Engineering Geology and Geotechnics - Lecture 4 Notes

Field Trip to Cragside

• Meeting Point: Turn circle next to Business School.
• Meeting Time: 9:00 AM, 6th March 2025 (Thursday).
• Details will be available on Blackboard.

Important Field Trip Information

• PPE Requirements:
  • Safety shoes
  • High-visibility vest
  • Hard hat
• No PPE = No Visit.
• No personal vehicles allowed.

Field Trip Tasks

• Walk-over survey
• Field vane test (2)
• Level survey (2)
• Ground penetrating radar
• Laser scanner

Module Overview

• Site Investigation Eurocode 7
  • Lectures (3), Seminars (3), TW1-3, TW2-4
  • Planning investigations: desk study
  • Soil and rock sampling and groundwater measurements
  • Field tests in soil and rock
• Module Overview
  • Lectures (1), TW1
• Engineering Geology
  • Lectures (4), Seminars (3), W4-5, 7-8, TW5, 7-8
  • Introduction to Earth
  • Weathering
  • Geological mapping
  • Geological structures
• Shallow Foundations
  • Lectures (3), Seminars (3), TW9-11, TW9-11
  • Bearing capacity
  • Settlement
  • Soil improvement
• EXAM Revisions
  • TW12
• Activity Week: TW6

Recap - Selecting Investigation Points

1. Investigation points arranged to assess stratification across the site.
2. Points for buildings/structures at critical locations relative to shape, structural behavior, and load distribution (e.g., corners of foundation).
3. Investigation area extends to where no harmful influence on neighboring area is expected.
4. Consider using equipment for continued monitoring during and after construction.
5. Points arranged to avoid hazards to structure, construction, or surroundings.

Recap - Quality Class & Sampling Category

Table outlining required sample quality class & sampling category for rock properties

Recap - Sample Type

• Site investigation practice divides collected samples into categories:
  • Disturbed samples:
    • Remoulded
    • Representative
  • ‘Undisturbed’ samples:
    • Block samples
    • Open-drive samples
    • Piston-drive samples
    • Rotary core samples (e.g., from a corebarrel).

Standards Complementary to EC7

• EC 7 Geotechnical design: Part 2 Ground Investigation and Testing
• EN ISO 14688 Identification and classification of soil
• EN ISO 22476 Field testing
• EN ISO 22475 Sampling methods and groundwater measurements
• EN ISO 17892 Laboratory testing
• EN ISO 18674 Geotechnical monitoring
• EN ISO 14689 Identification and classification of rock

Standards Complementary to EC7 - EN ISO 22476 Field Testing

• Part 1 Electrical cone and piezocone penetration test (CPT)
• Part 2 Dynamic probing
• Part 3 Standard penetration test (SPT)
• Part 4 Ménard pressuremeter test
• Part 5 Flexible dilatometer test
• Part 6 Self-boring pressuremeter test
• Part 7 Borehole jack test
• Part 8 Full displacement pressuremeter test
• Part 9 Field vane test (FVT and FVT-F)
• Part 10 Weight sounding test
• Part 11 Flat dilatometer test
• Part 12 Mechanical cone penetration test (CPTM)
• Part 14 Borehole dynamic probing
• Part 15 Measuring while drilling
• Part 16 Borehole shear test

Recommended Reading

• Bond, A. J., and Harris, A. J. (2008). Decoding Eurocode 7, London: Taylor & Francis, 598 pp.
• BS EN 1997-2: 2024
• BS EN ISO 22476

Lecture Outline

• Context
• In situ tests
  • Penetration resistance
  • Strength and/or compressibility
  • Permeability
• Geophysical investigations
• Monitoring
• Summary

Context

• Aim of field techniques is to determine physical properties of the ground.
• Some soils can be sampled, then laboratory testing determines soil/rock parameters.
• Other ground types are difficult to sample and test successfully.
• Field tests overcome limitations of lab testing.

Context - Routine Investigations

• Cost and effectiveness are major factors; soil types influence balance between sampling and in-situ testing.
• In complex projects, strength and stiffness are determined using both field and lab techniques.

