Shaft sinking in the UK is generally carried out using circular precast segmental linings which were originally developed after the Second World War as an alternative to the original and more expensive cast-iron linings. They are typically used to provide access for tunnelling operations, after which they can then be converted to permanent access chambers.

More recently, they are increasingly used to form structures such as storage chambers and pumping stations, where they can offer a cost-effective solution to more expensive alternatives such as in situ construction within a piled cofferdam.

The advantages are that the permanent works materials are, in effect, used as the temporary works during construction and also that the construction ‘footprint’ is kept to a minimum (this is an important consideration in urban areas). For small-diameter shafts (up to 4m in diameter), full-circle segmental rings are available on the market.

As circular structures, segmental shafts normally require no additional bracing during construction because all the ground loads are evenly distributed to produce only compressive loads in the lining. Also, the risk of settlement is kept to a minimum, as there are no large temporary working spaces to be backfilled on completion.

Pre-cast shaft linings were originally designed to mirror the earlier cast-iron linings so that they had a rib and recessed panel appearance. As such, they had limited use where depths exceeded around 20m; they are no longer readily available. These linings have now been replaced with solid units, normally 1,000mm wide, in standard diameter ranges from 4 to 25m, with different manufacturers having their own bolting systems. The individual segments are cast to very exacting tolerances to ensure accurate alignment between components. The design of these units normally incorporates a sealing system which makes use of hydrophilic strips or rubber gaskets, the latter being factory fitted.

Increasingly, these segments are being manufactured using fibre reinforcement which, if monofilament polypropylene fibres are used, have a beneficial effect in terms of fire resistance and also, more generally, makes it easier to break out openings and attach fixings. Although the majority of manufacturers produce a standard range of products to a set design suitable for the vast majority of projects, they will also provide a design service and special manufacture of linings to suit more demanding or specific design conditions. In addition, as part of their service, manufacturers are able to provide specialist items such as corbel units to accommodate landing slabs and also complete roof slabs designed to the engineer’s loading requirements.

A more recent development, often used in conjunction with segmental shaft construction, is the use of a sprayed concrete lining (SCL). SCL is used to replace the segmental lining and can incorporate reinforcement and/or lattice arches or, alternatively (and now more commonly), fibre reinforcement. Typically, segmental ring construction might be used for the upper levels of the shaft where ground conditions and surface features make this appropriate, with a change to SCL in the underlying more stable strata.

For permanent works, the use of a secondary in situ lining may be required to create a smooth finish. SCL can be used in conjunction with either sprayed-on or sheet-membrane waterproofing. One considerable advantage of a sprayed concrete lining is that openings required in the shaft lining can be formed by using additional framing reinforcement but without the need to introduce expensive temporary works support while the portal structures are constructed.


Alternatives to shaft-sinking methods not covered in this article include:

  • Sheet-piled cofferdams
  • Diaphragm walls
  • Contiguous and secant walls.


Shaft-sinking methods are broadly subdivided into two categories: underpinning and caisson sinking.


Underpinning involves the excavation and erection of each ring of the segmental lining beneath the previously constructed ring. As each ring is completed, cementitious material is injected behind the lining to fill any voids present and secure it in the ground, ready to support the next ring, which is bolted up from beneath. Different manufacturers have their own bolting systems for doing this.

This method is normally used in firm, self-supporting ground or where ground treatment processes have created stable ground conditions. However, it can also be used to recover a situation where a shaft being sunk as a caisson has become stuck, although additional ground stabilisation processes will probably be required in this instance.

The initial segmental ring is placed in a pre-dug excavation and keyed in to a concrete collar cast around it, normally by inserting dowels through the grout holes. It is vital during this process that this initial ring is constructed within the correct tolerance and also fully supported.

The grouting process requires the base of each ring to be sealed. There are geotextile hoses available that can be fixed behind the ring before it is built; after building, the hose is inflated with grout to seal the annulus before void grouting begins. It is, however, more common to push excavated material in under the ring once it has been built to achieve the same result: this is the so-called ‘fluffing up’.

When using this method, care must be taken not to damage the seals. It is also good practice to form small voids in the previously grouted annulus up to the grout holes of the ring above to release any trapped air as grouting takes place.

Excavation of shafts constructed by underpinning is commonly carried out by 360° excavators, initially working from the surface and then lowered into the excavation as it becomes deeper (Figure 1). Alternatively, there is a range of pole grabs available (some telescopic) that can be attached to excavators and used from the surface. (Figure 4) With the advent of zero-tailswing models, it is possible to get machines into all but the smallest of shafts. It is very important to accurately trim the excavation to the correct profile; this avoids overbreak and excessive grout use.

