Waste Landfill sites produce high lateral forces and frictional drag, which has proved problematic for existing traditional style shafts.

The development of a pre cast concrete landfill shaft has overcome the unique problems encountered due to its high strength and jointing system.  Additionally being a wet cast product, the smooth exterior finish reduces frictional drag. 

The standard shaft, 1200mm in diameter, can be constructed in traditional chimney style or by the caisson sinking method.

The units have a 120mm wall.  Being thicker than standard manhole units and are jointed using 3 No 20mm diameter high tensile bars.  The bars run vertically through the shaft wall with sequential units being bolted on as the landfill proceeds.

Pre-cast base sections are available.

Additionally in order to reduce punching shear on the formation a telescopic base section can be utilised. 

The base units are generally perforated for leachate percolation and subsequent pumping.

Advantages of the system include:

•  Speedy erection by site personnel
•  Restricted joints
•  Shaft system designed for purpose
•  Sealed watertight joints
•  Construction to any depth
•  Sections can be perforated with 75mm diameter holes

• Bottom sections can be supplied with fittings to connect to either an in-situ base or to a manufactured precast base slab.

• Cover slabs can be supplied with or without access to suit clients requirement, with the option of fittings to fix the slab to the top section.

• Base slabs can be manufactured with shaftfixings and sized to suit clients requirements.

• The base slab, complete with fittings, ready to accept the base unit can be factory produced tsuit the clients requirements.

• Base unit fittings attached to a template ready fo in-situ casting into the base slab are available on request.

• TA200 sealant and bolts for placing in-situ slab and fixing to shaft section are available on request

• Telescopic base system available.

Shaft weights and dimensions

• Sections are jointed utilising 3 number 20mm vertical tie bars, nuts and washers, sheradize coated to BS4921 1988.

• Joint sealant – 50mm x 15mm TA200 Butyl sealant.

• Sections can be supplied perforated – usually 16 No 75mm perforations per metre depth.

• Holes cut in bottom section for inlet pipes can be preformed to suit client requirements.

Typical base slab

Telescopic base system

Laboratory testing

Crushing strength test

Reinforced shaft sections are designed and tested in accordance with BS5911-3 to works proof and maximum loads of 20kN/m and 25kN/m respectively.

Results
Proof load 50kN/m 2.5 times BS requirements
Max load 100kN/m 4.0 times BS requirements

Shear test

Two sections axially aligned with sealant placed within the joint profile.  Sections joined using 2 x 6 number tie bars, washers and nuts – each tensioned to 7.5kg force per metre of torque.

Results
Shaft section failed in crushing, prior to reaching failure in shear – load applied in excess of 100kN/m.

Bending test

Two sections jointed with 3 number 20mm Æ
Tie bars and sealant.  Central load applied to subject units to bending force.

Results
Units reached an ultimate bending moment of 75kN/m.

Procedure for installing landfill shafts

A    Components

•  Ensure the contents of each load are carefully checked against the delivery note.

• The tie bars, socket nuts, washers and TA200 Butyl sealant should be stored in a secure clean dry environment until required.

B     Lifting

•  Ensure suitable craneage and lifting attachment are available for offloading and placing the sections.

•  3 No lifting loops will be required to utilise the cast in threaded inserts for lifting of shaft units.

C    General

• Where the shaft is constructed in line with the landfill, the cover slab can be placed on top of  the shaft temporarily for safety purposes and to stop waste accidentally entering the shaft
  


• The bottom shaft section, which can be perforated or plain with or without holes cut for lateral drain attachments, should be placed on the designed level concrete base, and securely fixed using thetie bars and sealant between the bottom section and base slab.



• The concrete base slab can either bemanufactured and supplied by CPM or cast in-situ, and should contain the necessary fittings to secure the bottom shaft section to it utilising the tie bars, washers and nuts.

• Each shaft section has cast in sockets for location of the threaded tie bars at designated positions.A washer and nut is then fitted to each tie bar and tensioned to 7.5kg force per metre torque.

• The top joint of each shaft section must be clean prior to locating the TA200 Butyl sealant.

• Prior to placing the next shaft section, the tie bars should be located into the socket nuts of the bottom shaft section.

• The lower joint of the second shaft section should be cleaned, lifted and positioned above the
  bottom shaft section and the tie bars engaged into the tie bar holes.

• Subsequent shaft sections should be installed after carefully cleaning the joints, fitting the sealant, tie bars, nuts and washers as previously described, until the required height is achieved.

• If required, locking fittings can be provided in the cover slab for security purposes, enabling thecover slab to be tied down to the shaft  section.

• A central perforated pipe can be installed if required and the annulus between the unit and the pipe filled with graded gravel.  Alternatively the shaft can be left empty in order that a pump can be lowered for water extraction if necessary.

• During construction, the fill around the shaft should be evenly compacted in layers.  Additionally,    a granular surround or other material is advisable to be placed around the shaft as infilling    continues, to reduce friction on the shaft due to landfill settlement.