Civil Engineering Reference
In-Depth Information
can cause the individual stones to move into unstable positions. Trees or tree limbs
often fall onto a wall and disturb the stability of the stones. It is not unusual to
step on a stone and have the wall collapse underfoot. In addition to suffering the
indignity of falling, a surveyor is likely to be injured by falling stones weighing a
hundred pounds or more.
When surveying, it is often necessary to climb over walls. The previous cau-
tions apply here as well. If it is necessary to climb over a wall, a person should
never jump off the wall unless he or she is sure of the ground condition of the
landing spot. Jumping into a pile of leaves covering a pile of stones is likely to
result in a twisted ankle or broken leg, particularly if the jumper is laden with a
backpack full of heavy equipment.
Another potentially dangerous situation arises when a steel tape is used to
measure along an electric stock fence. Electric stock fences are installed to keep
livestock enclosed. Electric fences can have voltages ranging from 3,000 to 10,000
volts. The voltage is not continuous but is only turned on for short intervals which
are called pulses. If an animal or person touches the fence during a live pulse, the
body acts as a conductor between the fence wire and the earth (ground) allow-
ing an electrical current to flow through the body. The current causes a non-
lethal but exceedingly painful shock. The equation E
=
I * R (Voltage
=
Current
* Resistance) gives the relationship between the voltage, current and resistance.
For a given voltage, the lower the resistance is the greater the current will be. The
lethality of the shock is dependent on the magnitude of current flow through the
body. Body resistance can vary between 1,000 and 10,000 ohms depending on
whether it is wet (sweating for example) or dry. So, the lower resistance of a wet
body will increase the current flow and the degree of electrical shock. For humans,
the electrical resistance of clothing, such as gloves and boots and whether they are
wet or dry also affects the current flow.
As we saw in Chap.
4
, it is customary when using a steel tape for the front per-
son to carry the tape reel and let the fully extended tape drag on the ground while
walking to the next point to be measured or set. In such cases, the rear person
holding the free end of the tape must be careful to throw the tape clear of the fence
when each measurement is completed, so there is no danger of the tape touching
the electric fence. Like the steel wire in an electric fence, a steel tape has low elec-
trical resistance so it is a very effective conductor of electricity. As the front person
is walking with his back to the dragging tape, he is unable to keep an eye on the
tape to be sure it does not touch the fence. It is therefore up to the rear person to
ensure that the tape stays clear of the fence as it is dragged along the ground. If
the tape does touch the fence, the person holding the tape reel is likely to receive
a shock (and immediately discover that his or her boots are probably not very well
insulated against such high voltages). Fortunately, the current of an electric stock
fence is very small, and the pulse width very short, so the contact will be unlikely
to cause any permanent physical harm. However the incident will certainly get the
person's full attention.
The author has measured along miles of electric fence without a single acci-
dent. However, one incident does come to mind where two crew members were