Segments are usually placed by crane using specially manufactured underpinning frames, supplied by the segment manufacturers. Annulus grout is typically mixed and pumped using special composite units driven by compressed air.

The segment manufacturers normally provide threaded grout sockets in their segments, and it is important to check that the grout-gun nozzle is compatible with the fittings supplied.

In addition to underpinning using segments, the same basic process can be used with SCL methods (Figure 2). Once the shaft has been excavated for the predetermined depth, the SCL is applied using a robot sprayer. This material is typically supplied ready mixed and retarded into a site-based silo, then discharged directly into the pump with an accelerator which is added at the nozzle.

Lining reinforcement can be provided in the form of mesh or prefabricated arches, but it is becoming increasingly common to use fibre-reinforced concrete, which speeds up the process considerably.

Shaft openings typically use steel reinforcement locally, and can be formed incrementally as the excavation proceeds without the need for temporary support. If required, sprayed waterproof or sheet membranes can be incorporated in the lining, normally by sandwiching them between two separate layers of SCL.

Caisson sinking

Caissons can be round, square or rectangular but the majority of caissons are circular. Caisson sinking typically involves constructing the first one or two rings of the shaft at ground level within a substantial reinforced concrete collar using a special cutting ring at the leading edge. Some manufacturers can supply the rings for caisson sinking with an external bolting system which eliminates the needs for bolt pockets within the shaft and also the need to gain access to the inside of the shaft during ring-building operations.

As with underpinning, it is essential that the initial rings are built accurately and held in position while the collar is concreted. These rings are surrounded by polystyrene sheets before concreting the collar to create a sleeve through which the shaft can slide. Sacrificial jacking bases are also positioned around the perimeter onto which the vertical shaft jacks are then fixed after concreting.

The number of jacks will depend on the diameter of the shaft and the depth to be sunk. (Figure 5) For shaft sizes up to around 10m-diameter, most segment suppliers manufacture their own precast cutting edges. Over this size, it is necessary to use a fabricated steel unit which must be designed to suit the sizes and fixing patterns of the rings to be used. For larger diameters and demanding ground conditions, it is essential to have a steel unit that can be welded on site to increase rigidity and prevent the shaft distorting during sinking.

The cutting edge (Figure 3) must provide an overcut to the rings to be used, so that an annulus is formed as the shaft sinks, enabling a lubricant to be introduced. This annulus is typically of the order 50mm. There are a number of products on the market suitable for this operation. The caisson is sunk by excavating from within and then letting the shaft sink in a controlled manner, almost always by the use of vertical hydraulic jacks, typically of 40t maximum capacity, positioned around the collar. The size, and hence weight, of this collar must be sufficient to counteract the anticipated jacking loads required. As the shaft sinks further, rings are added at the surface. Specially designed working cages are needed for this operation (Figure 4).

The annulus created by the cutting edge is kept filled with a thixotropic material such as bentonite or one of a range of synthetic products currently available to support the excavated ground and to minimise friction. On completion of sinking, this material is replaced by the injection of cementitious grout in one operation to lock the caisson into position and to replace the lubricant with solid material to minimise settlement. During sinking, a constant check must be kept on the verticality and square of the shaft, and corrections made on the jacks to keep it within tolerance.

Once a caisson becomes badly out of alignment the consequences can be severe, including getting it stuck and/or segment damage. In this regard, careful attention should be paid to the lubrication process, particularly where there is a risk of ground coming into the caisson. In addition, a careful analysis of the ground conditions should include a determination of the likelihood of large obstructions, such as boulders blocking the cutting edge.

Caisson excavation can be carried out ‘dry’ or ‘wet’ depending on ground conditions. If the ground is naturally stable or has been stabilised, excavation can be carried out from the surface or from within the shaft. If the conditions are unstable and/or waterlogged, excavation must be carried out with the shaft flooded to the prevailing hydrostatic level. In these circumstances, the excavation plant normally used is either an excavator-mounted pole grab, (Figure 5) where special telescopic models can reach depths of around 20m, or a rope-operated digging grab which is mounted on a crawler crane.

It is becoming more common to use caisson sinking, even in stable ground, because the method eliminates the need for the trimming process required when underpinning. The method also minimises the need for personnel to be in the shaft, as the ring building takes place at the surface.

The same plant is used for mixing and pumping the lubricant and for the final grouting operation, as has been described for underpinning.

