🏔️Intro to Geotechnical Science Unit 3 – Soil Compaction & Site Investigation

Key Concepts and Definitions

• Soil compaction involves increasing soil density by reducing air voids between soil particles, which improves soil strength and stability
• Compaction effort refers to the mechanical energy applied to the soil to achieve a desired density, typically measured in terms of the number of passes or the weight of the compaction equipment
• Optimum moisture content (OMC) represents the water content at which maximum dry density can be achieved for a given compaction effort
• Proctor compaction test determines the relationship between moisture content and dry density of a soil sample under a specified compaction effort (Standard Proctor or Modified Proctor)
  • Standard Proctor test applies a compaction effort of 600 kN-m/m³
  • Modified Proctor test applies a higher compaction effort of 2,700 kN-m/m³
• California Bearing Ratio (CBR) measures the resistance of a compacted soil sample to penetration, which helps assess the soil's suitability for pavement design
• Site investigation encompasses the process of collecting and analyzing soil and site data to determine the geotechnical properties and conditions relevant to a construction project
• Geotechnical site characterization involves determining soil stratigraphy, groundwater conditions, and engineering properties of soil layers

Soil Properties and Classification

• Soil classification systems, such as the Unified Soil Classification System (USCS) and the AASHTO system, categorize soils based on their particle size distribution and plasticity characteristics
• Particle size distribution describes the proportions of different sized particles (gravel, sand, silt, and clay) in a soil sample, determined through sieve analysis and hydrometer tests
• Atterberg limits (liquid limit, plastic limit, and shrinkage limit) define the moisture content thresholds at which fine-grained soils transition between liquid, plastic, semi-solid, and solid states
  • Liquid limit (LL) is the moisture content at which a soil transitions from a plastic to a liquid state
  • Plastic limit (PL) is the moisture content at which a soil transitions from a semi-solid to a plastic state
• Plasticity index (PI) quantifies the range of moisture contents over which a soil exhibits plastic behavior, calculated as the difference between the liquid limit and plastic limit (PI = LL - PL)
• Soil permeability refers to the ability of a soil to allow fluid flow through its pore spaces, which depends on factors such as particle size, void ratio, and soil structure
• Shear strength of a soil represents its resistance to shearing stresses, influenced by factors such as cohesion, friction angle, and effective stress
• Consolidation is the process by which a saturated soil undergoes volume change due to the dissipation of excess pore water pressure over time, resulting in settlement

Compaction Theory and Principles

• Compaction theory aims to understand the factors influencing soil compaction and the relationships between compaction effort, moisture content, and achieved dry density
• Proctor's compaction theory states that for a given compaction effort, there exists an optimum moisture content (OMC) at which maximum dry density can be achieved
• Compaction curve is a graphical representation of the relationship between moisture content and dry density for a specific soil and compaction effort, typically obtained from Proctor compaction tests
  • The peak of the compaction curve represents the OMC and the corresponding maximum dry density
• Compaction energy is the mechanical work done per unit volume of soil during the compaction process, which depends on factors such as the weight and type of compaction equipment, number of passes, and lift thickness
• Overcompaction occurs when a soil is compacted at a moisture content significantly higher than the OMC, leading to a decrease in dry density and potential stability issues
• Undercompaction refers to the situation where a soil is compacted at a moisture content lower than the OMC, resulting in a lower achieved dry density and potential settlement problems
• Field compaction control involves monitoring and adjusting the compaction process to ensure that the required dry density and moisture content are achieved consistently across the construction site

Compaction Methods and Equipment

• Static compaction applies a steady, slow-moving load to the soil surface, suitable for small areas or when a high degree of control is required (smooth drum rollers, pneumatic tired rollers)
• Dynamic compaction involves the application of impact forces to the soil surface, effective for compacting large areas and achieving deeper compaction (vibratory rollers, rammers)
• Sheepsfoot rollers have protruding studs or feet that penetrate the soil surface, suitable for compacting fine-grained soils (clays and silts) and achieving higher dry densities
• Smooth drum rollers provide a uniform, smooth compaction surface, ideal for compacting granular soils (sands and gravels) and finishing the surface of fine-grained soils
• Vibratory plates and rammers are handheld or walk-behind equipment used for compacting soil in confined spaces, trenches, or around structures
• Factors influencing the selection of compaction equipment include soil type, required compaction level, lift thickness, site accessibility, and project scale
• Quality control during compaction involves monitoring the achieved dry density and moisture content using field testing methods (nuclear density gauge, sand cone test) and adjusting the compaction process as needed

