📖 Geotechnical Instrumentation & ADAS Glossary¶

Comprehensive technical glossary and reference dictionary covering subsurface geotechnical instrumentation, sensor physics, automated data acquisition systems (ADAS/ADAQS), wireless telemetry, and field monitoring engineering. Grounded in international standards (ISO 18674, ASTM, BS 5930) and John Dunnicliff's foundational principles.

1. Subsurface Sensors & Measurement Devices¶

Piezometers & Pore Water Pressure¶

Piezometer: An instrument installed in soil, rock, or backfill to measure pore water pressure or groundwater elevation.
Vibrating Wire Piezometer (VWP): A diaphragm-based sensor where water pressure deflects a flexible diaphragm connected to a tensioned steel wire. Changes in diaphragm deflection alter wire tension and its natural resonant frequency (\(f\)). Highly resistant to electrical noise and long cable runs.
Standpipe Piezometer (Casagrande): An open-tube piezometer consisting of a porous ceramic or plastic tip connected to a riser pipe. Measures hydrostatic water level directly using an electric water level indicator (dip meter). Slower hydraulic response time in low-permeability clays.
Pneumatic Piezometer: A diaphragm sensor operated by gas pressure. Gas (usually nitrogen) is pumped down a supply tube until it balances pore pressure and opens a valve, venting through a return tube. Zero fluid volume change, negligible time lag.
Drive-In / Push-In Piezometer: A robust, pointed-tip piezometer pressed directly into soft clay or silt formations without pre-drilling a borehole.
High-Air-Entry (HAE) vs Low-Air-Entry (LAE) Filter:
- HAE Ceramic: Fine pore size (typically 1–2 µm) with high bubbling pressure, preventing air entry under negative pore water pressures (suction) in unsaturated soils.
- LAE Carborundum/Polyethylene: Coarse pore size (typically 50 µm), designed for rapid saturation in standard saturated groundwater measurements.
Multi-Level Piezometer Array: Multiple VWP sensors positioned at discrete elevations within a single borehole to capture vertical hydraulic gradients, perched aquifers, and seepage profiles.

Inclinometers & Lateral Displacement¶

Inclinometer Casing: Special grooved plastic (ABS) or aluminum tubing installed in a borehole, slurry wall, or embankment, with internal orthogonal keyways that guide probe orientation.
Traversing Probe Inclinometer: A wheeled torpedo containing dual orthogonal servo-accelerometers or MEMS sensors, lowered into grooved casing on a graduated control cable to record tilt profiles at regular intervals (e.g., 0.5 m).
In-Place Inclinometer (IPI): A string of articulated tilt sensor pods permanently suspended inside grooved casing at critical depths to provide continuous, automated lateral displacement logging.
ShapeArray (SAA / Flexible 3D Array): A jointed string of miniature rigid segments interconnected by flexible joints, containing triaxial MEMS accelerometers that measure continuous 3D displacement profiles in real time.
Horizontal Inclinometer: An inclinometer system utilizing horizontally installed grooved casing to measure vertical settlement and heave profiles beneath embankments, tanks, or landfills.
Spiral Twist Survey: An auxiliary probe run used to measure rotational twist in inclinometer casing keyways over deep boreholes, correcting azimuth errors in lateral deflection calculations.

Extensometers & Settlement Systems¶

Multipoint Borehole Extensometer (MPBX): An assembly of anchors fixed at varying depths inside a borehole, linked via rigid rods or fiberglass tendons inside protective sleeves to a reference head at the surface. Measures axial ground movement and zone deformation.
Magnetic Settlement Gauge: An access pipe surrounded by external spider magnets anchored to the soil. A portable reed-switch probe lowered down the pipe sounds a buzzer when passing each magnetic target, yielding point-specific vertical settlement.
Liquid Level Settlement Cell: A differential pressure sensor or overflow cell linked via liquid-filled tubing to an external reservoir, measuring vertical elevation changes of subterranean structures or foundation slabs.
Settlement Plate: A rigid steel or timber plate placed on original ground prior to fill placement, connected to a vertical riser pipe extended as embankment height increases.
Tape Extensometer: A portable high-precision steel measuring tape with an in-line micrometer tensioning head, used to measure convergence across tunnel walls and excavation struts.

