Any water that leaks into a structure like a shaft usually needs to be actively managed so that the required environment can be maintained. In cases of significant water leakage, it is often necessary to introduce extensive water management systems to control the environment and discharge the groundwater. These water management efforts can be costly, take up project time, are difficult to operate and maintain, and take up value space both above and below ground.
Some shafts can be externally tanked with a waterproof membrane; however, this can sometimes prove to be difficult, impractical, and costly. In many cases there is a need to provide resilience to inflow control by alternative means.
Traditional methods of underground shaft construction, such as cast-in-place linings, can require expansive excavations and heavy temporary supports. Linings are typically placed in sections, creating cold joints in the lining that will eventually lead to water ingress through the structure if differential pressure exists between the outside and inside of the shaft.
Some shaft sinking techniques require that the local groundwater table to be lowered to enable the construction in “dry” conditions without significant groundwater inflows. The introduction of pumping wells to lower the groundwater can have significant impacts on adjacent infrastructure and can be the cause of settlement and property damage resulting in reputation damage and costly repairs.
Six key benefits to segmental shaft method
However, there is another option — a way to combine “watertightness” with temporary and permanent ground support for shaft construction. Jacked pre-cast segmental shaft construction can often be completed through water-bearing stratum without the need for a dewatering campaign thereby limiting the risk of settlement, movement, and damage because of the works. Pre-cast segmental shafts utilize a pre-cast gasketed lining system, which allows the shaft lining to be constructed as it is progressively sunk.
Where the geology is appropriate, the use of hydraulic jacks to push pre-cast shaft linings into the ground has provided a quick, reliable, and efficient means to construct shaft structures. This means that shafts can be constructed through water bearing strata, sealed, and then drained, without employing extensive dewatering campaigns and effectively limiting settlement to surrounding structures.
The pre-cast rings are manufactured off-site, ensuring a high-quality product, and come equipped with factory fitted water seals (gaskets) that provide an immediate watertight seal during construction when the rings are bolted together. The compressed gaskets limit the amount of water ingress to an acceptable level for use in the transportation, water, and utility industries.
Segmental shafts are commonly used in Europe. Through our work in the UK, we have found six key benefits to utilising this method.
- Increased safety for construction personnel, by significantly limiting the requirement for a work crew in the shaft during construction.
- Reduced or negated construction de-watering requirements, as shafts can be sunk without the need to lower the water table locally, mitigating the risk of inducing settlements on adjacent structures.
- Reduced construction schedule risk by minimizing cast-in-place concrete, managing water inflows, and providing greater confidence in advance rates even through poor ground.
- Efficient construction, through combined temporary and permanent lining resulting in reduced excavation sizes and a shorter overall construction schedule.
- High quality and consistency of final product due to offsite segment production. No need for on-site or laboratory material testing during construction.
- No need for a primary or secondary cast-in-place lining, which reduces on-site waste streams including concrete waste, falsework, formwork, and shoring.
Further, pre-cast segmental shafts can be used in conjunction with other techniques when needed, including underpinning, rock bolts, mesh and shotcrete, sprayed membranes, or continuous secondary (slip-formed) lining systems.
Local considerations dictate
In Europe, pre-cast facilities are commonplace and are located close to worksites. At present in North America, such practices are not as well established in most markets and as such it may be necessary to transport segments longer distances or even import them. However, as no material testing is typically required on site the risk of needing to replace defective sections of work is low, thus mitigating any increased haulage costs. Establishing a pre-cast site with franchised pre-cast forms and technologies could be considered for projects which benefit from an economy of scale.