8. PILE INSTALLATION AND CONSTRUCTION CONTROL
8.4 INSTALLATION OF HAND-DUG CAISSONS
The construction of hand-dug caissons has been described in detail by Mak (1993) and outlined in Section 4.4.3.
Guidance notes on standard good practice on the construction of hand-dug caissons are published by the Hong Kong Institution of Engineers (HKIE, 1987). This document covers key aspects of construction considerations as well as supervision and safety.
8.4.2 Assessment of Condition of Pile Base 8.4.2.1 Hand-dug caissons in saprolites
For hand-dug caissons founded in saprolites, insitu tests that can be carried out to assess the condition of the founding material upon completion of excavation include plate loading tests (Sweeney & Ho, 1982) and continuous penetration tests using a GCO probe (a lightweight probing test) (Evans et al, 1982). Ku et al (1985) suggested that at least three penetration tests should be made in the base of each hand-dug caisson to assess the degree and depth of any softening.
In carrying out the GCO probing test, standard equipment and testing procedure as detailed in Geoguide 2 : Guide to Site Investigation (GCO, 1987) should be adopted. The tests should be undertaken to at least 1 m below the pile base and the results reported as the number of blows for each 100 mm penetration (designated as the GCO probe blow count, Np).
Evans et al (1982) suggested that Np is roughly equivalent to SPT N value. This approximate correlation enables an assessment of whether the base condition is consistent with the design assumptions.
Core drilling may be carried out through tubes cast into a pile with the use of a triple tube core barrel to assess the condition of the base interface. The coring is typically extended to not less than 600 mm below the pile base. It is important that attention is given to the use
of an adequate flushing medium and its proper control for success in retrieving the core.
8.4.2.2 Hand-dug caissons in rock
The discussion given in Section 8.3.3 concerning machine-dug piles founded in rock is also relevant to hand-dug caissons. Thomas (1984) suggested that closed circuit television inspection can be carried out to confirm the interface condition for hand-dug caissons.
For hand-dug caissons bearing on rock, the base should be inspected to examine if there are sub-vertical seams of weaker rock or weathered material. Where present, these should be excavated to sufficient depth below the bottom and the local excavation plugged with suitable grout or concrete, prior to commencement of concreting of the pile shaft.
8.4.3 Potential Installation Problems and Construction Control Measures 8.4.3.1 General
There are a number of case histories in Hong Kong involving the use of hand-dug caissons in unfavourable ground conditions. In these cases, the hand-dug caissons were abandoned part way through the contract and replaced with an alternative pile type (Mak et al, 1994).
Potential problems during concreting relate to the quality of the concrete and adequacy of the reinforcement cage, together with the procedure of concrete placement.
Reference may be made to Section 8.3.5.
8.4.3.2 Problems with groundwater
The construction of a hand-dug caisson below the groundwater table might induce piping failure (i.e. hydraulic base failure). In coastal reclamation sites where the groundwater table is high and soft or loose superficial deposits extend to considerable depths, excessive inflow and bore instability may occur, leading to ground loss and settlement around the site (Mackey & Yamashita, 1967b), and possible casualties within the hand-dug caissons.
Sudden base failure, probably due to an excessive differential hydraulic head between the outside and the inside of the excavation has also been observed in very dense granitic saprolites with average SPT N values of about 70 to 80 prior to construction.
It is often difficult to assess the porewater pressure distribution and seepage gradients because of the heterogeneity of the weathering profile and possible presence of structural discontinuities including relict joints, erosion pipes, fault and dykes. As reported by Morton et al (1980), the measured differential heads between the inside and the outside of a caisson can be between 10% and 97% higher than that estimated based on the assumption of an isotropic, homogeneous aquifer and a simplified flow pattern.
Heavy seepage flow into the bottom of a caisson may cause weakening of the soil through slaking, leaching and dispersion. Loosening (or possible damage of bonding
between soil grains) of initially dense to very dense saprolites can take place under significant groundwater flows, as observed by Haswell & Umney (1978).
