Page 199 - Moving the Earth_ The Workbook of Excavation

Source: MOVING THE EARTH

CHAPTER 5

DITCHING AND DEWATERING

DITCHING

Drainage by ditching is a very ancient type of excavation, and even drainage tunnels were built in
prehistoric times. The purpose of this work was generally reclamation of land for agriculture.
Modern advances consist largely in the use of machinery for ditching, some improved types of
pipe, and use of pumps to dewater areas that cannot be readily drained by gravity flow.
Most dry ditching is done by backhoes or wheel, ladder, or drag ditchers. In soft swamps, draglines
may be best. Clamshells are used for deep and tricky work. Shallow ditches in dry ground may be
dug by hoes, ditchers, front shovels, draglines, graders, and dozers of straight, angle, and other
varieties.
Investigation. Before excavation of any kind is begun, the site should be inspected carefully for
any conditions requiring precautionary measures. This is especially important when one is work-
ing in a developed area with buildings and existing utilities. Survey adjacent properties before
excavation and, if possible, before bidding for the work. Record all defects such as cracking and
settlement, so that after the excavation any claim of damages can be assessed as having been
caused by the work or as a preexisting condition.
The location of existing underground utilities, namely, sewer pipes, electric lines, telephone, fuel,
water, and gas, must be determined before excavation. The contractor should contact the utility owners
and ask them to establish the location. When necessary, discuss removal, relocation, or service inter-
ruption with the utility owner before excavation. If the utility line is paralleling the trench and within
the risky triangular wedge by a distance less than the depth plus half of the trench width, extra
precautions must be taken to ensure no damage to that utility.
Vacuum Excavation. In the utility business there is a procedure known as potholing to discover
where an existing utility line is located. Many government agencies are adopting regulations that
require potholing so that new construction does not damage or knock out existing utility lines. The
preferred method for potholing is now by vacuum excavation.
The portable equipment for this is like a large size vacuum cleaner using either high-pressure
water or air, up to 1410 cfm, to excavate a hole with the soft ground material being collected in a
large tank. Depending on the machine used and soil conditions, a 12-inch square, 5-foot deep pot-
hole can be completed in 20 minutes or less. Most vacuum excavation machines are capable of
digging much deeper, but utility potholes usually do not have to go more than six feet deep.
Vacuum excavation was developed more than 50 years ago, but its use has been emphasized
by the extensive use of horizontal directional drilling (HDD), which was discussed in Chap. 1 and
detailed in Chap. 20. In connection with HDD there are two main functions for vacuum excava-
tion: (1) removal of drilling fluid that escapes from the HDD’s pilot hole during drilling, back
reaming and product installation and (2) potholing, i.e., uncovering existing buried pipe or cable
so HDD crews can visualize their exact location. The vacuum excavation machines are available

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