Engineering Geology and Geotechnics - Lecture 2 Notes

Module Overview

  • Lectures and Seminars Schedule:
    • Weeks 1-3 (TW1-3, TW2-4): Planning investigations, including desk study, soil and rock sampling, groundwater measurements, and field tests.
    • Weeks 4-5 & 7-8 (W4-5, 7-8, TW5, 7-8): Introduction to Earth, weathering, geological mapping, and geological structures (Engineering Geology).
    • Weeks 9-11 (TW9-11): Bearing capacity and design of shallow foundations (Spread Foundations in EC-7).
  • Exam revisions in TW12, with activity week in TW6.

Recap - Eurocodes

  • Eurocodes include EN 1990, EN 1991, EN 1997, and EN 1998.

Recap - Limit State Design

  • Serviceability Limit States (SLS):
    • Concerned with the functioning of the structure under normal use, the comfort of people, and the appearance of construction.
    • Examples: deformation resulting in loss of function, vibration causing discomfort, and cracking affecting appearance or durability.
  • Ultimate Limit States (ULS):
    • Deal with the safety of people or structures, defining conditions under which the structure may fail.
    • Examples: collapsing, overturning, and excessive deformation.
  • Categories of Ultimate Limit States:
    • Equilibrium Limit States (EQU).
    • Uplift Limit States (UPL).
    • Hydraulic Failure Limit States (HYD).
    • Structural Limit States (STR).
    • Geotechnical Limit State (GEO).
  • Verification Methods for Limit States:
    • Design by Calculation (DbC): Uses an arithmetical approach with partial factor method.
    • Design by Prescriptive Measures: Relies on past experience and standard charts.
    • Design by the Observational Method: Combines calculation, experience, and monitoring.
    • Design by Experimental Models and Load Tests: Includes centrifuge modelling and load tests on piles.
  • Design Approaches:
    • Design Approach 1: Factors applied to actions alone (Combination 1) and mainly to material factors (Combination 2).
    • Design Approach 2: Factors applied to actions (or effects) and resistances simultaneously.
    • Design Approach 3: Factors applied to structural actions (not geotechnical actions) and material properties simultaneously.

Actions, Ground Properties, and Design by Calculation

  • Actions:
    • Weight of soil and rock
    • Stresses in ground
    • Earth pressures
    • Free water pressures
    • Ground water pressures
    • Seepage forces
    • Dead and imposed loads
    • Surcharges
    • Mooring forces
    • Removal of load or excavation of ground
    • Traffic loads
    • Indirect actions
  • Ground Properties:
    • Soils
    • Rocks
    • Other materials
    • Geometrical data
  • Design by calculation
    • Limiting values for movements
    • err on the side of safety
  • Calculation models
    • Analytical
    • Semi-empirical
    • Numerical
    • may include simplifications
  • Serviceability limit states
    • Loss of static equilibrium (EQU)
  • Limit states
    • Rupture or excessive deformation of
    • structure or ground
  • Ultimate limit states
    • (STR and GEO)
    • Uplift (UPL)
    • Heave by seepage of water (HYD)
  • Design Approach 1
  • Design Approach 2
  • Design Approach 3

Introduction to Site Investigation (SI)

  • This lecture focuses on Site Investigation within Eurocode 7.

Lecture Outline

  • Topics covered:
    • Site investigation.
    • Desk study.
    • Walk-over survey.
    • Preliminary investigation.
    • Design investigation.
  • Reading Materials:
    • Bond, A. J., and Harris, A. J. (2008). Decoding Eurocode 7.
    • BS EN 1997: Eurocode 7:
      • Part 1: General rules (EC7 Part 1).
      • Part 2: Ground investigation and testing (EC7 Part 2).

Site Investigation (SI) Defined

  • Ground investigation (GI): Exploring geotechnical and geo-environmental properties of the ground (soil, rock, groundwater).
  • Site investigation (SI): Includes legal and environmental aspects in addition to ground investigation.

The Significance of SI

  • Ground problems delay 30% of construction projects in Britain.
  • 50% of over-tender costs on road projects are due to inadequate ground investigation or data interpretation.
  • Deficiencies in ground investigation lead to:
    • Conservative or inappropriate design assumptions.
    • Additional costs arising from problems encountered on site.
    • Delays in completion.
    • Expensive remedial action later.

Cost-Benefit Analysis of Ground Investigation

  • ‘You pay for a GI, whether you have one or not’.
  • Example: The Chicago flood (1992) due to a breached utility tunnel caused by misplaced pilings.

