SMALL REPLACEMENT PILES

3.1 Mini piles

The construction of mini piles only requires relatively small mechanical plants. This provides a solution to construction works for sites in hilly terrain with restricted working space or accessibility difficulties. It also minimises the environmental disturbance. In cases of hilly terrain, the drilling equipment can be hung from the end of jib with main body of the crane at level area.

Minipiles have been in use in Hong Kong since 1970’s and the most common mini-pile in Hong Kong nowadays is the 219mm diameter hollow section with four numbers of T-50 reinforcement bars. This arrangement provides an allowable working load of approximately 1400kN. In recent years, the loading capacity were increased to 2350kN by using 3 numbers of 63.5mm diameter high yield thread bar (grade 555/700 i.e. fy = 520MPa) in a 273mm casing. The configuration of this minipile is shown in Figure 14.

Figure 14: Configuration of a minipile with 63.5mm diameter high strength threadbars

Conventionally, minipiles are not designed to resist bending moments in view of the limited bending capacity of the reinforcement bars. To resist lateral loads, they normally have to be raked so that their own axial load capacity can be utilised. Nonetheless, approval from Buildings Department has been given to a development in Central to allow the casing to provide the bending resistance in SEC 2/2008 (BD 2008). 3.2 Prebored H Piles

Conventional driven H piles could be damaged by underground boulders and generates significant nuisance to the neighbourhood. Prebored H Piles were therefore introduced in the 1980s to overcome these problems. The pile shaft is formed by boring and steel H section is inserted afterwards. Better than minipiles and similar to driven H-piles, prebored H-piles possess higher axial load capacity and bending resistance.

Overburden Drilling with EXcentric bit (ODEX) method was widely adopted for shaft boring works. The Odex system consists of a bottom pilot bit and an eccentric “swing out” reamer bit which rides along its shaft. The bottom pilot bit and the eccentric reamer are connected with a casing shoe with guide device ahead of the casing tubes bottom. The eccentric reamer swings out and drills a hole slightly larger to allow the progression of the casing by gravity. ODEX uses compressive air as flushing medium. Exhaust air from the hammer is blown directly into the ground at the centre of the cutting head, which may bring excessive cuttings as well as loose soils to the ground surface, causing ground loss or wash out zone around the drill bit. It is sometimes unavoidable that some cuttings would be flushed up outside the casing as the drillhole is made slightly larger

than the casing.

In 2008, two serious incidents of ground movement occurred during the preboring works. Significant ground subsidence occurred next to a construction site at Queen’s Road, Central. Another incident happened in Sheung Wan - “A multi-storey crack up to 15cm wide opened up on Tuesday between the Lee Hing Building at 98 Connaught Road West and a taller structure, standing wall-to-wall with it next door.” (SCMP 2008).

Since 2008, the ODEX method is generally not allowed in urban areas, especially near old buildings. In order to minimise ground loss and the risk of overbreaking, the concentric drilling system is often adopted. It creates less disruption to the ground and greatly reduces ground loss.

Concentric drilling system consists of a hammer operating inside a steel casing, a centre bit and a ring bit. The hammer is driven by high pressure air, usually at 7 to 18 bars. With the rotary motor and hammer in action, the drill bit advances simultaneously and drives the steel casing downwards. High pressure air vents out through an orifice in the drill bit to air-lift the soil and rock cuttings out of the bored hole, usually through gap between the hammer and the steel casing. The return flushing air is guaranteed to be forced up within the casing pipe immediately after exiting the pilot bit. This made concentric drilling better at minimising the disturbance to the surrounding ground during the drilling process (Wong et. al. 2011b). However, the ring bit is left in place and sacrificial.

The ODEX and Concentric drilling system (Robit) are shown in the schematic diagrams in Plate 12 and Figure 15 respectively.

Plate 12: Annotated photo of ODEX system Figure 15 : Schematic diagram of Robit system (with courtesy of www.holeproduct.com )

3.3 Shaft grouted prebored H-piles

Prebored H-piles are conventionally socketed in bedrock and their loading capacities are mainly derived from rock socket friction. However, for developments in area with extremely deep rockhead, construction of pile to reach bedrock is impractical, such as a development project within the Central - Sheung Wan Strip mentioned in section 2.3.1.

The development for a 25-storey residential tower is located in the Mid-levels where bedrock is over 80m below ground level. The layout plan and geological profile of the site is shown in Figure 16 and Figure 17. The restricted site access and limited working space inhibits the use of large diameter bored piles. Conventional small diameter friction piles do not have the necessary capacity to support the building. In view of this, shaft grouted frictional pre-bored steel H-piles were adopted. The configuration of the Shaft grouted prebored H-pile is shown in Plate 13 and Figure 18.

Reamer Casing Shoe Pilot Bit Ring Bit Pilot Bit Casing Tube

Figure 16: Site layout plan

Figure 17: Geological profile of the site Plate 13: Details of Trial Pile TP2 Strain Gauge Tube-A-Manchette (TAM) pipe

Figure 18: Typical section of a shaft grouted frictional pre-bored steel H-pile

Shaft grouted frictional prebored steel H-pile is not a recognised foundation under the current list of Recognised Types of Piles in Buildings Department. Loading test on 2 full scale instrumented trial piles were carried out to determine the shaft resistance and verify the design assumptions. The pile load profile of the two trial piles are shown in Figure 19 and Figure 20.

Figure 19: Pile load profile of TP1 Figure 20: Pile load profile of TP2

The results are divided into 8 groups (Zone A – Zone H) according to the SPT-N value. The shaft resistance of the bottom portion (Zone E - H) of the trial piles was mobilised to a very limited extent and therefore not shown for clarity. The depths and the corresponding N values of Zone A - D are shown in Table 7.

Table 7: The depth and the corresponding N values of different zones T-P01 Level (mPD) Average N value T-P02 Level (mPD) Average N value From To From To Zone A 32.4 28.8 26 Zone A 32.4 29.2 19.5 Zone B 28.8 20.8 14.3 Zone B 29.2 21.2 23.3 Zone C 20.8 12.8 40 Zone C 21.2 13.2 36 Zone D 12.8 4.8 61 Zone D 13.2 9.2 50.5

The trial piles TP1 and TP2 have achieved shaft resistance up to 12.3N kPa and 12.6N kPa respectively under 300% and 200% working load, as shown in Figure 21 and Figure 22. The trial piles show consistent results. The shaft resistance contributed by the colluvium layer was not taken into account in the design. In order to determine the friction value without the influence of the colluvium layer, the corresponding portion was sleeved.

Similar to the shaft grouted bored piles mentioned previously, the trend in Figure 21 is linear, which suggests that the shaft resistance of the piles was not fully mobilised.

The settlement of the piles is shown in Table 8. The actual settlement of the trial pile TP1 at 300%WL (18,000kN) is less than a quarter of the allowable.

Table 8: Actual settlement at100%, 200% and 300%WL for TP1 & TP2 Pile

Mark

Pile Head Settlement Allowable Settlement under

Peak Load 1x WL (6,000kN) 2x WL (12,000kN) 3x WL (18,000kN)

TP1 5.5mm 10.8mm 16.8mm 71.9mm TP2 10.5mm 21.0mm -- 92.3mm

The designed friction coefficient and the test results are summarised in Table 9.

Figure 21 : Summary of frictional coefficient among different zone for TP1

Figure 22 : Summary of frictional coefficient among different zone for TP2

Table 9: Summary of Designed Friction Coefficient and mobilised Friction Coefficient Friction Coefficient Capping

Ultimate Friction Designed Mobilised T-P01 T-P02 CDG 5N** 12.3N* 12.6N* 200kPa (SPT-N value capped at 40) * The shaft resistance of pile was not fully mobilised

** A FOS of 3.0 was adopted for estimating the working load capacity

The actual shaft resistance of T-P01 mobilised under 300% working load is shown in Table 10. Although the pile capacity was not fully mobilised, the shaft resistance is mobilised up to around 300kPa, which is significantly greater than the approved ultimate frictional force of 200kN. This suggests that there should be room for increasing the allowable friction.

