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