Structural & Load Monitoring¶

Load Cell (Center-Hole / Annular): An instrument placed beneath anchor nuts on tiebacks, rock bolts, or foundation struts to measure axial tensile or compressive load. Most commonly utilizes multiple vibrating wire strain gauges arranged in parallel.
Vibrating Wire Strain Gauge (VWSG): A sensor welded to steel structures or embedded directly in concrete (sister bar / embedment gauge) to record micro-strain (\(\mu\varepsilon\)) caused by structural loading or thermal expansion.
Earth Pressure Cell (Total Pressure Cell): Two circular or rectangular steel plates welded together with an incompressible hydraulic fluid between them, connected to a pressure transducer. Installed at the soil-structure interface or within embankment fill.
Tiltmeter: An instrument anchored to retaining walls, historic structures, or bridge piers to detect angular rotation. Utilizes electrolytic bubbles or high-precision MEMS sensors.
Crackmeter / Jointmeter: A displacement transducer anchored across concrete joints or rock fissures to monitor relative joint opening, closing, and shear movements.

2. Automated Data Acquisition Systems (ADAS) & Telemetry¶

Signal Conditioning & Protocols¶

ADAS / ADAQS: Automated Data Acquisition System — an autonomous setup comprising sensors, data loggers, power modules, and communications infrastructure for unattended geotechnical monitoring.
Pluck Frequency Excitation: The standard method for vibrating wire sensors, in which an electromagnetic coil applies a rapid swept-frequency impulse to pluck the steel wire into mechanical resonance, followed by reading the induced sinusoidal signal.
Spectral Analysis / VSPECT: Advanced signal conditioning technology that performs Fast Fourier Transforms (FFT) on the raw vibrating wire return signal, isolating the true fundamental resonant frequency from external electrical noise.
4–20 mA Current Loop: An analog transmission standard where sensor signals are modulated as electric current. Highly immune to cable resistance over long distances.
RS-485 / Modbus RTU: A balanced differential serial bus standard capable of multidrop networking up to 32+ digital sensors over distances up to 1,200 meters.
SDI-12: A low-power, serial digital interface standard widely used in environmental and geotechnical sensors operating at 1200 baud.
CAN Bus / CANopen: A robust vehicle-grade digital bus protocol adopted in high-speed and underground tunneling instrumentation.

Wireless Networks & Field Hardware¶

Data Logger: An electronic instrument that powers attached sensors, digitizes analog signals, processes calibration polynomials, and stores timestamped records in non-volatile flash memory.
Relay Multiplexer: An expansion module utilizing solid-state or sealed reed relays to sequentially switch multiple sensor channels into a single data logger measurement port.
LoRaWAN (Long Range Wide Area Network): Low-power, long-range wireless protocol operating on unlicensed sub-GHz bands (868 MHz EU, 915 MHz US/Americas, 923 MHz AS). Excellent penetration in dense urban environments and deep pits.
Wireless Mesh Network: A self-forming, self-healing network topology where sensor nodes act as relays to route data packets to a central gateway, providing operational resilience against line-of-sight obstructions.
Cellular IoT (NB-IoT & LTE-M): Low-power cellular communication standards built for battery-operated IoT sensors, transmitting small data packets directly to cloud servers via commercial telecom towers.
Satellite Telemetry (Iridium SBD): Low-earth-orbit satellite data transmission used for mission-critical remote dams and mining sites where terrestrial cellular networks do not reach.
Solar Power Module & MPPT: Photovoltaic solar array combined with a Maximum Power Point Tracking (MPPT) charge controller and sealed AGM or LiFePO4 battery for autonomous field operation.

