This document is an addition to the reservoir construction methodology Doc n° MF4-M50 “Mould reservoir construction”. Every step of the reservoir DB construction is similar to a simple reservoir except those described below. Before starting, consult Doc n° MF4-M40 “Distribution box construction methodology” to acquire all the principles of distribution box construction.

All pipes must be installed at ground level and inside foundation masonry and rocks filling. First define if the DB is going to be a quarter or a half one based on the number of outlets required

The DB inlet and outlets are made in PVC with a Tee and a drainage. The vertical pipe length is equal to the outside reservoir height (including foundations) + 50 cm (for a total of 1.9, 2.9 or 3.9m). If the DB has two connected outlets use two tees on the same horizontal pipes:

Then to help installing those pipes and the overflow, fill Format n° MF4-F53 “RDB pipes preparation before casting (half slab)” or Format n° MF4-F54 “RDB pipes preparation before casting (quarter slab)”. These documents will produce a drawing that will materialize pipe positions in the foundation:

• Tees and elbows 45cm from the edge of foundation (65cm from center), ~5cm from inside concrete wall at the end
• Inlets and outlets have 20cm in between each pipes
• Outlets are parallels top each other and can run in both direction
• Reservoir overflow opposite to outlets (to have space on top for reservoir door and DB)

This rules are meant to position pipe on the side of the reservoir to help maintenance operations¹⁾ and DB casting. The best way to prepare is to draw a 65cm radius circle in the middle of the foundation to place inlet and outlets every 20cm. Here under an example with a double outlet, for more details on how to read DBs sketch consult Doc n° MF4-M40 “Distribution box construction methodology”:

The same overflow & washout pipe is used for the DB and the reservoir as in the drawing below. Special care should be taken to positioned the pipe in the direction of the chosen drainage point.

Then the foundations can be build maintaining these pipes in position. All drainages must be at least 50cm out to allow installation of the drainage maintenance assembly (See Doc n° MF4-M15 “General construction standards” § Connect PVC with HDPE fitting for a method to safely connect a HDPE fitting on a PVC pipe).

The position of the 60×60 cm reservoir door is slightly different to that of a simple reservoir. It should still be installed above the overflow/washout pipe for handling purposes, but out of the way of the DB box and with sufficient space for the door to open.

Before casting of the top slab, the PVC pipe of the reservoir outlet has to be installed as follow. If the outlet to reservoir is small (≤32mm) and the inlet flow important (>0.3l/s), it is necessary to use a supplementary Overflow / Washout ⌀75mm installed in the same way:

Possible position of outlets to reservoir and overflow

When the top slab reservoir is finished, place Tee and elbow to form the DB outlets. Place a coupling for reservoir outlet & washout (if necessary). For details on this installation consult Doc n° MF4-M40 “Distribution box construction methodology” § Bottom slab casting:

Use spare part of outside mould as curved formwork (quarter or half) and wooden or metal sheet for the straight part. Cast a concrete slab without reinforcement in 1:2:3 concrete, minimum thickness is 10cm and adjusted to couplings tees and elbows level. Good practice standards about concrete handling and production can be found in Doc n° MF4-M13 “concrete production methodology”:

After 24 hours install inside formwork to cast the walls of the DB, 40cm high:

Build a concrete slab cover (10 cm thick) with small door (40x40cm). Details for door installation can be found here: Doc n° MF4-M15 “General construction standards” § Metal door installation:

The DB can be built before removing the formwork. But the mason must take extra care not to break the pipes passing through the reservoir.

Place a tee on the DB inlet, the tee height must be exaclty at outlets height:

The level of all the outlets have to be checked because the accuracy of the yield repartition depend on it. Every level should be +/- 1mm the same, check using a spirit level:

Visual inspection on the crest flow of each outlet have to be done as a second level check, they should all look the same: