By Scott Sloan
This is the first e-newsletter for 2014 for the Centre for Geotechnical Science and Engineering (CGSE). The Centre aims to publish regular updates throughout the year to augment the annual report which will be distributed in May.
2014 is already shaping up to be another busy and productive year with many CGSE members engaged in writing grant applications as well as finalising the submission of the annual report to the Australian Research Council. A major focus for the year ahead will continue to be work at Australia’s first National Soft Soil Test Facility, which includes completion of the construction of the test embankments, the installation of advanced monitoring instruments, the undertaking of extensive in situ testing, and state of the art sampling.
Following the workshop held during December 2013, I am pleased to report that there has been a further increase in staff exchanges between the nodes. The Newcastle node has already welcomed visitors from UWA this year: James Doherty and Antonio Carraro. Daniel Wilson from the Newcastle node has recently been at UWA while Majid Nazem is now there and will spend several months collaborating with colleagues.
The UWA node will shortly be conducting Self Boring Pressuremeter (SBPM) testing at Ballina and all nodes are currently carrying out extensive laboratory work on the soil samples obtained from this site. The University of Wollongong has been collaborating actively with other nodes, with a large number of joint papers under review, and was also successful in procuring a LIEF grant worth a total of $1.72M for the cyclic testing of rail infrastructure. This facility sits squarely in the focus of the CGSE and will be a significant boost to our understanding of the complex behaviour of rail infrastructure which is built on soft soils. The CGSE is cementing its cross-node research activity and we look forward to the outputs that will come about as a result of these burgeoning partnerships.
Finally, I am pleased to report that a number of our staff have recently been honoured for their outstanding contributions to the field of geotechnical science and engineering. Professor Harry Poulos AM FAA FTSE, a Partner Investigator in the CGSE, has been elected as a Foreign Associate of the prestigious US National Academy of Engineering for his “contributions to understanding foundation structures and ground support interactions”. He was one of 11 new Foreign Associates from Germany, Brazil, UK, Israel, Canada and China elected by the NAE in 2014. Harry has been a Fellow of the American Society of Civil Engineers since 1996, and is a Fellow of the Australian Academy of Science and the Academy of Technological Sciences and Engineering. Winthrop Professor Mark Randolph FAA FTSE FRS FREng of the UWA and a Chief Investigator in the CGSE, was named the 2013 Western Australian Scientist of the Year. Mark was the founding director the highly successful Centre for Offshore Foundations at UWA, and is a world authority on the analysis and design of offshore structures for the oil and gas industries.
The 2013 CGSE annual workshop, hosted by the UWA node and held at Bunker Bay in Western Australia, was well attended with approximately 80 participants including members of the Advisory Board, Chief Investigators, Research Associates and PhD candidates. The program included two keynotes: the 8th Terzaghi Oration ‘Protecting society from landslides – the role of the engineer’ delivered by Advisory Board member Professor Suzanne Lacasse of the Norwegian Technical Institute; and the 2nd McClelland Lecture on ‘Analytical contributions to offshore geotechnical engineering’ delivered by our own Chief Investigator Professor Mark Randolph. The full program, held over two days included a number of presentations by centre members, 3 Minute Thesis presentations presented by our PhD candidates, and a number of directed breakout sessions covering: Industry Impacts, Field Testing – Methods and Interpretation, Soft Soil Constitutive Models & Laboratory Testing and Georisk. The workshop proceedings are available here: (PDF 1.4MB)
Establishment of the National Soft Soil Test Facility at Ballina, NSW is now well under way with the site preparation, laying of the access tracks, and construction of the first instrumented test embankment all completed in 2013. The first test embankment included an innovative international collaboration, funded by the Indian Jute Board, involving the installation of jute prefabricated vertical drains (PVDs) in a portion of the embankment for the purpose of a comparison with conventional PVDs. Unlike conventional PVDs, these drains are biodegradable and will disappear from the soil profile once they have done their job of accelerating the settlement. This aspect of the project, led by Professor Buddhima Indraratna and Richard Kelly, is thus investigating environmentally friendly methods of construction on problematic soils, and will investigate the installation and long-term performance of these PVDs which are made of natural fibre.