3. Sensor Physics & Mathematical Reductions¶

Vibrating Wire Reductions¶

Natural Frequency Equation: The fundamental resonant frequency \(f\) of a clamped wire under tension \(\sigma\):

\[ f = \frac{1}{2L} \sqrt{\frac{\sigma}{\rho}} = \frac{1}{2L} \sqrt{\frac{E \cdot \varepsilon}{\rho}} \]

Where \(L\) is wire length, \(\rho\) is density, \(E\) is Young's modulus, and \(\varepsilon\) is wire strain.
Digits (Linear Period Units): A linearizing parameter directly proportional to wire strain:

\[ \text{Digits} = \frac{f^2}{1000} \]
Linear Calibration Equation:

\[ P = G \cdot (R_0 - R) \]

Where \(G\) is the gauge factor (calibrated linear coefficient), \(R_0\) is baseline reading in digits, and \(R\) is current reading.
Polynomial Calibration Equation (Second-Order):

\[ P = A \cdot R^2 + B \cdot R + C \]

Provides higher measurement precision over broad pressure ranges.
Thermal Compensation (\(K_T\)):

\[ P_{corr} = P_{raw} + K_T \cdot (T - T_0) \]

Corrects for differential thermal expansion between the vibrating wire and the stainless steel sensor body.
Barometric Compensation:

\[ P_{net} = P_{measured} - (B - B_0) \]

Essential for sealed piezometers to isolate genuine groundwater changes from ambient atmospheric barometric swings.

Inclinometer Data Reduction¶

Deflection Increment (\(\delta_i\)):

\[ \delta_i = L \cdot \sin(\theta_i) = C \cdot (A_0 - A_{180}) \]

Where \(L\) is probe wheelbase (typically 500 mm), \(\theta_i\) is tilt angle, and \(A_0, A_{180}\) are conjugate runs.
Check Sum (Zero Offset Error):

\[ \text{Check Sum} = A_0 + A_{180} \]

A constant value across all depths confirming instrument stability and absence of mechanical contamination in wheels.
Cumulative Displacement (\(D_k\)):

\[ D_k = \sum_{i=1}^k \delta_i \]

Summation of incremental displacements calculated from the stable bottom anchor upwards to the surface collar.

4. Engineering Risk & TARP Frameworks¶

TARP (Trigger Action Response Plan): A systematic emergency and risk management framework that establishes defined operational actions based on observed instrumentation thresholds:
- Level 0 (Green / Baseline): Normal behavior within predicted design limits. Standard monitoring frequency.
- Level 1 (Amber / Alert): Measurement exceeds statistical baseline or initial safety margin. Monitoring frequency doubled; engineering site inspection dispatched.
- Level 2 (Orange / Review): Significant trend acceleration or structural stress approaching design tolerance. Senior engineering review, verification of backup instruments, equipment standby.
- Level 3 (Red / Action Required): Critical threshold breached; imminent structural failure or uncontrollable seepage. Immediate work stoppage, site evacuation, and execution of emergency remedial protocols.
Velocity of Displacement (\(v = \frac{d\delta}{dt}\)): Rate of ground movement over time. An acceleration in displacement velocity is the primary precursor to catastrophic slope and excavation failures.
Phreatic Line (Seepage Line): The upper boundary of water seepage flow through an earth dam embankment, at which pore water pressure equals atmospheric pressure.
Piping (Internal Erosion): Progressive removal of soil particles by seepage flow through embankment dams or foundations, leading to internal conduits and dam breaching.

