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Lecture 8 Geophysical Survey and Site Investigation
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Lecture No. 5 Geophysical Survey and Site Investigation
Objectives of Soil Investigation
Site Investigation
To undertake survey and investigation activities required for obtaining data with respect to the physical site condition which will affect the construction project.
Objective of Soil Investigation
2. To obtain soil geotechnical parameters
1. To obtain site specific information
o To establish soil stratigraphy
o To establish potential problems associated with the site during construction and operation such as punch-through geohazards (offshore), slope stability, liquefaction (onshore)
o To obtain sufficient & reliable information to permit safe & economic design of installation of permanent works
Objective of Soil Investigation
2. To obtain soil geotechnical parameters
1. To obtain site specific information
o To enable selection of foundation type, calculation of bearing capacity and calculation of loads acted by soil
Traditional
Borehole,
Sampling and
Laboratory Test
Shallow
Geophysical
Method
SITE EXPLORATION AND CHARACTERIZATION
Insitu Testing
Method of Site Investigation
Traditional
Borehole,
Sampling and
Laboratory Test
Shallow
Geophysical
Method
SITE EXPLORATION AND CHARACTERIZATION
Insitu Testing
• Test Execution: Time consuming
• Sample Visual Observation: Yes
• Automation: Manual sampling, minor automation in test execution, computerized
data and visualization, minor during data interpretation
• Application: Calculation based on ‘soil mechanics’ approach
Method of Site Investigation
Traditional
Borehole,
Sampling and
Laboratory Test
Shallow
Geophysical
Method
Insitu Testing
• Test Execution: Faster than traditional method
• Sample Visual Observation: No
• Automation: Less automated test execution, minor computerized, interpretation and
visualization
• Application: Calculation based empirical correlation
SITE EXPLORATION AND CHARACTERIZATION
Method of Site Investigation
Traditional
Borehole,
Sampling and
Laboratory Test
Shallow
Geophysical
Method
Insitu Testing
• Test Execution: The fastest
• Sample Visual Observation: No
• Automation: Highly automated test execution, interpretation and visualization
• Application: Empirical correlation to determine soil type and layering system
SITE EXPLORATION AND CHARACTERIZATION
Method of Site Investigation
Planning of Soil Investigation
Sources of information for site investigation
Pervious use of site
Geologic condition
Slope stability
Topography
o Previous problem with foundation, damage, old map
o Depth and rock type, weathering stage, mining
condition
o Surface configuration, adjacent building (type of
foundation, damages, construction method), rock outcrop, road
Buried services o Water, sewer, electric, telephone, gas, oil tank, heating
duct
Sources of Information for Site Investigation
Slope stability
Material and facilities available locally
o Aggregate, fill, dump, electricity
Hydrological data o Range of tide, river level & discharge, wave & current
condition.
Climatic condition o Flooding, soil erosion, earthquake, swelling and
shrinkage, permafrost
Planning of Site Investigation: Location
Keywords: water main, pond, road, building
Chicken and egg problem
1. Which come first, chicken or egg?
2. Which come first,
(a) Soil exploration (and obtain soil parameter) and use soil exploration method to suit our foundation problem.
Or
(b) We have “foundation problem” and plan soil investigation program to suit our foundation problem.
Testing Program and Loading Path
Soil investigation programs should suit the foundation problems not the other way around!!!
MODULE CONTENT
6. TYPE OF MONITORING TOOLS
5. BASIC LABORATORY TESTS
4. BASIC SOIL MECHANICS
3. OPERATIONAL ASPECT OF SI
2. INTRODUCTION TO GEOPHYSICAL SURVEY
1. OBJECTIVE & PLANNING
Geophysical Survey
Geophysical Survey
• Bathymetry Survey (seabed mapping) • Echo sounder: to obtain water depth
• Sonar: to identify seabed feature (single beam sonar, interferometer sonar, side scan sonar)
• Seismic Survey • Shallow seismic profile
• High resolution survey conducted during O&G exploration can be used to obtain spatial picture to a certain depth
Geophysical Survey
Bathymetry Survey (Seafloor Mapping)
Single Beam & Interferometer Sonar
Principle:
Calculate travel time of reflected sound and calculate the distance
Typical Result
Geophysical Survey
Side Scan Sonar
Basic Principle
Tow Vehicle for Various Frequency and Shape
Data Acquisition and Processing System
Output: Seabed Features Interpreted Image
Verification Through Photo or Video Image
Backscatter Intensity Overlain with Sea Bottom Information
Overlain with isopach contour of sediment thickness from seismic interpretation
Geophysical Survey
Seismic Sub bottom Profiling
Basic Principle Seismic Source: the Boomer
Receiver: Hydrophone
Typical Result
Tools & Equipments Seismic Source: the Air gun
Tools & Equipments Seismic Source: the Water gun
Equipment Selection
Equipment selection is based on the accuracy and required depth of penetration. It is known that high frequency will result high resolution but at limited depth.
