, driven steel piles may be suitable as they have good energy-absorbing characteristics.
In the case of large lateral loads (e.g. tall buildings), piles with a high moment of resistance may have to be adopted.
5.2.4 Effects of Construction on Surrounding Structures and Environment
The construction of piles can have damaging or disturbing effects on surrounding structures and environs. These should be minimised by the use of appropriate pile type and construction methods. The constraints that such effects may impose on the choice of pile type vary from site to site, depending on ground conditions and the nature of surrounding structures and utilities.
Vibrations caused by piling are a nuisance to nearby residents and could cause damage to utilities, sensitive electronic equipment and vulnerable structures such as masonry works. Large-displacement piles are likely to produce greater ground vibration than small- displacement and replacement piles.
Construction activities, including percussive piling, are subject to the provisions of the Noise Control Ordinance (HKSARG, 1997). Percussive piling is banned within the restricted hours, i.e. from 7 p.m. to 7 a.m. on weekdays and whole day on Sundays and public holidays. It is only allowed in other times on weekdays provided that the generated noise level at the sensitive receivers does not exceed the acceptance noise level by 10 dB(A) (EPD, 1997). The use of diesel hammers, which are very noisy and prone to emit dark smoke, had been phased out for environmental reasons.
Excavation of hand-dug caissons below the groundwater table requires dewatering.
The resulting ground movements may seriously affect adjacent utilities, roads and structures supported on shallow foundations. Closely-spaced piles below the groundwater may dam groundwater flow, leading to a rise in groundwater levels (Pope & Ho, 1982). This may be particularly relevant for developments on steeply-sloping hillsides, especially where grouting has been carried out, e.g. in hand-dug caisson construction. The effect of rise in groundwater on adjacent underground structures like MTR tunnels, e.g. increase in buoyancy, should also be considered.
Installation of displacement piles will result in heave and lateral displacement of the ground, particularly in compact fine-grained sandy silts and clayey soils (Malone, 1990), and may affect adjacent structures or piles already installed. The use of replacement piles will obviate such effects. Should displacement piles be used for other reasons, prefabricated piles,
as opposed to driven cast-in-place piles, may be considered as they offer the option that uplifted piles can be re-driven.
Spoil and contaminated drilling fluid, for replacement pile construction, especially those arising from reclamation area, cause nuisance to surrounding environment and would need to be properly disposed of (EPD, 1994).
5.2.5 Site and Plant Constraints
In selecting pile types, due consideration should be given to the constraints posed by the operation of the equipment and site access.
Apart from mini-piles, all other piles require the use of large piling rigs. The machine for jacking piles carries heavy weights. These may require substantial temporary works for sloping ground and sites with difficult access.
Headroom may be restricted by legislation (e.g. sites near airports) or physical obstructions such as overhead services. In such case, large crane-mounted equipment may not be appropriate. Special piling equipment, such as cranes with short booms and short rectangular grab, are available to construct barrette piles in area with restricted headroom.
Alternatively, mini-piles will be a feasible option.
The construction of replacement piles may involve the use of drilling fluid. The ancillary plant may require considerable working space. On the other hand, prefabricated piles similarly will require space for storage and stockpiling. These two types of piles may therefore cause operational problems on relatively small sites.
5.2.6 Safety
Safety considerations form an integral part in the assessment of method of construction. Problems with hand-dug caissons include inhalation of poisonous gas and silica dust by workers, insufficient ventilation, base heave, piping, failure of concrete linings and falling objects (Chan, 1987). Their use is strongly discouraged in general.
Accidents involving collapse or overturning of the piling rigs, which can be caused by overloading, swinging loads, incorrect operation, wind gusts or working on soft or steeply- sloping ground, can result in casualties. Serious accidents may also occur when loads swing over personnel as a result of failure of chain or rope slings due to overloading, corrosion or excessive wear.
Notwithstanding the safety risks and hazards involved in pile construction, it should be noted that most of these can be minimised provided that they are fully recognised at the design stage and reasonable precautions are taken and adequate supervision provided.
Vetting of contractor's method statements provides an opportunity for safety measures to be included in the contract at an early stage.
5.2.7 Programme and Cost
The design engineer frequently has a choice between a number of technically feasible piling options for a given site. The overall cost of the respective options will be a significant consideration.
The scale of the works is a pertinent factor in that high mobilisation costs of large equipment may not be cost effective for small-scale jobs. The availability of plant can also affect the cost of the works. Contractors may opt for a certain piling method, which may not be the most appropriate from a technical point of view, in order to optimise the material, equipment and plant available to them amongst the ongoing projects.
The cost of piling in itself constitutes only part of the total cost of foundation works.
For instance, the cost of a large cap for a group of piles may sometimes offset the higher cost of a single large-diameter pile capable of carrying the same load. It is necessary to consider the cost of the associated works in order to compare feasible piling options on an equal basis.
A most serious financial risk in many piling projects is that of delay to project completion and consequential increase in financing charges combined with revenue slippage.
Such costs can be much greater than the value of the piling contract. The relative vulnerability to delay due to ground conditions, therefore, ought to be a factor in the choice of pile type.