Premier Tech Aqua Installation Guidelines
Conder Underground Tanks: Granular Surround
Below are the installation guidelines for the Premier Tech Aqua underground GRP tanks suitable for granular surround.
These notes are not specific or site-related instructions for installing your septic/silage tank – if you have any questions or concerns about any part of the installation procedure, please give Drainstore a call on 01773 767 611.
Drainstore offer installation services
Our drainage specialists are here to help with any installation issues you are facing. Call us on 01773 767611 and we can provide expert guidance, or you can schedule a free site visit.
Transportation, unloading, and storage of tanks
- Tanks must be held down during transportation using nylon straps. Do not use chains, cables or wire ropes as they may damage the tank.
- Do not over-tighten the straps, otherwise you risk causing deformation of the tank shell.
- The tanks are best lifted by using a crane with webbing lifting straps or slings – chains, cables and wire ropes should not be in contact with the tank.
- For tanks longer than 8 meters, we recommend using a lifting beam.
- Smaller tanks can be lifted with other suitable site equipment, as long as the list if controlled with greater care so as not to damage the tank.
- Not all tanks will have their centre of gravity at the centre of the tank, so lifting straps must be arranged to ensure stability during lifting.
- Move tanks by lifting and setting – do not drag or roll them. This also applies when removing them from the delivery vehicle.
- Place the tanks onto a smooth, level surface to avoid causing point loads on the shell of the underground tank.
- You can prevent rolling of the tank by using tyres, sandbags or similar to chock it.
- In high wind conditions, consideration should be given to strapping down the tanks to prevent damage.
Pre-Installation Inspection of the Tank
- Tanks should be visually inspected prior to installation. Check for fractures to the shell or ribs, delamination, scratches and abrasions deeper than 1.5mm, stress cracks, or star crazing. Special considerations should be given to the strap positions.
- Any damage should be reported to the delivery driver.
- Do not undertake any unauthorised repairs, as this will invalidate the warranty of your septic tank or silage tank.
- Check the invert depth is correct, the tank is the correct grade for granular surround, and that the pipe orientations are correct.
- Where present, all fixings (nuts, bolts, screw fixings etc.) should be checked and retightened to correct for any movement during transport.
- Excavations should be planned with due regard to Health and Safety requirements, and should be either shorted or battered back to a ‘safe’ angle. If you are in doubt, call our drainage experts on 01773 767611.
- The excavation should allow a minimum of 450mm clearance between the tank sides//ends and the excavation wall or face of shoring. Clearance of a minimum 450mm is also required between adjacent tanks.
- Soils with low bearing capacity (equivalent to less than 12 SPT blow counts) will require all tank clearances to be increased to half the tank diameter.
- Ground instability at the formation level (e.g. running sand) may necessitate over-excavation and stabilisation with hardcore or blinding concrete.
- Geotextile material may be required to prevent migration of the tank backfill material.
Buoyancy and Anchoring
- Where the depth of cover over the tank (tank crown level to finnal ground level) exceeds 70% of the tank diameter, the
tank will not require mechanical anchoring to prevent uplift movement. This for the worst case condition of an empty tank with the tank excavation ooded to ground level.
- If the depth of cover is less than 70% of the tank diameter, then mechanical anchoring is required.
- There are two methods of mechanical anchoring:
- a) Reinforced Concrete Anchor Slab
- A reinforced concrete anchor slab of minimum thickness 200mm, sized to cover the excavation area. The slab should incorporate Premier Tech Aqua webbing anchor straps.
- b) Concrete ‘Deadman’ Anchors
- A reinforced (minimum two 20mm steel bars) concrete beam (pre-cast or in situ) along each side of the tank, of equal length to the tank. The beams should incorporate Premier Tech Aqua webbing anchor straps.
- a) Reinforced Concrete Anchor Slab
‘Deadman’ Anchor Sizing
|Tank Diameter (mm)||Minimum Height (mm)||Width (mm)|
The concrete ‘deadman’ anchors should not lie in tank shadow i.e. below the projection of the tank diameter. If the maximum water table level (under all conditions) can be confirmed as being lower than the final ground level, then the requirement for anchoring can be reviewed. Please contact us for details. In this situation the potential for the creation of a perched water table by the flow of groundwater into the backfill needs to be considered.
Primary Backfill Specification
- Primary backfill material should be free-flowing granular material and can be:
- a) Pea Gravel
- Naturally rounded aggregate with particle size not less than 3mm and not greater than 18mm, compacted to a relative density of >70%.
- Pea gravel shall be clean and free flowing, free from large rocks, dirt, sand, roots, organic materials or debris.
- Upon screening analysis the backfill material shall have no more than 5% by weight passing a 2.38mm sieve.
- b) Crushed Stone or Processed Stone
- Crushed stone/gravel or processed stone with particle size not less than 3mm and not greater than 12mm, compacted to a relative density of >40%. Dry density must be at least 1,500 kg/m3.
