Experimental investigation of soil-structure interaction associated with extruded concrete lined small-diameter tunnels

Ni, Zinan (2023). Experimental investigation of soil-structure interaction associated with extruded concrete lined small-diameter tunnels. University of Birmingham. Ph.D.

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Abstract

Extruded concrete lining (ECL) has been used for tunnel construction since the 1960s, but has had limited use in recent years. It could offer an effective solution for smaller diameter tunnels (non-man entry), such as those used for power cables, as it: removes the need for automated segment lining installation (a possible weak point in a non-man entry system); and the need to use jacking systems to move the pipe barrel forward (which can result in the build-up of significant stresses, or stress concentration in parts of the lining, if not properly aligned, which can result in spalling or other failure mechanisms, compromising the integrity of the lining). Currently, the one of main challenges is the lack of understanding of soil-structure (ground-lining) interaction associated with ECL operation (others relating to the logistical issues of extruding a lining away from the access shaft, etc). The aim of this research is to investigate the soil-structure interaction during the operation of the ECL for small-diameter tunnelling using a primarily laboratory-based method.

A number of objectives were accomplished in order to achieve this aim; these are listed below and described after:

• to conduct a critical literature review on the ECL technique to establish the gaps in knowledge;
• to develop a two-dimensional (2D) plane strain laboratory experimental apparatus, with the ability to simulate differential loading (K_0) conditions to investigate the soil-structure interaction effects associated with scaled concrete tunnel linings during curing;
• to select materials for the experiment and to test their characteristics, then conduct a series of tests to assess the performance of the developed experimental apparatus;
• to establish the relationships between the concrete curing rate, ECL deformation, ECL strain, earth pressures and ground displacement by conducting a series of 2D plane strain experiments;
• to develop a plane strain finite element model to validate the experimental setup;
• to synthesise and analyse the experimental and numerical data, and to compare the similarities and differences between the data collected using different methods.

The thesis has, for the first time, built up a comprehensive literature review on the state-of-the-art of the ECL technique, possible ground-lining interactions, and the potential market for the ECL technique in the current tunnelling industry; from which a number of experimental ideas were proposed (as part of the methodology) to investigate the soil-structure interactions and one of these was developed into a novel 2D plane strain laboratory apparatus.

The novel 2D plane strain laboratory apparatus was developed to provide a controllable environment for testing model ECL under simulated differential loading (e.g. K\(_0\) conditions). The apparatus comprised based and top plates, moveable sidewalls (forming a test box), hydraulic rams and a formwork to cast the ECL, and was equipped with a pneumatic-hydraulic system to offer differential loading (K\(_0\) conditions) on the soil (tests were conducted using “fraction B Leighton Buzzard” sand). A clear Perspex top plate secured the soil in place (the plate being supported by an external frame to prevent significant vertical movements of soil) and allowed for the use of particle image velocimetry of the soil to determine relative movements (using identifiable targets). The following instruments were used in tandem to perform measurements in the apparatus: (i) force sensitive resistors (FSR) and soil pressure sensor (SPS) were used to measure the pressures inside, (ii) linear strain conversion transducer (LSCT) was used to obtain the movement of hydraulic jacks, (iii) strain gauges were buried inside the model ECL tunnel to detect strain variations of the ECL, and (iv) particle image velocimetry (PIV) was used to observe displacements of the soil. Whilst developed for a specific purpose, the apparatus could be used for all kinds of tunnelling or buried pipe investigations and need not be limited to the investigation of ECLs.

Two self-compacting concrete mixes were designed for this study and, with the use of a novel foldable formwork, model ECL could be cast then loaded and struck inside the apparatus under continuously stable loading without any pause of experiment. Consequently, tunnelling construction stages from the placement of the concrete and the curing of tunnel linings were simulated continuously. The concrete lining had a diameter of 290mm and a wall thickness of 30mm.

The duration of the experimentation varied, as did the duration of curing prior to the folding of the inner formwork, with the strength development of concrete during the experiment. The experimental results were compared with the results obtained from ABAQUS finite element models. The design, development, and testing of the 2D plane strain laboratory apparatus have been proven successfully by a series performance test on the apparatus. A parametric study on the soil-structure interaction associated with ECL operation was then successfully carried out and the results of the investigation are outlined in the following conclusions.

• The setting time of concrete has a vital effect on the interface between soil and the ECL. A faster setting concrete would result in a stiffer ECL at striking, and hence the ECL is not prone to deform under surrounding ground stresses.
• The timing of striking can change the deformation of ECL. Striking sooner would reduce the stiffness gained, hence the ECL is prone to deform in respect to the ground stresses.
• Stress redistribution is more noticeable on the ECL experiencing more deformation than those less deformed. The soil-structure interface changed accordingly with the deformation of ECL in sand.
• The degree of stress redistribution is linked to the number of cracks in the ECL. If an ECL has experience considerable stress redistribution owing to deformation, it is less likely to crack under extreme loading conditions happened at the early age.
• The numerical model successfully validated the novel laboratory apparatus being able to simulate a plane strain condition. The numerical results appeared to show the agreement in trend of deformation and stress of ECL that have been found in the experiment. Occasionally, the numerical results indicated approximately 50% variations at values comparing to those obtained from the experiment. This suggested that the model would benefit from refinement in the future to provide better results.

This research offers insights into the relationships between the concrete curing, tunnel deformation, stresses and soil displacement in the constructional period of ECL in particular with small-diameter tunnelling. The understanding of the soil-structure interaction was improved by the key findings above, but several points were raised to recommend future work.

• The research focused on plain un-reinforced concrete due to the limit on budget and timeframe. Future investigation on reinforced concrete, such as steel fibre reinforced concrete (SFRC), which shows recent popularity in the ECL applications (Table 2.1), would be an obvious next step for their better resistance and resilience to tensile stress.
• Further study on the other aspects of soil-structure interaction is recommended, such as long-term deformation of the ECL, the prediction of the cracks under extreme loading conditions, and the soil-structure interaction in the longitudinal direction of tunnel.
• Lack of field data comparison is a major obstacle to the study, but a refined numerical model could be established by conducting more sensitivity tests on the meshing of current numerical model.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):