Hydraulic Structures: Fourth Edition

Laboratory studies using lean concretes representative of those employed

in dam construction suggest that is a complex and variable parameter,

and that its value can approach unity (Leliavsky, 1958; Butler, 1981).

Having regard to this and to the control over interstitial pressures exerted

by local factors which are quite indeterminate, it is prudent to specify that

1.00 for all analytical purposes. Note that Puis thus comparable in

order of magnitude and importance with water load, Pwh.

Referring to the two-dimensional profile of Fig. 3.1 and plane X–X,

the nominal plane area is defined by the section thickness, T, i.e. AhT.

Assuming1.00 and no pressure relief from drains, equation (3.6) then

becomes

PuT (^) w
z 1 
2
z 2
 (3.7)
withPuacting through the centroid of the pressure distribution diagram at
distancey 1 from the heel, and
y 1 

T

3

 (m). (3.8)

In modern dams internal uplift is controlled by the provision of verti-

cal relief drains close behind the upstream face. Formed drains rise the full

height of the dam from an inspection gallery located as low as practicable

in relation to the tailwater level.

The mean effective head at the line of drains, zd, can be expressed as

zdz 2 kd(z 1 z 2 ) (m). (3.9)

The empirical coefficient kdis a function of relief drain geometry, i.e.

diameter, spacing and location relative to the upstream face. Given an effi-

cient drainage system analysis is commonly based on assuming that

kd0.33 (Moffat, 1976; USBR, 1976). Relief drain geometry and effi-

ciency are considered further in Section 3.5.3. Modern drains are typically

ofc. 200–250 mm diameter at 3.0–4.0 m centres.

Base and foundation uplift are controlled by a similar system of

relief drains drilled from the inspection gallery to depth within the under-

lying rock. The standard provision of a deep grout curtain upstream of the

line of relief drains and below the upstream face (Section 3.5.2), intended

to limit seepage, also serves to inhibit pressures within the foundation. Its

effectiveness in the latter capacity is much less certain than that of an effi-

cient drain system, and its influence on pressure is therefore normally dis-

regarded.

2 z 2 z 1



z 2 z 1

126 CONCRETE DAM ENGINEERING