The construction of the control test embankment, without PVDs will be completed in the 2nd quarter of 2014. Detailed site investigation for this second embankment is currently being undertaken and includes advanced in situ testing, geophysics testing, and bore hole sampling. In addition to the planned site investigation, a program involving undertaking 25 cone penetrometer tests along two grid lines, with varied spacing intervals, is currently being completed for the purpose of stochastic analysis of the soil variability. Staff from the UWA node will be visiting the test site to undertake piezoball testing using their load frame which offers a variable rate of penetration. Additionally, staff from the UWA will undertake testing with their self-boring pressuremeter.
Geophysical testing will comprise Electrical Resistivity Imaging (ERI) and Surface-Wave Testing (SWT), using the Multi-Channel Analysis of Surface Waves (MASW) method, as was previously undertaken under the footprint of the first embankment. The ERI will be conducted using a novel in-house method using electrodes connected to two inclinometer casings installed 56m apart. The method allows the electrodes to be positioned at various depths along the inclinometer casing, thus providing the ability to generate a 3-dimensional ERI tomography of the embankment subsurface as well as conventional inclinometer measurement. The survey can be replicated during embankment settlement and compared with the original survey to detect changes in the void ratio.
Sherbrooke block sampling is programed to be undertaken on the test site in the latter part of 2014. This is the highest quality sampling technique currently available, and involves the extraction large diameter undisturbed samples using a tool which incorporates specially-designed cutters that employ water jetting technology. Experts from the Norwegian Geotechnical Institute (NGI) will be visiting to assist with the sampling procedure. These samples will provide the highest possible quality specimens for advanced laboratory testing, as well as a base line for comparing the level of sample disturbance with conventional open tube and tube and piston samples. Benchmark laboratory testing is currently underway with all three nodes undertaking a program of triaxial and one-dimensional compression testing using standardised procedures on samples from the same depth. The results will provide a basis for comparing the laboratory test outputs from the different nodes.
CGSE Partner Investigator Professor Harry Poulos AM FAA FTSE, Senior Principal Coffey Geotechnics has been elected a Foreign Associate of the US National Academy of Engineering for his work on ‘ understanding foundation structures and ground support interactions’. (Read more)
CGSE Researcher Winthrop Professor Mark Randolph was named Western Australia’s Scientist of the year in 2013, while Professor Shassad Hossain won the Woodside Early Career Scientist of the Year.
Professor Randolph’s outstanding contribution to meeting the scientific and engineering challenges of the offshore oil and gas industry was formally recognized during the State’s celebrated Science Awards. His leadership and geotechnical engineering expertise attracts many world-leading companies to the CGSE, as well as academics who seek his expertise in finding practical solutions to complex problems.
CGSE researcher James Hambleton was selected as one of ten ‘2014 New Face of Civil Engineering Professionals’ by the American Society of Civil Engineers (ASCE).
The New Faces program promotes the achievement of young civil engineers by highlighting their contributions to and impact on society. James will be recognised for this honour at the ASCE’s annual Outstanding Projects and Leaders (OPAL) Gala on March 20, 2014, in Arlington, Virginia.
James has been with the CGSE since the beginning. His research focuses on the advancement of computational, analytical and experimental techniques in geomechanics, and he is an expert in plasticity theory. Applications of this research includes the enhancement of field tests for identifying soil strength and the development of prediction and mitigation techniques for land damage caused by off-road vehicles.
During 2013, Cholochat Rujikiatkamjorn received a number of significant awards which recognise his research contributions to the field of geotechnical engineering. The first of these awards, the prestigious Trollope Medal of the Australian Geomechanics Society, is for an outstanding contribution to geomechanics through recent research and publication of outcomes by a young professional. Cholochat also received a 2013 Young Member Award from the International Society of Soil Mechanics and Geotechnical Engineering for his academic achievements and contributions to the field of geotechnical engineering by a young member less than 36 years of age. Finally, he was was awarded a 2013 Endeavour Fellowship to pursue collaborative studies with China. Cholochat is based at the University of Wollongong node, and is the holder of an Australian Research Council Early Career Researchers Award (ECRA) for the project Ground Improvement Methods for Soft and Problematic Soils, which is funded for three years with a total value of $672,251.
Key researchers: Xiaowei Feng, Michael Cocjin, Susan Gourvenec and Mark Randolph
The increasing focus on deep-water developments in the offshore industry has led to renewed interest in the design of shallow foundations, often referred to as mudmats. Mudmats, which have historically been used to provide temporary stability for fixed-jacket platforms in relatively shallow water, are now widely employed for deepwater pipeline infrastructure, such as pipeline end terminations (PLETs) and pipeline end manifolds (PLEMs) (Fig. 1). The loads applied to mudmats have increased as subsea developments move into deeper water and as subsea components have taken on processing roles. The lifting capability and installation technology of current pipe-laying vessels have become insufficient to handle the ever increasing size of mudmats providing the drive to optimise mudmat design.