Context - Information Sources

• Information obtained in the field in at least three ways:
  • In situ soil tests
  • Geophysical investigations
  • Field monitoring instrumentation

Penetration Tests

• Tests ability of soil to resist insertion of an object (probe, cone, penetrometer).
• Provides insights into soil compaction, shear strength, and stiffness.
• Common methods:
  • Standard Penetration Test (SPT)
  • Cone Penetration Test (CPT)

Standard Penetration Test (SPT)

*What is SPT?

• SPT involves driving a split-barrel sampler into soil at a specific depth using a standardized weight and hammer.
• The number of blows required to drive the sampler a set distance is recorded and used.
• Equipment: hammer, drill rod, split spoon, etc.
• In accordance with EN ISO 22476-3

Standard Penetration Test (SPT) - How to Conduct

• Borehole Preparation: 10 to 30 meters deep
• Driving the Sampler:
  • Repeated blows of a standard weight ($63.5 kg$) falling through a height of $760 mm$
  • Sampler driven in increments of $15 cm$ at a time, three increments in total ($45 cm$).
• N-Value:
  • Blows required to penetrate the first $15 cm$ is N0, the last $30 cm$ is the N-Value.

Standard Penetration Test (SPT) - Quiz

*For example, if it takes $10$ blows to drive the first $15 cm$ and $20$ blows to drive the next $30 cm$, what is the N-value?

• The N-value is $20$.

Standard Penetration Test (SPT) - What’s For?

• Quantitative evaluation of:
  • The density index,
  • The soil bearing resistance (effective angle of shearing resistance), and
  • The settlement of foundations (Young's modulus of elasticity).
  • -- 4.6.4.2, BS EN 1997-2:2007

Standard Penetration Test (SPT) - Advantages

• Simplicity: Relatively simple and quick to perform in the field.
• Inexpensive: Well-established and inexpensive test.
• Historical data: Large amount of historical data available.
• Return samples: useful for general soil classification.

Standard Penetration Test (SPT) - Disadvantages

• Mainly for Coase soil (e.g. sand)
• Disturbance: The test can disturb the soil sample, which may affect the accuracy of certain soil property analyses.
• Depth Limitation: SPT is generally effective for shallow depths (typically up to 30 meters), and less effective in deeper soil profiles where other tests may be preferred.

Cone Penetration Test (CPT)

*What is CPT?

• CPT involves pushing a cone-shaped probe into the ground at a constant rate, typically $2 cm/s$ ($0.79 in/s$), while measuring resistance to penetration.
• Typically conducted using a truck-mounted CPT rig, allowing for greater depth penetration and more accurate results compared to SPT.
• Borehole preparation typically required to allow insertion of the probe, like SPT.
• The tests shall be carried out and reported in accordance with EN ISO 22476-1

Cone Penetration Test (CPT) - Probe Components

• CPT probe consists of:
  • Cone tip
  • Friction sleeve
  • Flexible porous element
  • Push rods

Cone Penetration Test (CPT) - Data Collection

• Collected data:
  • Tip resistance ($q_c$)
  • Friction resistance ($f_s$)
  • Pore water pressure ($u_2$) (if so, CPTu)
• Continuous data (profiles of soil behavior at various depths)
• In accordance with EN ISO 22476-1

Cone Penetration Test (CPT) - What is SCPTu?

• The SCPTu integrates seismic sensors (geophones or accelerometers) into the standard CPTu equipment, which allow the measurement of shear wave velocity ($V_s$) of the soil while conducting the penetration test.
• Shear wave velocity is a crucial parameter in seismic site characterization and helps assess the soil's stiffness.

Cone Penetration Test (CPT) - Pros and Cons

• Fast and continuous profiling
• More consistent and reliable than SPT
• Mainly for sand, silt, clay, and peat
• Penetration can be restricted in gravel, soft and hard rock
• No soil samples returned, but small samples can be acquired with CPT rig
• Complements drilling and sampling; ideally, CPT testing is done first, followed by drilling and sampling

Field Vane Test (FVT)

*What is FVT?

