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GEOTECHNICAL ENGINEERING CONCEPTS AND
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Triaxial test advantages - CORRECT ANSWER-Controlling drainage enables soils of low permeability to be consolidated and pore pressures measurements made. Volume changes in consolidation - CORRECT ANSWER-Lateral strains are zero in an oedometer, whereas lateral strains can take place in a triaxial. Triaxial test measurements - CORRECT ANSWER-Both the fully drained and fully undrained responses can be measured by varying the drainage conditions and/or loading rate of the ram. Specimen area correction in triaxial test - CORRECT ANSWER-Depending on whether the specimen volume increases or decreases during testing the cross-sectional area must be corrected using Equation 5.19 or 5.20. Stress invariants vs principal stresses - CORRECT ANSWER-Stress invariants represent the average principal stress and principal stress difference, allowing Mohr circles to be reduced to a single point.
Mean stress invariant and shearing - CORRECT ANSWER-A change in the deviatoric stress (or principal stress difference) is required to cause shearing, otherwise the stress state is isotropic and only causes volume change. Different stress invariants and failure criteria - CORRECT ANSWER-Equations shown in Figure 5.12 are used to move between different failure criteria as defined by different stress invariants. Back pressure in saturation - CORRECT ANSWER-Back pressure is used to drive pore air pressure into solution in the pore water, but the cell pressure must be just higher than the back pressure to ensure a negligible change in effective stress. Effective stresses in coarse-grained soils - CORRECT ANSWER-Due to high permeability pore pressures rarely increase above ambient conditions due to either shearing or isotropic loading. Interlocking in coarse-grained soils - CORRECT ANSWER-These three factors (density, grading, and particle shape) influence the degree of interlocking. Source of soil strength - CORRECT ANSWER-The only source of soil strength is particle interlocking. Soil strength - CORRECT ANSWER-Comes from particle interlock (measured by dilatancy), friction resistance between particles (true angle of friction) and work required to rearrange particles.
Ultimate stress state in triaxial testing - CORRECT ANSWER-Excessive specimen deformation makes it difficult to accurately measure (or reach) the ultimate stress state. Overconsolidated clays - CORRECT ANSWER-Can soften due to an increase in volume during shearing, with softening becoming more pronounced the more overconsolidated the specimen is. Undrained shear strength - CORRECT ANSWER-Depends on the way pore pressure changes during shearing, dictated by the direction of major principal stresses. Macro-structure/fabric in soil testing - CORRECT ANSWER-If soil strength depends on macro-structure/fabric, then large specimens need to be tested to account for this. Sensitivity in undrained strength - CORRECT ANSWER-Defined as the ratio of the undrained strength in the undisturbed state to that in a remoulded state, ranging between 1-4 for most clays and up to 100 for quick clays. Residual strength - CORRECT ANSWER-Involves a preferred orientation or translation of particles in a narrow shear zone, not continuous deformation of an entire soil specimen. Residual conditions in geotechnical structures - CORRECT ANSWER-Typically only reached for instances of slope instability, where straining along a narrow shear zone has taken place for a long time.
Critical state framework - CORRECT ANSWER-Can be used for both fine- and coarse-grained soils, unifying shear and deformation. Isotropic Compression Line (ICL) - CORRECT ANSWER-Represents an upper boundary to possible volume and effective stress states for a given soil. Critical State Line (CSL) - CORRECT ANSWER-Can be assumed parallel to the Isotropic Compression Line for most conventional situations. Shearing associated with pore pressure changes - CORRECT ANSWER- Undrained shearing is associated with changes in pore pressures, while drained shearing is associated with changes in effective stress and volume. Disturbed samples - CORRECT ANSWER-Can provide a large amount of information to describe the soil via empirical correlations. Undisturbed sample testing - CORRECT ANSWER-Expensive and should be used to validate properties critical to the design. Critical State Friction Angle - CORRECT ANSWER-Independent of initial void ratio in coarse soils. Plasticity Index - CORRECT ANSWER-Influences critical state friction in fine- grained soils. Relative Density - CORRECT ANSWER-Defines maximum friction angle in coarse-grained soils.
