RainSafe Tank Installation

A completed RainSafe tank under a finished car park, shown in cutaway

Before you start

Welcome

Congratulations on selecting RainSafe for your stormwater management system.
Your tank has been designed to provide the required storage volume, design flow-rate, loading capacities, maintainability and primary water requirements.
This module takes about 30 minutes and follows the installation from the delivery lorry to the final geotextile. There are twenty questions along the way, and you need 16 out of 20 to pass.

Before you start

Why the guidelines matter

Failure to adhere to these guidelines may result in damage to the chambers, and in inadequate hydraulic or structural performance of the tank.
The tank is buried. Once the stone and the surface are on top of it, nothing inside is visible and nothing is easy to reach.
Remediation after backfilling has been carried out is costly and time-consuming.

Every mistake on this job is cheap to fix before the stone goes in, and expensive afterwards.

A site team gathered around a drawing, one signing a record sheet

Before you start

Who needs to know this

For the requirements to be met and the tank to be certified, all installers and machine operators involved in the installation must be familiar with the guidelines.
They must agree to strictly follow the instructions, and provide a record of having done so.
That record is part of the certification of the tank, not a formality to be caught up on later.

A drawing marked with minimum dimensions

Before you start

These are minimum requirements

These guidelines are general, and provide the minimum requirements for correct installation of your system.
The project drawings may ask for more than the minimum. Where they do, the drawings govern.
Any deviation from the steps and measures detailed in this guide, whether because of site-specific constraints or any other requirement, must be approved by CubicM3.

A site constraint is a reason to ring CubicM3, not a reason to improvise.

A CubicM3 representative talking a crew through the guidelines on site

Before you start

The pre-installation briefing

A CubicM3 or RainSafe representative will provide a pre-installation briefing to run through these guidelines.
The briefing is where to raise anything the drawings do not cover: awkward levels, restricted access, unusual traffic loading, or a sequence that does not suit the programme.
Bring questions to it. It is much cheaper than finding the answer halfway through backfilling.

Before you start · Question 1 of 20

When does putting a mistake right become costly and time-consuming?

Remediation after backfilling is costly and time-consuming. Up to that point the chambers are visible and reachable; after it, putting anything right means taking the stone back out.

Before you start · Question 2 of 20

Site constraints mean you cannot achieve the spacing shown on the drawings. What do you do?

Any deviation from these steps and measures, for site-specific constraints or anything else, must be approved by CubicM3. The guidelines are the minimum requirement for the tank to perform and be certified.

A flatbed lorry carrying palletised stacks of RainSafe chambers

Unloading the chambers

Delivery on pallets

Your RainSafe chambers will arrive at the site on pallets.
The chambers have been designed to pack efficiently, to minimise storage space requirements and to protect the chambers in transit and in the stockpile.
They should remain on their pallets until they are ready for installation.

A telehandler with fork extenders lifting a tall pallet of chambers

Unloading the chambers

Unloading the pallets

Pallets should be unloaded carefully, using fork-extenders where necessary.
The stacks are deeper than a standard set of forks. Without extenders the load is carried on the tips, which is how pallets get dropped.
Banding straps should only be removed just before installation.

The straps are what holds a tall stack together. Cut them early and the stack is no longer a stack.

A pallet stack standing square on level ground beside one leaning on soft ground

Unloading the chambers

Tall, heavy and easy to topple

The palletised stacks are tall and heavy.
The pallets should be placed on flat, level ground to avoid the risk of toppling. Soft or sloping ground under one corner is enough to start a stack moving.
When unloading chambers, they should be lifted off the stack carefully by at least two operatives.

A crossed-out illustration of a chamber being pushed off the top of a stack

Unloading the chambers

Never push, never topple

The chambers should never be pushed off the top of the stack.
Stacks should never be pushed over to make unloading quicker.
Doing either could cause serious injury or even death, and result in significant damage to the chambers.