IMPROVED DESIGN APPROACH FOR MUDMATS UNDER MULTI-DIRECTIONAL LOADING
Due to pipeline expansion and contraction, it is commonplace that mudmats are subjected to six degrees-of-freedom loading, including vertical load (V), biaxial horizontal load (Hx, Hy), biaxial moment (My, Mx) and torsion (T) , or V-H2-M2-T loading. Uniaxial ultimate limit states and failure envelopes in V-H2-M2-T space have been defined and a design method has been proposed to collapse the loading in six degrees-of-freedom into two-dimensional load space (as shown in Fig. 2). The resulting failure envelopes are described by a single algebraic expression (Feng et al. 2013).
RECOMMENDATION ON INTERNAL SKIRT SPACING
Mudmat embedment is achieved by skirts – a thin peripheral section protruding beneath the mat that penetrates the seabed confining a soil plug. Internal skirts are also often provided to ensure that the confined soil plug displaces as a rigid body – thus mobilising maximum capacity. If insufficient internal skirts are provided, a failure mechanism may occur within the soil plug, reducing the load carrying capacity of the mudmat. The critical number of internal skirts required to ensure plug integrity for embedded rectangular mudmats under V-H2-M2-T loading, for varying foundation embedment ratio and soil strength heterogeneity was determined by finite element analysis. The results are presented as design charts such as shown in Fig. 3 (Feng & Gourvenec 2013, Mana et al. 2013).
ENHANCING MUDMAT CAPACITY THROUGH CONSOLIDATION
A time lag of months may exist between installation of mudmat foundations and operation of the pipelines that they support. Consolidation of the soil beneath the mudmats will lead to an increase in shear strength of the soil and associated increase in load carrying capacity of the mudmat in all degrees of freedom. The effect of consolidation on mudmat capacity under V-H2-M2-T loading is being explored through stress-pore fluid, elasto-plastic finite element analysis.
ENHANCING MUDMAT CAPACITY WITH CORNER PINPILES
A novel approach to reduce the plan area of mudmats is to incorporate pinpiles at each corner, resulting in a hybrid subsea foundation (HSF) where the load is shared between mudmat and piles (Fig. 4). The piles provide significant increase in capacity, particularly in respect of sliding and torsional modes of failure. Centrifuge modelling and has investigated the response of the system (Gaudin et al. 2012, Dimmock et al, 2013). Numerical analysis addressing the load sharing mechanisms between the mudmat and the pile group is being undertaken.
An alternative novel approach to reduce the size of mudmats is to design a mobile mudmat – one that moves to absorb load rather than remaining stationary to resist loads. A suite of centrifuge tests investigating the sliding capacity of a model PLET on the surface of very soft clay (Fig. 5) has recently been completed. This study explores the long-term benefits of soil reconsolidation around the foundation in terms of increasing the sliding capacity of the foundation (Cocjin et al. In prep).
Optimisation of shallow foundation for deepwater infrastructure, or ‘mudmats’, has been addressed through:
Aspects of the work presented were financially supported by Subsea7. Xiaowei Feng and Michael Cocjin are supported through the Lloyds Register Foundation.
Cocjin M, Gourvenec S, White D and Randolph MF (In preparation). Observations on the sliding behaviour of a shallow foundation.
Dimmock P, Clukey EC, Randolph MF, Gaudin C and Murff JD (2013). Hybrid subsea foundations for subsea equipment. J. Geotechnical and Geoenvironmental Engng, ASCE, aop June 2013.
Feng X and Gourvenec S (2013). Optimal shear key interval for offshore shallow foundations. Proc. of the 32nd OMAE, Nantes, France.
Feng X, Randolph MF Gourvenec S and Wallerand R (2014). Design approach for rectangular mudmats under fully three-dimensional loading. Géotechnique.
Gaudin C, Randolph MF, Feng X, Clukey EC and Dimmock P (2012). Centrifuge modelling of a hybrid foundation for subsea equipment. Proc. 7th OSIG, SUT, London, 411-420.
Mana D, Gourvenec S, Martin, C (2013). Critical skirt spacing for shallow foundations under general loading. Journal of Geotechnical and Geoenviromental Engineering, , 139(9), 1554-1566.