5. Field Installation & Grouting Best Practices¶

Fully Grouted Method (Mikkelsen / Contreras): Installation technique where piezometers are placed directly into a borehole and encapsulated completely in a cement-bentonite grout without a traditional sand filter pack or bentonite pellets. Enabled by the extremely low volume displacement of modern diaphragm sensors.
Grout Mix Proportions: Specific ratio of water to Portland cement and sodium bentonite powder designed to match the permeability and modulus of the surrounding ground formation:
- Soft soil: Water:Cement:Bentonite \(\approx\) 2.5 : 1.0 : 0.3 to 0.4 by weight.
- Medium to stiff rock: Water:Cement:Bentonite \(\approx\) 1.5 : 1.0 : 0.1 by weight.
Bentonite Pellet Seal: Highly compressed sodium bentonite tablets tamped above sand packs in traditional Casagrande installations, swelling upon hydration to form an impermeable hydraulic seal.
Telescoping Coupling: Slip-joints placed along inclinometer or settlement casing strings to prevent axial compression and buckling when large vertical settlements occur.

6. Tailings Storage Facilities & Tailings Dam Safety¶

Tailings: The finely ground waste solids left after extracting metals or minerals from ore, transported and deposited as a slurry.
Tailings Storage Facility (TSF): The complete impoundment system — embankment(s), deposited tailings, supernatant pond, decant, and reclaim works — that stores mine tailings.
Tailings Dam: The engineered embankment that retains the tailings deposit; often raised incrementally over the mine life and frequently built from the tailings themselves.
Embankment Raise (Dam Raise): An incremental addition to the height of a tailings embankment, placed by the upstream, centerline, or downstream method.
Downstream Construction Method: Each raise is placed downstream of the previous one, so most of the structure rests on foundation — generally the most robust method.
Centerline Construction Method: Raises are placed on the original centerline; intermediate in risk.
Upstream Construction Method: Each raise is placed on the upstream (deposited) side, so the embankment rests on loose, saturated tailings — the highest-risk method, prone to liquefaction.
Beach (Tailings Beach): The gently sloping surface of deposited tailings between the discharge point and the supernatant pond.
Beach Geometry: The slope and length of the beach, which control where the phreatic surface sits and how much freeboard is retained.
Supernatant Pond: The body of clarified water above the settled tailings, from which water is recovered or decanted.
Decant (Decant Tower / Decant System): The structure and piping used to remove supernatant water and control pond level and freeboard.
Freeboard: The vertical distance between the pond surface and the lowest point of the embankment crest; a primary safeguard against overtopping.
Spigotting / Deposition: Discharging the tailings slurry from the embankment crest to build the beach by hydraulic deposition.
Thickened / Paste / Filtered Tailings: Dewatered tailings (progressively higher solids content) that reduce free water and lower liquefaction risk.
In-Pit / Paddock Storage: Storing tailings in a mined-out pit or a ring-dyke paddock instead of a valley impoundment.
Liquefaction: Loss of shear strength when a saturated, loose (contractive) soil is loaded or shaken and pore pressure rises toward near-zero effective stress.
Static Liquefaction: Liquefaction triggered by a static load (a raise, rapid pond rise, or foundation weakness) rather than an earthquake — the dominant catastrophic mechanism in tailings failures.
Seismic Liquefaction: Liquefaction triggered by cyclic earthquake shaking.
Pore-Pressure Ratio (\(r_u\)): The ratio of measured pore pressure to the initial vertical effective stress (\(r_u = u / \sigma'_v\)); rising values signal approaching liquefaction.
Excess Pore Pressure: Pore pressure above the steady-state (hydrostatic) value, generated by loading, construction, or shaking.
Flow Failure / Run-out: A liquefied mass that flows rapidly and travels long distances, causing extreme downstream damage.
Toe Drain: A permeable zone or drain at the downstream toe that collects and controls seepage, keeping the phreatic surface low.
Turbid Seepage: Seepage carrying fine soil particles (cloudy discharge) — a warning sign of internal erosion (piping).
Consequence Category: A classification of a facility by the potential downstream impact of failure (often A/B/C), which sets the intensity of monitoring and review.
Closure (Mine Closure): The post-operational phase in which the facility is decommissioned and made safe, requiring continued surveillance until the long-term steady state is demonstrated.
InSAR / A-DInSAR (Satellite Radar Interferometry): A remote-sensing technique using repeat satellite radar passes to measure millimetre-scale ground displacement over a whole facility with no on-site instruments.