- The material shall be clean and free flowing, free from large rocks, dirt, sand, roots, organic materials or
- The material should be washed or screened to remove fine particles.
- Upon screening analysis the backfill material shall have no more than 5% by weight passing a 2.38 mm sieve.
- a) Pea Gravel
- Use of other then specified backfill/bedding materials will
void the tank warranty.
- All backfill material shall be free of ice and snow at time of installation. Backfill material shall not, during placement, be frozen or contain lumps of frozen material.
Primary Backfill Installation
- Tanks must be installed with Primary Backfill only within the region immediately surrounding the tanks.
- The tank bedding depth, using primary backfill, i.e. directly below the tank, must be a minimum of 300mm below the tank to natural ground. This can be reduced to 200mm if a reinforced concrete anchor slab is used.
- The Primary Backfill must extend a minimum of 450mm outward from the tank sides and ends.
- Compaction should be by lightweight rollers of vibratory plate compactor until the minimum cover (with load) depth has been achieved. Compact evenly around the turret extensions to reduce the risk of distortion.
- The use of geotextile barrier fabrics surrounding the Primary Backfill material is considered good installation practice. The fabric must be chosen to allow the flow of water in and out of the excavation but prevent the movement of fine soil particles into the Primary Backfill material.
Secondary Backfill Specification
- Secondary backfill shall not be used adjacent to the tank.
- Secondary backfill may be used only at a distance of 450mm from the tank walls.
- The following are approved as Secondary Backfill materials:
- a) Coarse Sand or Gravel
- Coarse sand or gravel containing rocks no larger than 36mm on the largest dimension.
- The material shall be clean and free flowing, free from dirt, clay, fine sand, roots, organic materials or debris.
- Upon screening analysis this backfill material shall have no more than 5% by weight passing 0.075mm
- During placement this backfill material must be compacted to 95% relative compaction.
- b) Select Native Backfill
- Clean native backfill, or clean selected backfill, containing rocks no larger than 36mm on the largest dimension.
- The material must be compacted to 95% relative compaction.
- The quality of this backfill material shall be such that it exhibits an ultimate bearing strength in excess of 170 kPa in the compacted state.
- a) Coarse Sand or Gravel
The minimum tank burial depths, with and without full load, are as follow:
Tank Minimum Cover (mm)
|Tank Diameter (mm)||With Live Load||Without Live Load|
The minimum cover with live load can be reduced by using a reinforced concrete slab above the tank, contact us for more information and guidance.
This grade of tank are designed to be installed below ground and completely surrounded with granular material.
Generally, the depth from finished ground level to the top crown of the main shell should be no more than two meters. This may vary dependent upon ground water conditions. Deeper inverts may be accommodated on a standard shell providing the water table level does not exceed two meters above the top crown of the main shell. For deeper burial with high water table conditions heavy duty shells are available. If the tank is installed outside these parameters it may suffer irreparable damage.
Should you be in any doubt regarding suitable shell application please contact us.
- Tanks must not be subjected to buoyant forces during installation, account should be taken of ground water levels and surface water run-off and their accumulation in the tank excavation. This applied event if tanks are mechanically anchored.
- The tank excavation should be maintained empty of water, by pumping or whatever suitable means, until the tank cover depth reaches a minimum of 300mm above the tank.
- If this is not achievable tanks may be filled with water as ballast until required conditions are achieved. If water filling is carried out during backfilling, water level inside tanks must not exceed the level of backfill material outside the tank.
Loose shafts should be sealed using silicon sealant sikaflex-291, or similar, prior to installation to prevent ingress of groundwater under high water table conditions. It is the installation contractor’s responsibility to ensure a watertight seal.
Tank Installation Procedure
- Excavation and anchorage provision in accordance with preceding information. Ground water must be pumped to give a dry excavation.
- Place primary backfill bedding material as described in preceding information. Ensure material is clean and contains no oversize material.
- Lift tank into position and align as required for connecting pipework, access shafts, etc.
- Secure anchor straps, if used.
- Connect any low-level pipework as required.
- Commence backfilling, with primary backfill material, in layers approximately 300mm, ensuring tank and any pipework is properly “haunched”.
- Continue backfilling, with primary backfill material, evenly around the tank to at least 300mm above the tank top, connecting any high-level pipework, as required. Mount and seal any turret extensions.
- Backfill evenly to grade using the same primary backfill material, or select secondary backfill material.
- Compaction should be by lightweight or vibratory plate compactor until “traffic” depth has been achieved.
- Compact evenly around the turret extension to reduce risk of distortion.
- Cut turret extensions to length and fit manhole cover and frame.
- Important: Ensure that no surface loadings are transferred from the cover direct to the tank. Cover frame construction should allow for ground movement (settlement).
- Inspect tank internally to ensure roundness is maintained and deflection does not exceed 1% of the tank diameter.
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