• FVT is an in-situ test used to measure the undrained shear strength of cohesive soils, e.g., clay.
• Assess how easily the soil can be sheared by applying a torque to a vane inserted into the ground.
• A rectangular vane, consisting of four plates fixed at $90°$ angles to each other.
• In accordance with EN ISO 22476-9

Field Vane Test (FVT) - How to Conduct

• The vane is inserted vertically into the soil at the desired depth.
• The torque (rotational force) is gradually applied to the vane until the soil starts to shear or fail around the vane.
• The maximum torque required to rotate the vane is recorded.
• Once the peak is reached, the device can be rotated rapidly to measure the undrained remoulded strength of the soil.
• Sensitivity can be calculated as the ratio of the peak to remoulded strength.

Field Vane Test (FVT) - How to Obtain Derived Value

• Obtain derived values for the undrained shear strength from field vane test results:
  • $C<em>u = μ C</em>{fv}$
    • Where:
      • $C_{fv}$ – undrained shear strength measured in FVT
      • $μ$ – correction factor
• The correction factor $μ$ shall be determined based on local experience (Annex I in BS EN 1997-2: 2007).

Field Vane Test (FVT) - Assumptions

• Penetration of the vane causes negligible disturbance, both in terms of changes in effective stress and shear distortion
• No drainage occurs before or during shear
• Soil is isotropic and homogeneous
• Soil fails along a cylindrical shear surface
• At peak and remoulded strength, there is a uniform shear stress distribution across the shear surface
• There is no progressive failure, so that at maximum torque the shear stress at all points on the shear surface is equal to the undrained

Field Vane Test (FVT) - Pros and Cons

• Relatively simple and cost-effective
• A long and established history of use
• Mainly for soft to medium clays, silts, and other cohesive soils; generally not suitable for soils with undrained strengths greater than about $75-100 kPa$ or for fibrous peats, sands or gravels.
• Most useful for shallow layers of soil
• Data need empirical correction

Plate Loading Test (PLT)

*What is PLT?

• Determine vertical deformation and strength properties of soil and rock masses in situ by recording load and corresponding settlement when a rigid plate is loading the ground.
• Test is carried out on a levelled and undisturbed surface either at ground level or on the bottom of an excavation at a certain depth.
• A plate is bedded on to the soil to be tested and load is applied via hydraulic load cell and a hydraulic jack.

Plate Loading Test (PLT) - Load Application

• The maximum load or settlement required is governed by the objectives of the test, as well as by the expected strength and deformation characteristics of the soil and the size of the loading plate.
• Load is applied to the plate in successive increments (a minimum of one fifth of the design loading) and held until the rate of settlement reduces to less than $0.02 mm$ per min.

Plate Loading Test (PLT) - Load Increments

• Load increments are applied either until:
  • Shear failure of the soil occurs
  • The plate pressure reaches two or three times the design bearing pressure proposed for the full-scale foundation.
• The tests shall be carried out and reported in accordance with EN ISO 22476-13

Plate Loading Test (PLT) - What’s For?

• Predict the behavior of spread foundations
• Derive geotechnical parameters of a homogeneous layer (the layer should have a thickness of at least twice the width or diameter of the plate)
• Calculate the plate settlement modulus ($E_{PLT}$)
• Derive Young’s modulus of elasticity ($E$)

Plate Loading Test (PLT) - Design Guidance

• Guide design methods, assuming that:
  • The size of the plate has been chosen considering the width of the planned spread foundation
  • A homogenous layer up to two times the width of the planned spread foundation exists
• Derive $c_u$ if penetration is conducted at a constant rate and sufficiently fast to preclude any drainage

In Situ Permeability

• Soil permeability (or hydraulic conductivity) can rarely be obtained with sufficient accuracy from laboratory tests. Thus, in-situ permeability test is common.
• Typical in-situ permeability tests:
  • Slug test
  • Constant head test
  • Falling head test

Slug Test

*What is slug test?

• Determine permeability of unconfined aquifers or shallow soil layers.
• This test involves introducing a "slug" of water (or air) into a well and measuring how the water level changes over time.
• It consists of:
  • Installation (a cased borehole or a simple open-tube piezometer well)
  • Slug insertion/withdraw
  • Measurement of change in water over time

Field Investigation Tests and Properties

Table outlining the property, clause, and testing method

Geophysical Investigations

• Subsurface exploration that involves the use of geophysical techniques to study the physical properties of the earth’s or other planets’ subsurface
• Typical examples:
  • Seismic Methods (reflection or refraction)
  • Electrical Resistivity Methods
  • Magnetic Methods
  • Electromagnetic (EM) Methods
  • Ground Penetrating Radar (GPR)
  • Borehole Geophysics
  • Ambient Noise

Ground Penetrating Radar (GPR)

*What is GPR?

