Principal Design Engineer
In many situations, pre-cast concrete segmental shafts deliver significant benefits over other methods, allowing risk to be better managed and mitigated when sinking through difficult ground conditions in a very controlled manner. Constructing a single-pass shaft liner which combines temporary and permanent works requirements can save time and money in addition to mitigating risk. From within a very confined work site at surface, in the appropriate ground conditions, this methodology can be utilized to construct shafts of up to 80-feet diameter with a depth of up to 230 feet.
When constructing a shaft, if one of the goals is to have it be watertight, failure to mitigate and manage groundwater inflows during construction can have several unintended consequences including:
- impacts to the durability of the shaft itself: carbonation and corrosion can lead to cracking, spalling and even failure of the lining systems in extreme circumstances, if left unchecked.
- groundwater carrying contaminants into the structure, which could impact the operation and maintenance of the shaft.
- groundwater inflows into the shaft which may need to be treated and disposed.
- inflows that may have an impact on the surrounding groundwater table, leading to settlement of existing infrastructure and buildings, and impacts to water supply wells.
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.
In very sensitive structures or environments, secondary waterproofing applications can also be considered to complement gasketed pre-cast segmental shaft construction to reduce water ingress further to the point of being essentially waterproof.
In cases where a shaft interfaces with other in-ground infrastructure, additional support and detailing is required. In some situations, switching construction methodologies at an interface may be more appropriate depending on the ground conditions. These interfaces may include:
- soft spots required to allow for the excavation of a tunnel into / through the shaft.
- launch frames and head walls for TBMs, pipe-jacking or micro-tunnelling.
- development of a starter or tail tunnel required for commencement of TBM drive.
- incoming feeder tunnels (e.g. in large sewer lift stations).
When designers look at shaft and tunnel systems, they should do so holistically, rationalizing interfaces, selecting appropriate complementary construction methodologies, design of temporary and permanent works items (including in shaft structures), undertaking ground movement impact assessments and any mandatory applications that have to be submitted.
With appropriate site investigation, ground characterisation and construction controls, this design and construction methodology has been successfully used to safely construct single-pass shaft liners through challenging ground conditions in water bearing stratum, while minimising the impact on adjacent surface and near surface infrastructure. Due to the benefits derived from this construction technique it has been used successfully in dense urban environments, confined worksites and even from within existing basements while the building above remains occupied and operational.
Ben Hodgetts, C.Eng., was a contributing author. He is a Senior Engineer with approximately 10 years’ experience in the analysis, design and constructability of underground structures in both temporary and permanent conditions.