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Martin Hill TM Void mitigation:
planning and construction
Presented at the BCP Infrastructure
Workshop
October 30, 2014
Sylvia R. Pope, P.G.
City of Austin, WPD, Environmental
Resources Management Division
Karst geohydrology
• Karst definition
– Karst refers to terrain that contains sinkholes, caves, sinking streams, and karren (bedrock sculpted by dissolution). Dissolution of rock, typically limestone or dolomite, forms these features.
• Edwards Group and Georgetown Formation
– Karst terrain that comprises the Recharge Zone of the Edwards Aquifer. Cave passages, fractures, void horizons transmit significant volumes of groundwater. Rapid recharge and travel time through the aquifer.
• Characteristics of formations that have lead to cave and sinkhole development
– Caves typically occur below contact with less soluble strata and along structural trends. Dolomitic strata may contain evaporite layers that are easily dissolved.
Why protect voids?
• It’s required by rules and regulations.
• The goal of void mitigation is to preserve the function of the void while protecting and isolating it from infrastructure.
• Mitigating voids helps preserve mesocaverns that may harbor endangered karst invertebrate species.
• Protecting caves and karst features maintains the water quantity and quality recharging the Edwards Aquifer. We are the ultimate consumers of that water.
• Voids convey water to the aquifer or to springs, even if they appear dry when intercepted by trenching.
Infrastructure planning
• Where to look for information?
• When to contact agencies during planning process? – During initial route selection
• Whom to contact for more detailed information? – Austin Water Utility, Wildlands Conservation
Division, BCCP Permit Coordinator, Kimberlee Harvey at 512-972-1686
– Watershed Protection Dept., Environmental Resources Management Division geologists (Sylvia Pope, David Johns, Nico Hauwert, Scott Hiers) at 512-974-2550
What permits are required?
• Are BCCP infrastructure permits are required? – If the project is within Karst Zones 1 or 2 and the City of
Austin/Travis County’s 10(a) permit area; yes.
• What City of Austin permits apply to construction of infrastructure projects? – Either small project, site plan or general permit. Refer to the
Administrative Criteria Manual or contact the Development Assistance Center (DAC)
• Are there additional requirements from other agencies ? – TCEQ may require a Water Pollution Abatement Plan or
Organized Sewage Collection System Plan, if in the TCEQ-defined Recharge Zone
• When to contact the permitting agency? – At the project concept stage
What types of projects require a
BCCP infrastructure permit?
• Wastewater lines and infrastructure
• Water lines and infrastructure
• Power lines and transmission facilities
• Stormwater control structures
• Drilling projects for geological investigations
• Roadways
• Gas or fuel pipelines
• Cable TV and communication infrastructure
• Other infrastructure, not otherwise specified
Geological data – Project Summary
• Geotechnical boreholes per AWU criteria
• Geophysical studies in proximity to BCCP
cave cluster along project alignment
• Karst survey per COA and TCEQ
requirements
• Hydrogeological study of BCCP caves
along project alignment (more from Nico
Hauwert, next presentation)
Project specific void planning
• Borehole void protocol – Triggers, notification and mitigation protocol within
presumed hydrogeologic basin of BCCP caves. AWU Wildlands, BCCP program and WPD
– downhole video logging for biological survey
– Bait and trap for biological survey
– Borehole grouting protocol
• Trenching void protocol – Trench inspection by Texas P.G. – consultant
• Void mitigation protocol – Standard COA and TCEQ void mitigation or
determined by BCCP Infrastructure Coordinator?
– Cave-sized voids require engineering design of a mitigation strategy specific to the void
Borehole
void
Mitigation
may be
required as a
condition of
BCCP
Infrastructure
Permit. This
example is
for an LCRA
power pole
project.
Construction phase: Reporting Voids
• Void and Water Flow Mitigation Rule (LDC 25-8-281
(D), COA ECM 1.12.0 and Item No. 658S of the SSM)
requires that work stop and the void be reported to
the City of Austin
• Report the void to the COA inspector (general permit
or CIP or site/subdivision or environmental) and to
ERM Watershed Protection Dept. hydrogeologists
• TCEQ also has solution feature discovery
notification requirements. ALWAYS report voids to
the TCEQ, even if you have been told that it isn’t
necessary by a TCEQ staff member
Construction phase: Mitigating Voids
• Submit site plan correction to COA Watershed
Protection Dept. (WPD) (LDC 25-8-281 (D)) and
partner departments
• Submit mitigation plan to TCEQ per the solution
feature discovery requirements
• Coordinate void mitigation design with COA WPD
and TCEQ to minimize conflicts (i.e., concrete
specification, encasement material)
• Mitigation measures are installed after approval is
given
Other potential considerations for Void
Mitigation
• USFWS-permitted biologist for evaluating voids intercepted during construction
• Equipment and standby time for biological evaluations
• Licensed geologist (Texas P.G.) for inspecting trenches greater than five (5) feet deep and all other voids intercepted during construction activities
• Equipment and standby time for geological evaluations
• Engineer to prepare and submit void mitigation plans
• Provisions for an engineer to conduct structural analysis and develop a specific void mitigation for a cave-sized void
These expenses may not be incurred if you don’t intercept voids but
budgeting for them decreases delays.
Initial geotechnical
investigation
borehole BH-7 for
a project near the
Martin Hill TM
intercepted a void
within the
presumed
drainage basin of
Fossil Garden
Cave. Specific
protocol for
biological
investigation was
required.
Void 2 – safety considerations of
investigation
Void 2 – safety concerns for inspection,
mitigation review ??, TCEQ enforcement
Questions for ERM
geologists?
• Contact Sylvia Pope at: – 512-974-3429
– Email [email protected]
• Contact Nico Hauwert at: – 512-974-2148
– Email [email protected]
• Contact David Johns at: – 512-974-2781
– Email [email protected]
• Contact Scott Hiers at: – 512-974-1916
– Email [email protected]