Geophysical Methods for Screening and Investigating Utility Waste Landfill Sites in Karst Terrain Gary Pendergrass, PE, RG, F.NSPE Principal Geological Engineer Kansas City Geotechnical Conference 2017 Overland Park, KS April 20, 2017
U.S. Karst Map
Primary Focus of UWL Siting in Karst Structural Stability Groundwater Monitoring
Formation of Karst Limestone has no appreciable primary permeability. Groundwater flow occurs along fractures (secondary permeability). Dissolution of limestone is a surface phenomenon.
Formation of Karst As dissolution of limestone progresses along vertical fractures, the fractures widen.
Formation of Karst Just as differing colors of strata indicate differing depositional environments and differing chemical constituents, certain stratigraphic zones can be preferentially soluble due to differing CaCO 3 content.
Formation of Karst This roadcut illustrates how preferential groundwater flow along horizontal fractures and preferential dissolution of limestone along bedding planes can produce horizontal solution cavities.
Formation of Karst Dissolution along vertical fractures/joints and dissolution along preferential bedding planes produce the vertical and horizontal components of karst systems.
Modes of Sinkhole Formation Solutional Sinkholes Collapse Sinkholes
Formation of Solutional Sinkholes Solutional sinkholes tend to be relatively stable. Many hold water and are utilized as farm ponds. Easily identified and avoided in landfill siting.
Formation of a Collapse Sinkhole
Formation of Collapse Sinkholes Discrete, isolated features that originate at the soil - solution cavity interface within a joint. Surrounding soil is typically undisturbed and intact. Basically a hole in the soil that propagates toward the surface. Typically triggered by a change in soil moisture content.
Stabilization of Collapse Sinkholes Locations susceptible to sinkhole collapse will be identified and stabilized prior to UWL development
Stabilization of Collapse Sinkholes
Groundwater Monitoring in Karst Limestone has very low primary permeability. Groundwater moves preferentially through joints and fractures.
Groundwater Monitoring in Karst A monitoring well sited in massive limestone would likely be a dry hole.
Groundwater Monitoring in Karst A monitoring well sited near a joint or fracture trend would more likely capture the groundwater moving through the system.
Goals in UWL Siting Identify areas susceptible to collapse sinkhole formation and pre-emptively stabilize There are no perfect sites, but all can be engineered Identify groundwater flow pathways beneath prospective sites and determine if site can be effectively monitored
Characteristics of Karst Karst is not swiss cheese. Karst is a very well-organized, logical system which develops and behaves in a very well-organized, logical fashion Karst processes are well understood Karst can be accurately characterized using appropriate investigative techniques.
Use of Geophysical Techniques for Karst Investigations Electrical Resistivity Tomography (ERT) ERT measures the electrical resistivity of earth materials ERT also provides an indication of moisture content
Use of Geophysical Techniques for Karst Investigations Multi-channel Analysis of Surface Waves (MASW) MASW indicates the nature and strength of earth materials MASW works very well in combination with ERT Top Rock ~52 feet
Elements of UWL Preliminary Site Screening and Investigation Review and compilation of existing site data Establishment of a 3D GIS model Site reconnaissance to delineate existing surficial karst features ERT screening survey (traverses on 100-foot centers, 100-foot depth) MASW soundings on 400-foot grid Confirmatory drilling & coring Development of a comprehensive site model Evaluation of prospective UWL sites Selection of a preferred UWL site
Site Reconnaissance Entire area of investigation was walked and inspected by GeoEngineers geologists in a systematic fashion. Distinct surficial karst features identified. Features mapped by walking the rims with ipad linked to GPS. Photo log/descriptions keyed to map numbers. Locations to be surveyed by licensed land surveyor. Suspected karst features those which were simple closed depressions (no visible eye) or altered (filled with rock, etc.) are further evaluated by comparison to closest ERT profiles.
Site Reconnaissance
Preliminary Geophysical Investigation
Preliminary Geophysical Investigation
Multi-channel Analysis of Surface Waves
Multi-channel Analysis of Surface Waves
Representative ERT Profile
Representative ERT Profile
Interpreted Joint Sets Karst Cultural Drainage
Schematic Cross-section
3D GIS Database/Model
3D GIS Database/Model
3D GIS Database/Model
3D GIS Database/Model
Preliminary Site Investigation Conclusions A stable utility waste landfill foundation can be ensured by detailed characterization of the subsurface, identification and proactive mitigation of potentially unstable areas, and appropriate landfill design which provides for stabilization of moisture content, stormwater diversion, and groundwater control A groundwater monitoring system which effectively monitors the uppermost continuous aquifer can be sited and installed on the basis of detailed ERT surveys, focused drilling/coring, and hydrologic testing
Elements of a Detailed Geophysical Investigation ERT survey (traverses on 20-foot centers, 160-foot depth) Processing of 3D ERT data MASW soundings on 200-foot grid Confirmatory drilling & coring Development of a comprehensive 3D GIS site model
ERT
Site Mapping
Top of Bedrock Map
Site 1 Horizontal Slices
3D ERT Model
3D ERT Model
3D GIS Modeling
USEPA Coal Combustion Residuals Rule On April 17, 2015, USEPA finalized the CCR Rule Response to TVA Kingston Plant CCR release in December 2008 The rule established requirements for disposal of CCR in utility waste landfills (UWLs) and surface impoundments.
Location Restrictions for Utility Waste Landfills The rule establishes five location restrictions to ensure UWLs are properly sited. Placement above uppermost aquifer (new units) Wetlands (new units) Fault Areas (new units) Seismic Impact Zones (new units) Unstable Areas (new and existing) The rule requires a qualified Professional Engineer (PE) to certify that the technical requirements of the rule are being met. Existing UWLs must be certified with respect to unstable areas by October 17, 2018.
U.S. Karst Map
Geophysical Investigation GeoEngineers has developed techniques to produce high resolution 3D imagery of UWLs using Electrical Resistivity Tomography (ERT) and Multi-Channel Analysis of Surface Waves (MASW).
Geophysical Investigation Marine ERT can be utilized to image beneath landfill ponds and closed surface impoundments
What we expect to see Higher resistivity (dryer) materials beneath the liner. Horizontal stratification of materials within the fill material. Karst features (sinkholes) would be represented as vertical anomalies.
What we expect to see Differential settlement would be represented by lateral offset in resistivity pattern. Lower resistivity anomalies at landfill toe (drainage anomalies). Lower resistivity linear anomalies representing solution-widened joints or fracture trends
UWL 3D ERT Profile
UWL Groundwater Monitoring System Proposed monitoring well location
Summary Geophysical techniques provide a sound basis, rationale and justification for UWL certification and provide the scientific documentation necessary for public acceptance.
Questions? Gary J. Pendergrass, PE, RG, F.NSPE Principal GeoEngineers, Inc. 3050 S. Delaware Avenue Springfield, MO 65804 417.799.2612 gpendergrass@geoengineers.com