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CPM offer a range of reinforced manhole units specially designed for sinking by the caisson method from DN 2000 to DN 4000 in varying depths.
Cutting shoes fitted to the base section of the chamber and a specially developed joint system enables shafts to be installed by the caisson method in the minimum of time, and is suitable for use in virtually all ground conditions.
A concrete cutting shoe is also available for use in certain ground conditions.
Advantages of system include:
• Fast, clean, accurate construction
• Immediate permanent shafts
• Fewer joints than segmented systems
• Minimal labour costs
• Suitable for jacking pits
| The caisson shaft sinking system was originally designed for use with microtunnelling, but is now more widely used for construction of pumping stations, wet wells and manholes, particularly in difficult ground conditions. |
Caisson chamber sections weights and dimensions
Chamber
section |
O.D. |
Available depth of section |
Wall
thickness |
Approx. weight |
Approx. weight of base section and cutting shoe |
m |
DN |
mm |
0.5 |
0.75 |
1 |
mm |
kg per
metre |
(0.80m overall depth) kg |
2000
2550
2740
3000
3660
4000 |
2260
2730
3060
3350
4030
4400 |
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:
: |
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● |
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● |
130
140
160
175
185
200 |
2130
2900
3580
4270
5300
6360 |
1690
2260
2660
3090
:
: |
Tongue and groove joint |
Flat and groove joint |
Chamber section |
Type of cutting shoe available |
Tie bars (max) |
Joint type |
DN |
steel |
concrete |
no. |
|
2000
2550
2740
3000
3660
4000 |
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● |
:
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:
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8
8
8
8
10
12 |
Flat & groove
Flat & groove
Flat & groove
Flat & groove
Tongue & groove
Tongue & groove |
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Notes
1. DN 4000 chambers are supplied in two half sections, each weighing
approximately 3180 kg per metre, for assembly on site.
2. The steel cutting shoes is fitted to the base section providing a
10mm overbreak.Cutting shoe with a larger overbreak can be manufactured
on request.
3. When a concrete cutting shoe is requested as an alternative to steel,
for use in certain ground conditions, this is done entirely at the clients risk.
4. The DN 3660 and DN 4000 steel cutting shoe is supplied in two sections,
and the DN 4000 concrete shoe in four sections(all supplied loose).
5. At least half of the vertical tie bars should be used equally spaced
around the shaft at all times. In certain circumstances such as a high
water table, deep shafts etc then all tie bars must be used and the use
of secondary sealant may also be advisable. e.g. hydrophylic material.
6. Purpose designed lifting brackets necessary for handling sections are
available, and can be purchased from CPM. |
Caisson cover slab weights and dimensions
To suit chamber section
DN |
Effective thickness |
Overall thickness |
Overall diameter |
Approx. weight |
mm |
mm |
mm |
kg |
2000
2550
2740
3000
3660
4000 |
185
185
185
200
260
260 |
200
200
200
225
275
275 |
2260
2830
3060
3350
3960
4500 |
1906
2766
3166
4590
7760
10040 |
Weights are based upon 675 sq access or similar
Caisson reducing slabs weights and dimensions
To suit chamber section
DN |
Effective thickness |
Overall thickness |
Overall diameter |
Approx. weight |
mm |
mm |
mm |
kg |
2000
2550
2740
3000
3660
4000 |
185
185
185
200
260
260 |
200
200
200
225
275
275 |
2260
2830
3060
3350
3960
4500 |
1660
2780
3180
4490
7380
9660 |
Reducing slabs to suit up to and including DN 4000 chambers are manufactured with a 900mm circular access, the overall diameter being equal to that of the chamber section for which they are supplied.
Caisson landing slabs
Landing slabs are manufactured to suit DN 2740 to DN 4000 section for use in deep manholes.
Caisson landing slabs weights and dimensions
To suit chamber section
DN |
Effective thickness |
Overall thickness |
Access diameter |
Approx. weight |
mm |
mm |
mm |
kg |
2740
3000
4000 |
200
200
260 |
200
200
275 |
900
900
900 |
2460
2960
7748 |
For DN 2740 and DN 3000 they are supplied in four sections and constructed by lowering into the shaft and locating in a specially produced recessed chamber section.
For DN 3660 and DN 4000 the slab is produced in sections and constructed by lowering into the shaft and locating on four corbels, bolted to the chamber section.
| Reducing slab with 900mm access |
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Sections of landing slab for
DN 2740 chamber |
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Landing slab positioned in
DN 2740 chamber section
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Lifting brackets in chamber section up to DN 3000. Cast in lift holes are provided in DN 3660 and DN 4000 sections
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Procedure for installing caisson shafts
Components
Ensure the contents of each load are carefully checked against the delivery note.
The tie bars, socket nuts and washers should be stored in a secure area until required.
Where EPDM seals are supplied – (DN2000-DN3000), these should be stored on a flat surface to enable them to recover and make the seals easier to fix to the sections during sinking. For DN3660 and DN4000 TA 200 butyl sealant is required for jointing.
Lifting
Ensure suitable craneage and lifting attachments are available for offloading and sinking the shaft.
