🏗️Civil Engineering Systems Unit 5 – Geotechnical Engineering Fundamentals

Key Concepts and Definitions

• Geotechnical engineering applies principles of soil mechanics and rock mechanics to the design of foundations, retaining structures, and earth structures
• Soil mechanics studies the behavior of soils under the influence of loading forces and soil-water interactions
• Rock mechanics focuses on the behavior of rock masses and the design of structures in rock, such as tunnels, slopes, and foundations
• Soil classification systems (Unified Soil Classification System, AASHTO) categorize soils based on their particle size distribution and plasticity characteristics
• Soil properties include grain size distribution, plasticity, compressibility, shear strength, and permeability, which influence the soil's behavior under loading and drainage conditions
• Effective stress principle states that the behavior of soil is controlled by the effective stress, which is the total stress minus the pore water pressure
• Consolidation refers to the gradual reduction in volume of a saturated soil due to the expulsion of water from the pore spaces under sustained loading
• Shear strength of soil is the maximum resistance to shearing stresses that the soil can offer, and it depends on the soil's cohesion and angle of internal friction

Soil Properties and Classification

• Soil texture describes the relative proportions of sand, silt, and clay particles in a soil
  • Sand particles range from 0.075 mm to 4.75 mm in size
  • Silt particles range from 0.002 mm to 0.075 mm in size
  • Clay particles are smaller than 0.002 mm in size
• Soil structure refers to the arrangement of soil particles and the presence of aggregates or voids
• Atterberg limits (liquid limit, plastic limit, and shrinkage limit) define the boundaries between different states of soil consistency based on its water content
• Soil plasticity index (PI) is the difference between the liquid limit and the plastic limit, indicating the range of water content over which the soil exhibits plastic behavior
• Soil classification systems, such as the Unified Soil Classification System (USCS) and the AASHTO system, group soils with similar engineering properties based on their grain size distribution and plasticity characteristics
• Soil permeability is a measure of the soil's ability to allow water to flow through its pore spaces, and it depends on factors such as grain size, void ratio, and soil structure
• Soil compressibility refers to the soil's tendency to decrease in volume when subjected to a compressive load, and it is influenced by factors such as soil type, initial void ratio, and stress history

Soil Mechanics Principles

• Effective stress ($\sigma'$) is the difference between the total stress ($\sigma$) and the pore water pressure ($u$), expressed as $\sigma' = \sigma - u$
  • Effective stress controls the shear strength and volume change behavior of soils
• Mohr-Coulomb failure criterion defines the shear strength of soil as a linear function of the normal stress on the failure plane, expressed as $\tau = c + \sigma' \tan \phi$, where $c$ is cohesion and $\phi$ is the angle of internal friction
• Consolidation theory describes the time-dependent volume change of saturated soils under loading due to the gradual dissipation of excess pore water pressure
  • Primary consolidation occurs when the excess pore water pressure dissipates and the soil skeleton carries the applied load
  • Secondary compression is the additional settlement that occurs after the excess pore water pressure has dissipated, due to the rearrangement of soil particles
• Soil compaction is the process of mechanically increasing the density of soil by reducing the air voids, which improves its strength and reduces its compressibility and permeability
• Soil bearing capacity is the maximum load that a soil can support without experiencing shear failure or excessive settlement, and it depends on factors such as soil type, foundation size and shape, and loading conditions
• Soil liquefaction is a phenomenon in which saturated, loose granular soils lose their shear strength and behave like a liquid when subjected to rapid loading or vibration, such as during an earthquake

Site Investigation and Soil Testing

• Site investigation is the process of collecting information about the subsurface conditions at a project site, including soil and rock properties, groundwater conditions, and potential geologic hazards
• Desk study involves reviewing available geological maps, aerial photographs, and previous site investigation reports to gain a preliminary understanding of the site conditions
• Site reconnaissance is a visual inspection of the project site to identify surface features, topography, drainage patterns, and any signs of instability or potential hazards
• Subsurface exploration techniques include drilling boreholes, excavating test pits, and performing cone penetration tests (CPT) to obtain soil samples and in-situ test data
  • Boreholes are drilled using various methods (auger drilling, rotary drilling, percussion drilling) to obtain soil samples and perform in-situ tests
  • Test pits are excavated using backhoes or excavators to expose the soil profile and collect bulk samples for laboratory testing
  • Cone penetration tests (CPT) involve pushing a instrumented cone into the ground to measure the cone resistance and sleeve friction, which can be correlated to soil properties
• Laboratory tests are conducted on soil samples obtained from the site to determine their physical and mechanical properties, such as grain size distribution, Atterberg limits, shear strength, and compressibility
  • Grain size analysis (sieve analysis and hydrometer analysis) determines the particle size distribution of soil
  • Atterberg limits tests (liquid limit and plastic limit) assess the plasticity characteristics of fine-grained soils
  • Direct shear test and triaxial shear test measure the shear strength parameters (cohesion and angle of internal friction) of soil under different stress conditions
  • Consolidation test determines the compressibility characteristics of soil by measuring its volume change under incremental loading

Foundation Types and Design

• Shallow foundations are used when the load-bearing soil is close to the surface and can support the structure without excessive settlement or bearing capacity failure
  • Strip footings are continuous linear foundations used to support load-bearing walls
  • Pad footings (isolated footings) are square or rectangular foundations used to support columns or posts
  • Raft foundations (mat foundations) are large, continuous slabs that distribute the load over a wide area, suitable for soils with low bearing capacity or structures with closely spaced columns
• Deep foundations are used when the load-bearing soil is at a considerable depth below the surface, or when the shallow soil layers are weak or compressible
  • Piles are long, slender elements driven or drilled into the ground to transfer loads to deeper, more competent soil layers or rock
    • Driven piles (displacement piles) are hammered into the ground, displacing the soil around them
    • Bored piles (replacement piles) are constructed by drilling a hole and filling it with concrete, often with reinforcing steel
  • Caissons (drilled shafts) are large-diameter, cast-in-place concrete foundations that transfer loads to deeper soil layers or rock through a combination of end bearing and skin friction
• Foundation design involves selecting the appropriate foundation type, size, and depth based on factors such as the structure's load, soil conditions, groundwater level, and environmental considerations
  • Bearing capacity analysis ensures that the foundation can support the applied loads without experiencing shear failure in the soil
  • Settlement analysis predicts the total and differential settlement of the foundation under the applied loads, considering both immediate (elastic) and long-term (consolidation) settlement
• Ground improvement techniques are used to enhance the properties of weak or problematic soils, enabling the use of shallow foundations or reducing the depth and cost of deep foundations
  • Soil replacement involves excavating and replacing weak or compressible soil layers with competent fill material
  • Preloading applies a temporary surcharge load to the ground surface to induce consolidation and improve the soil's strength and stiffness before construction
  • Ground reinforcement techniques, such as geotextiles and geogrids, are used to improve the soil's tensile strength and reduce settlement

Earth Pressure and Retaining Structures

• Earth pressure is the lateral pressure exerted by soil on a retaining structure, and it depends on factors such as soil type, moisture content, and drainage conditions
  • Active earth pressure occurs when the retaining structure moves away from the soil, allowing the soil to expand and mobilize its shear strength
  • Passive earth pressure occurs when the retaining structure moves towards the soil, compressing it and mobilizing its full shear strength
  • At-rest earth pressure is the lateral pressure exerted by the soil when no lateral movement of the retaining structure occurs
• Retaining walls are structures designed to support the lateral pressure of the soil and maintain a difference in ground elevation
  • Gravity walls rely on their own weight and the weight of the soil above the heel to resist the lateral earth pressure
  • Cantilever walls consist of a vertical stem and a base slab, with the stem acting as a cantilever to resist the earth pressure
  • Counterfort walls have vertical ribs (counterforts) connecting the stem to the base slab, providing additional resistance to bending moments in the stem
  • Buttress walls have vertical or inclined supports (buttresses) on the exposed face of the stem to resist the earth pressure
• Mechanically stabilized earth (MSE) walls use reinforcing elements (strips, grids, or sheets) embedded in the soil to create a composite soil-reinforcement structure that can support itself and the retained soil
• Sheet pile walls are constructed using interlocking steel or aluminum sheets driven into the ground, often used for temporary excavation support or in waterfront structures
• Anchored walls use ground anchors (steel cables or rods) to transfer the lateral loads from the retaining structure to a more stable soil layer or rock at a greater depth

