Environmental Engineering Reference
In-Depth Information
wall deflects, the ground surface behind the wall subsides, and significant movements can
result in detrimental settlements of adjacent structures. The problem is particularly seri-
ous in soft to firm clays and in clays underlain by sands or silts under water pressure.
Wall Design Criteria
The design of a braced excavation in soft ground is based on calculations of earth pressure
magnitudes and distributions that can be determined only by empirical or semiempirical
methods. Calculations of wall deflections and backslope subsidence cannot be made with
the classical theories, although the finite element method is used in current analysis for
such predictions. As a rule, however, the engineer cannot be certain of the real pressures
and deflections that will be encountered, especially in soft ground. The engineer also may
have little control over construction techniques that will influence wall performance.
A wall may become unnecessarily costly if the design provides for all contingencies.
Because the nature of wall construction provides the opportunity for strengthening by plac-
ing additional braces, anchors, or other supports, initial design can be based on relatively
low safety factors but with provisions for contingency measures. This approach to design
and construction is feasible only if construction is monitored with early warning systems.
Construction Monitoring
Elements to be monitored during construction include:
Movements of the backslope area and adjacent structures, heave of the excava-
tion bottom, and movements in the support system
●
Groundwater levels and the quality and quantity of water obtained from the
dewatering system
●
Strains and loads in the support system
●
Vibrations from blasting, pile driving, and traffic
●
Instrumentation
As illustrated in
Figure 4.37,
vertical deflections of the wall, excavation bottom, backslope,
and adjacent buildings are monitored by precise leveling (a). In important structures,
measurements should be made with tiltmeters (e) or pendulums (f). Inclinometers (L) are
installed immediately behind the wall to measure its lateral deflections. Excavation clo-
sure can be measured with convergence meters (g) and backslope subsidence with verti-
cal extensometers (k) or shallow buried strain meters (h).
Loads in anchors and braces are monitored with strain gages (v or w) or load cells (t),
or checked periodically by jacks. Strain gages must be protected against temperature
changes, or the temperatures must be monitored, to provide useful data. Pressure cells (s)
installed behind the wall have the advantage that they can be installed between supports.
Groundwater variations are measured with piezometers (r).
An immovable bench mark is necessary for reliable measurements of deflections of adja-
cent ground, structures, and the excavation bottom.
4.5.4
Earth Dams
Objectives
An earth dam as actually constructed may differ from that designed because of the neces-
sity of using earth materials for fabrication, which may vary in properties and distributions