7. Dam Performance Monitoring (ASCE MOP-135 scope)¶

Dam Performance: The observed behavior of a dam, its foundation, and appurtenant structures compared with expected behavior.
Expected vs. Measured Behavior: The comparison at the heart of monitoring; a large, unexplained, or trending difference is an anomaly.
Performance Signal: The difference between measured and expected behavior used to judge whether the dam is performing acceptably.
Potential Failure Mode: A credible way in which a dam could fail (e.g., overtopping, internal erosion, slope instability, foundation seepage); the basis for choosing instruments.
Uplift: Upward water pressure acting on the base of a concrete dam or within a foundation, reducing stability.
Appurtenant Structures: Auxiliary works such as spillways, outlet works, and decants that support the main dam.
Crest Settlement / Displacement: Vertical or horizontal movement of the dam crest, a key deformation indicator.
Surveillance Plan: The documented program stating what is monitored (visually and by instrument), how often, and the response to each anomaly.
Independent Review: Periodic review of monitoring data and the program by a party independent of the operating team, to catch normalized trends.

8. Monitoring Program Planning & Reliability (Dunnicliff)¶

Geotechnical Instrumentation: The measurement of soil, rock, foundation, and structural behavior to confirm performance and detect change.
Systematic Planning Approach: Dunnicliff's structured process (the "hub") that defines the geotechnical questions first, then selects parameters, instruments, locations, and frequency.
Geotechnical Question: A specific question the monitoring must answer (e.g., "will the wall exceed allowable movement?"), which dictates what is measured and the decision supported.
The Chain of 25 Links: Dunnicliff's metaphor that monitoring success is a chain from defining the project need to a reading that informs a decision; break any link and the chain fails.
Recipe for Reliability: The set of ingredients (selection, procurement, installation, calibration, reading, maintenance, data handling, people) that make monitoring trustworthy.
Observational Method: A design approach in which the design is refined during construction based on monitored performance.
Transducer: A device that converts a physical quantity (pressure, displacement, load) into an electrical or pneumatic signal.
Accuracy vs. Precision: Accuracy is closeness to the true value; precision (repeatability) is closeness of repeated readings to each other. Neither alone guarantees a good measurement.
Hysteresis: The dependence of an instrument's output on the direction of change (loading vs. unloading).
Error Budget: The combined effect of all individual uncertainties (sensor, cable, readout, environment) that bounds the total measurement uncertainty.
Calibration Factor / Zero / Span: The slope of the input–output relationship (calibration factor), the reading at zero input, and the full-scale range (span).
Baseline Reading: The reference reading(s) taken after installation and settlement, against which all future change is judged.
Piezometric Head: The equivalent water-surface elevation corresponding to a measured pore pressure.
Hydraulic Time Lag: The delay of a piezometer's response to a change in pore pressure, governed by the permeability and geometry of the surrounding ground and filter.
Saturation / De-airing: The procedure of removing air from a piezometer's filter and connecting tube so it responds correctly to pore pressure; a critical, often-neglected installation step.
Convergence: The shortening (closure) of the distance between points in a tunnel or excavation, indicating ground movement.
Telltale: A simple device that indicates relative movement across a joint or section.
Sister Bar: A vibrating-wire strain gauge attached parallel to a rebar so that it measures the rebar's strain when embedded in concrete.
Overcoring: A stress-relief technique in which a borehole instrument is isolated by overcoring to back-calculate the in-situ rock stress state.
Flat Jack: A thin hydraulic jack inserted in a slot in rock to measure stress relief and estimate in-situ stress.
Borehole Pressure Cell (BPC): A device grouted into a borehole to monitor changes in rock stress around an opening.
Contact Stress Cell: A pressure cell placed flush against a soil–structure interface to measure contact stress.

