Shaft sinking in the UK is generally carried out using circular pre-cast segmental linings, originally developed after the war as an alternative to the original and more expensive cast-iron linings. They are typically used to provide access for tunnelling operations where they can then be converted to permanent access chambers. More recently, they are increasingly being used to form storage chambers and pumping stations etc 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 (an important consideration in urban areas).
Specification clauses for shaft construction and breakouts from shafts can be found in the BTS Specification for Tunnelling (2010).
For small-diameter shafts in the range up to 4m, full-circle segmental rings are available but high unit weights need to be considered when specifying the construction plant required.
As circular structures, segmental shafts normally require no additional bracing during construction since all the ground loads are evenly distributed to produce only compressive loads in the lining. Another advantage is that, properly constructed, the risk of settlement is kept to a minimum; the construction process should ensure that the exposed ground is supported with the minimum of delay and there are no large temporary working spaces to be backfilled on completion.
Precast shaft linings were originally designed to mirror the earlier castiron linings so that they had a rib and recessed panel appearance. As such, they had a limited use in the context of depths exceeding around 20m and are no longer readily available. These linings have now been replaced with solid units, normally 1,000mm wide, in standard diameter ranges from 4m 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 utilising hydrophilic strips or rubber gaskets, the latter being factory fitted.
Increasingly, these segments are being manufactured using fibre reinforcement which has a beneficial effect in terms of fire resistance and also 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. Additionally, 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 sprayed concrete lining (SCL). SCL is used to replace the segmental lining and can be used in conjunction with reinforcement and/or lattice arches or, alternatively (and now more commonly), with 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, 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 with SCL is that openings etc required in the shaft lining can be formed by the use of additional framing reinforcement but without the need to introduce expensive temporary works support while the portal structures are constructed. On the other hand, openings in shafts built with segmental rings generally require considerable temporary support during construction (particularly at deep levels) as the inherent compressive strength of a shaft ring is lost once an opening is formed in it, until the permanent works are completed.
Major alternatives
Alternatives to shaft sinking methods that are not covered in this article include sheet piled cofferdams, diaphragm walls and contiguous and secant walls. For information on these methods, please consult the full publication (see box, p.44).
Common methods of construction
Shaft sinking methods are broadly subdivided into two categories: underpinning and caisson sinking. See the BTS (2004) publication, Tunnel Lining Design Guide, for more details.
Underpinning
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 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 to do 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 situation.
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 built within the correct tolerance and fully supported, as any settlement during the casting of the collar could have serious repercussions in keeping the rest of the shaft vertically aligned. Once the first ring of the shaft is fixed in the ground, it is common to fix plumbing brackets around the top of the ring to check verticality as the shaft is sunk.
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 built to achieve the same result: socalled ‘fluffing up’. When using this method, care must be taken not to damage the seals. If an excavator is being used it is possible to fit a purpose-made blade for this purpose. 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. Alternatively, there are a range of pole grabs available (some telescopic) that can be attached to excavators and used from the surface. With the advent of zero tail swing models, it is possible to get machines into all but the smallest shafts. It is very important to accurately trim the excavation to the correct profile; this avoids overbreak and excessive grout use.
Traditionally carried out by hand, adherence to current HAVS (hand arm vibration syndrome) regulations can make this a time-consuming process. One method of overcoming this is to reverse the bucket on the shaft excavator so that it can dig upwards to assist the trimming process. Segments are usually placed by crane using specially manufactured underpinning frames, supplied by the segment manufacturers.
Grouting normally uses bagged cementitious material supplied shrink-wrapped and on pallets for weather protection and ease of handling by forklift. It can be mixed and pumped in special composite units driven by compressed air. The segment manufacturers generally provide threaded grout sockets in their segments, and it is important to check that the grout gun nozzle is compatible with the fittings supplied.
As well as underpinning using segments, the same basic process can be used with SCL methods. Once the shaft has been excavated for the pre-determined length, the SCL is applied using either a hand-held nozzle or a robot sprayer.
