2.Read and translate the text: Principles of soil drainage

Since drains laid below ground are fairly inaccessi­ble, their operation must be as trouble-free as possible. To achieve this you need to follow these principles in their construction:

1. Ensure that the drains are watertight so that waste does not leak out and contaminate the ground.

2. Lay drains on even gradients to make sure that the water carries solid matter away smoothly.

3. Lay drains in straight lines. If a drain changes direction, then insert an inspection chamber.

4. Construct a manhole if several pipes come together to join a single drain run.

5. Construct manholes at 90 metre intervals on straight runs.

6. Construct a manhole at the boundary of the property before the drain joins the public sewer.

7. Use soil drains that are at least 100 mm in diameter.

8. Surround drains under buildings in at least 150 mm of concrete.

9. Backfill trenches for drains near or below foun­dation level with concrete.

10. Join branch drains to the main drain at an oblique angle.

11. Insert a trap at every inlet to a drain other than a soil pipe.

Pipe sizes and falls. A minimum 100 mm drain is usually adequate for a domestic plot. About 20 small houses can be connected to a 100 mm drain because only one or two properties will discharge water at the same time. If required, a larger pipe such as one with a 150 mm diameter is available.

The typical gradients for the waste fall are in Table 1. The gradient, which is a downhill slope, determines the speed of the effluent dis­charge. When the drains are laid to the correct fall, then the velocity of the water flow will keep the drain free of solids so that it is self-cleaning. The fall of the drain should follow the natural slope of the ground to reduce the amount of excavation needed.

Table 1 Recommendations for gradients

Pipe diameter (mm)	Gradient
100	1:40
150	1:60
225	1:90

A drain run may be steeper when short branches are connected to manholes. Generally, it is cheaper to make drain runs as short as possible with a minimum of manholes.

Calculating the depth of a drain. It is cheaper to connect the sanitary fittings to the main sewer by the shortest possible route. Your objective is to find the invert levels for both ends of the drain run. You begin by measuring the drain run from the house to the plot boundary. The following example shows you how to complete the calculations.

Example:

• Find the invert level nearest the house:

If, you want 400 mm cover over the pipe and the outside diameter of the drain is 125 mm, then the invert will be 525 mm below ground. Where the ground level is 150 mm below the finished floor level or datum, add 150 mm.

• Find the invert level at the lower end of the drain run:

Drain run = 30 metres

Invert level below datum at head of drain =675 mm

Fall to drain in mm over 30 metres = 30 000/40 = 750 mm

Invert at lower end of drain = 675 + 750 = 1425 mm below datum.

The depth of the drain is related to the site datum level, not ground level. You must still make sure that the drain has enough ground cover.

Table 2 describes the guidelines for the mini­mum depth in different locations.

Table 2. Minimum depth for rigid pipes

Pipe size	Fields or gardens	Driveways and minor roads
(mm)	(mm)	(mm)
100	400	700
150	600	1000

You should lay flexible pipes at least 600 mm deep in fields and gardens and 900 mm under roads and driveways.

If pipes are closer to the surface than recom­mended, then they must be protected. You should surround rigid pipes with concrete 150 mm thick and provide expansion joints at 5 metre intervals. You should surround flexible pipes with concrete or protect them by putting them under precast concrete paving slabs that rest on 75 mm granular fill material.

Notes:

1. other than – кроме, за исключением