Table 10: The mobilised shaft resistance of the trial piles (T-P01)

Zone Mobilised Shaft Resistance (kPa) SPT-N value Friction Coefficient

A 293.5 26.0 11.3 B 176.0 14.3 12.3 C 202.1 40.0 5.1 D 306.6 61.0 5.0 4 DISPLACEMENT PILES 4.1 Percussive piles

Before the 1970s, displacement piles, such as Precast Reinforced Concrete Piles, Precast Prestressed Spun Concrete Piles and Driven Cast-in-place Concrete Piles were widely used in Hong Kong. However, due to various reasons, they have phased out. Nowadays, only Driven H Piles are still in common use.

The major drawback of percussive piles is the noise and vibration during percussion, as well as the emission of excessive black smoke, which lead to neighbourhood’s resistance. The use of diesel hammers is restricted by the Noise Control Ordinance (EPD 2006). Nowadays, displacement piles are often driven by hydraulic hammers (shown in Plate 14).

4.2 Jacked piles

Jacked piles have been widely used in China and Southeast Asia, but it is relatively new to Hong Kong. Jacked piles were first officially used in Hong Kong in 2000 as part of the foundation strengthening works of two existing building blocks in Tin Shui Wai.

Jacked piles are installed into the ground by hydraulic pressure. As shown in Plate 15, the steel H-pile was being pressed into ground by hydraulic jacks with kentledge as counterweight. The jacking process is quiet and vibration-free. Virtually no nuisance would be made during the jacking process. This makes jacked piles particularly suitable for noise and vibration sensitive areas.

Plate 15: Pile jacking machine at Hollywood Road Site

Jacked piles are not common in Hong Kong as the size of the jacking machine has limited its use in urban area. It might be difficult to install the piles near site boundary. Moreover, underground obstructions could cause serious problems to the jacking process if they are deep under the ground.

Following jacked piling in Tin Shui Wan, a private residential development in Hollywood Road also adopted jacked piles in the foundation system. The area became very vulnerable to any drilling or vibrating operation after significant ground settlement and horizontal movement had been recorded during foundation works at a site opposite the road. Therefore, the jacking technique was adopted in order to reduce the vibration and minimise the adverse effects on the surrounding structures. Steel H-piles were jacked to the approximate founding level using a jacking machine. However, the final set was still done by drop hammers to fulfil the statutory requirements.

Since 2000 to 2003, jacked piles were used in several private and HKHA developments (Chan, 2005). The construction methods of jacked piles in these projects were similar to that adopted in the Hollywood Rd project. Pile buckling was also reported by Chan (2005). This was caused by the formation of voids around the H-pile during pile jacking. This can be solved by filling the voids with sand during the jacking process.

From the basics of soil mechanics, pre-loading is an effective way of reducing the creep settlement as the settlement during reloading would be much smaller. Li and Lam (2011) reported that, in the Lee On Estate project, a preloading force of 2.3WL had significantly reduced the residual/creep settlement of jacked piles during static load test under 2WL. The results satisfied both the total pile head settlement and residual settlement criteria. With the development of the pre-loading technique, jacked piles that are solely installed by jacking, without involving drop hammers in the final set, were successfully installed in ArchSD and HKHA projects.

Recently, jacked piles without final set by drop hammer were also adopted in a private development at Tai Po (BD 2011). The termination criteria for jacking were slightly different from that stipulated in the particular specification for Jacked Steel H-piles of ArchSD (ArchSD, 2008). The termination criteria of jacked piles set

Hydraulic jack

out by the particular specification for Jacked steel H-piles (ArchSD 2008) and the summary decisions of the Structural Engineering Committee SEC 02/2011 (BD 2011) are summarised in Table 11.

Table 11: Comparison of termination criteria of jacked piles Criteria Architect Services Department

- Particular Specification Buildings Department (BD) - SEC 02/2011 Preloading Stage 1: 2.3WL Stage 2: 2.2WL 3.05WL Holding time

Stage 1: Suitable hold time determined by the Contractor

Stage 2: Hold until the rate of settlement is less than 5mm in 15 minutes

Hold until the rate of settlement is less than 5mm in 15 minutes

(minimum 30 seconds)

Despite the fact that fully jacked piles had been approved by the Buildings Department in a private development project, jacked pile remains unlisted in the recognised pile type under the central data bank of the Buildings Department. The acceptance of jacked pile is still considered on a case by case basis.