Introduction to Geotechnics

Soils and Foundations

• Geotechnics is introduced

Course Details

• Imen BEL HADJ ALI is the instructor
• The academic year is 2019/2020

Geotechnical Problems

• Landslides in Algeria and Mexico (2010)
• Landslide in Ain Drahem (2012)
• Cracking of a retaining wall because of significant land pressure
• Leaning Tower of Pisa had foundations implanted in a soil layer with insufficient bearing capacity
• Cracks on walls due to foundation design issues

Soil-Related Issues

• Landslides
• Structure instability
• Instability and pathologies on retaining walls

Geotechnical Solutions

• Site recognition should be done before any construction project
• A geotechnical study is required
• Geotechnical studies involve analyzing slope stability
• Geotechnical studies also dimensioning structure foundations, computing settlement and dimensioning retaining walls
• Site investigation is done with a geotechnical campaign involving in situ and laboratory tests

Plan Summary

• Definition and historical overview of soils
• Examples of projects requiring geotechnics
• Steps and methods of a geotechnical campaign

Geotechnics Definition

• Geotechnics includes all activities related to applications of soil mechanics, rock mechanics, and engineering geology
• Soil mechanics models the behavior of soils, including their deformability and resistance to materials
• Rock mechanics models rock behavior, including deformability and material resistance
• Engineering geology traces the history of the earth, detailing material nature, structure, and evolution over time

Soil Mechanics Study

• Soil mechanics studies the physical and mechanical properties of soils
• Soil mechanics studies the interaction between land and surrounding structures
• Soil mechanics studies the interaction between land and the new structure to be built

History of Geotechnics

• Soil mechanics is a young science
• Coulomb (1773) is credited with the first foundations
• Terzaghi initiated modern soil mechanics (1925)
• The science of soil mechanics began in the early 20th century
• Soil was mainly used as a construction material in prehistoric times
• Road, canal, and bridge construction became important between 3000 and 300 BC
• Structures became larger in Roman times (300 BC to 300), leading to ignored foundations
• Frequent wars during the Middle Ages (300 to 1400) caused structures to become more important
• Soil settlement caused structural damage and the Leaning Tower of Pisa (1174-1370) is an example of the building issues

Renaissance Period (1400-1650)

• Art developed significantly
• Artists such as Leonardo da Vinci and Michelangelo became great engineers

Modern Period (1650-1900)

• Significant scientific advances
• Coulomb presented his earth pressure theory in 1776
• Darcy followed with his flow law in 1855
• Collin published a book on landslides in 1846
• Mohr presented his theory on stresses in soil with his famous Mohr circle in 1882
• Boussinesq published his theory on elasticity with a proposed solution for soil stress calculations

1900-2000 Period

• Terzaghi's work in 1925 began the modern era of soil mechanics
• Many geotechnical engineers contributed to the development of soil mechanics and technology
• The development of unsaturated soil theory and numerical methods were part of these advances

Soil Study

• Examination of soil is necessary for all projects

Soil Construction

• All construction is done with soil or rock, whether it is a building, bridge, road, or tunnel
• Soil behavior affects the safety, economy, and project success

Geotechnical Study Projects Examples:

• Slopes, dams, building foundations, services, roads and marine construction
• Analyzing the system of transferring structural efforts to soil through foundations is required for structures
• Buildings and structures of art are structures of this kind
• Dimensioning foundations requires a geotechnical campaign to determine level of bedrock and predicted soil settlement

Soil Usage

• Soil itself is a construction material
• For example, earthworks such as the Matemale Dam in France

Retaining Structures Support Masses

• Retaining structures support soil masses unable to provide slide stability under geometrical conditions

Reinforcing Soils

• Unstable soils require geotechnical study to make them stable
• Soils with low mechanical performance require geotechnical evaluation to improve resistance

Unstable Soil Studies

• Slope and embankment stabilities should be studied during the construction of roads and earth dams
• The mass of soil must be stable under its weight and imposed loads

Stability Calculations for Landslide Prevention

• Using slope stability calculations and soil properties to propose solutions for landslide prevention
• Steps used to avoid landslides:
  • Terracing: lightening the head, softening the slope, or substitution of the unstable mass
  • Implementing drainage devices
  • Adding strengthening materials: ground anchors, tie rods, micro piles, row piles, or metal profiles Introduction of resistant elements (anchorage tie rods, bolting by bars, micro-piles, rows of piles or metallic profiles).
  • Geotextiles: Synthetic permeable textiles that play an essential role in stabilizing the soil by providing better deformability before any failure.

Dealing with Low Mechanical Performance Soil

• There are techniques to use to upgrade the soil's performance
• First technique: Preloading and drainage
• Second technique: Dynamic compaction is reserved for large sites without nearby residents
• The procedure employs dropping a mass from a great height multiple times on the same spot
• The shock wave remanies soil in the impact zone and decreases the volume between particles
• Third Technique: Vibrocompaction
• Fourth Technique: Solid injection
• Solid injection is used to treat sandy, silty, clayey, humid, or saturated soils
• It involves injecting a highly viscous mortar under pressure into a grid specifically established based on the nature of the soil and the objectives to achieve static horizontal densification
• Fifth Technique: Ballasted Columns

Soil Improvement Techniques

• Preloading + Drainage
• Tourbe evolutive, argiles tres molles
• Vibrocompaction
• Argiles-limons compressible
• Compactage Dynamique
• all types of soil, more effective for Cailloux and Remblais a blocs
• Injection Solide
• all types of soil
• Colonnes Ballastees
• all types of soil
• Plots Ballastees Pilonnes
• all types of soil

Geotechnical Study Procedures

• A site investigation should begin with a preliminary enquiry to visit the site and consult the terrain database (topographic map, geological map, aerial photograph, previous geotechnical survey).
• Soils are studied in two stages: summary geotechnical study (APS phase) and detailed geotechnical study (APD phase)

Geotechnical Analysis

• A geotechnical survey is conducted through field and laboratory tests

Field and Laboratory

• Field tests: coring, pressuremeter test, penetrometer test, and piezometer test
• Laboratory tests: sieving, Casagrande apparatus, triaxial test, and oedometer test
• The auger fitted with a core sampler extracts the soil core
• The core is useful for defining a soil section and its sampling point
• To determine the terrain stratigraphy, multiple cores are sampled at different points and depths
• The pressuremeter and penetrometer measurements calculate the bearing capacity of soil layers
• The measurements calculate the structure's foundation dimensions, terrain calculations, and soil settlement calculations