Dewatering during caisson construction can cause extensive groundwater drawdown resulting in excessive ground settlement and may result in damage to surrounding utility services and structures. Chan & Davies (1984) observed that the average settlement of buildings supported on piles founded in completely weathered granite is 2 to 3 mm for every metre head of drawdown.
The water discharged from the pumps should be collected in a sedimentation tank and checked regularly to determine the quantity of fines being removed. This would assist in the identification of zones with excessive loss of fines and give an early warning of the possibility of subsidence or collapse of caisson rings in that area. Such ground loss may also lead to excessive settlement of the ground surface.
8.4.3.3 Base heave and shaft stability
Excessive differential head or hydraulic gradient and unstable ground could lead to collapse of the excavated face, rapid inflow of mud and water, and heaving of the caisson base. In extreme situations, voids can be created in the ground adjacent to the caissons and can lead to formation of sinkholes if ground loss is excessive.
The rate of base heave has been found to be variable between sites, and between piles in any one site (Shirlaw, 1987). In some cases, heave occurs quickly and can only be recognised by counting the number of buckets of arising for each working shift. The mechanism of base heave is generally thought to be related to slaking, swelling and softening of the soils which are a function of the degree of weathering and can be promoted by stress relief and high seepage gradient (Chan, 1987). Alternatively, the bonded structure of the saprolites may collapse as the material starts to yield under low effective stresses and therefore softening in situations where the material is in a metastable state (Lam, 1990).
Some weathered granites have been observed to exhibit a pronounced tendency for swelling and loosening at low effective stresses (Stroud & Sweeney, 1977; Davies & Henkel, 1980). Mackey & Yamashita (1967a) observed that the zone of loss of soil strength was as much as 9 m away from the caisson. A possible cause of significant base heave and shaft instability could be improperly backfilled site investigation boreholes or the presence of old wells.
If excavation has to proceed below the apparent rock surface where caisson rings will not be constructed, the risk of caisson instability arising from the presence of weathered rocks outside the unsupported shaft possibly under a high water head should be carefully considered. Local grouting of the soil-rock interface may be necessary in order to minimise this problem.
8.4.3.4 Base softening
It is common for softening to occur rapidly in granitic saprolites in the base of
excavations below the water table (Philcox, 1962; Mackey & Yamashita, 1967a). The susceptibility to softening is related to the degree of weathering. Some completely weathered granites swell rapidly when the effective stress is reduced to a low value (Davies & Henkel, 1980).
Evans et al (1982) observed significant softening of a caisson base down to a depth of 0.8 m, about 70% of the shaft diameter. The degree of softening increased with the length of time between completion of excavation and commencement of concreting. It was further observed that upon concreting, re-compression of the softened base took place to a depth of about 50% of the pile diameter over a period of 10 days. Grouting of the pile base was carried out at a maximum pressure of 300 kPa but the re-compression of the softened material was not significant in this instance. If there are lengthy delays to the placement of reinforcement and concrete, consideration may be given to constructing a concrete plug at the bottom of the pile in order to limit the effects of stress relief.
Endicott (1980) reported similar findings of base softening but found from loading tests on short length concrete plugs that the base stiffness was satisfactory, with the load resisted by shaft resistance. However, to improve confidence level and alleviate the concern of long-term behaviour of caissons with a soft base, the pile base was grouted to achieve a given probe test resistance.
Even in the situation where the general groundwater table has been drawn down, some disturbance to the shaft of the bore will be inevitable due to stress relief and possible seepage gradient built up around the pile. This is highlighted by the results of horizontal plate loading tests in completely decomposed granite reported by Whiteside (1986). In these tests, the disturbed zone appeared to be fully re-compressed at a stress level ranging from 400 to 500 kPa, and it is notable that this stress level is substantially in excess of the vertical effective stress and the likely pressure of the wet concrete.
8.4.3.5 Effects on shaft resistance
In difficult ground conditions, forepoling stakes may be driven into the ground ahead of the excavation to provide temporary support prior to the casting of concrete liner for each lift. These timber stakes are typically left in the ground and could potentially result in reduced shaft resistance.
