OSD for Below-Ground Storage



Here is an example of above ground storage:

  • Underground Tanks
  • Pipe Package


Below Ground Storage (Underground Tank)


Below Ground Storage (Underground Storage Tank) Diagram


Below Ground Storage (Pipe Packages)


The main advantages of below ground storage are

  • out of sight
  • occupy less physical space
  • will not cost inconvenience with ponding of water  that could result using above ground storage
  • suitable for difficult topography
In difficult topography, the only feasible solution may be to provide all or most of the required storage volume below-ground.

However, it should be recognised that below-ground storages are:

  • More expensive to construct
  • Difficult to inspect for silt and debris accumulation
  • Difficult to maintain
  • Can be dangerous to work in and may be unsafe for property owners to maintain


Underground Tanks


(a) Basic Configuration

Below ground storage can be configured into almost any geometrical plan shape.

Storage tanks can be connected both ‘in-line’ and ‘off-line’ to the stormwater conveyance system depends on the design objectives.



(b) Structural Adequacy

Storage tanks must be:

  • Constructed from durable materials that are not subject to deterioration by corrosion or aggressive soil conditions.
  • Designed to withstand the expected live and dead loads on the structure including external and internal hydrostatic loading.
  • Considered the buoyancy in its design to ensure that the tank will not lift under high groundwater conditions.

(c) Horizontal Plan

The area that the storage facility will occupy will depend on height and width limitations on the site.

A rectangular shape offers certain cost and maintenance advantages, but space availability will sometimes dictate a variation from a standard rectangular plan.

It may be necessary on some site to design irregular shaped tanks.

(d) Bottom Slope

The floor slope should not be > 10% and not be >2% for all parts of the storage to provide good drainage.  (2% > x > 10%)


(e) Ventilation

It is very important to provide ventilation for below-ground storage systems tominimise odour problems.

Ventilation may be provided though the tank access opening(s) or by separate ventilation pipe risers.

Inflow and outflow pipes should not be consider in the design because their contribution is unreliable.

(f) Overflow Provision

An overflow system must be provided to allow the tank to surcharge in a controlled manner if the capacity of the tank is exceed due to a blockage if the capacity of the tank larger than the storage design ARI.

(g) Access Opening

Below-ground storage should be provided with opening to allow access by maintenance personnel and equipment .

An access opening should be located directly above the outlet for cleaning when the storage tank is full and the outlet is clogged.

A permanently installed ladder or step iron arrangement must be provided below each access opening if the tank is deeper than 1200 mm.


Pipe Packages


 Figure 19.9 Basic Layout Of A Pipe Package Storage



(a) Basic Configuration

  • Consisting 1 or more parallel rows of buried pipes connected by a common inlet and outlet chamber
  • Size is determined by the storage volume requirements and the physical availability of space on the site
  • No need to installed the pipe in a straight line along its entire length but it must fit in any site limitation


(b) Minimum Pipe Size And Longitudinal Grade

The minimum pipe size to facilitate inspection and cleaning is 900 mm diameter

Pipe should be laid at minimum longitudinal grade of 2% to avoid standing pockets of water which can occur due to lack of precision during construction


(c) Low Flow Provision 

Although sediment will settle out inside pipe packages, the extent of deposition can be reduced by installing one of the pipes lower than the others as shown in Figure 19.9

To keep the other pipes from filling during low flows, the difference in level between the low flows confined wholly within the low flow pipe

Confining low flows to one pipe will help the system to become self cleansing


(d) Inlet Chamber

The site drainage system is connected to the pipe package though an inlet chamber at the upstream end

The chamber must be large enough to permit easy access to all of the pipes by maintenance personnel and equipment


(e) Outlet Chamber

At the downstream end, the pipe package is connected to the municipal stormwater drainage system though an outlet chamber

The chamber must also be large enough for maintenance access

Flow through the outlet chamber may be controlled by one of the primary outlet


(d) Overflow Provision

A secondary outlet overflow system must be installed at either the inlet or outlet chamber to prevent water from surcharging at stormwater inlets or manholes upstream during storms larger than the storage design storm

On steep sites or for long pipe lengths, the overflow should preferably be located at the inlet chamber as illustrated in Figure 19.10 (a). While Figure 19.10 (b) illustrates an overflow located at the outlet chamber.

Figure 19.10 Pipe Package Secondary Outlet



(e) Access Openings

Access opening are required at both chambers

  • to facilitate normal cleaning
  • for maintenance of a pipe package.
  • provide access for ventilation
  • provide lighting
Two openings should be installed in each chamber if more than three parallel pipes are used.

The maximum distance between access openings shall not exceed 30 m.

For a long pipe packages an additional access openings along each of the pipes may be required.




Editor :- Dilah – 13 Feb 14




Back : Home                        Next : OSD for Composite Storage