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Geotechnical Investigations for Tunneling

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Geotechnical Investigations for Tunneling Breakthroughs in Tunneling September 12, 2016 Geotechnical Site Investigations For Tunneling Greg Raines, PE Objective To develop a conceptual model adequate to estimate the range of ground conditions and behavior for excavation, support, and groundwater control. support Typical Phases of Subsurface Investigation Phase 1: Planning Phase – Desk Top Study/Review Phase 2: Preliminary/Feasibility Design – Initial Field Investigations Phase 3: Final Design – Additional/Follow-Up Field Investigations Final Phase: Construction – Continued characterization of site Typical Phases of Subsurface Investigation Phase 1: Planning Phase – Desk Top Study/Review Review: Geologic maps Previous reports/investigations Aerial photos Case histories Develop conceptual geologic/geotechnical model (cross sections), preliminarily identify technical constraints/issues for the project. Plan subsurface investigation program. Identify/Collect Available Geotechnical Data in the Project Area Bridge or control Information can include: structure • Geologic maps • Data from previous reports • Drill hole data • Preliminary mapping Compile available local data into a database for further evaluation. Roads or Residential Canals Area Geologic Profiles – Understand Geologic Setting and Collect Specific Data Bedrock Surface Elevation Maps Aerial Photo / LiDAR Interpretation Aerial Photo Diversion Tunnel Use digital imagery/LiDAR to map local features prior to field mapping. Dam LiDAR Field Geologic Mapping Field Geologic Mapping Structural Data Collection (faults, folds, shears, contacts, alteration zones) Mapping at the scale of the project/features ! Geomechanical Mapping of Discontinuities (Kinematic Data Analysis) Tunnel Alignment Courtesy of Rocscience Develop Preliminary Geologic / Geotech Conceptual Model for the Project Goal is to: • identify the primary technical considerations, • limitations/constraints, • scope, • purpose, • geotechnical targets, • and site conditions that must be resolved. Typical Phases of Subsurface Investigation Phase 2: Preliminary/Feasibility Design – Initial Field Investigations Implement the field drilling/CPT, sampling, testing, geologic/geomorphic mapping, surface & down-hole geophysics, and laboratory testing program. This scope can often change based on the findings during the investigation. This phase is often where the bulk of the site characterization effort is applied. Typical Phases of Subsurface Investigation Phase 3: Final Design – Additional/Follow-Up Field Investigations Target specific geologic/geotechnical features, conditions, or issues that may impact the project design or areas where design modifications have been developed. Often specialty field or laboratory testing is performed (e.g. in-situ stress measurement, tri-axial testing, etc..). Subsurface Exploration Conventional Drilling Cone Penetrometer Testing (CPT) Test Pits Sampling & Testing Geophysical Methods 15 A lot of Borings for Shallow Soil Jobs Deep Rock Tunnel – Fewer Borings Establish Drill Hole Configurations (Depth, Location, Vertical, Inclined, Horizontal) Drill Rig Access and Logistics Surface Water Air Support Underground/confined areas Types of Rigs – Solid Stem Some typical methods to drill the subsurface include: • Auger • Mud/Air Rotary • Mud/Air Rotary with Casing Advance Hollow Stem • Reverse Circulation • Sonic • Becker Penetration • Large Diameter • Cone Penetration Tests (CPT) • Rock Core Each of these methods have pros and cons and are well suited for specific exploration programs, depending on both the subsurface conditions as well as the data needs for the program. Types of Rigs – Auger Solid Stem Hollow Stem Pros: Cons: • Simple, widely available • Depth limitations • No water, mud, or air • Gravels, cobbles limitations • Accommodates most sampling types • Flowing/running ground limitations • Hole collapse (solid stem) Types of Rigs – Mud / Air Rotary Pros: Cons: • Widely available • Location of G.W. in drill hole • Accommodates most sampling types • Mud impact to sensing/testing zone • Efficient in most ground conditions • Cobble/boulder limitations Types of Rigs – Reverse Circulation Schramm (dual tube RC air percussion rig) Pros: Cons: • Efficient • Logging cuttings • Can identify water zones (air) • Sampling limitations • Large diameter (nested wells) • Larger equipment/support requirements • High depth capacity Types of Rigs – Sonic Pros: • Efficient • No water, mud or air • Accommodates most sampling types • Nearly full sample recovery • Can detect water in recovered core • Advance through most geotechnical conditions Cons: • Not widely available for investigation purposes • Potential for sample disturbance • Comparatively