At GTL the importance of understanding the existing soil make-up before undertaking pile design is firmly recognised.
The risk of what is in the ground is always an uncertainty that makes both project managers and engineers nervous. The key to an economic and technically proficient foundation design is good quality ground investigation information. Identifying the site’s particular geohazards means that GTL can design out some of the risk associated with piling, both in terms of cost and programme delays.
The risk below
The type of soils investigation on the rail is a particularly hot topic of discussion and one in which GTL often find themselves engaged. It is GTL’s opinion that the effect of a good front-end ground investigation is huge and the ability to maximise the available soils information for a site is paramount to the success of even the smallest of pile installations. GTL is mindful that helical piles may not be the best value option for the client on all sites. In response to this, GTL has developed a series of site investigation specifications for particular trackside structures that are suitable for the robust design of helical piles and other foundation types. This allows pile design to be both financially and technically sound, giving reassurance to the client that the piles, as designed, can be installed and will perform as required. It is GTL’s opinion that a good ground investigation – coupled with a static pile analysis – allows GTL, as pile designers, to understand the interaction between piles and soils, and to estimate predicted settlements and deflections.
GTL carries out a static analysis on all their piles based on conventional geotechnical techniques and formulas. The pile is designed to meet the performance requirements of the structure, both in load and settlement. A static design ensures a sound understanding of the geotechnical conditions, the interaction of the pile with the ground and its effects on existing structures. Furthermore, GTL’s transparency of design allows any civil engineer with basic geotechnical knowledge to understand and appreciate the design. GTL’s design approach and capabilities have a proven track record in the oil and gas industry where serviceability performance requirements are often more onerous than in the rail environment.
The unique way that GTL designs means that they can take onboard the risk in the ground where geohazards are known and provide fixed-price capabilities, making GTL a one-stop shop for helical pile design, supply and installation. GTL sees that this is a significant market advantage and something that is attractive to principal contractors and the overall client in Network Rail.
Quality of installation
GTL endeavours to be an industry leader by providing a high quality service at a competitive price. A pile design is only as good as the quality of the installation; unlike the design, the installation cannot be Category 3 checked. GTL has introduced several on-site solutions that ensure the quality of installation is maintained, providing confidence for the client.
First, GTL offers a full in-house installation service; larger projects are carried out by their pre-qualified partners. All of the installation partners come from a piling background, and are trained and monitored by GTL. Where GTL is required to use the client’s installation crew, a senior GTL piling supervisor is in attendance to monitor and record the pile installation. GTL is committed to the ongoing training of their own and their partners’ installation crews, with all in-house crews and partners being regularly audited.
Second, GTL is constantly looking at innovative ways to develop their installation equipment and the way they monitor pile installation. One innovative idea that GTL introduced to the industry is the Wireless Rotary Torque Transducer (WRTT), which transmits installation torque data to a handheld PDA. This data can then be downloaded to a laptop and forms part of GTL’s pile installation records. The WRTT is used primarily to monitor and ensure that the pile’s torsional capacity is not exceeded, leading to piles failing in installation; it is not used to confirm pile set. The torque data provides a qualitative assessment of the soils during installation, which is compared against the existing soil data. This recognised correlation between soil stiffness and torque helps GTL to continually develop their pile designs and installation techniques.
Axe level crossing
The recently completed helical pile installation by GTL at the Axe level crossing near South Chard is a perfect demonstration of how good geotechnical information can lead to a successful completion of a project to the satisfaction of the principal contractors and the overall client, Network Rail. GTL designed four number helical piles that were 9.0m deep, which were installed and tested within the tight constraints of a night possession. Installation torques and settlements were as predicted in the Approved Form B submission and everyone involved hailed it a very successful installation.
Research and development at the Serpentine Gallery, Hyde Park
Research and development is recognised by GTL as a crucial factor in their success. In addition to enabling GTL to consistently improve their performance by adding value, improving quality and remaining technically superior to their competitors, they recognise it as being essential in maintaining their market position.
GTL is committed to ongoing research and development of their in-house technical capabilities and to understanding the performance of helical piles in different soil types. Since 2007, GTL has designed, supplied and installed helical piles to support various Serpentine Pavilion installations. In 2009 a suite of CPT tests was carried out to confirm the soil properties, as previous soil investigations were found to be lacking in sufficient detail to justify designs to the local authority building control.
GTL has carried out a suite of tests in conjunction with Arup Geotechnics to understand the failure mechanism (in tension and compression) and settlement performance of helical piles. This enables GTL to provide more cost-effective and efficient helical pile designs. GTL has performed cyclic load tests on their helical piles to understand the effects of repeated load reversals from compression to tension. This data allows GTL to improve its understanding of the effects on load capacity and settlement performance from repeated high frequency load reversals.
On the same site, further geotechnical investigations will be carried out using super heavy dynamic probes. The new data will be used to provide the beginning of an established correlation between super heavy dynamic probes in clay for future helical pile designs in the rail environment.
If you would like to discuss this article, any forthcoming projects, or hosting a CPD seminar then please contact Gavin Rixon at GTL’s head office:
Tel: 01403 741 166 Email:email@example.com