With wet caissons, it is normally necessary to seal the base with the shaft submerged. This is because dewatering, once the shaft has reached its depth, might cause the base to heave or ‘blow’ under hydrostatic pressure. Even if dewatering is a possibility, sealing the base in wet ground conditions can be extremely difficult. The depth of the so-called concrete ‘plug’ must be sufficient to provide enough resistance to the hydrostatic uplift in conjunction with the weight of the shaft rings and the weight of the collar. The latter is normally attached to the shaft, once sunk to its final position, by fixing dowel bars through the top rings into the collar designed to provide the shear resistance required.

The concrete plug is placed by tremie methods, almost always using concrete pumps. To provide a key, it is usual to install recessed panel rings in the plug location or, alternatively, corbel rings. Segment manufacturers usually supply these as part of their shaft segment range. The plug must be left in place for a minimum of five days to cure before dewatering begins. Preparation of the surface following dewatering can then commence, usually by placing a regulating blinding, to allow construction of the structural base above. Some engineers like to incorporate a dowelled connection between the plug and the structural base in the design to ensure against any possibility of separation along the boundary.

Current practice in the UK for calculating temporary resistance to uplift normally ignores any resistance provided by grouting the annulus or the shear resistance of the ground at the base, and usually assumes a groundwater level at ground level. On top of this, a safety factor of the order of 1.05 is typically applied.

Where the base of the shaft is founded in stable ground, or where the base has been rendered temporarily stable by dewatering/pumping or another ground stabilisation process, as an alternative to a deep plug it is possible to provide uplift resistance by under-reaming. The shaft is first stabilised by normal annulus grouting and the cutting edge is usually removed, which in the case of a steel fabricated unit, can be reused. The base excavation is then under-reamed, using temporary supports if required, to extend it beyond the shaft footprint. Once the reinforced concrete base has been cast, the passive resistance of the undisturbed ground above the toe is mobilised to counteract uplift.

Circular caissons formed of pre-cast segments can be sunk incorporating an external concrete surround or ‘jacket’. As each ring (or possibly pair of rings) is added, it is surrounded by in situ concrete using a steel shutter prior to further sinking. This concrete is usually fibre reinforced but can incorporate mesh reinforcement and is typically 250–500mm thick. This process requires modification of the shaft jacks to give them additional overhang so that they only bear on the pre-cast segments and not the jacket.

The advantages of this system are:

  • It increases the weight of the shaft and thus can reduce the depth of the anti-flotation plug required, which may be desirable to avoid sinking the shaft into more challenging ground conditions.
  • It adds an additional measure to ensure water-tightness.
  • It adds rigidity to the shaft to counteract any tendency for the shaft to distort.
  • It can be used to incorporate vertical injection tubes to the cutting edge area that can be used for jetting during sinking or final grouting operations.
  • Most importantly, it enables any portals required at the shaft base for follow-on tunnelling operations to be constructed without the need for internal temporary bracing. The pre-cast linings at the portal positions can be broken out while leaving the intact jacket to take the circumferential hoop loads and maintain the structural integrity and water-tightness of the shaft during portal construction. On completion, exit seals can be fitted to the portals prior to the launch of the tunnelling equipment, which, if correctly tooled up, can then cut through the fibre-reinforced concrete lining provided by the jacket during the launch procedures.

There are a number of ground stabilisation processes that can be used to aid shaft sinking and to reduce construction risks. It is worth bearing in mind that if the shaft construction involves excavating, moving and disposing of large amounts of saturated material, particularly in urban areas, it may be prudent to consider ground stabilisation on environmental grounds to lessen the impact. Likewise, if the shaft is to be sunk using sump pumping to control groundwater, the issues of silt separation and discharge facilities should be seriously considered; very exacting standards are normally demanded from licensing authorities before such discharges can be accepted into surface water disposal systems. If deep well dewatering is being considered, there are issues to be addressed with regard to abstraction and discharge licenses.

The use of such processes needs to be considered and decided upon at the construction planning stage, as installation is more difficult to achieve once construction has started; is likely to be less effective; and can be very disruptive and costly.

Precast roof slabs

Most shafts require some form of roof slab for the completed structure. As an alternative to costly in situ construction, often requiring expensive temporary formwork support, most shaft segment manufacturers will provide a precast solution as part of the service they offer. This can also have the benefit of time savings, as the manufacture takes place off site, with the installation itself normally taking one or two days. The manufacturer will typically design the slab to the engineer’s requirements as part of this service. However, the whole process normally takes around 8–10 weeks, so early planning for this option is advisable.


British Standards Institution (BSI) (2019) BS 6164:2011. Code of practice for health and safety in tunnelling in the construction industry. BSI, London, UK.


This article is extracted from chapter 15 of Peter F Pallett and Ray Filip (eds.) (2018) Temporary Works: Principles of design and construction, Second edition. ICE Publishing. © Thomas Telford Ltd, used with permission. A print copy of the book is available for purchase at