For most operations, this material is supplied ready-mixed and either discharged directly into the pump or held in a re-mixer on site. As with most operations involving SCL, the material is supplied retarded and an accelerator is added at the nozzle.
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. 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 membranes can be incorporated into the lining, normally by sandwiching them between two separate layers of SCL.
Caisson sinking
Caissons can be round, square or rectangular but must be of a regular cross-section with no protrusions which would cause drag leading to the danger of lock-up. The majority of caissons are circular, however.
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. As with underpinning, it is vital 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 before concreting within the collar onto which the shaft jacks are then fixed. 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 which can be welded on site to increase rigidity and prevent shaft distortion during sinking.
The cutting edge 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 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 with specially designed working cages needed for this operation.
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 onto 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. Caissons have far less danger of becoming stuck in fine-grained homogeneous soils than in, say, gravels or boulder clay, where alternative methods might be more appropriate.
Caisson excavation can be carried out ‘dry’ or ‘wet’ depending on ground conditions. If the ground is naturally stable or has been stabilised by, for example, dewatering, excavation can be carried out from the surface or from within the shaft using excavation plant described for underpinning above.
If the conditions are unstable and/or waterlogged, or where the hydrostatic conditions could cause the base of the excavation to ‘blow’, 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, where special telescopic models can reach depths of around 20m (see image below), or a rope-operated digging grab mounted on a crawler crane. With the latter, the digging ability is governed by the hardness of the material and the submerged weight of the grab. In hard material it is possible to add weights to the grab; if this is not successful, measures such as pre-auguring or the use of chisels suspended from the shaft crane must be considered.
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 as for the final grouting.
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 tremmie 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 can then commence, usually by placing a regulating blinding, to allow construction of the structural base above.
Current practice in the UK in 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 it 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 re-used.
The base excavation is then under-reamed, using temporary supports if required, to extend it beyond the shaft footprint. Once the reinforced concrete base is cast, this mobilises passive resistance of the undisturbed ground above the toe to counteract uplift.
There are a number of ground stabilisation processes that can be used to aid shaft sinking and to reduce construction risks; these are discussed in detail (see full publication).
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 considered and decided upon at the construction planning stage as installation is more difficult to achieve once construction has started, likely to be less effective and can be very disruptive and costly.
Pre-cast 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 pre-cast 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 1-2 days. The precast manufacturer will typically design the slab to the engineer’s requirements as part of this service. However, the whole process normally takes of the order 8-10 weeks, so early planning for this option is advisable.
Principles of design
The design of a shaft, the method of sinking and the selection of the lining depend on many factors. Shaft projects can vary from small, simple schemes to large and complex systems, and the final use will affect the design.
The most important factor which influences shaft design is the type of ground and whether it is unstable or competent and self-standing when excavated. The presence of groundwater exacerbates the unstable ground conditions and imposes hydrostatic pressures which increase linearly with depth in both unstable and competent strata.
Once ground conditions are known from site investigations, the basic parameters for excavation and muck removal, groundwater control, ground stability control and lining installation can be evaluated and the shaft lining type chosen (visit http://www.alanauld. co.uk/index.php for more details).
Calculation of active pressures from the ground on to the walls is covered in the full publication. These figures are also used to give upward pressures to check the stability of the base in the temporary as well as the permanent condition. Note that it is the situation before the base is installed that is the critical condition, and that particular dangers are created by the hydrostatic forces.
Structural design of pre-cast units follows standard reinforced concrete or fibre-reinforced design as appropriate, but is a service provided by the supplier.
Design of plan-circular structures in the ground is by using hoop compression, suitably factored to make allowance for non-uniformity of ground loading, and allows a very light efficient structure to be used. Openings in hoop compression attract very large concentrated loads around their edges.
It is always prudent to design out any plan shape with intrusions or a non-uniform cross-section, not only for the high stresses at angles but because of the danger of such shapes becoming stuck when being sunk as a caisson