EC7: Site Investigations

  • Main purpose: reduce and control ground-related risks in construction.
  • Properly conducted investigation provides:
    • Basis for economic and safe design.
    • Meeting tender and construction requirements.
  • Saves money by avoiding:
    • Redesign during construction.
    • Over-conservative design.
    • Delays and litigation costs.
    • Foundation problems post-construction.

EC7: Planning Geotechnical Investigations

  • Geotechnical investigations shall be planned to ensure relevant geotechnical information and data are available at various project stages.

Desk Study

  • Essential component of ground investigation; must always be carried out.
  • Should be carried out at the start of site investigation.

Purpose of Desk Study

  • Provide as much information as available without commissioning new ground investigation work.
  • Wider role than just determining soil and rock conditions; aims to determine the position of services and structures and identify potential hazards.

Desk Study - Information Sources

  • Rock and groundwater properties: Geological maps and descriptions.
  • Soil properties: Previous investigations at the site and in the surroundings; Geotechnical memoires and handbooks.
  • The National Geotechnical Properties Database (NGPD) holds data from commercial site investigations, including:
    • Borehole details.
    • Lithological description.
    • Field and laboratory geotechnical tests.
    • Engineering.
    • Environmental chemical testing.
  • Previous land uses: Historical maps and local records of the site (e.g., National Library of Scotland Maps, Ordnance Survey Archive).

Desk Study - Legislation

  • Restrictions and status of the area:
    • Tier 1: Strongest protection (e.g., Sites of Special Scientific Interest).
    • Tier 2: Includes local wildlife sites and ancient woodland.
    • Tier 3: Weakest protection but can have development restrictions (e.g., national parks).
  • Wetlands are designated under the Ramsar Convention.

Desk Study - Aerial Photos and Topography

  • Aerial photos: Captured from aircraft, drones, or helicopters providing detailed, localized views.
  • Topography: Topographical maps and Digital Elevation Models (DEMs).
    • A digital representation of the Earth's surface topography.
    • Satellite imagery, aerial photography, radar (like NASA SRTM - Shuttle Radar Topography Mission), LiDAR (Light Detection and Ranging).

Desk Study - Weather and Climate

  • Weather and climate variations: Flood maps and records.

From Desk Study to Walk-Over Survey

  • Limitations of desk study information:
    • Resolution.
    • Frequency of revision.
    • Purpose of the information.
    • Reliability of the data sources.
    • Availability of the information.
  • Walk-over survey should be done after a substantial part of the desk study is completed.

Walk-Over Survey

  • Carried out after the desk study and preliminary plans for ground investigation site work.
  • Purpose: glean extra information on the geology and on likely construction problems.
  • A direct inspection of the site and the surrounding area.

Purpose

  • Confirm information collected during the desk study.
  • Collect additional information about the site.
  • Assist in planning subsequent phases of the investigation.
  • Identify possible health & safety issues.

Walk-over surveys operations

  • Site inspection.
  • Local enquiries.

Site inspection

  • Thorough visual examination of the site and its surroundings making full use of maps and aerial photographs.
    • Topography: abrupt changes in slope; valley bottoms or depressions; evidence of overburden on slopes; excavations at the base of the slope; signs of landslip, eg tilting trees, posts or walls; signs of subsidence; evidence of imported soil including
    • Soils and rocks: basic ground type; evidence of peat, silt or other highly compressible material ; cracks, or signs of shrink-swell behavior; sudden changes in conditions, e.g., soil to rock
    • Water features: presence of springs, ponds, wells, ditches or streams; signs of flooding; a high water table indicated, e.g. by waterlogged ground
    • Vegetation: species, height and condition of the trees, hedges and scrub; presence of deep-rooted trees (can impact foundations); type and condition of vegetation on land adjoining the site
    • Site access: condition of access roads, bridges, and footpaths; identify manholes, utility poles, drainage pipes, and overhead cables; verify the potential location for boreholes
    • Existing structures: signs of cracks in buildings,; signs of failure, cracks, or bulging in retaining walls & earthworks; buried structures & previous foundations; thorough assessment of damage in neighbouring structures
    • Potential risks and hazards for health and safety: signs of contamination, e.g., discolored soil, dead vegetation, chemical odors, oily residues; discolored standing water or sheens on water surfaces.

Local enquiries

  • Getting local knowledge on the site can prove invaluable.
  • Who to enquire:
    • Local builders and civil engineering contractors
    • Local authority engineers and surveyors
    • Local statutory undertakers
    • Local archives
    • Local inhabitants
    • Local clubs and societies
    • Schools, colleges and universities

Desk Study & Walk-Over Survey - Cost-Effective Data Collection

  • Both provide large quantities of invaluable information at negligible cost.
  • Include not only the site but also its surrounds.
  • Essential components of ground investigation; must always be carried out.

