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 Keeping
water out of the quarry floor is a challenge facing aggregate producers
working in many conditions, but especially those working in heavily
karstified limestone deposits.
Both long- and short-term aspects of an aggregate operation can
impact the problem. For example, a quarry in and of itself can
create relatively abrupt and unnatural hydraulic gradients in the
surrounding ground by forming a massive open space beneath the
natural piezometric surface. Short-term disturbances such as
blasting, excavation, and the alteration of regional piezometric
conditions by pumping may further impact water conditions.
While many limestone quarries are located and engineered to avoid
known karstic locations, aggregate producers should be prepared to
deal with water infiltration of some extent. Remedies for dealing
with water range from simple pumping to the installation of grout
curtains. Although aggregate producers are familiar with the former,
the latter may also be a viable option under the correct conditions.
Grout curtains installed in virgin karst have a finite effective
life — the length of which depends on the rock mass characteristics,
the intensity and quality of any grouting conducted, and the
prevailing hydraulic gradients. Unfortunately, this life cannot be
precisely predicted. Such grouting operations cannot be guaranteed
to comprehensively treat a very karstic rock mass. Under long-term
service conditions, seepage may eventually result in open channels
through features in the karst filled with residual clay or other
erosional or weathering materials.
The first installment of this two-part series addresses what to
do when water flow velocity or volume reaches a level that demands
action. Such interludes are typically highly stressful for all
parties, especially given the consequences of failure. They
invariably present a technical scenario that is extremely
challenging to resolve. It is at such times that logic is often lost
in the rush, and the “ready, aim, fire” syndrome kicks in. The
second installment will focus on construction techniques and
considerations.
Crisis management
The following eight-step sequence reflects three fundamental
stages in implementing any successful remedial grouting operation:
• Exploration and situation assessment;
• Responsive
execution; and
• Verification and monitoring of performance.
Sudden, significant, and obvious changes to the preexisting
structural and hydrological regimes characterize a karst-related
flow event. Flow or seepage rates may increase substantially — by an
order of magnitude or more — the flow may be discolored, new seepage
entry and exit points develop (e.g., “eddies” and “boils”),
piezometric surfaces drop, or surface manifestations may occur in
the form of depressions in embankments and sinkholes in overburden.
At such times, normal facility operations are interrupted or
suspended, and depending on the severity of the situation, a
fundamental safety situation may be declared and a wide range of
technical, operational, managerial, financial, and statutory bodies
may become involved. Time will be of the essence in order that
resolution is achieved as quickly and cost effectively as possible,
and that any safety-related issue is correctly and firmly managed.
The following steps reflect the approach the author has developed
over the course of several such events.
Step 1. Appoint a project manager to act as a coordinator of
the short-term emergency and the subsequent long-term remediation
efforts. This manager should be from the ranks of the facility owner
and should have long and direct experience with the construction and
operation of the site and with the modus operandi of the ownership.
The manager should be relieved from his prior routine duties as much
as possible and should be fully empowered to seek further
assistance, both from internal resources and external consultants.
Establish a separate “mission control” room for collecting and
analyzing all data and for holding all technical meetings. Formally
record every meeting.
Step 2. Evaluate exactly what the situation is, via analysis of
all available data sources, but at this time paying special
attention to documenting verbal accounts from witnesses. Such
accounts can be of great benefit in subsequent analysis, but their
value depends on their accuracy and completeness, both of which will
rapidly recede with time.
Step 3. Implement all necessary short-term measures which
legally, administratively, or practically have to be taken. From the
technical viewpoint, this may include installing additional, simple
instrumentation (to help quantify the issue, e.g., structural
movement monitoring, flow measurements); increasing the frequency of
reading existing instrumentation; inspecting the site; relocating
equipment that is threatened by inundation; or installing extra
pumping capacity. These actions help create a baseline, mitigate the
impact, identify if the situation is deteriorating further, and help
the project manager determine the level of imminent danger.
Step 4. Design and conduct a focused program of new site
investigation. The purpose of the investigation is to establish the
exact path of the flow (typically it is in a massive conduit as
opposed to in a widely dissipated “delta”), its rate and velocity,
and the nature of the rock around the conduit. (A conduit in a zone
surrounded by other clay-filled karstic features that have not, as
yet, been “flushed out” will represent a severe problem during
subsequent remediation and service.) This study will permit a
remedial design to be conducted and priced. It will also highlight
if the flow has the potential to create further distress to
overlying or adjacent structures. During this time, maintain the
instrumentation reading schedule of Step 3.
The site investigation should comprise the following two
complementary tasks:
- Desk study: review all relevant construction records;
historical performance data; instrumentation data; regional,
local, and site geology; climactic and seismic records; aerial
photographs; personal recollections; and published technical
papers.
- Field study: install investigation holes by the fastest and
most economical method to try to physically locate the conduit.
This should be done as far “upstream” as possible.
These holes can then be instrumented to provide ongoing data on
groundwater levels, chemistry, temperature and pH; can be used for
various types of geophysical testing, such as seismic tomography; or
can be used as grout holes in subsequent remediation. Other types of
geophysical testing, such as Ground Penetrating Radar, Spontaneous
Potential, Electrical Resistivity (Dipole-Dipole or Wenner
Schlumberger), and magnetic or gravimetric surveys, can be
conducted. Dye testing, if properly conducted, can be extremely
useful.
It may happen that despite best efforts and intentions, the exact
source or path of the flow cannot quickly be determined with
accuracy. Perseverance is essential: do not commence the subsequent
steps until Step 4 is satisfactorily concluded.
