Value engineering (VE) is a current buzz phrases within the industry. It is a systematic process which either identifies and eliminates unnecessary costs or increases utilisation. Although the main focus of VE tends to be aimed at permanent works, there is no reason why temporary works schemes should not also benefit from its use.
In my experience, designers of temporary works earthwork support systems often adopt a conservative approach, due in the most part, to the provision of poor information in the first instance. By investing a relatively short amount of time in the compilation of a thorough and detailed design brief, including detailed site ground information, these designers can immediately begin to consider the use of more optimistic soil parameters. This could subsequently allow the use of lighter duty support systems and in some cases, fewer of them.
It is also prudent to note that the design of temporary works support systems, are generally designed as a separate entity to the permanent works and are often carried out by different engineers from different departments even different companies. This discontinuity puts the temporary works designer (TWD) on the back foot as the permanent design is normally completed, signed off and in most cases, started before the temporary works is given any thought. With no room for compromise, the additional cost incurred by an overcomplicated construction sequence, will lie within the temporary works remit.
I have no doubt that by having the TWD involved at an earlier stage, it would help in reducing the cost of constructing the project. The connection detail between hydraulic struts and capping beams is one example of where design cooperation can be of benefit. Integrated or cast in corbels can vastly simplify this vital detail but requires forethought, planning and early commitment. Connections such as these can replace the need for dozens of bolted fixings. Post drilling holes for such a large amount of bolts, accurately, in heavily reinforced concrete and often at height, is not an easy task.
All too often, the additional benefits of using proprietary systems are ignored. With equipment rental, the key element to an economical system, is obviously reducing the hire durations for the equipment. Large cost savings can be achieved by reducing and simplifying the construction program with speed of installation/removal and without the need for on-site fabrication.
Despite the desire to use as little material as possible for the structural earthwork support system, frames and struts spanning excavations do need to be fairly robust as they are susceptible to accidental loading, strikes and construction imperfections. However, it is when we have to consider the additional loads due to potential temperature increases that design loads can go through the roof and in my opinion can lead to quite uneconomic design. In some cases, particularly if the retaining wall is considered to be “stiff”, design loads due to temp effects can double those from ground pressures alone. On the recent Tyne Tunnel project for example, where we were commissioned to supply temporary propping to the north side cut and cover approach, the specified temperature range was -10°C to + 43°C. I am not even sure if 43°C has ever been recorded in Newcastle! But even with a 30% reduction for wall stiffness; this still doubled the already considerable prop design loads. This required a larger steel section to be used for some props which further exacerbated temperature effect due to its increased cross section and stiffness. The strut tubes ended up being almost twice the size than if temperature effects had been ignored, which made for a very congested working area in places.
Whilst I am not for a minute advocating ignoring temperature effects, a sensible approach using observational principals could be adopted in this area. For example, where hydraulic struts are used that have a slightly reduced stiffness when compared to the steel alternative (due to compression of fluid within the ram itself) the flexibility of the overall support system will increase and consequently reduce load effects due to an ambient temperature rise.
To warrant this, a limited amount of prop load measurement was done on the Tyne Tunnel project, using both strain gauges and pressure measurements. It was quite obvious from this, that measured loads typically only reached 50% of anticipated design loads; even throughout a relatively warm summer.
Another often mentioned construction process is the Observational method, the aim of which is to achieve greater overall economy without compromising safety. Value management and observational methods when applied together can offer potentially large cost savings to construction parties. Having an effective load monitoring system in place is a key element of the observational construction methods and you can read more on this in a previous blog entry on load monitoring systems.
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