The table shows frequency range of various seismic sources.
Identification of Geohazards
Types of Geohazards
• pockmark, fault, volcanoes, shallow gas, subsea landslide etc. are interpreted through integration of data from bathymetry, ground truth (seabed image, grab sample, sediment core) and sub bottom profile (shallow or high resolution)
Aspect of Offshore Structure Affecting Soil
Investigation
Typical Characteristics of Offshore Structures
• Unique construction method (and stress path)
• Dominated by wave load, soil behavior governed by long term cyclic behavior
• Rapid (short) construction period (and load build up), mainly governed by undrained loading
Typical Characteristics in Offshore Structures
Loads • Self weight from structure
• Environmental loads (wind, current, wave, seismic)
Rate of Loading • Rapid during installation (self
weight, piling, spudcan, GBS)
• Periodic (wind, current and wave)
Implications
o Cyclic loads are transferred into foundation load
o In most cases undrained loading (piling, spudcan, anchor, GBS, wind, current, wave, seismic)
o Governed by remoulded or residual shear strength due to large strain during installation (pile, spudcan & anchor)
Immediate Interests
o Remoulded and residual shear strength
o Effect of strength anisotropy (inherent and stress induced)
o Effect of loading rate
Long Term Interest
o Strength degradation & permanent deformation under cyclic load
o Strength gain (pile set-up, thixotropy)
Basic Soil Parameter for Fixed Structure
CLAY SAND ROCK
• General description
• Layering system
• Grain size distribution
• Water content
• Total unit weight
• Atterberg’s limit
• Shear strength (torvane, pocket penetrometer, fall cone, UU. Etc.
• Remoulded shear strength
• Sensitivity
• Stress history
• Organic material content
General description Layering system
Grain size distribution Water content
Max/Min density Relative density
Drained shearing resistance
Stress history
Angularity Carbonate content
Organic material content
General description RQD
Water absorption Total unit weight
Unit weight of solid block Unconfined compression
strength
Mineralogy Carbonate content
Parameters for Specific Design Issues
Foundation stiffness
Cyclic displacement
Permanent displacemeent
Bearing capacity
o Compressibility
o Permeability
o Permanent shear strain and pore pressure under combined average and cyclic shear stresses for triaxial
and simple shear stress paths
o Cyclic shear strain as function of cyclic shear stresses
under combined average and cyclic shear stresses for triaxial and simple shear stress paths.
o Initial shear modulus
o Damping
o Cyclic shear strain as function of cyclic shear stresses
under combined average and cyclic shear stresses for triaxial and simple shear stress paths.
o Initial shear modulus
o Monotonic shear strength under different stress path
o Cyclic shear strength under combined average and cyclic shear stresses for triaxial and simple shear
stress path
o Angle of shearing resistance (sand)
Parameters for Specific Design Issues
Skirt penetration
Liquefaction potential
Soil reaction stress
o Initial shear modulus
o Cyclic shear modulus degradation curves
o Damping
o Coefficient of reconsolidation (sand)
o Undrained anisotropic monotonic shear strength.
o Remoulded shear strength (or sensitivity)
o Drained angle of shearing resistance (sand)
o Residual interface angle of shearing resistance (sand)
o CPT resistance (sand)
o Seabed topography and objects in the seafloor.
o Boulders in the soil within the skirt penetration depth
o Monotonic and cyclic shear strengths.
o Compressibility under virgin loading and unloading
o Cyclic and permanent shear strains and permanent pore pressure under combined average and cyclic
shear stresses for triaxial and simple shear stress paths.
Parameters for Specific Design Issues
Pile capacity & drivability
o Monotonic and cyclic shear strengths.
o Compressibility under virgin loading and unloading
o Cyclic and permanent shear strains and permanent pore pressure under combined average and cyclic
shear stresses for triaxial and simple shear stress paths.
Seabed erosion / scour o Permeability
Scope of Geophysical Survey for Platforms
Minimum survey area Minimum depth
Usually 1 km X 1 km in shallow water, 2 km X 2 km in deep water. Possible extension to 5 km X 5 km in areas with geohazards to incorporate possible platform location shifts etc. o High resolution / ultra high
resolution seismic survey for shallow geology and fault offset
analysis
o Line spacing 100-200 m
o 3D exploration sesmic data for
regional geohazard analysis and drilling hazard analysis to
approx. 100 m depth
o Side scan sonar, line spacing
100-200 m depending on water depth
Means of survey
o Swath bathymetry, preferably
multibeam
Subsurface information
Seabed features
Seabed topography
Similar to recommended for geotechnical data
Scope of Geotechnical Survey for Platforms
10 nos of continuous CPT
3 nos of BHs with continuous sampling to 15 m, thereafter sampling with less than 0.5 gap
Scope of Work Penetration
1 no of BH with continuous sampling down to 15 m, thereafter sampling with less than 0.5 m gaps to 0.5 X to 0.7 X platform diameter, followed by alternate sampling and CPT with less than 0.5 m gaps.
