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VERIFICATION OF GEOTECHNICAL GROUTING- · PDF fileSeismic Refraction –Detects contrast...
Transcript of VERIFICATION OF GEOTECHNICAL GROUTING- · PDF fileSeismic Refraction –Detects contrast...
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VERIFICATION OF GEOTECHNICAL GROUTING- STRATEGIES AND
PLANNING
Michael J. Byle, D.GE, F. ASCE
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Verification of Geotechnical Grouting was published By the ASCE Committee on Grouting in 1995
It is still available and still has useful information
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OUTLINE
• Overview of grouting methods
• Identifying Basis for Verification
• Guidelines for planning verification
• Methods for verification
• Summary
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Overview of Grouting Methods
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Permeation Grouting
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Permeation Grouting
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Rock Joint Grouting
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Permeation Grouting
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Compaction Grouting
Compacted zone Water Table
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Compaction Grouting
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Jet Grouting
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Deep Mixing
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Deep Mixing
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Hot Melts
• Injection of a molten solid that solidifies in the ground
• Predominantly Bitumen
• Used to control high volume water flows
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Hot Bitumen Grouting
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Verification is Important
• Verification provides a basis for:
– Measuring success
– Payment for performance based contracting
– Modifying the means and methods during construction
– Satisfying third parties
• Insurance companies
• Financial institutions
• Owners
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Planning for Verification
Verification Objective
1. Liquefaction resistance
2. Cutoff of Seepage
3. Resultant bearing capacity
4. Settlement Reduction
5. Structural support (i.e. underpinning)
Factors Monitored
1. Soil Stiffness
2. Presence of grout
3. Grout Stiffness
4. Overall ground strength
5. In-Place grout quality
6. Resultant Permeability
7. Size and shape of injected mass
8. Temperature
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Using Correlated Properties
Desired Properties
• Defining limits of grout intrusion
• In-situ grout compressive strength
• Settlement
Correlated Properties
• Electrical conductivity/resistivity
• Magnetic properties
• Seismic wave velocity
• Blow count resistance
• Ground movement
• Acoustic emissions
• Stiffness
• Static penetration resistance
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Using Correlated Properties
Desired Properties
• Hydraulic Cutoff
• Permeability Reduction
• Sealing Leaks
Correlated Properties
• In situ permeability
• Groundwater gradients
• grout permeability
• Electrical conductivity/resistivity
• Acoustic emissions
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Basis for Verification
• In situ values will differ from ex situ test results
• Variability in the formation will result in variability of the grout injection and properties
• Recognize that in many instances it will not be possible to measure the property of concern
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Consider Verification During Pre-Design
• Subsurface Characterization – Forms the baseline from which to assess
improvement
– Provides insight into appropriate verification methods
– Identifies parameters essential for verification
• Consider multiphased investigation to permit focused investigation prior to final design
• Helps to select verifiable grouting methods
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Verification Considerations in Investigation
– Characterize site variability or outstanding uncertainty of site conditions
– Special parameters needed for verification (electrical, chemical, geophysical, etc)
– Improve detail of characterization to anticipate potential verification problems
– May use special tools and techniques • Geophysics – seismic, resistivity, gravimetric, etc.
• In situ tests – CPT, SPT, DMT, Permeameters, etc.
– Investigation may need refinement as the design develops
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Investigation Should Identify Parameters for Verification of Grouting
Understanding how grouting may alter any of these parameters is essential to planning verification
• Permeability
• Grain Size Distribution
• Compressibility
• Void Ratio/Relative Density
• Electrical Resistivity
• Seismic Wave Velocity
• Soil Contrasts
• Soil/Groundwater Chemistry
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Basic Fundamentals to Begin Planning Verification
• Physical limits of strata
• Groundwater level and flow
• Range of variability
• Stratification
• Subsurface structures
• Obstructions (boulders, rubble, etc.)
