BTSYM | WORKSHOP REPORT
● Pipejacking differs from segmentally-lined tunnels in that the entire tunnel linings are pushed along the whole route rather than, like segment, be placed stationary in set locations.
● Sprayed concrete lined (SCL) tunnels (for sequential excavations and supports) offer flexibility in geometry and are more suitable for short tunnels and those with variable alignment and shapes.
● Drill & Blast is a flexible excavation method that tends to be chosen more for tunnels in more remote, unpopulated mountainous areas with high strength rocks.
● Shafts - to be break up tunnelling routes, intermediate shafts can be adopted; there are many options available to choose from (such as precast segments, sprayed concrete, diaphragm walls (D-Walls), rock bolts, etc) depending on the required geometry and ground conditions.
LEARNING IN A ‘CON-FUSION’ WAY Given the inherently multi-disciplinary nature of tunnel engineering, the author’s personal experience shows that an engineer can go through periods of confusion in the learning process, during which one feels even less clear or confident than before. This is reckoned to be a natural (even essential) step in the whole learning and growth process, which can unfold in the following stages: ● ‘Conceptualisation’: You learn the specifics on an isolated knowledge node; you think you have mastered it, but the knowledge is actually gained in a vacuum, disconnected from the broader context.
● ‘Confusion’: You have collected many isolated nodes of knowledge and started to sense that they are somehow related, but sometimes apparently conflicting; you start attempting to put these nodes together but only to find these muddled up in your mind and difficult to separate.
● ‘Fusion’: Eventually, you start to internalise the knowledge pieces as a coherent framework. The once-disconnected nodes form an integrated structure, allowing you to apply them fluidly and purposefully toward a common goal in a single combined strand.
As an example, to illustrate the point, the author attempted to confuse the workshop attendees with a commonly used ‘linear-elastic-perfectly-plastic’ stress- strain relationship for three different materials: concrete; steel; and, soil. The constitutive models for these materials are
typically taught in different modules or classes, and bear extremely similar resemblance in appearance, which can give rise to confusions. Below are some key points on how these materials are very different to each other in meaning and context, such as their stress-strain relationships being derived from quite different types of forces: ● Reinforced concrete: the stress-strain curve applies to uni-axial compressive stresses only; the concrete must also be reinforced with at least minimum code-
32 | August 2025
required amount of reinforcement to demonstrate the ductility as depicted. Behaviour under tension is distinctively different – unreinforced concrete in tension is extremely brittle; also, the shape of the curve differs between bar-reinforced and fibre- reinforced concrete.
● Steel: The stress-strain is for uni-axial tension only. When plotted on the same diagram as concrete, it shows on the opposite (negative) quadrant. The stress-strain curve for steel in compression does not exist, as the failure mechanism is different. The ultimate strain is an order of magnitude higher than that of concrete, which means steel is a lot more ductile than concrete. The composite action between steel and concrete in reinforced concrete in tension makes the line non-linear and much more complicated.
● Soil: The stress-strain curve applies to shear only – the relative sliding between particles in contact along an interface plane. The position of the failure line (horizontal) is typically defined by the Mohr-Coulomb criteria, dependent on the compressive stress perpendicular to the interface (the confining stress); it is therefore not a set value but changes in accordance with the confining stress.
● Soil: It should also be noted that the volumetric behaviour (change in size) of soil is distinctively different from its shear behaviour (change in shape) – the drained bulk stiffness can increase as the soil gets compressed more, especially for the first time, and the undrained bulk stiffness (with water virtually incompressible, so all volumetric compression is resisted) approaches infinity in relative terms.
‘GROUND-STRUCTURE’ INTERACTION It’s uniquely imperative for a tunnel engineer to appreciate that the ground is both loading and resistance. The extent to which it plays each role is determined by ‘ground–structure interaction.’ In civil engineering courses, the structural engineering
and geotechnical engineering modules are often taught separately. ‘Soil-structure’ interaction, which is where these two meet together and work in a combined strand, is usually less well taught in classes – and where done so it usually related to surface structures and foundations. This ties exactly into my point on ‘con-fusion’ and the developing ‘knowledge tree’ as civil engineers find ways to understand how these work together. A further challenge in appreciating their interaction
can arise outside civil engineering. In the workshop, we discussed ‘structure’ as referring
to those man-made elements such as tunnel linings and props, and while covering both primary and secondary linings it was mainly the former (as it is usually the one that interacts with the ground). The following was explained in essence: ● The ground can provide resistance to its own loading; ● The structure shares responsibilities in providing resistance with the ground;
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