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19 June 2015 |   ByP Jones
Fugro Author

Curtis Island, off the northern Queensland coast is the location of three of Australia’s major LNG development projects. Applying the  benefits of its Osterberg Cell® bi-directional load test method to the construction of a jetty at one of these developments, Fugro performed two push-out load tests on sacrificial piles.

The objective was to optimise the pile design for the driven tubular steel piles supporting the jetty structure. The Osterberg Cell push-out test offers significant time and cost benefits over conventional static load test methods as it requires no complicated external reaction system at the pile head.

The specification was for two nominal Ø 510 mm  Osterberg Cells with a combined push-out capacity of 2,000 tonnes for the first test pile, representative of the main jetty structure,  with predominantly axial compression loads.  The second test pile, representative of the dolphin structures with predominantly tensile loads, was specified with two nominal Ø 405 mm  Osterberg Cells with a combined push-out capacity of 1,260 tonnes.

Assembly of O-Cell instruments into reinforcing cage
Assembly of O-Cell instruments into reinforcing cage.

Tolerating Tilt

A major technical attribute of the Osterberg Cell is its ability to tolerate tilt (up to 5%) during pressurisation. Without this tilting tolerance, mechanical contact points could occur resulting in internal friction, which would lead to undesirable consequences such as unreliable calculation of the loading force. The side-by-side Osterberg Cell arrangement allowed the tremie pipe to be placed through the Osterberg Cell loading assembly to the base of the reaction pile, allowing concrete  to be poured in a single continuous operation. 

The load applied by the Osterberg Cell loading assembly acts in two opposing directions, counteracted by the pile soil resistance above and below. The push-out test loads the test pile from the pile base upwards in its entire length rather than from a balanced soil resistance point along the test pile shaft. A sacrificial reaction pile constructed below the Osterberg Cell loading assembly provides resistance in the opposing downward direction and a discontinuity in the reinforcing cage, coinciding with the tip of the test pile, and allows a fracture to form in the surrounding concrete when loaded. This creates two discrete pile sections: the upper test pile and the lower reaction pile. To assess load distribution along the shaft of the test pile, strain gauge instrumentation was utilised. While the reaction pile was not part of the test pile, supplemental strain gauge and extensometer instrumentation was installed to obtain important geotechnical data about the  variable mudstone bedrock.

Installation and Concreting

Hoisting a single 42-metre long reinforcing cage instrumented for an Osterberg Cell test was not feasible yet concreting was to be completed within 12 hours of final excavation; this was in order to minimise the effects of shaft wall instability or base degradation within the uncased, water-supported mudstone. To meet this requirement, the full length reinforcing cage was pre-assembled from three fully instrumented cage sections in a nearby partly drilled out steel pipe pile the day before installation and concreting. With the full length tremie pipe and an inclinometer casing conduit pipe also pre-assembled within the reinforcing cage, the challenge was met with several hours to spare! 

For the test equipment set-up and to perform the test, a pre-assembled work platform was lowered onto the test pile. With embedded instrumentation and external gauges connected to an automated data acquisition system with connection to a computer,  the Osterberg Cell movement curves could be viewed in real-time. Pile head movement was monitored separately using a digital level referenced to a fixed staff on the adjacent test pile and cross-checked to a shoreline total station while the Osterberg Cell loading assembly was pressurised using water and a high pressure pump. Eliminating the use of hydraulic oil was a clear benefit in this environmentally sensitive area with its strict conditions on water pollution. 

Optimising Pile Design

Each load increment was held constant  (within 0.5%) until an Osterberg Cell expansion creep rate of 0.25 millimetres  per hour was met. Geotechnical related time effects are often ignored but can have a significant effect on the load-displacement behaviour. This was dramatically illustrated on the final load increment of one test pile - the steadily decreasing creep rate, indicating a stabilising upward displacement, became an accelerating creep rate as the test pile ultimate friction capacity was reached. Both test piles were successful in providing geotechnical data to verify soil strength design parameters and optimise the pile design for the intended pile capacity.

Did you know?

Osterberg Cell® technology was developed by Dr. Jorj Osterberg over 25 years ago and is continually advanced by Fugro.

 

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