Page 227 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.29

If only one side of the stream is accessible to machinery, the logs may be pulled across from
that side by the use of a cable through a snatch block anchored on the opposite bank. The block
should be placed high, if possible, to prevent the end from digging into the bank.
Concrete. Concrete bridges consist in general of two abutments supporting a slab. The slab usually
includes guardrails, and supporting ribs or stringers which may be flat or arched. The abutments
are usually continued into wing walls to direct the stream through the opening and to protect the
embankment against sliding or erosion.
Even small structures are quite heavy and require that the abutments rest on solid footings. The
flow of the stream should not be restricted, as it might then scour out the material against the abutments
and undermine them. Abutments must be strong enough to resist the horizontal thrust of the fill
behind them.
Reinforcement should be used throughout the structure, and is particularly important in the slab
and its ribs.
The forms for the slab must be supported on a temporary wood or steel bridge of considerable
rigidity.
Bridges should be engineered for the site and conditions. Construction should not be attempted
without experienced supervision.

CULVERT DESIGN

A culvert may be made of almost any structural material. Reinforced concrete or corrugated metal
pipe, and poured reinforced concrete are standard for highways and railroads. Tile and plain con-
crete may be used for light service. Log and timber construction are usual in pioneer and military
roads.
Water passages (barrels) may be round, arched, rectangular, or in special shapes. More than
one may be used.
Capacity. A culvert serves to carry the water from a drainage area or watershed of a certain size.
This water includes surface runoff of rain and melted snow and ice, and whatever groundwater
comes to the surface within the area.
The size of culvert opening should be determined by the amount of rain which is likely to fall
in the watershed within a certain period, and the character and slope of the ground so far as they
affect the percentage of water that will run off, and the speed of its flow.
Additional factors to consider are the opening required by normal stream flow before it rains; the
extent to which the opening may be restricted by silting; the velocity of water in the culvert; the extent
to which water not passed through it can pond against the embankment before overtopping it or
damaging property behind it by flooding; and the probable damage from overtopping.
Runoff. Rate of runoff is determined by intensity of rainfall, the size and shape of the watershed,
and the slope, plant cover, and permeability of the soil.
Rainfall is measured in inches, and its intensity in inches per hour, although the period of mea-
surement may be less than an hour. For example, a rainfall of 3 inches might fall at the rate of 6
inches per hour for 30 minutes. In calculating runoff, an adjusted or equivalent rate can be used
which makes allowance for variations in rate and duration.
Each watershed has a period of concentration, at the end of which the runoff is assumed to be
at a maximum. This is the time required for water to flow from the farthest point in the shed to the
culvert. If rainfall is continuous, and ground conditions are unchanging, the runoff at the culvert
will increase from the beginning of the rain until it includes water from the whole area, after
which it will continue at the same rate.
This period will be longer for long, narrow watersheds than for square or round ones of similar
area.