Chambers are light enough to lift and large enough to kill someone when they fall from height.

Unloading the chambers · Question 3 of 20

When should the banding straps come off a pallet of chambers?

Banding straps should only be removed just before installation. Until then they are the only thing holding a tall, heavy stack together.

Unloading the chambers · Question 4 of 20

How should a chamber be taken off the top of a stack?

Chambers are lifted off the stack carefully by at least two operatives. They are never pushed off the top, and stacks are never pushed over — either can cause serious injury or death and will damage the chambers.

An excavator forming a rectangular tank bed, with level pegs set out

Tank excavation

Excavate to the drawings

Excavate the tank bed to the dimensions and levels shown in the drawings, and prepare the sub-grade.
The dimensions include the working space around the rows, not just the footprint of the chambers themselves.
Levels matter as much as plan size: the whole tank is built off this surface.

An engineer testing the bearing capacity of an excavated formation

Tank excavation

Check the sub-grade

The quality and bearing capacity of the sub-grade must be inspected and confirmed to meet design requirements.
What counts as adequate depends on the expected traffic loading above the tank and on the total cover depth over it.
Refer to the RainSafe CSTB-ATec document for detailed guidance.

Inspected and confirmed, not assumed. The whole structure is carried by this formation.

A soft patch of formation being dug out and replaced with compacted stone

Tank excavation

Ground improvement

At this stage, ground improvement measures should be taken where necessary.
This is the moment for it. Once the liner and the bedding stone are down, improving the ground underneath means taking the tank back apart.
What is required will come from the design: the inspection of the formation decides whether it is needed.

A sharp stone being lifted out of the formation, with fine bedding material alongside

Tank excavation

Protect the liner

Any protruding sharp objects should be removed to protect the tank liner.
Rock, old services, reinforcement and broken concrete all puncture a liner under load, and the puncture is not found until the tank is in service.
If necessary, a protective layer of fine bedding material should be placed prior to installation of the liner.

Tank excavation · Question 5 of 20

Which document gives detailed guidance on sub-grade quality and bearing capacity?

The CSTB-ATec document carries the detailed guidance on the sub-grade, assessed against the expected traffic loading and the total cover depth. Table 2 of the Installation Guide is a different thing: it governs plant working above the tank.

Tank excavation · Question 6 of 20

Why are protruding sharp objects removed from the excavation?

Sharp objects puncture the liner, and the puncture is not discovered until the tank is in service. Where removing them is not enough, a protective layer of fine bedding material goes down before the liner.

A roll of liner being unrolled across a tank bed from a spreader beam

Placing the tank liner

Laying the liner

Place a non-woven geotextile or impermeable liner over the prepared tank bed, as specified for your tank.
Use of a spreader beam is highly recommended. It keeps the roll level and takes the drag off the material as it comes off.
Dragging a liner across a formation is how it picks up damage and how it ends up out of position.

A flat lapped joint beside a wrinkled one marked with a cross

Placing the tank liner

Wrinkles and overlaps

Avoid wrinkles, and ensure minimum joint overlaps are maintained.
A wrinkle is a fold that concentrates load and gives stone a point to work against.
An overlap that has crept below the minimum is a joint that will open under fill.

Flat and fully lapped before any stone goes on. It cannot be corrected later.

A perforated under-drain laid along the base of the lined tank

Placing the tank liner

Under-drains

Install the under-drain or drains, as per the tank design.
Position, fall and connection all come from the design drawings for your tank.
Note where they run. They are the one thing in the base that the bedding stone and the plant placing it can damage.

Placing the tank liner · Question 7 of 20

What is highly recommended when placing the tank liner?

A spreader beam keeps the roll level and takes the drag off the material, so the liner goes down flat and in position rather than being hauled across the formation.

Placing the tank liner · Question 8 of 20

Which pair of defects must be avoided in the liner?

Wrinkles concentrate load and give stone something to work against; an overlap below the minimum is a joint that opens under fill. Extra overlap and extra liner are not defects.

