Holdfast Training Services Trial Blog

Since June 2016, one of my tasks has been to track progress for the solution to the bearing capacity issue faced on the Rail Bridge cast-in-situ permanent cased bored piles.  The testing methodologies to find a solution have given me experience in a full repertoire of sequential testing procedures. Bit of a lengthy blog but gives a good insight into pile testing. I would be interested to see whether you would take the side of the sub-contractor or the client?

Background

The new Rail Bridge through the Airport East site is supported by 6 large diameter permanent cased piles, P1 and P2 are 1200mm dia supporting Abutment A, P5 and P6 are 1200mm dia supporting Abutment B and P3 and P4 are 2500mm dia at the central pier, refer Figure 1 below. All the piles are designed in toe bearing alone and have 500mm rock sockets within Class IV sandstone.

The SLS design axial load for the abutment piles is 7.4 MN. For analyses of high-strain dynamic tests, this value was increased to 9.0 MN to account for potential future negative skin friction on the piles. The ULS design axial load for the abutment piles is 10 MN.  With a geotechnical strength reduction factor of 0.52 applied, a design ultimate geotechnical strength of 19.2 MN is required (= 10 MN / 0.52).

Figure 1 – Plan view of Rail Bridge pile layout

Concerns over the fractured sandstone allowing groundwater to seep into the piles required a wet tremie pour to be adopted. Following the completion of steel casing installation (Figure 2) and following excavation, water was pumped into the casing up to the water table level to maintain positive head prior to the concrete pour. The positive head aimed to reduce the in flow of sediment from ground water flow.

Figure 2 – Permanent casing being driven during an airport possession, note the railway line has been slewed to allow for works. 

Sonic logging tubes were attached to the reinforcement cages, these were kept 200mm off the base of the reinforcement cage, 300mm from the toe of the pile.

During installation of P3 and P4, the 2500mm dia piles, the 16mm thick permanent casing buckled, resulting in a bespoke reinforcement cage being required.

Prior to the concrete pours the reinforcement cage was removed from the hole to allow the base to be cleaned. The piling rig using a cleaning bucket removed any loose material at the pile base. This also agitated any sediment causing it to go into suspension prior to the pour.

The tremie was lowered below the water line to the base of the pile. As the level of the concrete in the pile increased the tremie pipe was withdrawn in sections ensuring that a minimum immersion of 2-3m of the tremie pipe remained in the concrete at all times.

Pile Testing 

Cross-hole sonic logging (CSL) was used to assess the integrity of the piles. A primary advantage of CSL is that it can assess the integrity of piles at depths which may be beyond the capabilities of sonic echo testing.  It is probable that a large number of piles will include defects of some sort. The important consideration is that these defects will not materially affect the structural performance of the pile. The CSL assessment detected 15 issues of ‘Poor/Defect’ anomalies at a depth of approximately 35.6m – 36.1m below the top of concrete in P2 and 4 issues of ‘Poor/Defect’ anomalies at a depth of approximately 35.6m – 35.7m within P6.  Further tomography assessment of P6 at 35.7m  concluded that only 40% of the cross-sectional area of the pile comprised of 40 MPa concrete.

Core drilling of the piles was then recommended.   On attempting to drill and core P5 the subcontractor made 4 attempts to reach the bottom of the pile without success using a percussion drill with a 100 mm diameter tungsten carbide head. The helical reinforcement was damaged during these attempts as it was struck by the carbide head. It is assessed the drill head maybe wandering during drilling due to potentially softer section of concrete due to removal and reentry of the tremie tube. P2 was successfully cored, the sample revealed what appears to be a 50mm layer of loose coarse aggregate from the segregation of concrete at the toe between sound concrete and the sandstone rock socket.

Figure 3 – Coring samples from P2.  Note the aggregate in Core 3. 

The toe of the core was cleaned using air to show the extent of the void.  The void was measured at 400 mm at the base of the pile and a photo is shown in Figure 4 from a CCTV recording of the core. The coring holes and toe void were grouted, and subsequent CSL showed some improvement.

Figure 4 – CCTV photo shows the void at the base of P2, slowly filling with ground water. 

P1, P2 and P5 were high-strain dynamically tested using a pile driving analyser (PDA) to check the mobilised capacity. The mobilised capacity of P1, P2 & P5 exceeded the required capacity.  A guide to structural integrity and performance of the pile is given by considering the energy of the hammer blow delivered to the pile by the 20 tonne drop weight (Figure 5).  The piles subsequent responding mobilisation capacity and estimated static deflection at mobilised capacity was much less than the allowed movement of 85 mm. The temporary compression of the piles under dynamic loading ranged from 8.1 mm to 9.9 mm with a 1 mm set. 

Figure 5 – High-strain dynamic testing hammer on P1. 

The pile resistance is subject to input data, primarily including Young’s Modulus and the Damping Constant.  Corrected values of Young’s Modulus are correlated with signal matching from CAPWAP (Case Pile Wave Analysis Program) testing which estimates the total bearing capacity of the pile.  The CAPWAP results showed the concrete over the length of the socket is a lower modulus than the upper concrete in the pile.

CAPWAP results showed a reduction in the axial stiffness at the base of the pile (i.e. the material at the base of the pile has a stiffness lower than that of “good” concrete). Also a reduction in the pile cross-section at the base of the pile (i.e. there are zones of contaminated or “poor” concrete at the base of the pile).

The Client – Roads and Maritime Services Conclusion 

‘Whilst the dynamic analysis provides a pile capacity at this point in time, the uncertainty of the size of the voids and / or quantum of the integrity issues, coupled with the poor integrity section not being fully contained within the sandstone rock socket or the steel pile casing, results in the Principal being unable to determine the extent of any future pile settlement over the design life of the structure.’

Conclusions & Recommendations

The pile should only be considered defective if it does not meet the SLS or ULS requirements. The client has rejected the piles due to the defects with in them despite them still passing the relevant criteria. They have decide to replace P1, P2, P5 & P6, this time using a dry tremie pour, by achieving a seal between the permanent casing and the rock socket.

The reason for failure of concrete in the toe of the rock socket has not been fully concluded.  After gaining a better level of technical knowledge on the subject by reading  Tomlinson & Woodward, Pile Design and Construction Practise, 5th Ed, Section 3.4.8 discusses the issues related with groundwater in pile boreholes. It mentions, ‘a strong ground water flow can wash away concrete completely’.  In cases of strong inflow, ‘the water must be allowed to rise to its normal rest level and topped up to at least 1.0 m above this level to stabilise the pile base.’ It does directly mention this with cased piles but, honeycombing of concrete could certainly be an issue.

Figure 6 – Example of a defective shaft of a bored pile caused by cement being washed out of unset concrete (Tomlinson & Woodward, 1977)

Despite the piles having permanent casings, ground water flow through fissures in the sandstone could have caused the grout to wash out of the concrete, leaving the aggregate in the base of the piles. Grout leakage could have occurred in the tremie pipe during delivery leaving insufficient grout to bond the aggregate.  The tremie pipe may have been too far from the toe of pile on concrete delivery causing separation of concrete on delivery.

A dry tremie pour with a high artesian head in the sandstone through fissures could cause the same issues to occur.  John Holland have now mitigated risk by the client providing both a methodology and load values required in the piles, with the sub-contractor paying for reworks.

Photo for Comments Section

Figure 7 – Plastic deformation of a pile casing driven past refusal