9. FHWA Highway Instrumentation (FHWA-HI-98-034 scope)¶

Golden Rule (instrumentation): Every instrument must be selected and placed to answer a specific geotechnical question — if there is no question, there should be no instrumentation.
Chain of 31 Links: The 21 planning links plus 10 execution links that must all hold for a monitoring program to succeed; one weak link can break it.
Embedment Earth Pressure Cell: A flat, fluid-filled cell embedded in soil to measure the total stress normal to its face.
Aspect Ratio (cell): The diameter-to-thickness ratio of an earth pressure cell; a high ratio reduces measurement error.
Soil/Cell Stiffness Ratio: The ratio that governs how much an earth pressure cell redistributes local stress, and thus its measurement error.
Observation Well: An open pipe with no subsurface seal; it creates a vertical connection between strata and is rarely suitable for performance monitoring.
Open Standpipe (Casagrande) Piezometer: A filter-tipped riser pipe; reliable but with a long hydrodynamic time lag.
Pneumatic Piezometer: A diaphragm balanced by gas pressure through twin tubes; short lag, no freezing, but operator-dependent.
Vibrating-Wire (VW) Piezometer: A stiff-diaphragm instrument read by the change in wire frequency; short lag and datalogger-ready.
Multipoint Piezometer: A single borehole containing several sensors to profile pressure with depth.
Settlement Platform: A surface plate (often with a riser) recording fill settlement as an embankment is built.
Subsurface Settlement Point: An anchor at depth (driven/grouted or Borros) recording settlement of a buried layer.
Liquid-Level Gage: A liquid-filled tube and cell that measures settlement or heave from the change in pressure or liquid level.
Series Extensometer: A stack of anchors in one borehole that resolves deformation into increments between adjacent anchors.
Horizontal Inclinometer: An inclinometer traversing a horizontal casing to give a settlement profile.
In-Place (Fixed) Inclinometer: A string of sensors left in casing for continuous, automated monitoring.
Shear-Plane Indicator: A device that detects the depth at which inclinometer casing shears.
Acoustic Emission (AE) Monitoring: Detecting the high-frequency sound from grain slip and breakage as early warning of developing instability.
Demec Gage: A mechanical surface strain gage measuring the change in distance between two reference discs.
Calibrated Hydraulic Jack: A center-hole jack used to apply and measure load in tensioning; it must be calibrated because fluid-pressure readings carry friction error.
Contact Earth Pressure Cell: A cell measuring stress or load at a soil–structure interface, at a pile toe, or at a shaft base.
Load Transfer (deep foundations): The distribution of axial load between shaft friction and end bearing, resolved from strain gages, sister bars, and telltales.
Ground Improvement: In-situ modification of ground properties — by grouting, densification, or drainage — to make it suitable for construction.
Acceptance Test (pre-/post-installation): A test confirming an instrument meets specification before installation and survived installation afterwards.
Installation Record Sheet: The as-built record of position, depth, seal, and the calibration values in force — the permanent record that makes data interpretable.
Cause-and-Effect Plot: A plot of measured change against an influencing factor (load, rainfall, fill height) to reveal relationships.
Implementation (final link): Acting on the interpreted data — the step that makes monitoring worthwhile.

