Structure and support · Below Ground & Retaining
Water-Retaining Tank Structure System
A reference account of the tank, pool and reservoir shell as a structure whose load case reverses with what is inside it, and of the joints, linings, penetrations and terminations that reversal governs.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What water-retaining structure is
A water-retaining structure is a shell in which the structure and the containment are the same element. A wall elsewhere in a building may be made resistant to water by something applied to it; here the walls and base are what holds the water, and any lining or finish on the wet face is working with the shell rather than instead of it.
Its defining feature is a load case that reverses. Full, the contained water presses the walls outward and holds the base down. Empty, with wet ground outside, the earth presses the walls inward and water beneath tries to lift the base. Both conditions are designed for at once, which is why emptying a tank or a pool is a structural decision rather than a cleaning one, and why the question of who may empty it belongs with the designer.
Joints are the point of the whole exercise. The shell is an element that must not leak, so every place a pour was interrupted, and every place movement was deliberately allowed for, is a planned discontinuity through the containing material. Waterstops, reinforcement continuity and the wet-face finish all follow where those joints are, and moving one of them moves the others.
The boundary of this entry is the fabric. Plant that treats, circulates, heats, doses or filters the contained water is a services matter and is not part of the shell, although the inlets, outlets and fittings it needs are holes made deliberately through the only element doing the containing.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- liquid-retaining structure
- water-retaining structure
- tank structure
- pool shell
- reservoir structure
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Contain a body of water within an element that is simultaneously the structure holding it.
- Resist ground and groundwater from the outside during the periods when the contained volume is reduced or absent.
- Resist flotation when the shell is empty and the ground around it is wet.
- Carry the discontinuities - construction joints, movement joints, penetrations and terminations - of an element that has to remain continuous against water.
- Present a wet face that can be cleaned, inspected and, where the finish permits, repaired.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportThe shell: walls and base acting as one
The structural element that also does the containing. Walls and base are continuous with each other rather than separate members meeting at a corner, because the corner is both a structural connection and part of the containment.
- Cavity or voidThe contained volume and its level
The space the shell exists to hold. It is a load case rather than an inert void: whether it is full, part full or empty changes what every other role is doing, so the operating condition of the tank is part of its structural description.
- Primary supportRestraint against flotation
Whatever holds an empty shell down when the ground around it is wet - the weight of the structure, the base geometry, the ground bearing on it, or a designed relief arrangement. It is the role that only matters in the condition nobody pictures when the tank is drawn.
- Jointing and sealingConstruction and movement joints with their waterstops
The planned discontinuities through an element that must not leak, and the profiled or hydrophilic strips cast into them to interrupt a path no surface treatment can reach. Where they fall is a decision about pouring, movement and containment at the same time.
- Control layerTreatment on the ground face
What is done to the outside of a buried shell, where the water is arriving from the ground rather than from within. It answers a different problem from the wet face and is commonly specified by a different party.
- Finish surfaceWet-face finish or lining
The surface in permanent contact with the contained water: tiling, an applied lining, or the structural surface left exposed. It is what is seen and cleaned, and it conceals the shell, so its condition and the shell's condition are separate questions.
- Service zonePenetrations for inlets, outlets and fittings
Every pipe, fitting and fixing passing through the shell is a deliberate hole through the containing element, and commonly through the wet-face finish as well. Where they are grouped, and how far they sit from a joint, is a fabric decision rather than a services one.
- Edge and terminationCoping, perimeter deck and overflow channel
Where the shell stops at the top. The water level, the overflow arrangement, the surface people stand on and the coping that terminates the wall all meet in this line, and it is the part of the structure most exposed to freeze-thaw and to wear.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The reversal is the system. A wall proportioned against water pushing outward is asked to resist ground pushing inward the moment the tank is emptied, and a base that the contained water was holding down is trying to float once that water is gone. Neither condition can be designed for alone, and an operating decision - drain it down for maintenance - moves the structure between them.
Move a joint and three other roles move with it. The waterstop line, the continuity of the reinforcement and the wet-face finish are all set out from the joint positions, so a joint relocated to suit how the pour was organised is a change to the containment rather than to the programme.
A penetration is a hole through the one element doing the containing, and commonly through the lining and the joint zone as well. Grouping them and keeping them clear of joints and corners is easier to detail than resolving each where a fitting happens to land, and every one added later is being cut into a structure that was complete.
The wet-face finish hides the shell it sits on, and water that gets past it travels before it appears. A damp patch outside a tank, or a falling level inside one, points to somewhere along a joint or a penetration rather than to the place the evidence shows up, which is why investigation here is a specialist exercise and not an inspection.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Shells are commonly encountered in these families, cast in place or assembled from units and stitched. What any of them has to do in an element carrying a reversing load case is a determination for the structural engineer.
Control layer
Resistance to water may be sought within the mix itself or through an applied layer on the ground face. Which route a particular structure takes is settled by a suitably qualified waterproofing designer rather than chosen from a list.
Jointing and sealing
Cast-in strips and applied sealants of these kinds are commonly met at joints in retaining and containing structures. Whether a given material can sit in permanent contact with the contained water is a documentation question for the manufacturer and the designer.
Finish surface
Wet faces are commonly finished in these families where the surface is to be seen and cleaned. Permanent immersion is a demanding condition and what it asks of an adhesive, a grout or a tile belongs with the product documentation.
Edge and termination
Copings, channel edges and the deck immediately beyond them are commonly formed in these families. This line is wet, walked on and exposed to frost at the same time, which is unusual among external terminations.
Protection
Metalwork built into or passing through a permanently wet structure is commonly met in these families. What the contained water does to a given metal, particularly where it is treated chemically, is a question for the designer rather than an assumption.