Exact Collapse Loads - CORRECT ANSWER-Theoretically possible using lower and upper bounds. Efficient Stress Distributions - CORRECT ANSWER-Can carry higher external loads in structures. Upper Bound Approach - CORRECT ANSWER-Does not require force equilibrium for analysis. Undrained Mechanisms - CORRECT ANSWER-Comprise straight lines and circular arcs in soil. Hodograph - CORRECT ANSWER-Diagram representing mechanism velocity, not acceleration. Dissipated Energy - CORRECT ANSWER-Calculated as force multiplied by relative velocity. Efficient Mechanisms - CORRECT ANSWER-Dissipate less energy during movement. Lower Bound Solutions - CORRECT ANSWER-Do not require kinematically admissible mechanisms. Fan Zone - CORRECT ANSWER-Mobilizes significant soil strength through stress discontinuities.
Bearing Capacity Factor (Nc) - CORRECT ANSWER-Generalizes bearing capacity equation for various scenarios. Drained Mechanisms - CORRECT ANSWER-Consist of straight lines and logarithmic spirals. Normality Principle - CORRECT ANSWER-No energy dissipated by shearing in soil mass. Associative Flow Rule - CORRECT ANSWER-Friction angle equals dilation angle, preventing energy loss. Nγ - CORRECT ANSWER-Varies due to dilation and soil-footing interactions. Peak Shear Strengths - CORRECT ANSWER-Used for bearing capacity of undisturbed soils. Ground Water Levels - CORRECT ANSWER-Fluctuate seasonally; highest level used for design. Characteristic Values - CORRECT ANSWER-Cautious estimates, not smaller than design values. Permanent Actions - CORRECT ANSWER-Not always more critical than variable actions.
Foundation Settlement - CORRECT ANSWER-Estimated using correlations from in-situ tests. Burland and Burbidge Method - CORRECT ANSWER-Settlement method based on Standard Penetration Test. Schmertmann Method - CORRECT ANSWER-Settlement method based on Cone Penetration Test. Design Values - CORRECT ANSWER-Used to determine deformation at SLS. Earthquake effects on body forces - CORRECT ANSWER-An earthquake can cause body forces in the 𝓍-direction to be non-zero. Constitutive models and equilibrium - CORRECT ANSWER-Any constitutive model (i.e., stress-strain relationship) can be applied as the equations of equilibrium and compatibility are independent of material properties. Mohr-Coulomb criterion - CORRECT ANSWER-The Mohr-Coulomb criterion is an example of a yield function. Shear strength and shear strain - CORRECT ANSWER-A hardening law can be used to define the relationship between the increase in yield stress and corresponding plastic strain component. Young's Modulus and Poisson's ratio - CORRECT ANSWER-Only the shear modulus (G) is independent of drainage conditions.
Normalization of G - CORRECT ANSWER-G is normalized by G0 as the relationship between G/G0 is independent of stress. Elastic model limitations - CORRECT ANSWER-Elastic models assume the soil is infinitely strong. Soil shear strength and frictional strength - CORRECT ANSWER-Interlocking can cause the frictional resistance to be higher than that predicted by considering friction alone. Cohesion intercept and angle of shear resistance - CORRECT ANSWER-The cohesion intercept and angle of shear resistance are mathematical constants defining a linear relationship between shear strength and effective normal stress obtained from strength testing. Soil strength and shearing velocity - CORRECT ANSWER-When shearing is fast and permeability is low, soil strength can be defined by the undrained strength; if shearing is slow and permeability is high, soil strength can be defined by the drained strength. Direct shear box test speed - CORRECT ANSWER-Direct shear box tests must be carried out slow enough to ensure there is a negligible increase in excess pore water pressure for this assumption to be true. Direct shear box test and pure shear - CORRECT ANSWER-Only an approximation to the state of pure shear is produced in the specimen and shear stress on the failure plane is not uniform as failure occurs progressively from the edges towards the centre.Ultimate Limit State (ULS)
Base Resistance - CORRECT ANSWER-Resistance from soil stress above pile base. Embedment - CORRECT ANSWER-Stress from overlying soil resisting failure mechanism. Surcharge Term - CORRECT ANSWER-Weight of soil contributing to base resistance. Adhesion Factors - CORRECT ANSWER-Factors showing scatter, affecting pile resistance calculations. Effective Stresses - CORRECT ANSWER-Higher stresses develop against displacement piles. Interface Friction Angles - CORRECT ANSWER-Friction angles for timber and concrete exceed steel. Shaft Friction - CORRECT ANSWER-Friction calculated differently for drained and undrained conditions. Eurocode 7 DA1b - CORRECT ANSWER-Guidelines factoring actions and resistance for piles. In-situ Tests - CORRECT ANSWER-Tests used to calculate shaft and base resistance.