slightly higher $/m Types of Rigs – Large Diameter Borings • Physical examination/mapping of the intact geologic conditions Types of Rigs – Horizontal Directional Drilling Pros: • Effective for bypassing critical or existing features • Minimal ground disturbances Cons: • Specialized equipment and crew needed • Limits on the stress radius of the pipe Types of Rigs – Cone Penetration Test (CPT) Pros: • Widely available • Efficient • No water, mud or air • Advance many holes in comparison Cons: • No samples • Push depth limitations Cone Penetrometer Testing Types of Rigs – Rock Core Test Pits Used for soil logging, sample collection and lab testing materials anticipated to be encountered. Types of Samples – Drive Samplers (SPT, MC, LPT) MC Liners Mod Cal SPT Energy Calibration Can be performed in most soil types, difficult in gravel-cobbles. Provides information relating to relative density, strength, and applicability of some ground improvement methods (e.g., soil mixing, jet grouting, chemical grouting) Types of Samples – “Undisturbed” Samples Shelby Tube Pitcher Sampler Typically applied to soft-medium, stiff cohesive soils in order to test strength, stiffness, Piston consolidation, etc… Sampler Excellent Fair to Good Poor to Fair Forms basis for many rock mass classification systems and provides information on the distribution/magnitude of rock fractures relating to rock mass strength, modulus, permeability, and Very Poor to Poor groutability. In Situ Testing – Pressure Meter / Dilatometer In Situ Testing – Hydraulic Conductivity Testing Packer Testing (single and double) Constant Head Falling Head Tunnel Alignment Relates to permeability and groutability of a formation (rock or soil) for parameters used in analysis, and design. Packer Tests Inflation Tube Inflatable Packer End Cap Test Zone Perforated Screen End Plug Packer Tests K(cm/s) 0.0E+00 2.0E-04 4.0E-04 6.0E-04 8.0E-04 1.0E-03 1.2E-03 1.4E-03 1.6E-03 1.8E-03 2.0E-03 1 2 3 4 5 Hydraulic Jacking and Fracturing Tests Jacking Fracturing Pressurization of borehole is done until fracturing of rock in borehole wall. The rock will generally fracture in the direction perpendicular to the minimum stress direction σ3. The fracture alignment is viewed using borehole televiewer or impression packer. The intermediate and vertical stresses are then calculated from the estimated minimum stress value Hazardous Gases & Contaminants Underground • Methane (CH4) • Hydrogen Sulfide (H2S) • Carbon Dioxide (CO2) • Gasoline Vapors • Chlorinated Solvents (PCE, DCE, & TCE) Typical Drill Hole Logs Rock Log Soil Log Typical Drill Hole Logs Soil: Lithology Rocks: Soil type (USCS) Rock Type Color Recovery, RQD, Consistency / GSI density Color Grain size Texture distribution Degree of General: Moisture weathering Drill rate Cementation Strength Rig Behavior Plasticity (clays) Hardness Circulation return Roundness Structure Depth to water Discontinuities: Instrumentation • Type Drill difficulties • Width Shift changes Roughness Code • Infilling Amount Testing intervals and Slickensided & Type results Polished • Surface Shape Smooth • Roughness • Spacing (Joint Rough Very Rough Sets) Data Compilation Summary Sheets Geologic Profiles – Understand Geologic Setting Geophysics Down hole geophysics Seismic reflection* Seismic refraction Resistivity Ground penetrating radar Seismic tomography* Bathymetry Magnetics* (*Not described in this presentation) Oriented Down-Hole Televiewer – Optical & Acoustic Optical Acoustic Down-Hole Televiewer Geophysics – Caliper & Gamma Records Records magnitude of deviations of natural gamma the borehole ray decay diameter indicative of indicating clay mineral intervals of bearing rock wash-out or and soil cave-in Suspension Logging Cross Hole Geophysics/Tomography Seismic Refraction and and Resistivity Resistivity Refraction Compression, P-Wave Marine Geophysics Seismic Reflection Profile Magnetic Survey 3D Resistivity Imaging Ground Penetrating Radar – (Example: Performed Inside Tunnel) Groundwater and Hydrogeologic Conditions Fractured Rock Tunnel Alignment Shaft Screen Clay Screen Highly permeable Screen sand-gravel Piezometers Typical stand-pipe Multi-level vibrating Fully Grouted Multi- piezometer wire piezometer level vibrating wire Multi-level Single piezometer Pump / Slug Tests Slug test assembly Typical Lab Testing • Soil – USCS classification – Strength & Modulus – Moisture/density – Plasticity – Gradation – Hydrometer Unified Soil Classification System (USCS) Grain Size Analysis 1” ½” #4 #10 #40 #200 Plasticity Testing (Atterberg Limits: PI, LL, PL) Hydrometer Test Stickyness Index Medium Low High EPBM Clogging Potential Density & Strength Specialized Lab Testing – Soil Abrasivity Typical Lab Testing • Rock – Rock classification – Compressive/shear strength – Tensile strength (Brazilian)
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