Clean angular stone beside rounded and dusty material marked with crosses

Placing the bedding stone

Only one kind of stone

Only clean, crushed, angular stone may be used within the tank.
It should conform to Clause 505 Type B, or Remblai 20-40 specification.
Rounded stone rolls instead of locking. Dirty or fine material blocks the voids the tank relies on to store and move water.

Clean, crushed, angular. Anything else changes both the structure and the storage.

Bedding stone being spread and compacted across the lined tank bed

Placing the bedding stone

Place and compact

Place and compact the bedding stone.
This is the layer the chambers sit on, so it is compacted before any chamber is set out on it.
Work it up evenly across the bed rather than in one corner at a time.

A staff measuring the depth of the bedding layer against a marked level

Placing the bedding stone

Depth to the design

The depth of bedding stone must be as per the design drawings, and should reach the minimum depth.
Check it across the bed, not at one point. A bed that is right at the edges and shallow in the middle is not a level bed.
If the formation needed improvement, confirm the depth again over the improved ground.

A tracked machine placing bedding stone, keeping clear of a marked under-drain

Placing the bedding stone

Equipment and under-drains

There is no restriction on the type of equipment used for the placement of the bedding stone.
The restriction on plant comes later, once the chambers are in and the fill is going on around them.
However, take care to avoid crushing or otherwise damaging the under-drain or drains.

Placing the bedding stone · Question 9 of 20

Which stone may be used within the tank?

Only clean, crushed, angular stone to Clause 505 Type B or Remblai 20-40 may be used within the tank. Angular stone locks together to form the structure; clean stone keeps the voids open for storage and flow.

Placing the bedding stone · Question 10 of 20

What restricts the equipment used to place the bedding stone?

There is no restriction on the type of equipment for the bedding stone — but the under-drains must not be crushed or damaged. The Table 2 restrictions apply later, to plant working above the chambers.

Chamber row positions marked out in line on the bedding stone

The Separator Row

Mark out the rows

Mark chamber row positions out clearly on the bedding stone.
Marking every row before the first chamber lands is what keeps the spacing between rows right across the whole tank.
It is far easier to correct a line on the stone than a row of connected chambers.

The Separator Row

Start with the Separator Rows

Installation of the RainSafe chambers begins with the Separator Rows, which connect to the inlet manholes.
The Separator Row takes the incoming flow, so it sets the position of everything built off it.
Build these first and the rest of the tank follows from them.

The Separator Row

The woven geotextile bed

The woven geotextile bed is laid out in the correct position, along the line marked for the Separator Row.
Woven, not non-woven: this is the strip the Separator Row is built on, and it is specified for strength.
Position it against the marks before any chamber goes on it.

A single continuous strip running the full length of a row

The Separator Row

Continuous, flat, no wrinkles

A continuous strip must be provided for the full length of the Separator Row.
This must be flat and free of wrinkles.
Continuous means exactly that: not a run of short pieces laid end to end under the chambers.

The geotextile extending a marked 300 millimetres past the edge of a chamber

The Separator Row

The 300 millimetre margin

Sufficient length and width must be provided to ensure the geotextile extends a minimum of 300 millimetres beyond the side edges and ends of the Separator Row chambers and end caps.
The stone placed on that margin later is what holds the geotextile in place.
That is what allows the Separator Row to be jetted and maintained over the life of the tank without pulling the bed out from under it.

The 300 millimetre margin is a maintenance detail, not a tolerance. It is why the tank can be cleaned.

The first chamber of a row set down with its end cap in place

The Separator Row

First chamber and end cap

The first chamber and end cap of the Separator Row are then placed in position.
Set them to the marks. The whole row, and the rows built off it, follow this first unit.
Check the position before connecting anything to it.