Where there is a tendency for high seepage gradients and base heave, the ground may be subject to softening around the hand-dug caisson and hence result in reduction in shaft resistance. If the bore is allowed to cave in, loosening of the surrounding ground will result.
Tests to evaluate the available frictional resistance of the caisson rings can be carried out from within caissons using a special jacking frame (Sweeney & Ho, 1982; Sayer & Leung, 1987).
8.4.3.6 Effects on blasting
Where blasting is used to break up obstructions or expedite excavation in rock, consideration should be given to assessing the effects on relatively green and mature concrete
in adjacent caissons, as well as on caisson ring stability where bore excavation is not complete.
8.4.3.7 Cavernous marble
Houghton & Wong (1990) discussed the potential problems associated with construction of hand-dug caissons in karstic ground conditions. The principal problem is the need for dewatering during construction, which could lead to sinkhole formation (Chan, 1994b). The use of hand-dug caissons in karstic marble is strongly discouraged.
8.4.3.8 Safety and health hazard
The particular nature and procedure adopted in hand-dug caisson construction have rendered this operation one of the most accident-prone piling activities in Hong Kong. The most common causes of accidents include persons falling into the excavation, falling objects, failure of lifting gear, electrocution, ingress of water/mud flow, concrete ring failure, and asphyxiation. Furthermore, the working environment constitutes significant health hazards arising principally from the inhalation of silica dust that may cause pneumoconiosis.
Concern for safety and health hazards must start at the design stage and continue until completion of the works. Training courses for workers and their supervisors should be promoted. General guidance aimed at site operatives is provided by the HKIE (1987).
8.4.3.9 Construction control
Precautionary measures which could be adopted to minimise the effects of groundwater drawdown and ground loss include the construction of a groundwater cut-off (e.g. sheet piles or perimeter curtain grouting coupled with well points or deep wells) which encloses the site, the use of recharge wells in the aquifer undergoing drawdown (Morton et al, 1981), and advance grouting at each caisson position prior to excavation. Reference may be made to Shirlaw (1987) on the choice of grout for caisson construction. Care should be taken to control the grouting pressures to avoid excessive ground movement.
Where deep well dewatering is deemed to be unwarranted, the use of pressure relief wells constructed prior to commencement of excavation may be considered to reduce the risk of high hydraulic gradients developing during construction. This is particularly relevant where there is a risk of artesian water pressure at depth.
The presence of old wells or underground stream courses will affect the effectiveness of the pre-grouting operation. In addition, where fractures are induced in the ground during grouting as a result of using an inappropriate grout type or lack of control of the grouting process, the permeability and hence the rate of softening may increase which could lead to base heave.
An alternative means of control is phasing of caisson construction sequence in order to limit ground movements and groundwater drawdown. Where caissons are sunk on a group
basis, one or two caissons may be advanced first to serve as deeper dewatering points for the other caissons.
Where poor ground is encountered, grouting may be carried out locally to help stabilise the soil for further excavation. Alternatively a steel casing may be installed through the soft ground. Any voids resulting from over-excavation or caving should be backfilled with concrete of similar quality as the lining.
Where significant base heave has been observed, the surrounding ground is likely to have been disturbed and both the shaft resistance and the end-bearing resistance may be affected. A careful review of the design for the affected caissons will need to be made.
The design of the linings should be examined for suitability and may need to be examined after construction, as for any other structural temporary works. In assessing the effects of blasting on relatively 'green' concrete, reference may be made to Mostellor (1980) who suggested limiting ppv values of 6, 13 and 25 mm/sec for a concrete age of 12, 24 and 48 hours respectively as a very rough guide.
In addition to ensuring strict compliance with safety requirements and implementation of precautionary measures, it is important that sufficient instrumentation comprising piezometric and movement monitoring of the adjacent ground and structures is included to control the excavation operation. The monitoring results should be regularly reviewed to assess the need for remedial measures.
Possible early signs of instability should be taken seriously and investigated thoroughly. Excessive excavation depths and hence the risk of base heave will be reduced if rational design methods are adopted to avoid overly-conservative pile designs.