American Society of Civil Engineers.

Transactions of the American Society of Civil Engineers (Volume 81) online

. (page 4 of 167)
Online LibraryAmerican Society of Civil EngineersTransactions of the American Society of Civil Engineers (Volume 81) → online text (page 4 of 167)
Font size
QR-code for this ebook


the equation for h can be ascertained for a model from the volume
of flow through the gravel, by making experiments with varying
quantities of water, and a corresponding variation of head, before
the sheeting is placed in position. For this experiment, however, the
gravel should be covered with an impervious material, so that the
entire discharge must pass through the gravel. After the sheeting
has been placed and the flow of water under it has been ascertained,
this volume will determine, from the results of the preceding experi-
ments, the proper value of s.

There is now to be considered the problem of determining the effect
of sheet-piling when the value of s which will obtain after the sheet-
piling is in place is not known.



Grunsky.




36 DISCUSSION ON DESIGNING AN EARTH DAM

Mr. Without any sheeting under a dam of impervious material the

hydraulic grade line may have any of the positions, a, l>, or c, Fig. 13.
When in the position indicated by the line, a, the outfall of the water
into a channel or other receiver below the dam takes place with a
more or less abrupt release of pressure, the dam is in danger. When
the hydraulic grade line is in the position, c, the flow in the gravel
under the dam is so small that water will not appear at the down-
stream toe of the dam. When in the position, h, all water appearing
at the down-stream toe will be under no pressure at the ground's
surface, and the water passing under the dam ceases to be a menace.

w. s. ^^

t

. previous ^ - ^^^"~?S~^^ '

Fig. 13.

Let it be assumed that, in the case of the finished structure, which
has the benefit of sheet-piling, the hydraulic grade line passes through
the down-stream toe, so that the average gradient of the line is
H -^ L. For this particular case, the effect of the sheet-piling can
be estimated as follows:

The average gradient throughout a distance of about d feet on

S + .s
either side of the sheeting, as already shown, will be — - — .

The value of H will be

if = (i — 2 cZ) s + /i (7)

// = {L — 2 cl) s + -
from which



ff = (I - 2 (!) s + jj— ^j (S)



H

'= S^



Online LibraryAmerican Society of Civil EngineersTransactions of the American Society of Civil Engineers (Volume 81) → online text (page 4 of 167)