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
A shell formed beneath or within a ground floor
Where a tank sits under a floor, the slab and the shell are two elements founded on the same ground with different jobs and different tolerance of movement. How they are separated, and whether the floor bears on the tank, are decisions taken before either is poured.
Read about Ground-Bearing Slab →Overflow, drain-down and discharge connections
Water leaving a shell deliberately has to reach a drainage system that can accept it, and an overflow only works if what it discharges into can take the flow at the moment it arrives. Where the connection is made is also a penetration through the containment.
Read about Buried Drainage Network →A shell holding collected rainwater
Where the contained water is collected rain, the storage element is this structure and the collection, filtration and draw-off around it belong to another system. The same reversing load case applies whether the water arrived from a roof or from a main.
Read about Rainwater Harvesting →Surround paving at coping level
A bound surround meeting a coping is a stiff surface against a structure that moves for reasons of its own, in a zone that is wet, walked on and commonly falls towards a channel. The joint between them is a detail rather than an incident.
Read about Rigid Paving →Guarding at a change of level around the shell
Where the surround stands above the water or above lower ground, guarding is anchored into the structure or the deck beside it. An anchorage near the top of a containing wall is a penetration in a sensitive zone, and where guarding is required at all is a jurisdictional question.
Read about Guarding System →Relief of groundwater outside the shell
Drainage around a buried shell changes the inward and uplift cases the structure was designed for. If the structure relies on that relief working, then the drainage has become part of the structural argument and its maintenance is a structural duty.
Read about Wall Base Drainage →Water treatment and circulation plant
The plant that filters, doses, circulates or heats the contained water is outside the scope of the fabric, but its pipework, fittings and access chambers are holes through the shell and the lining. What the treated water does to the materials it touches is a question for the plant designer and the fabric designer together.
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- This is one of the few structures where water is both the design action and the thing being contained, and where it may arrive from either side. How water is expected to be resisted, and which face is doing the resisting in each operating condition, is a matter for a suitably qualified waterproofing designer.
- Movement
- The shell changes temperature with what it contains and with the ground it sits in, and it is restrained by its own geometry. Where movement is permitted to occur, and what the containment does at that point, are settled together rather than one after the other.
- Durability
- Surfaces here are permanently wet, intermittently wet, or wet and then frozen, depending on where they sit. The wet face, the coping line and the buried face are three different exposures on one structure and are commonly discussed as though they were one.
- Maintenance and access
- Draining a shell to inspect or repair it puts the structure into its other load case, so access for maintenance is a structural question. How a tank is emptied, how quickly, and under what conditions belongs in the designer's instructions rather than in an operator's judgement.
- Buildability
- Much of what governs performance is buried in the pour: waterstops, reinforcement continuity and the treatment of each joint. These are inspected once and then covered, and there is no later opportunity to look at them.
- Documentation
- Joint positions, waterstop types, penetration locations and the operating conditions the structure was designed for are only recoverable from records. Anyone later adding a fitting, changing the water level regime, or emptying the shell is relying on that record.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- What operating conditions has the shell been designed for, including the condition where it is empty?
- Who is permitted to drain this structure, and what has to be checked before it happens?
- Where are the joints, and were they positioned by the containment argument or by how the pour was organised?
- Which face is being relied on to resist water arriving from the ground, and who designed it?
- What penetrations are needed, are they grouped, and how far do they sit from joints and corners?
- How does the coping, deck and overflow line deal with water, wear and frost at the same time?
- If groundwater relief is part of the design, who maintains it and what happens if it stops working?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- The structure is not a box sealed by a coating. The shell is the containment, and an applied layer or a lining works with it rather than in place of it.
- It is not the same problem as a basement. A basement keeps water out of occupied space and is propped by its floor plates; this shell holds water in, has no floor plates propping it, and is loaded from opposite directions at different times.
- Emptying it is not a cleaning task. Removing the contained water removes a load the structure was relying on and exposes it to ground pressure and to uplift, which is why the operating regime is part of the design.
- Where a leak appears is not where it originates. Water travels along joints, behind linings and through the surrounding ground before it becomes visible, and tracing it is a specialist investigation.
- Filtration, dosing and circulation are not part of this system. They are services that pass through it, and every one of those passages is a hole in the containment.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What load cases has this shell been designed for, and does the set include empty with high groundwater?
- What is the flotation check based on, and what does it assume about the ground around the structure?
- Where are the movement and construction joints, and what waterstop arrangement is specified at each?
- What is the wet-face finish expected to do structurally, and what is it expected to do only visually?
- How were the penetrations positioned, and what is the procedure for adding one later?
- What is the drain-down procedure for this structure, and who authorises it?
- What record will be handed over showing joints, penetrations and the design operating conditions?
What this page does not do
- This entry describes how the structure is organised. It is not a design, a specification, or a substitute for structural and waterproofing design by qualified professionals.
- Nothing here states or implies a thickness, reinforcement arrangement, water depth, joint spacing or capacity; all of those are project-specific determinations.
- Emptying, refilling or altering the water level regime of a water-retaining structure can change the actions on it and should be referred to the designer.
- Requirements for guarding, access, water quality and discharge around such structures vary by jurisdiction and use, and verify them against local requirements.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Commonly built alongside
Systems routinely present in the same building without a junction recorded between them and this one. Where two systems do meet, the junction is set out above instead.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Basement · Substructure
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Below-Ground Structures, Earth Retention and Water Control
Assemblies built against retained ground, where lateral earth pressure and the water held in that ground decide the design together rather than one at a time.
Browse all below ground & retaining entries →