Rock strength testing
Uniaxial compressive strength, softening coefficient, density and water absorption — for rock classification and design.
Licensed service · EL-845/2026
Rock is generally regarded as a reliable foundation, but rocks with the same name — limestone or sandstone, for instance — vary widely in strength and can lose strength when saturated. Laboratory tests establish the actual strength of the rock and allow it to be classified correctly.
Why rock is tested
The key parameter of rock and weak rock is uniaxial compressive strength (UCS). It underpins the strength classification of the rock, foundation design, the stability of cuttings and slopes, and the difficulty category of drilling and excavation. If the report contains only a visual description of the rock, the designer has to assume its strength conservatively — that is, low.
Weak rocks — marls, clayey sandstones, some limestones — lose a noticeable part of their strength when saturated. This is expressed by the softening coefficient: the ratio of strength in saturated and dry condition. If the rock beneath a foundation may become wet, the designer uses the saturated strength.
When it is required
- where foundations bear on rock or weak rock;
- in mountainous and foothill areas — for design of cuttings, retaining walls and slopes;
- when the thickness of the weathered zone and the change in strength with depth must be assessed;
- when planning the volume and difficulty of drilling and excavation;
- for a preliminary assessment of rock as a construction material.
Parameters determined
| Parameter | Method | Purpose |
|---|---|---|
| Uniaxial compressive strength (dry) | A regular specimen (cylinder or cube) is compressed in a press until failure | Strength classification, foundation design |
| Uniaxial compressive strength (saturated) | The specimen is saturated with water before testing | Strength under wet conditions, softening coefficient |
| Softening coefficient | Ratio of saturated to dry strength | Assessment of water resistance |
| Density and particle density | Measurement of specimen volume and mass, pycnometer method | Porosity, load calculations |
| Water absorption | Weighing the specimen before and after saturation | Open porosity and susceptibility to weathering |
| Point load strength | An irregular lump or core piece is loaded between two conical platens | Indirect strength estimate where regular specimens cannot be prepared |
Where needed, tensile strength (by the splitting method) and elastic modulus can also be determined. Test methods and specimen size requirements are selected in line with the applicable standards and codes. [YOXLA]
Samples required
- Rock core — cylindrical rock samples recovered by core drilling. Intact, crack-free lengths are selected for testing.
- Block sample — a piece of rock from a trial pit, cutting or outcrop; specimens are cut from it in the laboratory.
- For each rock type and weathering zone, enough samples are taken for statistical processing.
- Samples are packed to prevent drying and cracking; top and bottom, borehole number and depth are marked.
Core drilling is carried out as part of borehole drilling and sampling.
Workflow
- Sample selection. Suitable lengths are selected from core or blocks and the rock is described.
- Specimen preparation. Specimens are cut on a rock saw and the end faces are ground flat and parallel; dimensions are recorded.
- Saturation. Specimens to be tested saturated are kept in water until constant mass.
- Testing. Specimens are loaded in a press until failure; failure load and failure mode are recorded.
- Processing and delivery. Strength values and the softening coefficient are calculated, processed statistically and issued in a test report.
Equipment used
- rock saws and grinding machines — for preparing regular specimens;
- hydraulic compression presses — for uniaxial compression tests;
- point load tester — for irregular lumps and core pieces;
- callipers, balances, drying ovens and saturation tanks.
Deliverables
- table of strength, density and water absorption by specimen;
- softening coefficient and conclusion on water resistance;
- strength classification of the rock;
- statistical processing by rock type and weathering zone;
- PDF and hard copy.
Duration
The duration depends on the number of specimens, how difficult the rock is to cut and whether saturation is required. The exact timeframe is stated in the commercial proposal.
What we need from you
- sample list: borehole number, depth interval, rock description;
- required parameters (dry, saturated, etc.);
- brief information on the structure and foundation.
What affects the price
- the number of specimens and the effort needed to prepare them;
- test scope: dry only, or also saturated;
- additional tests such as point load or tensile strength;
- urgency.
Prices are on the laboratory soil testing prices page; for an exact calculation, send a quote request.
Ground conditions in Azerbaijan
The shell limestones and sandstones lying close to the surface on the Absheron Peninsula are variable in strength: strong layers often alternate with weakly cemented ones on the same site. In the foothills of the Greater and Lesser Caucasus, marls and clay shales occur, and their strength drops sharply when saturated.