Step 5. Assuming the situation is to be positively rectified,
as opposed to merely being monitored or managed by other means
(e.g., ongoing pumping from the quarry floor), the project manager
and his or her advisors develop the design for remediation.
At this stage, seek input from specialty contractors and other
specialists and review the technical literature for case histories
of similar nature. It is essential that the design clearly
identifies the “measure of success” of the project in terms of, for
example, the residual flow rate or piezometric levels at various
locations.
It is common to find that few grouting contractors will have faced
such a severe problem before, and unfortunately, most will tend to
initially underestimate the difficulty of the remediation.
Considerable amounts of time and money have been lost by using local
contractors in haste, using simple and conventional methods which
are later proved to be wholly inadequate. It is also usually the
case that such contractors have been hired on a “cost plus” or “time
and materials” basis and so may not be highly motivated to achieve a
quick and definitive solution, even if they did possess the
technological resources.
Step 6. With the design and budget approved, hire the
contractor. This should be done on the basis of “best value” as
opposed to “low bid,” although the two may be the same. Emphasize
the contractor’s experience, expertise, and work plan, as opposed to
price. Hiring the wrong contractor will certainly lead to
disappointment and dispute over schedule, performance, and cost, and
indeed, inappropriate construction methods may worsen the situation
and make further remediation attempts even more challenging.
Step 7. Execute the work. During this phase, all data relating
to the contractor’s operations (e.g., drilling, water testing, and
grouting records, and progress) and to the impact on the overall
structure/bedrock system (e.g., flow characteristics, piezometric
levels, structural movements, changes in groundwater chemistry, and
temperature) must be collected and studied in real time by the
project manager and his team in “mission control.” Only in this
responsive, integrated fashion can the effect and effectiveness of
the work be revealed progressively, and a sound engineering basis
created upon which to instruct changes to the program if required,
such as additional or deeper holes or different grout mixes. Such
data are also invaluable in the ongoing process of reevaluating the
soundness of the design (Step 5). This step is in place until the
remediation has been completed and a short-term (e.g., seven days)
confirmation period has successfully elapsed. Prepare a fully
comprehensive “as built” report covering all the relevant data from
Steps 1 through 7 as soon after the remediation as practical.
Step 8. Long-term monitoring. Many — if not all — the
piezometers and other monitoring devices installed beforehand should
still be functional at this point. The project manager must
establish a regular schedule for reading these instrumentation
sources, analyzing their data, and for conducting any relevant
revised site or structural inspections. Establish a database and a
well-defined series of protocols to follow if certain
instrumentation trigger and threshold levels are reached, or if any
significant flow or pressure aberrations reoccur. These protocols
should include details of the responsible person(s) to be notified,
and appropriate emergency response plans.
The most effective a grout curtain in karst will ever be is
immediately after its construction. In service, as the full
hydraulic gradient is being placed on the curtain (i.e., as the
quarry is pumped dry) pockets of ungrouted or ungroutable weathered
material will be exposed to pressures which may prove sufficient,
over time, to cause such pockets to “blow out.” This will occur
despite the very best efforts of the design and construction teams.
However, there is no predictive capacity as to how severe this
increase in residual permeability will be or how fast it will
develop. Clearly, such deterioration will depend on the nature of
the karst (i.e., how much erodible material remains), the applied
hydraulic gradient, and the length of time over which it acts.
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References
Bruce, D.A. (2003). “The Basics of Drilling for
Specialty Geotechnical Construction Processes.” Grouting and Ground
Treatment, Proceedings of the Third International Conference,
Geotechnical Special Publication No. 120, Ed. L.F. Johnsen, D.A.
Bruce, and M.J. Byle, American Society of Civil Engineers, pp.
752-771.
Bruce, D.A. and Gallavresi, F. (1988). “The MPSP System: A New
Method of Grouting Difficult Rock Formations.” ASCE Geotechnical
Special Publication No. 14, “Geotechnical Aspects of Karst
Terrains.” pp. 97-114. Presented at ASCE National Convention,
Nashville, Tenn. May 10-11.
Bruce, D.A., W.G. Smoak, and C.C. Gause. (1997). “Seepage Control: A
Review of Grouts for Existing Dams,” Proceedings of the Association
of State Dam Safety Officials, 14th Annual Conference, Pittsburgh,
Pa., Sept. 7-10, Compact Disc.
Cadden, A.W., D.A. Bruce, and R.P. Traylor. (2000). “The Value of
Limited Mobility Grouts in Dam Remediation.” Association of State
Dam Safety Officials Annual Meeting.
Bruce, D.A., R.P. Traylor, and J. Lolcama. (2001). “The Sealing of a
Massive Water Flow through Karstic Limestone.” Foundations and
Ground Improvement, Proceedings of a Specialty Conference, American
Society of Civil Engineers, Blacksburg, Va., June 9-13, Geotechnical
Special Publication No. 113, pp. 160-174.
Chuaqui, M. and D.A. Bruce. (2003). “Mix Design and Quality Control
Procedures for High Mobility Cement Based Grouts.” Grouting and
Ground Treatment, Proceedings of the Third International Conference,
Geotechnical Special Publication No. 120, Ed. L.F. Johnsen, D.A.
Bruce, and M.J. Byle, American Society of Civil Engineers, pp.
1153-1168. |
Donald A. Bruce, Ph.D. is president of
Venetia, Pa.-based Geosystems, L.P. He can be reached at
724-942-0570.
(part 2 of article click here)
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