Trenches
Author/Compiled by
Beat Stauffer, Dorothee Spuhler (seecon international gmbh)
Adapted from
Water Distribution Operator Training Handbook
Executive Summary
Before an underground water pipe can be installed, a trench must be excavated. Trench
dimensions depend on pipe size, soil, and climate. Mostly, the excavation is done by
hydraulic backhoes, as they are easy to operate and provide good control. Costs depend on
the local conditions (machinery, soil, etc.). It is very important that safety regulations are
properly implemented to avoid accidents.
Advantages
Pipes are protected from climate (e.g. frost), traffic, unauthorised abstraction, etc.
Disadvantages
Expensive initial costs
Costly maintenance/repairs if pipe is damaged
Dangerous work in deep trenches (cave-in)
Factsheet Block Title
Introduction
Factsheet Block Body
The most expensive part of pipe installation (either sewer pipes, e.g. conventional or drinking
water pipes), is the excavation of the trenches. Therefore, it should be carried out as
economically as possible.
Most trenching for water main installation is done by hydraulic backhoes, which are easy to
operate and provide excellent control. The size of the machine should be chosen proportionally
to the size of pipe, and respectively to the size of the trenches. Smaller trenches can be made
manually if machines are not available, but many labourers are necessary to do it in a reasonable
time.
A hydraulic backhoe excavates a trench (left) and people at work (right). Source:
HOWSTUFFWORKS (2012)
Preparations
Before the work can begin, a plan for the project must be prepared. The plan should include at
least the following details:
The location and depth of the main that is to be installed
The location of valves, hydrants, and all fittings that can be anticipated
The location and depth of all sewer and gas pipes, electric, telephone, television and
streetlight cables in the line of the new construction
Details of any other obstructions that must be protected or avoided
Factsheet Block Title
Basic design principles
Factsheet Block Body
Trench width
If work is done by a backhoe, the appropriate bucket width to use is based on pipe size, trench
depth and local experience concerning soil conditions. On the other hand, the trench width
should be minimised to save time and possible site damages, yet, as already mentioned, wide
enough to allow proper installation, i.e. there should be enough room to make up the pipe joints,
tamp backfill under and around the pipe, and maintain the trench wall.
Generally, the trench width should be no more than 0.3 to 0.6 m greater than pipe diameter.
Especially under paved areas, it is important to keep the trench width as narrow as possible to
minimise the traffic load that will be transmitted down through the backfill and exerted on the
pipe. If there are unusually heavy surface loads, special installation recommendations should be
considered.
Trench depth
In warmer climates, the depth is designed to sufficiently spread surface loading and protect the
pipe from damage. The minimum cover for mains is typically 0.8 m, and 0.5 m for water
services. In colder areas, the depth is dependent on the maximum depth of frost (knowledge of
local conditions necessary). If the pipes are too shallow in such climates, water services will
freeze at the point of connection. If adequate burial depth is not possible, the main can be
insulated (e.g. closed-cell styrofoam insulation).
Excavation
The excavated material should be piled at least 0.6 metres from shallow trenches. For deeper
trenches, it must be piled even further away. This minimises the risk that dirt or stones are kicked
into the construction down to a worker or that excavated material falls back. Furthermore, there
must be enough space for the labourers to walk along the construction. A safe deposit distance
also prevents someone from tripping and falling into the trench. Moreover, the weight of the
excavated material must be considered. If it is not deposited correctly, it could lead to a cave-in
of the trench.
The bottom of the trench should be buried as close as possible to the specified grade so that it
provides a continuous, even support for the pipe. After the pipe has been placed, high points
must be levelled out and low points should be filled to provide a good support for the pipe.
Factsheet Block Title
Cost considerations
Factsheet Block Body
The costs for excavation depend on various criteria. Rocky ground, for example, slows down the
excavation, while very sandy and unstable ground must be shored, which requires extra material
(see picture above) and knowledge. Furthermore, of course, the size of the trench (depth, width)
affects the costs. For very deep and wide trenches, heavy machinery is necessary.
Factsheet Block Title
Safety
Factsheet Block Body
The excavation should not be started much ahead of pipe laying. The main reason for this is that
the longer the construction stays open, the more likely it is to have a cave-in (especially during
heavy rainfall). At the end of the day, it is best to be completely backfilled or at least protected
by barricades and lights. If there is traffic and the trench cannot be filled, it should be covered
with heavy steel plates (HUBEN 2012). If the trench is very deep or soil conditions are bad, the
construction should be protected to prevent a cave-in (see picture). Read more about trenching
safety in IHSA (1991).
Vertical shores to prevent a cave-in.
Source: TRENCHTECH n.y.
Applicability
Basically, trenches are required wherever underground water pipes will be installed. As
described above, the depth and width of the trench depends on piping size and local conditions.
If trenches are very deep or excavated ground is unstable, special safety regulations are required
(e.g. vertical shores).
Library References
Further Readings
Important Weblinks