10. Foundation Engineering & Soil Mechanics (USACE / FHWA public-domain scope)¶

Effective Stress (\(\sigma'\)): The stress carried by the soil skeleton — total stress minus pore-water pressure; the stress that actually controls strength and deformation.
Pore-Water Pressure (\(u\)): The pressure of water in the voids; positive below the water table and negative (suction) above it.
Total Stress (\(\sigma\)): The stress carried by the soil skeleton and the pore water together; \(\sigma = \sigma' + u\).
Void Ratio (\(e\)): The volume of voids per unit volume of solids.
Porosity (\(n\)): The volume of voids per unit total volume.
Degree of Saturation (\(S\)): The fraction of the void volume filled with water.
Atterberg Limits (\(LL\), \(PL\), \(SL\)): The water contents marking the transitions between the solid, plastic, and liquid states of a fine-grained soil.
Plasticity Index (\(PI\)): The range of water content over which a soil is plastic; \(PI = LL - PL\).
USCS (Unified Soil Classification System): The two-letter engineering classification of soils (e.g. CL, SP, GW) based on gradation and plasticity.
Compaction Curve: The plot of dry unit weight against water content; its peak defines the optimum water content.
Optimum Water Content (OMC): The water content at which a given compactive effort produces the maximum dry unit weight.
Proctor Test (Standard / Modified): The laboratory compaction test that defines the moisture–density relationship used for field control.
Darcy's Law: The linear relation between seepage velocity and hydraulic gradient, \(v = ki\).
Coefficient of Permeability (\(k\)): The constant linking flow velocity to hydraulic gradient for a given soil.
Flow Net: A graphical solution of Laplace's equation, drawn as orthogonal flow lines and equipotential lines.
Exit Gradient: The hydraulic gradient at the downstream surface; if it exceeds the critical value, piping begins.
Quick Condition / Piping: The state in which upward seepage reduces effective stress to zero and the soil boils.
Consolidation: The time-dependent compression of a saturated soil as pore water drains and effective stress rises.
Coefficient of Consolidation (\(c_v\)): The parameter governing the rate of primary consolidation.
Compression Index (\(C_c\)): The slope of the virgin portion of the \(e\)–\(\log\sigma'\) curve.
Preconsolidation Pressure (\(\sigma'_p\)): The maximum effective stress a soil has ever sustained.
Overconsolidation Ratio (\(OCR\)): The preconsolidation pressure divided by the present effective stress; \(OCR = 1\) is normally consolidated.
Mohr–Coulomb Criterion: Failure when the shear stress reaches \(\tau = c' + \sigma'\tan\phi'\).
Cohesion (\(c'\)): The shear-strength intercept of the failure envelope at zero effective normal stress.
Angle of Internal Friction (\(\phi'\)): The slope of the Mohr–Coulomb envelope, reflecting interparticle friction and interlocking.
Undrained Shear Strength (\(c_u\) / \(s_u\)): The shear strength of a saturated clay measured without drainage.
Bearing Capacity (\(q_{ult}\)): The maximum pressure a foundation soil can carry before shear failure.
Allowable Bearing Pressure (\(q_{all}\)): The design pressure obtained after applying a factor of safety and settlement limits.
Settlement: The vertical downward movement of a foundation under load.
Differential Settlement: The difference in settlement between points of a structure; it governs distortion and damage.
Skin Friction: The shear resistance developed along the shaft of a pile or drilled shaft.
End Bearing: The resistance mobilised at the base of a deep foundation.
Negative Skin Friction (Downdrag): Down-drag on a pile when the surrounding soil settles relative to the pile.
Pile Group Efficiency: The ratio of the capacity of a pile group to the sum of the single-pile capacities.
Lateral Earth Pressure (At-Rest / Active / Passive): The horizontal stress a soil exerts on a retaining structure — least when the wall yields (active) and greatest when the wall is pushed into the soil (passive).
Coefficient of Lateral Earth Pressure (\(K_0\), \(K_a\), \(K_p\)): The ratio of horizontal to vertical effective stress in the at-rest, active, and passive states.
Factor of Safety (slope): The ratio of available shear strength to the shear stress required for equilibrium.
Standard Penetration Test (\(SPT\), \(N\)-value): A driven split-spoon test whose blow count indexes soil density and strength.
Cone Penetration Test (\(CPT\)): A pushed cone giving continuous tip resistance and sleeve friction with depth.
Liquefaction: Loss of strength in saturated loose sand under cyclic loading when pore pressure approaches the total stress.
Expansive Soil: Clay that swells on wetting and shrinks on drying, causing heave and cracking.
Collapsible Soil: Soil that undergoes sudden volume reduction on wetting at constant load.