60
Geotechnical News December 2011
THE GROUTLINE
grout on the subsurface conditions, the
long term stability of the grout mate-
rial, and time dependent processes that
can undermine the effectiveness of the
grouting solution.
Managing Site Uncertainties
Uncertainties associated with site
conditions can be effectively managed
through proper site investigations
in advance of the grouting program
design. The investigation must be
tailored to each site and should include
the following:
1. Geologic Assessment
2. Hydrogeological Assessment
3. Subsurface Exploration
4. Karst Assessment
The Geologic Assessment must in-
clude an evaluation of the formations
present including identification of the
rock type,behavior, and layering; fre-
quency of fractures and orientations;
strike and dip of the rock beds, loca-
tions and orientation of dikes, faults,
and unconforming contacts; presence
and types of karst features present, and
geomorphologic history of the materi-
als at the site. Often times this can be
accomplished by a review of technical
publications and maps together with a
site visit. The literature review should
include review of previous studies and
maps of the project site, and review of
well drilling logs, a review of historic
and current aerial photos can provide
input on the distribution of karst fea-
tures and changes in them over time as
well as, enable fine tuning of the geo-
logic data through identification of lin-
eaments and other features.
A key factor in the geologic as-
sessment is to develop a geologic site
characterization. This characterization
must include an evaluation of the dis-
tribution of identified karst features,
faults, fractures and other features that
can be related to karst activity or rock
variability. The geologic assessment
should characterize the type and dis-
tribution of karst features and the ex-
pected pattern of weathering.
The formation of karst features is
always water related. The history of
groundwater conditions over geologic
time ultimately produced the karst con-
ditions that remain within the rock at
present. Except in the case of highly
soluble formations, dissolution of rock
is unlikely to be a significant ongoing
problem; however, the current hydro-
logic conditions are likely to be a sig-
nificant factor in the occurrence of ac-
tivity in sinkholes and subsidence. The
Hydrogeological Assessment must as-
sess these items, from a geologic, his-
toric, and current perspective. The Hy-
drologic Assessment should include an
assessment of groundwater levels and
flows, from both a static and dynamic
perspective. The nature of groundwa-
ter flow should also be addressed, as
to whether the karst voids are highly
interconnected forming a ‘water table’
behavior or whether the voids form
isolated conduits producing erratic wa-
ter levels and potentially high velocity
flows.
It is essential that the subsurface
investigation be planned to consider
the conditions identified in the geo-
logic and hydrogeologic assessments
to ground truth the conditions expected
that should be treated by grouting. The
subsurface explorations in karst are
frequently conducted to refusal at the
top of rock. This can be misleading,
since karst is often pinnacled and may
contain ‘floaters’, boulders that are
remnants of prior collapse or differen-
tial weathering. The subsurface investi-
Figure 5. Crosshole seismic tomography image of sinkhole.
