Discussion

The paper by Boden et al. (2018) shows horizontal coring is an excellent ground investigation system for tunnels in igneous rock. It costs more than vertical drilling but gives better value for money since, if you drill from the portals, all the core is relevant to tunnel design and construction. Crucially it passes through any fault zones and areas where rock types change. In contrast, with vertical boreholes, the core taken from overburden is of little value, and only the core retrieved from tunnel level is really useful. I believe the 1997 Cheung Ching Tunnel was the first in Hong Kong where the whole length was covered by horizontal drilling from the portals.

For tunnels in sedimentary materials, normally it is more important to know the levels of the various strata, and horizontal drilling is of less benefit. However, vertical drilling is like painting a football pitch into the ground. The white lines at each end represent vertical drill holes; if you have a generous client who is prepared to pay for holes at 50 m centres, you can paint on the half-way line also. The grassy areas are all a mystery until you actually excavate.

My experience with flood control and drainage of fast growing cities has taught me that it is not so much the planning, design and construction that pose the problems, but rather the maintenance and upkeep of the system. Many systems fail when water bodies, waterways and hydraulic structures are not properly maintained and when solid waste and illegal building are not properly controlled. I believe Dolman and Ogunyoye (2018) should have stressed this in their paper as a major point of interest and care.

Our understanding of water challenges in cities, gained from looking back into history and at our current systems, is that they are more complex and multi-faceted than the contributor suggests. Although maintenance and upkeep is a relevant aspect of urban water management, we need to recognise that many current systems are products of single-purpose, infrastructure-driven, solutions. It is therefore important that we first evolve and adapt our systems into those that are resilient in the face of complex and changing conditions, as opposed to just maintaining an increasingly failing system.

With the above in mind, our paper focused on the understanding of the broad challenges, and how we can start a transition into more integrated management of the functions as well as the social and political contexts. The urban water management transitions framework (Wong and Brown, 2008) offers a way forward to transition towards ‘water-sensitive’ cities.

The water-sensitive city framework identifies six distinct states of the urban water system as a city evolves in response to socio-political drivers. The first three stages of the embedded continuum describe the evolution of the water system to provide essential services, such as secure access to potable water (water supply city), public health protection (sewered city) and flood protection (drained city). These are followed by the waterways city, water cycle city and ultimately a water-sensitive city. They describe the anticipated evolution of the urban water system to: deliver higher-order services such as social amenity and environmental protection, provide reliable water services under constrained resources, and ensure inter-generational equity and resilience to climate change.

Considering more than 50% of cities are not fitted with a sewer system or storm drainage, developing cities have the potential to leapfrog towards greater water sensitivity through the provision of multi-functional and multi-purpose water infrastructure. Yet many developed cities have historically heavily invested in single-purpose systems, and consequently must keep investing in the maintenance and upkeep of these systems. The sustainable water usage and water-sensitive city transition can be promoted in coherence with other transitions, such as energy and circular economy.

The paper by Mahmoudi Moazam et al. (2018a) is an interesting and potentially valuable work on old plain concrete arch bridges. However, they say, ‘the bridges behaved linearly under vertical loading’ (Mahmoudi Moazam et al., 2018a: p. 135). While some published load–deflection plots are presented as smooth curves, the reality is a series of straight lines, with stiffness changing as hinges form in the arch (Harvey, 2012, 2013).

They also say, ‘this study presents a comprehensive model for precise assessment of plain concrete arch bridges’ (Mahmoudi Moazam et al., 2018a: p. 137). If accuracy is what is meant, that has not been demonstrated. There is no way of knowing whether the deformations measured are from the cause assumed in the analysis, or that the stiffness of foundations against rotation can be effectively computed from tests that do not impose rotation. Furthermore, the model uses smeared-crack behaviour, but masonry bridges tend to crack in discrete locations.

In Figure 4(a) for bridge km-23, there are step changes in measured deflection at about 500 kN and 1100 kN. The flattening of the curve over 6000 kN also indicates the bridge has suffered permanent damage, which is surely inappropriate unless it has been taken out of service and will be demolished.

A lot of research has been performed in the field of masonry arch bridges and we are aware that some of the results are questionable. However, we believe research that shows stone arch bridges can have a linear behaviour under vertical static loading is valid. The response of masonry arch bridges is of course affected by various factors including length of spans, number of spans, geometric characteristics and mechanical properties. Figures 9 and 10 in the paper by Marefat et al. (2017) indicate that masonry arch bridges behave non-linearly too, with changes in stiffness occurring as hinges form in the arch. For more clarification please see our paper Mahmoudi Moazam et al. (2018b).

We believe the best way to predict the seismic behaviour of masonry arch bridges is through calibrating a numerical model with the results of an experimental study. We used the smeared crack model as this has been used in other valuable studies such as that by Fanning and Boothby (2001). The km-24 bridge modelled under the 1990 Manjil earthquake demonstrated linear behaviour, so the crack model was not important in this case.

The Figure 4 plots resulted from loading the bridge over a period of 3 d. The loading was paused at night and creep occurred, creating steps in the plot. The static tests indicated that despite the existing levels of damage and deterioration, both bridges were still able to carry their service loads very safely.

The study clearly showed that bridges with shorter spans and higher-quality material (such as km-24) will not reach their structural capacity under strong ground motion. Field test observations also indicated that such bridges are over-designed. On the other hand, bridges with longer spans and moderate material quality (such as km-23) are more vulnerable. Bridges with low-quality material are highly vulnerable to earthquake damage and retrofitting is clearly necessary.

It is good to see from the paper by Zhou et al. (2018) that woven arch bridges are being built again. However, woven systems imply combinations of an n segment basic system and an n+1 segment auxiliary system. A three-segment basic system (Figure 3) is clearly simplest for temporary support, but for two systems to work together they need to fit exactly or have local flexibility to redistribute load.

Also, given the poles will inevitably have different diameters, they would either need to be set at different levels, thus exerting a rotational force on the cross-beams, or the cross-beams would need notching. The paper makes no reference to abutments or how the arches are supported on them. These arches are such that the flexural interaction needed would permit very little movement in the abutments.

The authors say the dead weight of the superstructure improves the stability of the arches but that does not seem right. The connection between the main members and cross-beams is little more than a pin and, if anything, the pin connection will become less stable as the load increases.

Finally, there are sketches of bracing systems, but no details of how they work.

Figure 3 is intended simply to show how the basic and auxiliary systems fit together and is not intended to be a series of structural mechanics diagrams. Each system is securely fixed at the ends through ground beams located on masonry abutments, and they limit the movement of each other though the cross-beams working in shear. ‘Woven’ should be seen as a spatial description of the timber members rather than a structural one: the overall strength comes from reliable connections between the system members.

As illustrated in Figure 3, the basic system is always a tri-segment system, while only the number of segments in the auxiliary system changes. Without restriction from the other system, both the basic system and auxiliary system can move if the connections between the system members fail. The cross-beams which link the longitudinal system members with mortise-and-tenon connections contribute much to the structural capacity, and it is the synergistic effect of putting the basic and the auxiliary systems together that makes the bridges work.

All the members are sourced from local fir trees so the diameters do vary. To keep a system’s members tight against the cross-beam of the other system, the end positions of the members are adjusted.

The mortise-and-tenon connections are designed to be in compression, such that the self-weight of the structure improves stability. Ground beams which connect the ends of the basic system are placed on masonry abutments so the arch thrust is transmitted into the abutments.

Lateral stability is provided by diagonal bracing connecting the basic system cross-beams to the king-posts at the outer ends of the ground beams.