General
Where the caisson is intended to be used as a permanent manhole and has initially been constructed to ground level or above, the top section(s) can be removed and replaced with a reducing slab and shaft sections and/or cover slab. This can be carried out within the ring beam but it is essential that purpose designed lifting equipment suitable for lifting by the internal blind lift holes is used |
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The area for excavation should be carefully checked for services to at least 800mm greater than the external diameter of the caisson shaft and these should be excluded from this area.
The excavation should be at least 1.0m deep and 400mm greater than the external diameter of the caisson unit being used.
The base unit complete with steel cutting shoe must be surrounded with 10 - 15mm polystyrene sheet. Masking tape can be used to hold the sheet in position and to ensure there are no gaps in the polystyrene cover.
The base unit can then be placed centrally in the excavation and accurately levelled.
Where inlet/outlet holes are to be cut in the shaft, the unit should be positioned such that where practicable the vertical tie bars are not impeded.
The base unit including the steel cutting shoe has “cast-in” sockets for location of the threaded tie bars at the designated positions. A washer and socket nut is then fitted to each bar and tensioned to 14kg force per metre torque.
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The top joint of the unit must be clean prior to locating the seal. The seal can be held in place using spots of adhesive or masking tape to ensure it is not displaced when positioning the next chamber unit.
Where the caisson shaft is to be used as a jacking or reception pit for microtunnelling, the depth of unit above the base unit should be selected so that the horizontal axis of the inlet/outlet pipes, as near as possible, coincides with the joint between two 1m unit, thus limiting the loss of strength within any single unit.
Before placing the next unit, the tie bars should be suitably positioned in order to locate into the socket nuts on the top of the base section. The tie bars should be fitted with a socket nut and bearing washer in place. Whilst the units will fit together in any position, it is advised to line up the “cast in” marks.
The bottom joint of the second unit should be cleaned, lifted and positioned above the base unit and the tie bars engaged in the socket nuts. If any part of the unit is likely to come into contact with the concrete being placed to form the ring beam it should be protected by polystyrene sheet as described earlier.
Small realignments of the unit may be necessary to ensure that all tie bars can be secured. The socket nuts should be tightened to 14kg force per metre torque and the units checked for level again.
The annulus between the units and the excavation should then be filled with Grade C20 concrete to form a ring beam. This provides both permanent ground support to the upper part of the excavation and a temporary guide collar for sinking the caisson shaft. Sockets for temporary handrailing may be cast in, if required. It is normally possible to commence sinking the shaft the following day, having dissolved the polystyrene wrapping using a suitable chemical.
The next unit should be placed in position after carefully cleaning the joints and fitting the seal and tie bars as previously described.
To sink the caisson, excavation within the units should be carried out using a single hydraulic grab mounted on a suitable circle slew excavator or similar. A radiused boring clamshell will facilitate more efficient excavation.
The caisson will sink under its own weight in very soft ground. In firmer soils it may be necessary using the arm of the hydraulic excavator through a baulk of timber or similar, to avoid damaging the joint, to exert pressure on the top of the uppermost unit.
The 10-15mm gap surrounding the unit within the in-situ concrete ring beam can be monitored visually as a guide the caisson is sinking vertically. It is recommended a spirit level is also used to ensure the shaft is vertical, and therefore, any initial error in the setting up of the base unit can be counteracted. Any misalignment can be corrected by applying load to the high portion of the top unit, taking care to avoid damaging the joint. |
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The excavation should be continued and chamber units added until the required depth is reached.
Where the caisson is to be used as a microtunnelling jacking shaft, it will usually be necessary to sink it at least 600mm below the final shaft base. The actual depth will depend upon the type of jacking set up, the thickness of the manhole base required and the type of ground conditions. The softer the ground the greater the depth of caisson required for stability.
A concrete base, where required, can be placed by hand if the shaft is dry, however, if the caisson has been sunk under water then it will be necessary to use tremi techniques. Under no circumstances should the shaft be pumped out prior to the concrete being placed and cured.
In the case of deep shafts or where a high water table exists it is recommended that all vertical tie bars should be used as well as a secondary sealant (Hydrophylic sealant or similar) to ensure water tightness at the joints. The secondary sealant should be placed either side of the seal and in contact with the concrete only.
The top of the uppermost unit, where practicable, should finish approximately 150mm above the ring beam to form a barrier in order to prevent surface water and small debris entering the shaft whilst work is being carried out, e.g. jacking or reception shaft for microtunnelling. If necessary, additional units can be temporarily installed (without the seal and bolts) above ground level to form a security barrier.
A standard M16 nut should be fitted in lieu of the socket nut on the last permanent unit, or elsewhere in the shaft where a reducing or cover slab is the next unit to be finally positioned. This enables the slab (when installed) to sit correctly on the chamber and provide an effective seal.
Where the caisson is used for microtunnelling, etc., upon completion of the work within the shaft, the top section(s) can be removed within the in-situ ring beam and replaced with a cover or reducing slab. It will be necessary to use the purpose designed lifting brackets to remove any temporary units, as previously described.
Grouting of installed caissons is not normally necessary, although where one is to be used as a microtunnelling jacking shaft, this may be appropriate at the jacking plate horizon. |
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