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Verification Tools and Techniques
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Grouting as Verification
oRange Detection Pipes for Jet
Grouting
oDeflection measurements during
grouting
oPressure/volume relationships o Aparent Lugeon Method
o LMG Pressure Response
o Digital real-time methods are now
commercially available
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Cylinder 1 Swing Tube
Hopper
Cylinder 2
Grout Line
Volume of Grout
1 Stroke
Process Repeated
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Volume of Grout
Cumulative Envelope
Courtesy Moore and Taber
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Volume of Grout
Hydraulic Fracture Envelope
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Direct Sampling
oCoring
oTest Excavations
oDirectly measure properties of
sampled materials
oSome materials difficult to sample
oMay not measure intended objective
oConsider spatial variability vs. sample size
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Coring
Coring success is material dependent
Core deviation from grout mass may be a
problem and should be validated with
inclinometer
Cored samples can be tested in laboratory
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Coring Jet Grouted Fill
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Physical Examination
Appropriate where the presence and
distribution of the grout is an effective
indicator of performance (i.e. inclusions)
Requires excavation of grout mass or
extraction
Usually destructive
Appropriate for Test Sections for Critical
Applications and for Support of Excavation
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Element
Extraction
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Direct Examination
Photo Courtesy of Sam Bandimere
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Test Excavation
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In Situ Modulus Tests
o Includes DMT and PMT
oMeasure deflection under lateral or radial
loading
oGood for some sands, silts, clays
oCan be correlated to density
oAffected by residual stresses
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Settlement Plates/Points
oMeasure elevation or change in elevation
at a given point
oUsually used to measure heave or
settlement
oAccuracy depends on the method of
survey
oWhere settlement control is the goal, this is very effective, but after the fact
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Load Testing
oUsually for structural elements
oCan be done on subgrades with
plates
oDirectly measure deflection under
loading to calculate modulus
oCan measure bearing capacity if failure is achieved
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Penetration Resistance
Commonly used - Cone Penetrometer
and SPT
Relatively inexpensive
Accuracy and repeatability dependent on
method and personnel
Widely accepted
Can be correlated to density
Not many correlations for modified
ground
Influenced by residual stresses
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Density Tests
Include - Sand Cone, Rubber Balloon,
Drive Cylinder Method, Nuclear
Densometer
Require excavation or exposed surface
Nuclear density gage relies on empirical
correlation of radiation transmission to
density Reliable but difficult to use
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Plate Load Test
Appropriate for applications where
changes in the strength of the soil,
grouted soil is measurable and useful for
evaluating grouting effectiveness Measurement is limited by the plate size
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Geophysical Methods
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Terrain Conductivity
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Terrain Conductivity
• Electro-magnetic method
• Coil induces alternating electric current in the ground
• Second coil detects magnetic field generated by alternating current in the ground
• Conductivity varies inversely to the ratio of the primary to secondary coil field strength and the square of the distance between the coils
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Ground Penetrating Radar
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GPR
• Pros – Identifies contrast in electrical properties of materials – Low cost – Good shallow resolution in granular soils – Voids are high contrast objects for GPR and show up well
• Cons – Limited depth
• Most soils - several feet • Optimal conditions – 10-15 feet
– Will not work in high attenuation soils • Clay • Micaceous soils and rock
– May not resolve grout
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Magnetic Survey
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Magnetics
• Utilize Magnetometers and Gradiometer to measure changes in the earth’s magnetic field
• The magnetic field intensity is measured on a grid, plotted and contoured
• Anomalies indicated by steep gradients or closed contours
• Interpretation can be difficult
• Same sensitivities as Terrain Conductivity
• Best for metallic or magnetically constrasting materials (additive to grout may be advisable)
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Resistivity
(+) (-)
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Electrical Resistivty • Applies voltage across two electrodes
• Measures resistance across inner electrodes
• Depth is dependent upon electrode spacing
– Larger spacings equal larger depth
– Result is average over depth
• Only useful where there are contrasts in the electrical properties of the materials to be tested (e.g. grout and soil, soil and rock, grout and rock, grout and groundwater, etc.)
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Micro-gravity • Measures the strength of the earth’s gravitational field at discrete points
• Plots results to identify areas of low gravity that imply deeper rock, or voids
• Test is sensitive to vibration
• Requires corrections for terrain
• Good for resolving large voids or areas of voids in otherwise sound rock
• Relatively high cost; readings are slow
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Seismic
Refraction
Reflection
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Seismic Refraction
– Detects contrast in stiffness of materials – Reasonable cost – Good resolution in most materials – Best if done as a before and after – Easy to identify large soft zones or voids
• Limitations – Cannot detect soft layers beneath stiff layers – Difficult to interpret if highly irregular surface (pinnacles can lead to
pulse inversion) – Resolution decreases with depth – May not be able to resolve grout – Traffic and construction produce noise that can limit quality of
results
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Seismic reflection
• Pros – Good resolution in most materials – Measures soil/rock stiffness – Easier to interpret (Voids are always low velocity zones) – Effective at greater depth than refraction – Can detect soft layers beneath stiff layers
• Cons – Cost – requires large number of data points – Difficult to interpret if highly irregular surface (pinnacles can lead to pulse
inversion) – Resolution decreases with depth – Small voids in high velocity material may be invisible – May not be able to resolve grout
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Borehole methods
• Virtually all of the methods discussed can be done in and between boreholes
– Vertically within boreholes (borehole logging methods)
– Diagonally from ground surface to borehole (up-hole or down-hole methods)
– Horizontally between boreholes (cross-hole methods)
• Many limitations of methods can be overcome by adjusting spacing or depth of boreholes
• Borehole drilling and sampling gives a true answer at each borehole that can be used to calibrate the geophysical data
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Crosshole Geophysics
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Crosshole Tomography
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TOMOGRAPHY
• A process of imaging based on wave phenomenon. (similar to a CAT Scan)
• A graphical numerical model is created to produce the same result as the data collected
• This requires a large amount of data
• A forward model is used to assess the likelihood of success – A model of expected conditions is created
– The ability of the system to detect the conditions is tested by simulation
– The resulting information is used to select the appropriate methods and arrangement for the field test.
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Resistivity tomography
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Seismic Tomography
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Multiple Tools
• There is no magic bullet
• Each site is unique
• Combining methods of verification can produce a superior result
• Grout monitoring together with a sampling method and a geophysical method can provide a more complete picture
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Planning for Verification
1. Identify need for grouting early, preferably during investigation stages
2. Identify goals of grouting and what it is possible and necessary to verify
3. Determine the mechanism and properties of the grout in-place as compared to the substrate
4. Select methods that can detect relevant properties
5. Use them in combination to best effect
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Planning for Verification
7. Use a qualified inspector
8. Record and report everything
9. Plan on the unexpected a) Evaluate the results as the work progresses
b) Adjust methods as needed to assure good results
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