• GPR is a non-invasive geophysical method used to investigate and map subsurface features by sending high-frequency electromagnetic waves into the ground and recording the reflected signals.
• Operate at frequencies between $10 MHz$ to $2 GHz$.
• The higher the frequency, the higher the resolution but the shallower the penetration.
• Map subsurface features such as rockhead, groundwater table, voids, fractures in rock and the extent of contaminated ground.

Ground Penetrating Radar (GPR) - Pros and Cons

• Non-destructive
• Real-time results
• It works best in dry, sandy soils or materials with lower conductivity (e.g., rock or concrete).
• Clay-rich soils and wet environments can absorb or attenuate the radar waves, limiting its use for deeper investigations.

Borehole Geophysics

• Borehole Geophysics refers to a set of geophysical techniques that are applied in and around boreholes to gather information about the subsurface conditions.
• Provide valuable data on the physical properties of the materials surrounding the borehole, such as soil or rock.
• Typical seismic profiling methods (to directly measure shear-wave velocity Vs):
  • Downhole testing
  • Crosshole testing
  • P-S logging

Ambient Vibrations

• Ambient vibrations or noises refer to the naturally occurring background vibrations present in the environment.
• These noise signals can be recorded using geophones.
• Using the horizontal-to-vertical spectral ratio (HVSR) technique to obtain the fundamental frequency of soil deposit (clause 10.7.2, BS EN 1997-2:2024).

Supervision, Monitoring & Maintenance

• Annex J of EN 1997-1 lists items that need to be considered during supervision of construction including (generalized):
  • Movements, yielding, stability of excavation walls and base; temporary support systems; effects on nearby buildings and utilities; measurement of soil pressures on retaining structures and of pore-water pressure variations resulting from excavation or loading
  • Safety of workmen with due consideration of geotechnical limit state
  • Ground-water flow and pore-water pressure regime; effects of dewatering on ground-water table; effectiveness of measures to control seepage inflow; corrosion potential
  • Effectiveness of control measures such as sub-horizontal borehole drains
  • Platform, collection frequency, processing, coverage, in situ, remote sensing, continuous, discontinuous, real time, rapid, post-processed, local, distributed

Ground Monitoring - Tilt Sensor/Inclinometer/Tiltmeter

• Measure the angle of inclination or tilt of an object or surface relative to the Earth's gravity.
• For example, installed in dam walls or high-rise buildings to monitor tilting and any early signs of potential collapse.
• During foundation monitoring, tilt sensors are used to measure the tilting of newly constructed buildings or structures.

Ground Monitoring - Extensometers

• Measure displacement or strain in materials, typically to monitor the deformation of structures, soil and rock.
• Extensometers measure the change in distance between two fixed points as a structure or material deforms.

Ground Monitoring - Geodetic GNSS

• Measurement of the relative locations of points up to several thousand kilometers apart with an accuracy of several millimeters
• For example, monitoring the deformation or settling of large infrastructure projects over time.
• Provide continuous long-term data
• Needs significant post-processing and a long-term dataset
• Systems can be expensive to install and maintain (need power)

Groundwater Monitoring - Piezometer

• Measure the pressure or head (elevation of the water table) in groundwater or other fluids within soil and rock.
• In geotechnical investigations, piezometers are used to measure pore pressure in soil and rock formations.

Summary

• In situ testing
  • Penetration resistance
    • Standard Penetration Test (SPT)
    • Cone Penetration Test (CPT)
  • Strength and/or compressibility
    • Field vane test (FVT)
    • Plate loading test (PLT)
  • Permeability
    • Slug test
• Geophysical investigations
  • Ground Penetrating Radar (GPR)
  • Borehole Geophysics
  • Ambient Noise
• Monitoring
  • Tiltmeter
  • Extensometers
  • Geodetic GNSS
  • Piezometer