Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 7 RETAINING WALL
7.4.3
Lateral Earth Pressure Due to Ground Water
In cases where ground water exists, the lateral pressure due to the water at any depth below the
ground water level is equal to the hydrostatic pressure at that point since the friction angle of water
is zero and use of either Equation
7.5 or 7.6 leads to a coefficient of lateral pressure for water, Kw
equal to
1.0. The computation of the vertical water pressure is based on triangular pressure
distribution that increases linearly with depth as illustrated in Figure 7.12. The lateral earth pressure
is added to the hydrostatic water pressure to obtain the total lateral pressure acting on the wall at
any point below the ground water level. For a typical soil
friction angle of 30 degrees, Ka = 1/ /3.
Since Kw = 1, it can be seen that the lateral pressure due to
water is approximately 3 times that due
the active lateral earth pressure. Thus, it is important to provide adequate drainage behind the wall
to reducee and control the ground water table build-up.
Figure 7.12 General Distribution of Combined Active Earth Pressuree and Water Pressure
7.4.4
Lateral Pressure from Surchage
Surcharge loads on the backfill surface near an
earth retaining structure also cause lateral pressures
on the structure. The
loading cases usually consist of:
• Uniform surcharge
• Point
loads
• Line loads parallel to the wall
• Strip
loads parallel to the wall.
Surcharge loads (vertical loads applied at the ground surface) are assumed to result in a uniform
increase in lateral pressure over the entire height of the
wall. The uniform increase in lateral
pressure for a uniform
surcharge loading can be
written as:
7-14
March 2009