PEC EPE Geological key terms

The intact rock plus all discontinuities, such as bedding planes, joints, and faults, that together behave as an engineering material at the scale of a foundation, slope, or tunnel.

The strength of a laboratory-sized rock specimen free of visible fractures, usually reported as unconfined compressive strength or point-load index and used as an upper bound for the rock mass.

The peak axial stress a cylindrical rock specimen sustains with no confining pressure, determined by compressing it at a controlled rate until failure; a primary input to rock mass classification.

A field or laboratory test in which a rock core or lump is broken between conical platens, giving a quick strength index that can be correlated with unconfined compressive strength.

Any planar or near-planar break in a rock mass, such as a joint, bedding plane, foliation, or fault, that interrupts mechanical continuity and commonly controls permeability and failure.

A group of roughly parallel joints formed by the same stress episode; several sets intersecting in a rock mass create blocky geometry that governs stability of slopes and underground openings.

A fracture along which the two sides have moved relative to each other, ranging from millimetres to kilometres of displacement; faults concentrate shear and often form weak, clay-rich seams.

Fine-grained, clay-rich material produced by grinding along a fault surface; it is weak, often slippery when wet, and can control sliding of foundations, slopes, and dam abutments.

Angular rock fragments cemented in a matrix along a fault zone, formed by brittle crushing; it is more permeable and less coherent than intact rock and is a common weak zone in excavations.

A core-logging index equal to the percentage of recovered core consisting of intact pieces 100 mm or longer, divided by the drilled length; it measures rock mass fracturing.

Systematic recording of drilled rock core, including recovery, Rock Quality Designation, lithology, weathering, and discontinuity descriptions, to build a factual picture of subsurface conditions.

The length of core retrieved from a drill run expressed as a percentage of the drilled length; low recovery signals very weak, weathered, or highly fractured ground that may be lost during drilling.

A classification score combining intact rock strength, Rock Quality Designation, discontinuity spacing and condition, groundwater, and orientation, used to estimate support and stand-up time for tunnels.

A rock mass classification using six ratios of block size, joint shear strength, and stress conditions to produce a Q value that guides tunnel support selection and span design.

A rock mass classification index based on the surface condition and structure of the rock mass, used with the Hoek-Brown criterion to estimate rock mass strength for design.

An empirical failure criterion for rock masses that relates principal stresses to intact rock strength, geological strength index, and a disturbance factor; widely used for slopes and tunnels.

A projection of the orientations of planes and lines, typically plotted on a lower-hemisphere equal-area net, used to analyse joint sets and identify potential wedge or planar failures.

Assessment of whether a slope or excavation can physically fail by planar sliding, wedge sliding, or toppling, based on discontinuity orientations relative to the face and on friction angles.

Sliding of a block along a single discontinuity that dips out of a slope face at an angle steeper than the friction angle but shallower than the face angle.

Sliding of a block bounded by two intersecting discontinuities whose line of intersection daylights in the slope face and plunges more steeply than the friction angle.

Detachment and movement of a rock block defined by intersecting discontinuities, common in underground openings and rock slopes where joint sets create removable blocks.

The resistance of a soil or rock surface to sliding, expressed through cohesion and friction angle; for discontinuities it depends on roughness, infill, and effective normal stress.

A linear envelope relating shear stress at failure to normal stress through cohesion and friction angle, commonly fitted to soil and rock discontinuity test data for design.

Total stress minus pore water pressure; it controls consolidation, shear strength, and deformation in soil and rock, and is the basis for analysing slope and foundation stability.

The pressure of water within the voids or fractures of a geological material; rising values reduce effective stress and can trigger slope failure or internal erosion.

An instrument installed in a borehole to measure groundwater pressure at a specific depth, providing data for effective stress analysis, slope stability, and dewatering design.

A simple piezometer consisting of a perforated tube with a response zone sealed by grout or bentonite, in which the water level is measured manually or with a transducer.

A drilled hole used to recover samples, log ground conditions, install instruments, or conduct in-situ tests; its design and logging quality strongly affect the reliability of ground models.

Field measurement of ground properties in place, such as pressuremeter, dilatometer, or plate load tests, avoiding the disturbance that accompanies sampling and laboratory testing.

A borehole device that expands a cylindrical probe against the borehole wall while measuring pressure and volume change, giving stiffness and strength parameters of the surrounding ground.

A borehole test in which a split spoon sampler is driven by a standard hammer and the blow count per 300 mm recorded; the result indexes soil density and strength.

The capacity of a geological material to transmit water, governed by pore and fracture geometry; it controls seepage, dewatering, and contaminant transport around engineering works.

A measure of how readily a fluid flows through a porous or fractured medium under a hydraulic gradient, combining the medium's permeability with fluid properties.

A water-pressure test in a borehole interval, commonly used in dam foundations, that records the water take per metre under pressure to express rock mass permeability in Lugeon units.

Injection of cement or chemical grout into voids and fractures to reduce permeability and improve rock mass strength, often verified by water-pressure tests after completion.

The vertical succession of weathered zones from fresh rock upward through slightly, highly, and completely weathered material to residual soil, mapped to select founding levels.

Soil formed in place by weathering of bedrock and left above the parent rock; it retains relict structures and can be weak, collapsible, or prone to erosion.

Assessment of whether a natural or engineered slope will remain in place under given groundwater and loading conditions, usually expressed as a factor of safety against failure.

The ratio of resisting forces or moments to driving forces or moments along a potential failure surface; values above one indicate theoretical stability, with required margins set by consequence.

The system of rock bolts, shotcrete, steel arches, or lining installed to control deformation and maintain stability of an underground opening, selected using rock mass classification.