Environmental Engineering Reference
In-Depth Information
Evaluation and Analysis
Basic Elements
The following elements should be considered:
Rock bedding, i.e., horizontal vs. dipping
Overburden thickness and properties and thickness variations, which are likely
to be substantial
Bedrock surface characteristics, i.e., weathered, sound, relatively sound and smooth
with cavities following joint patterns from the rock surface (see Figure 6.22), or
highly irregular in configuration and soundness (see Figure 10.17)
Cavities within the rock mass — location, size, and shape
Arch characteristics, i.e., thickness, span, soundness, and joint characteristics and
properties
Groundwater depth and withdrawal conditions, i.e., present vs. future potential
Analysis
Analysis proceeds in accordance with a normal foundation study (except where founda-
tions may overlie a rock arch, then rock-mechanics principles are applied to evaluate the
minimum roof thickness required to provide adequate support to foundations). A gener-
ous safety factor is applied to allow for unknown rock properties.
10.4.4
Support of Surface Structures
Avoid the High Hazard Condition
Project relocation should be considered where cavities are large and at relatively shallow
depths, or where soluble rock is deep but overlain by soils subject to raveling. The decision
is based on the degree of hazard presented, which is directly related to the occurrence of
groundwater withdrawal or its likelihood in the future. Groundwater withdrawal repre-
sents very high hazard conditions. In fact, the probability and effects of groundwater with-
drawal are the most important considerations in evaluating sites underlain by soluble rock.
Foundation Treatments
Dental Concrete
Cavities that can be exposed by excavation can be cleaned of soil and filled with lean con-
crete (see Figure 6.22), which provides suitable support for shallow foundations.
Grouting
Deep cavities that cannot be reached by excavation are often filled by grout injection, but
the uncertainty will exist that not all cavities and fractures have been filled, even if check
explorations are made subsequent to the grouting operations. Grouting has the important
advantage of impeding groundwater movement and therefore cavity growth, even where
pumping is anticipated.
Deep Foundations
Deep, heavily loaded foundations, or those supporting settlement-sensitive structures,
when founded on, or rock-socketed in, soluble rock, should be proof-tested, whether
grouted or not (see discussion of explorations in Section 10.4.3) .
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