Site Investigation Techniques

• Desk study involves gathering and reviewing existing information about the site, such as geological maps, aerial photographs, and previous site investigation reports
• Site reconnaissance is a visual inspection of the site to identify surface features, topography, drainage conditions, and potential geotechnical hazards
• Subsurface exploration techniques are used to obtain soil samples and gather information about the soil profile, stratigraphy, and groundwater conditions
  • Drilling methods (hollow stem auger, rotary wash boring) involve advancing a drill rig to retrieve soil samples at various depths
  • Sampling techniques (split spoon sampler, thin-walled tube sampler) are used to collect representative soil samples for laboratory testing
• Cone Penetration Test (CPT) is an in-situ test that measures the resistance of the soil to the penetration of a cone-shaped probe, providing continuous data on soil strength and stratigraphy
• Standard Penetration Test (SPT) involves driving a split spoon sampler into the soil using a standardized hammer, with the number of blows required to advance the sampler (N-value) used as an indicator of soil strength
• Geophysical methods (seismic refraction, electrical resistivity) help characterize subsurface conditions by measuring the physical properties of the soil and rock
• Groundwater monitoring involves installing piezometers or monitoring wells to measure groundwater levels and assess the potential impact of groundwater on the construction project

Field and Laboratory Testing

• Field testing methods are performed on-site to evaluate soil properties and compaction quality
  • Nuclear density gauge measures the in-situ density and moisture content of compacted soil using gamma radiation and neutron scattering
  • Sand cone test determines the in-situ density of compacted soil by excavating a small hole, filling it with calibrated sand, and measuring the volume of the hole
• Dynamic Cone Penetrometer (DCP) is a portable device that measures the penetration resistance of soil layers, providing an estimate of the CBR value
• Plate load test applies a load to a steel plate placed on the soil surface to determine the bearing capacity and settlement characteristics of the soil
• Laboratory testing is conducted on soil samples collected during site investigation to determine their physical and engineering properties
  • Moisture content test measures the amount of water present in a soil sample by oven-drying the sample and calculating the weight loss
  • Specific gravity test determines the ratio of the density of soil solids to the density of water
• Consolidation test applies a series of loadings to a saturated soil sample to measure its compressibility and estimate the magnitude and rate of settlement
• Direct shear test measures the shear strength parameters (cohesion and friction angle) of a soil sample by applying a normal stress and a shear force
• Triaxial test determines the shear strength parameters of a soil sample under controlled drainage and confining stress conditions

Data Analysis and Interpretation

• Soil profile development involves organizing and presenting the data collected during site investigation to create a visual representation of the subsurface conditions
• Stratigraphy interpretation involves identifying the different soil layers, their thicknesses, and the boundaries between them based on the soil profile and laboratory test results
• Geotechnical parameter selection involves determining representative values for soil properties (shear strength, compressibility, permeability) based on field and laboratory test results
  • Statistical analysis (mean, standard deviation, coefficient of variation) helps assess the variability and reliability of the test results
  • Engineering judgment is applied to select conservative and appropriate parameter values for design purposes
• Settlement analysis predicts the magnitude and rate of settlement expected under the applied loads, considering the compressibility and drainage characteristics of the soil layers
• Bearing capacity analysis determines the maximum load that the soil can support without excessive settlement or shear failure, based on the shear strength parameters and foundation geometry
• Liquefaction potential assessment evaluates the susceptibility of saturated, loose granular soils to liquefaction under seismic loading conditions
• Slope stability analysis assesses the stability of natural or engineered slopes, considering the soil properties, groundwater conditions, and external loads

Practical Applications and Case Studies

• Shallow foundations (spread footings, mats) transfer loads from the structure to the near-surface soil layers, requiring proper site preparation and compaction to ensure adequate bearing capacity and minimize settlement
• Deep foundations (piles, drilled shafts) transfer loads to deeper, more competent soil layers or bedrock when the near-surface soils are weak or compressible
• Retaining walls (gravity walls, cantilever walls, mechanically stabilized earth walls) are designed to resist the lateral earth pressures and maintain the stability of soil masses
• Pavement design involves selecting the appropriate pavement structure (flexible or rigid) and determining the required layer thicknesses based on the subgrade soil properties, traffic loads, and environmental conditions
• Embankment construction requires proper selection and compaction of fill materials to ensure stability, minimize settlement, and prevent erosion
• Landfill design and construction involve site selection, liner systems, leachate collection, and monitoring to minimize the environmental impact and ensure the long-term stability of the waste containment facility
• Geotechnical earthquake engineering focuses on the seismic design of foundations, retaining structures, and earth structures, considering the dynamic soil properties and potential for liquefaction
• Case studies provide valuable insights into the application of geotechnical principles and the challenges encountered in real-world projects
  • Failure case studies (slope failures, foundation failures, dam failures) highlight the importance of thorough site investigation, proper design, and quality control during construction
  • Successful case studies demonstrate the effective use of geotechnical investigation techniques, innovative design solutions, and construction best practices in challenging site conditions