Correlation Factors - CORRECT ANSWER-Factors reducing with more in-situ tests conducted. Load Testing - CORRECT ANSWER-Verifying design assumptions for deep foundations. Preliminary Piles - CORRECT ANSWER-Piles loaded to failure, unlike working piles. Tension Anchors - CORRECT ANSWER-Used to provide reaction for load tests. Precision Levelling Equipment - CORRECT ANSWER-Tools for accurate displacement measurement in tests. Constant Rate of Penetration Tests - CORRECT ANSWER-Tests with specific penetration rates for soil types. Maintained Load Test (MLT) - CORRECT ANSWER-Test where DVL is exceeded only in second stage. Hyperbolic Extrapolation Technique - CORRECT ANSWER-Method to estimate failure loads from incomplete tests. Empirical Relationships - CORRECT ANSWER-Relationships for pile loads based on load tests.
Cavity Expansion Theory - CORRECT ANSWER-Modeling method for boreholes or pile shafts. Rotational Slip - CORRECT ANSWER-Failure mechanism not solely due to weak layers. Slope Failure Shape - CORRECT ANSWER-Controlled by discontinuities like fissures or weak layers. Undrained Shearing - CORRECT ANSWER-Only moment equilibrium considered in stability analysis. Stabilising Moments - CORRECT ANSWER-Both stabilising and destabilising moments affect stability. Deep Clays Stability - CORRECT ANSWER-Controlled by slope height, not slope angle. Slice Effective Weights - CORRECT ANSWER-Determine mobilised strength in slope stability analysis. Indeterminate Problem - CORRECT ANSWER-Requires assumptions for exact solutions in rotational slips. Pore Water Pressure Ratio (ru) - CORRECT ANSWER-Higher ratio leads to lower factor of safety.
Bishop Routine Solution - CORRECT ANSWER-Balances only moment equilibrium in slope stability. Seepage Forces - CORRECT ANSWER-Can be included in plane strain analysis of slips. Advanced Limit Equilibrium Methods - CORRECT ANSWER-Enable analysis of any failure surface shape. Effective Stress Analysis - CORRECT ANSWER-Uses actual pore water pressures, not failure conditions. Fill Water Content - CORRECT ANSWER-Higher content leads to greater excess pore pressures. Overconsolidated Clays - CORRECT ANSWER-Require critical state strength for stability assessment. Flexible Retaining Walls - CORRECT ANSWER-Designed to resist soil pressures, not buckling. Gravity Retaining Wall - CORRECT ANSWER-Typically not used as a temporary retaining structure. Active State - CORRECT ANSWER-Plastic equilibrium with vertical major principal stress.
Routine Design - CORRECT ANSWER-Not based on fully passive states. Gravity Retaining Wall Mass - CORRECT ANSWER-Can utilize mass of retained soil. Bearing Capacity - CORRECT ANSWER-Critical for gravity retaining wall stability. Tension Cracks - CORRECT ANSWER-Do not develop in purely frictional materials. Sheet Pile Walls - CORRECT ANSWER-Example of retaining wall using passive resistance. Diaphragm Walls - CORRECT ANSWER-Cast in bentonite slurry to prevent soil collapse. Filter Cake - CORRECT ANSWER-Forms when bentonite slurry seeps into soil. Wet Concrete Placement - CORRECT ANSWER-Requires tremie pipe to prevent segregation. Contiguous Piles - CORRECT ANSWER-Concrete facing prevents erosion between piles. Secant Wall - CORRECT ANSWER-Continuous wall formed by overlapping piles.
Lateral Pressure Distribution - CORRECT ANSWER-Dictated by number of anchors or props. Passive Resistance - CORRECT ANSWER-Additional support in anchored and propped walls. Deformation of Flexible Wall - CORRECT ANSWER-Assumed in limit pressure development. Arching Effect - CORRECT ANSWER-Causes deviation in earth pressure distributions. Linear Approximations - CORRECT ANSWER-Often underestimate pore water pressures. Steady State Seepage - CORRECT ANSWER-Changes effective unit weight of soil.
