Laboratory simulation of field tube sampling using transparent soil, glass tubes and PIV (Particle Image velocimetry)

In this proposed research project an innovative physical modelling system using transparent soil, glass tubes and computer based image recognition technology will be established to investigate the tube sampling disturbance to soil. Transparent soil is a mixture of amorphous silica powder with pore fluid matching its Refraction Index, which is semi-transparent and has engineering properties similar to clay. Tube sampling is the most widely used sampling technology to obtain soil specimen from the ground during site investigation.Site investigation is important because the biggest source of uncertainties and risks for the construction of civil engineering projects lies in the ground. It is observed that among projects that were delayed, nearly half of the cases were due to unforeseen ground problems and not much improvement has been made during the past three decades (Tyrell et al., 1983; NEDO, 1988; Chapman & Marcetteau, 2004). As a result, projects are often built at costs over the initial budget and it is estimated that across the European Union, about 50 billion euro is spent each year due to such problems (Chapman, 2008).Though it is of critical importance to obtain accurate and representative ground information, the task itself is very challenging for several reasons. Our knowledge of the ground mainly comes from studying soil samples, which by volume are usually less than 1/1,000,000 of the ground affected by construction (Clayton et al., 1995). Most of the time soil samples are retrieved by pushing tube samplers into the ground, a process called tube sampling. Though better sampling techniques do exist, the required technical and financial support makes them impractical for many projects, so tube sampling is still the most widely used sampling method around the world. It has long been recognized that this sampling process might cause significant disturbance to the soil, so the soil samples obtained do not truly reflect the in-situ soil state. Without understanding the tube sampling disturbances, it is impossible to interpret the laboratory test data properly and obtain the correct engineering property values. It has been a primary concern among geotechnical engineers, and though extensive research has been done, our understanding on this problem is still incomplete. Due to the practical difficulties, the real movement of soil during tube sampling has never been measured in the past.The proposed research project aims to visualize and measure the whole-field movement of soil during tube penetration for the 1st time. The process of field tube sampling will be simulated in the laboratory in ways similar to those of Santagata et al. (2006), and transparent soil and glass tubes will be used in this project. The process will be recorded using digital photography, from which the soil movement can be measured very accurately using Particle Image Velocimetry (PIV). Incremental and accumulated strain paths can be derived from the displacement data (White & Bolton, 2004). Glass tubes of various shapes will be tested in normally consolidated and over-consolidated transparent soil to investigate the effects of sampler design parameters like area ratio, cutting-shoe geometry and inside clearance. The investigation will also be extended into the post-sampling stage to include disturbances during storage and extrusion. The obtained data will provide valuable insights into the whole process from in-situ tube sampling to laboratory testing, and offer practical guidance on the design of the tube samplers, laboratory handling of soil samples, and the interpretation of laboratory tests. It will reduce the financial risks associated with the building of infrastructure, promoting the wealth of the nation and welfare for the general public.

Who will benefit from this research? Sampling disturbance is a problem present in almost every construction project. Even after several decades' research, it is still unresolved and continues to provide significant scientific and technological challenges. New breakthroughs on this topic, likely to be achieved in this proposed research project, will make fundamental contributions to engineering science and have a positive and lasting impact on both academia and the civil engineering industry. This section will focus on explaining the benefits to the civil engineering industry, a crucial sector of the overall UK economy. Currently there is urgent demand for new insights into tube sampling disturbance: a topical and heated debate is on-going regarding the implication of Eurocode 7 to geotechnical sampling in the UK. The key issue is that according to BS EN ISO 22475-1:2006, samples obtained by U100, the most widely used sampler in UK, are no longer suitable for shear strength and compressibility testing. Though a new type of thin-wall open drive sampler (UT100) was developed (Gosling & Baldin, 2010) to meet this challenge, only limited trials were done and recent trials in Scotland resulted in crumpling and deformation of the cutting shoe (Reid and Taylor, 2010). According to the latter paper the UT100 is not suitable for the Scottish area, and currently the only alternative is to use the much more expensive rotary coring technique. Given the limited budget for ground investigation, this will probably result in reduced extent of sampling and actually increase the margin of errors. Reid and Taylor stated 'there is a need for a comparable sampler to the U100 which meets the standard set by Eurocode 7'. Due to the importance and pertinence of the topic, the outputs generated by this research project will benefit every stakeholder in ground engineering: ground investigation contractors, testing laboratories, consultant engineers, clients and the general public. How will they benefit from this research? To retrieve high quality samples Ground Investigation contractors need to select the most suitable sampler for the ground. Experimental investigation on sampler design parameters and the influence of over-consolidation can offer immediate guidance on this aspect. The images and vector plots produced will also greatly facilitate the training of field crew because it shows directly what is happening to the soil during their work, an insight they can never get on site. Soil testing laboratories will benefit from the knowledge of post sampling disturbances. Quantifying soil disturbance during this stage, which to-date has largely been ignored, will lead to the systematic evaluation from sampling to testing. It is the starting point for laboratories to develop improved test procedures too. It is also possible that new re-consolidation process can be designed to ameliorate the influence of sampling and post-sampling disturbances, resulting in more realistic reflection of the in-situ state. Consultant engineers often find it almost impossible to assess the range of error in ground investigation reports. Consequently there is a tendency to adopt over-conservative design schemes to avoid risks, resulting in increased cost of the building. Better quality sampling and testing, together with the holistic review on the sampling and laboratory disturbance, will help them in better interpreting the geotechnical parameters, leading to more cost effective design without jeopardising the safety of the infrastructure. This in general will lead to a more efficient civil engineering sector, promoting the wealth of the nation and welfare for the general public. What will be done to ensure that they benefit from this research? Various methods like journal papers, magazine articles, conference presentations, workshops, project websites and video clips will all be used - for more details please see 'Pathways to Impact'.