Slope Stability Analysis

• Slope stability refers to the resistance of an inclined surface, such as a natural slope or an engineered embankment, to failure by sliding or collapsing
• Factors affecting slope stability include soil type and properties, slope geometry, groundwater conditions, external loads, and vegetation
• Failure modes in slopes can be categorized as translational slides (planar failure surface), rotational slides (curved failure surface), or flow-type failures (liquefaction or rapid loss of shear strength)
• Limit equilibrium methods are used to analyze the stability of slopes by comparing the driving forces (gravity and external loads) to the resisting forces (soil shear strength) along a potential failure surface
  • Method of slices divides the sliding mass into vertical slices and calculates the factor of safety based on the force and/or moment equilibrium of each slice
    • Ordinary method of slices (OMS) satisfies only the force equilibrium in the vertical direction
    • Bishop's simplified method satisfies the vertical force equilibrium and the moment equilibrium, assuming zero interslice shear forces
    • Janbu's simplified method satisfies the vertical and horizontal force equilibrium, assuming a correction factor for the interslice shear forces
    • Spencer's method satisfies both the force and moment equilibrium, assuming a constant interslice force inclination
  • Infinite slope analysis is a simplified method for analyzing the stability of shallow, planar failures in long, uniform slopes
• Finite element methods (FEM) provide a more advanced approach to slope stability analysis by considering the stress-strain behavior of the soil and the progressive development of failure
• Slope stabilization measures are used to improve the stability of slopes and prevent or mitigate slope failures
  • Slope geometry modification involves reducing the slope angle or height to decrease the driving forces
  • Drainage improvements, such as surface and subsurface drains, lower the groundwater table and reduce the pore water pressures in the slope
  • Retaining structures, such as walls or piles, can be used to support the slope and prevent sliding
  • Soil reinforcement techniques, such as geotextiles, geogrids, or soil nails, increase the shear strength of the soil and resist the driving forces
  • Vegetation and bioengineering methods use plants and their root systems to stabilize the slope surface and improve the soil's resistance to erosion

Geotechnical Considerations in Construction

• Site preparation involves clearing and grubbing the site, removing unsuitable materials (e.g., organic soil, debris), and grading the surface to the desired elevations and slopes
• Excavation support systems are used to ensure the stability of temporary excavations and protect adjacent structures and utilities
  • Soldier pile and lagging walls consist of vertical steel piles (soldier piles) driven or drilled into the ground, with horizontal timber or concrete lagging placed between the piles to retain the soil
  • Secant pile walls are formed by constructing overlapping reinforced concrete piles, creating a continuous wall to support the excavation
  • Diaphragm walls (slurry walls) are constructed by excavating a narrow trench, filling it with bentonite slurry to stabilize the trench walls, and then placing reinforced concrete to form a continuous wall
• Dewatering is the process of removing groundwater from an excavation or construction site to allow for safe and efficient construction activities
  • Sumps and pumps are used to collect and remove water from localized areas within the excavation
  • Well points are small-diameter wells installed around the perimeter of the excavation, connected to a header pipe and pumped to lower the groundwater table
  • Deep wells are larger-diameter wells drilled to greater depths, suitable for dewatering in deep excavations or in soils with low permeability
• Soil compaction is performed to improve the soil's strength, stiffness, and resistance to settlement, typically using mechanical equipment such as rollers, compactors, or vibratory plates
  • Field compaction control is achieved by measuring the in-situ dry density and moisture content of the compacted soil and comparing it to the maximum dry density and optimum moisture content determined from laboratory tests (e.g., Standard Proctor test, Modified Proctor test)
• Quality control and quality assurance (QC/QA) programs are implemented to ensure that the geotechnical aspects of construction meet the project requirements and design specifications
  • Field testing, such as in-situ density tests, plate load tests, and penetration tests, is performed to verify the soil properties and compaction levels
  • Laboratory testing of soil samples taken from the site is conducted to confirm the soil classification, strength, and compressibility parameters
  • Monitoring of settlement, pore water pressures, and lateral movements using instrumentation (e.g., settlement plates, piezometers, inclinometers) helps to assess the performance of foundations, embankments, and retaining structures during and after construction
• Geotechnical instrumentation and monitoring are used to observe the behavior of soil, rock, and structures during construction and throughout the life of the project
  • Piezometers measure pore water pressures in soil and rock, helping to assess the effectiveness of dewatering systems and monitor the stability of slopes and embankments
  • Inclinometers measure lateral movements and deformations in soil and rock, often used to monitor the performance of retaining walls, slopes, and embankments
  • Strain gauges and load cells measure the stresses and strains in structural elements, such as piles, anchors, and retaining walls, to verify their design and performance
  • Automated data acquisition systems (ADAS) collect and transmit data from various geotechnical instruments, allowing for real-time monitoring and early detection of potential issues