Preliminary and Design Investigations

  • The findings of the desk study and walk-over survey inform the choice of test methods and sample collection.
  • Site investigation points shall be marked on the site before the investigation process commences, their location and elevation shall be surveyed and entered in a site plan on completion of the investigation.
  • The ground investigation is carried out in two phases: preliminary and design. Often both are carried out together.

Five key questions

  • Five key questions should be answered before a ground investigation commences to provide direction to the process:
    • What is the purpose of the investigation?
    • What information is required and when?
    • What areas and depth of the ground are to be investigated?
    • What is the time required for the investigation?
    • What is the estimated cost?

Preliminary Investigations

  • Objective: Obtain sufficient data to:
    • Assess site stability.
    • Evaluate impacts of proposed works on the site.
    • Identify the potential for fill materials to be sequestered.
    • Consider potential foundation types and ground improvement methods.
    • Provide an initial ground model.
    • Compare alternative sites.
    • Identify borrow areas.
    • Plan the design investigation.

Design Investigations

  • A more comprehensive investigation than the preliminary investigations, involves the use of:
    • Field investigations
    • Laboratory tests
  • To assess:
    • Ground conditions
    • Groundwater conditions
    • Contamination

Ground investigation

  • Ground investigations shall provide a description of ground conditions relevant to the proposed works and establish a basis for
    the assessment of the geotechnical parameters relevant for all construction stages.
  • The information obtained should be sufficient to assess the following aspects, if possible:
    • The suitability of the site with respect to the proposed construction and the level of accepted risks;
    • The deformation of the ground caused by the structure or resulting from construction works, its spatial and temporal distribution;
    • The safety with respect to limit states (e.g., bearing resistance);
    • The loads transmitted from the ground to the structure and the extent to which they depend on its design and construction
    • The foundation methods (e.g. ground improvements, whether it is possible to excavate, etc.);
    • The sequence of foundation works;
    • The effects of the structure and its use on the surroundings;
    • Any additional structural measures required (e.g. support for excavation, etc.).

Design Investigations - Assess Ground, Groundwater and Contamination

  • Involves field investigations and laboratory tests to assess ground conditions, groundwater conditions, and contamination.

Groundwater Investigation

  • Groundwater investigations shall provide all relevant information on groundwater needed for geotechnical design and construction.
  • Groundwater investigations should provide, when appropriate, information on:
    • the depth, thickness, extent and permeability of water-bearing strata in the ground;
    • the elevation of the groundwater surface and variation over time;
    • the pore water pressure distribution;
    • the chemical composition and temperature of groundwater.
  • possible harmful effects of the groundwater on excavations or on slopes (e.g., slope
    failure and collapse);
  • any measures necessary to protect the structure (e.g., waterproofing);
  • the scope for and nature of ground water-
    lowering work;
  • the effects of groundwater lowering,
    desiccation, impounding, etc. on the
    surroundings (Groundwater Dependent
    Ecosystems, e.g., wetlands);
  • the capacity of the ground to
    absorb water during construction
    work;
  • whether it is possible to use local
    groundwater for construction
    purposes.

Contaminated Site Investigation

  • Around 70% of ground investigations are carried out on previously-used, or brownfield, sites and more than 50% of all sites investigated are contaminated in some way, giving rise to geoenvironmental and geotechnical issues.

Contaminated Site Investigation - Assess Risks and Remedial Measures

  • Information should be sufficient to assess:
    • The type and extent of ground contamination on and in the vicinity of the site.
    • The effectiveness of measures taken to contain or remedy contamination.

Possible contaminants by industry

  • Chemicals: Acids; alkalis; metals; solvents; phenols
  • Petrochemicals: Hydrocarbons; phenols; acids; alkalis and asbestos
  • Metals: Heavy metals and asbestos
  • Energy: Combustible substances; phenols; cyanides; sulphur compounds; asbestos
  • Transport: Combustible substances; hydrocarbons; asbestos
  • Mineral extraction: Heavy metals; gases (methane); leachates
  • Water supply: Metals; microorganisms
  • Other: Metals; organic compounds; methane; toxic, flammable or explosive substances; microorganisms

Lecture Summary

  • Site investigation – applying EC7
  • Desk study – datasets, benefits and limitations
  • Walkover survey – verifying Desk Study analysis
  • Preliminary investigation – initial findings
  • Design investigation

Seminar Information

  • Every Thursday
  • Lecture from 13:00-15:00 at ELA003
  • Seminar from 15:00-17:00 at ELB001