SPECIAL PROTECTION
GROUND LOADS
Where a rigid pipe of:-
a) less than 150mm diameter has less than 300mm depth of cover,
or
b) 150mm or more diameter has less than 600mm depth of cover,
it should be surrounded with concrete either 100mm or the diameter
of the pipe, whichever is greater, in thickness and have movement
joints, at not more than 5m centres.
Where a flexible pipe has less than 300mm depth of cover under an
area other than a vehicular area, it should have concrete paving
slabs laid as bridging on granular or other flexible filling at
least 75mm above the top of the pipe. Where a flexible pipe has
less than 600mm depth of cover under a vehicular area it should
have a reinforced concrete slab laid as bridging in a similar manner.
FIGURE 4
Protection For Flexible Pipes
FIGURE 5
Bedding For Flexible Pipes
1. Provision shall be required to prevent
ground water flow
in open trenches.
2. The barrel of the pipe shall have continuous bearing on
the floor of the trench or the granular fill.
The trench should not be opened too long in advance of
pipe laying and should be backfilled as soon as possible. It is
essential to ensure that the sides of the trenches are adequately
supported in accordance with the requirements of BS6031.To minimise
a possible hazard, a trench should be open for the minimum time
practicable.
At the crown of the pipe and for 300mm, or one pipe diameter if
greater, above it the width of the trench within any timbering should
be as narrow as is practicable, but not less than the outside diameter
of the pipe plus sufficient extra width (usually about 150mm) on
each side of the pipe to provide access for making the joints and
placing and compacting sidefill. Above this level, the trench may
be of any convenient width.
If the "as-dug" material is suitable for use as bedding,
the bottom of the trench may be trimmed to form the pipe bed. Otherwise,
the trench should be excavated to an adequate depth below the invert
level of the pipe to allow for the necessary thickness of bedding
material. The thickness of bedding under the barrel of the pipes
should be a minimum of 100mm, but in very wet or soft conditions
or where the trench bottom is very irregular, it may be necessary
to increase this thickness. Bedding should be properly compacted
and finished so as to provide uniform support for the pipe. It is
essential that bricks or other hard materials are not placed under
the pipes for temporary or permanent support.
Material to be used for bedding and surrounding the pipes should
be selected granular material, either available locally or, if necessary,
brought to the site. Suitable materials are described in Table 1.
TABLE 1
SUITABLE MATERIAL FOR BEDDING AND
SURROUNDING PIPES
 |
|
Nominal
pipe size (mm) |
Material (complying with the
requirements of BS882:Part 2) |
| 110 |
10mm, nominal single-sized aggregate |
| 160 |
10mm or 14mm, nominal single-sized aggregate or 14 to 5 graded
aggregate |
| 220 and over |
10mm, 14mm or 20mm, nominal single-sized aggregate, or 14
to 5 or 20 to 5 graded aggregate |
Alternatively, granular material in accordance with the following
materials for bedding recommendations and having a particular size
not exceeding that in Table 1 depending on pipe size, may be used.
SPECIAL PROTECTION -SETTLEMENT
A drain which runs under a building should be surrounded by at least
100mm of granular or other flexible filling.
It is recommended that a drain, which passes through a wall or foundation,
should either:-
a) pass through an opening giving at least 50mm clearance all round
as shown in Fig 7(a); or
b) be built in with, on each side, flexible joints within 150mm and
rocker pipes of maximum length 600mm as shown in Fig 7(b).
c) wall protection sleeves are available shown Fig 7(c).
FIGURE 7
PIPES PENETRATING WALLS
A drain which is at a level lower than the foundations of a building:-
i) where the trench is within 1m of the foundations, it should be
filled with concrete up to the level of the underside of the foundations;
or
ii) where the trench is more than 1m from the foundations, it should
be filled with concrete to a level, below the level of the underside
of the foundations, equal to the distance from the foundations less
150mm.
Flexible pipes must be wrapped in polythene before surrounding in
concrete. The minimum thickness of the concrete surround should be
150mm or the diameter of the pipe, whichever is greater.
(Note: where a drain is to pass under a foundation it should be supported
on piles, or where the ground is unstable, specialist advice should
be sought on the required protective measures).
SPECIAL
PROTECTION -
SURCHARGING
Where a drain is liable to surcharge, protective measures as described
in standard BS8301: 1985 should be used.
MATERIALS FOR BEDDING
1 VISUAL EXAMINATION
Examine the material and reject any which contains pieces with sharp
edges.
2 PARTICLE SIZE
The maximum particle size should generally not exceed 20mm. The
presence of an occasional particle between 20mm and 40mm is acceptable
provided the total quantity of such particles is only a very small
fraction. If particles over 40mm are present, the material should
be rejected.
The following test will ensure compliance with this recommendation.
A weighed representative sample of the material, about 50kg of the
proposed material (about 2kg) should be sieved, using test sieves
of 19mm and 38mm nominal mesh size (see BS410).
Note 1: To obtain a representative sample, about 50kg of the proposed
material should be heaped on a clean surface and divided with a
spade down the middle. One of these halves should then be similarly
divided, and so on until the required sample is left.
Note 2: In the sieving, clumps of material that break up under light
finger pressure may be helped through the sieve, but considerable
force should not be used to squeeze oversize clumps through the
mesh.
The material is not recommended if:
a) any particles are retained on the 38mm sieve,
or
b) more than 5% by mass of the sample is retained on the 19mm sieve.
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