1.5 X platform
diameter
Sample Testing
o Index testing
o Triaxial tests
o Oedometer test
o Permeability tests
o Simple shear test
o CAUE, CAUC, CADE and
CADC triaxial tests
o Shear wave velocity
measurement by bender element
o Resonant column test
o X-ray photograph to determine soil layering
within the tube
o Radioactive core logging
Gravity platform
50 m or 1.5 X platform diameter
50 m
Scope of Geotechnical Survey for Platforms
1 no of BH with sampling every meter down to 15 m, thereafter sampling with less than 0.5 m gaps to 30 m, followed by alternate sampling and CPT with less than 0.5 m gaps, or
2 nos. BHs: one with sampling only and one with near continuous CPT
Scope of Work Penetration Sample Testing
o Index testing
o Testing for pile capacity and drivability and for
bearing capacity
Piled platform
At least to pile penetration + 4 pile diameters or pile penetration pile group diameter, whichever is the greater
Continuous CPT at a location 5-10 m from main borehole
30 m
Scope of Geotechnical Survey for Platforms
1 no of BH with samples at every meter down to 15 m, thereafter sampling with less than 0.5 m gaps
Scope of Work Penetration Sample Testing
o Index testing
o Testing for static bearing capacity
Jack-up rig
30 m or anticipated spudcan penetration + 1.5 X spudcan diameter, whichever is deeper
1 no. Continuous CPT at a location 5-10 m from main borehole and/or at each leg location
20 m
Anchor Structures
CLAY SAND ROCK
• General description
• Grain size distribution
• Water content
• Total unit weight
• Atterberg’s limit
• Shear strength (torvane, pocket penetrometer, fal cone, UU. Etc.
• Remoulded shear strength
• Sensitivity
• Stress history (OCR)
• Organic material content
• Carbonate content
General description Grain size distribution
Water content Max/Min density
Relative density Drained shearing
resistance
Stress history (OCR) Angularity
Carbonate content Organic material content
General description RQD
Water absorption Total unit weight
Unit weight of solid block Unconfined compression
strength
Mineralogy Carbonate content
Parameters for Specific Design Issues of Anchor Structures
Corrosion
Liquefaction
Slope stability
Scour / erosion
o Strain rate effect
o Cyclic response
o Permeability
o Strength anisotropy
o Electrical resistivity
o Geochemical test
o Bacteriological analyses
o Cyclic response
o Coefficient of reconsolidation
o Permeability
Suction anchor installation in clay& skirt penetration
o Undrained anisotropic shear strength
Slope stability
o Strain rate effect
o Cyclic response
o Permeability
o Strength anisotropy
Parameters for Specific Design Issues of Anchor Structures
Long term holding capacity of suction anchors in clay
Liquefaction potential
Suction anchor installation in sand / skirt penetration
o Strain rate effect
o Cyclic response
o Anisotropic monotonic and cyclic
o Shear strength
o Thixotropic regain
o Consolidation characteristics
o Permeability
o Cone resistance
o Drained angle of shearing resistance
Suction anchor capacity
o Strain rate effect
o Cyclic response
o Permeability
o Strength anisotropy
Parameters for Specific Design Issues of Anchor Structures
Soil pile friction in carbonate soil
o Sand compressibility
o Crushability
Anchor piles o Elastic modulus
Scope of Geophysical Survey for Anchor Structures
Full extend of anchor spread
Full extend of anchor spread
Minimum survey area Minimum depth
Full extend of anchor spread
o Sub bottom profiler, or
o High resolution seismic survey
o Line spacing 100-200 m
o Side scan sonar, line spacing
100-200 m depending on water depth
Means of survey
o Swath bathymetry, preferably
multibeam
Subsurface information
Seabed features
Seabed topography
More than depth required for geotechnical data
Scope of Geotechnical Survey for Anchor Structures
1 no of BH/core and/or 1
No. of CPT per anchor
1 no of BH/core and/or 1 No. of CPT per anchor
Scope of Work Penetration
1 no of BH/core and/or 1 No. of CPT per anchor
Sand: 5-10 m
Soft clay: to 20 m
Sample Testing
o Anchor designs are
generally not sensitive to displacements. Hence
emphasis in laboratory tests is generally on
determination of static soil strength (peak and
remoulded) and anchor-soil interface frictional
resistance
Pile
Vertically loaded anchor (VLA)
Drag anchor
Pile penetration + 4 pile diameter
Soft soil: to 50 m
Other soil: to depth of fluke + 5 m
1 no of BH/core and/or 1
No. of CPT per anchor Gravity base
1.5 X width (unskirted) or depth of skirt + 1.5 X width
1 no of BH/core and/or 1
No. of CPT per anchor Suction caisson
Depth of caisson + 1 diameter
Recommended