An end cap being cut on its moulded guide with a reciprocating saw

The Separator Row

Cutting the inlet

Following the cutting guides on the end cap, cut a hole for the inlet pipe using a reciprocating saw.
The inlet hole invert should always be at the bottom of the end cap.
Insert the corrugated inlet pipe, ensuring a minimum of 300 millimetres protrudes into the chamber.

Invert at the bottom, and 300 millimetres of pipe inside the chamber.

Two chambers being joined, large corrugation over small, with locating buttons aligned

The Separator Row

Building out the row

Build out the Separator Row, following the direction guides shown on the tops of the chambers.
Fit the large end-corrugations over the smaller end-corrugations on the other end of the chamber.
To ensure a firm connection, slot the locating buttons into the corresponding sockets. Repeat this for the entire length of each row.

An end cap cut at the top to receive the overflow bypass pipe

The Separator Row

The overflow bypass end

The end cap at the far end is cut to receive the overflow bypass pipe at the top of the end cap.
Top, not bottom: the bypass only takes flow once the tank is full, which is the opposite of the inlet.
The end cap is then placed to close off the Separator Row.

Non-woven geotextile being laid over the full length of a completed Separator Row

The Separator Row

Cover the Separator Row

A layer of non-woven geotextile is placed over the entire length of the Separator Row.
Non-woven over the top; woven underneath. The two do different jobs and are not interchangeable.
Entire length means end cap to end cap, with no gaps left open to the stone above.

The Separator Row · Question 11 of 20

How far must the woven geotextile extend beyond the Separator Row chambers and end caps?

A minimum of 300 millimetres beyond the side edges and the ends. The stone placed on that margin holds the geotextile in place, which is what allows the Separator Row to be maintained later.

The Separator Row · Question 12 of 20

Where should the invert of the inlet hole be cut in the end cap?

The inlet hole invert should always be at the bottom of the end cap. The overflow bypass is the opposite: that is cut at the top of the end cap at the far end of the row.

The Separator Row · Question 13 of 20

How do two chambers connect to each other?

Large end-corrugations fit over the small ones, following the direction guides on the tops of the chambers, and the locating buttons slot into the corresponding sockets to make the connection firm.

The Separator Row · Question 14 of 20

What goes over the completed Separator Row?

Non-woven geotextile goes over the entire length of the Separator Row. Woven material is the bed underneath it; the two are specified for different jobs.

Several completed chamber rows running parallel across the tank bed

Building out the tank

The remaining rows

By connecting adjacent chambers, build out the remaining rows.
The same connection applies throughout: direction guides followed, large corrugations over small, locating buttons home.
Then cap off all rows with an end cap.

Bypass pipework being assembled between rows at the end of the tank

Building out the tank

Overflow bypasses

Cut-outs for overflow bypasses should be made, and bypass pipes assembled and connected.
Make the cut-outs to the drawings for your tank: the bypass arrangement is part of the hydraulic design, not a site detail.
Check every connection before the stone goes near it.

A cross-section showing stone columns between chamber rows carrying load

Building out the tank

Spacing is structural

Take care to ensure that the spacing between rows matches the design.
The stone columns between chamber rows form an essential part of the overall structure. It is therefore essential for the structural integrity of the tank that, in all cases, at least the minimum required separation between chambers is achieved or exceeded.
Similarly, the correct spacing between the edges of rows and the side walls of the excavation must be ensured.

The gaps between the rows are not spare ground. They are columns, and they carry the tank.

Building out the tank · Question 15 of 20

Why does the spacing between chamber rows matter so much?

The stone columns between the rows are structural. The minimum separation must be achieved or exceeded in all cases, and the same care applies to the spacing between the outer rows and the excavation side walls.

Building out the tank · Question 16 of 20

The rows are built slightly closer together than the design shows, to fit within the excavation. Is that acceptable?

In all cases, at least the minimum separation must be achieved or exceeded, because those stone columns carry the structure. If the excavation will not allow it, that is a deviation for CubicM3 to approve, not a site decision.