3.3.8 Quality and properties of rocks and rock masses General assessment

(1)P In assessing the quality and properties of rocks and rock masses, a distinction shall be drawn between the behaviour of rock material as measured on undisturbed core samples and the behaviour of much larger rock masses, which include structural discontinuities such as bedding planes, joints, shear zones and solution cavities. Consideration shall be given to the following characteristics of the joints:

  • spacing;
  • orientation;
  • aperture;
  • persistence (continuity);
  • tightness;
  • roughness, including the effects of previous movements on the joints;
  • filling.

(2)P In addition, when assessing the properties of rocks and rock masses, the following items shall be considered, if relevant:

  • in situ stresses;
  • water pressure;
  • pronounced variations in properties between different layers.

(3) Estimates of rock mass properties, such as:

— strength and stiffness,

— jointing, especially in fractured zones,

— water permeability of the joint system,

— deformation properties of weathered rock,

may be obtained by using the concept of rock mass classification described in EN 1997-2.

(4)P The sensitivity of rocks to e.g. climate or stress changes, shall be assessed. Consideration shall also be given to the influence of chemical degradation on the performance of rock foundations.

(5) In assessing the quality of rocks and rock masses, consideration should be given to the following features:

  • some porous soft rocks degrade rapidly to soils of low strength, especially if exposed to weathering;
  • some rocks exhibit high solution rates due to ground-water causing channels, caverns and sinkholes, which may develop to the ground surface;
  • when unloaded and exposed to the air, certain rocks experience pronounced swelling due to the absorption of water by clay minerals. Uniaxial compressive strength and deformability of rock materials

(1)P In assessing the uniaxial compressive strength and deformability of rock materials the influence of the following features shall be considered:

  • the orientation of the axis of loading with respect, for example, to specimen anisotropy, bedding planes, foliation;
  • method of sampling, storage history and environment;
  • number of specimens tested;
  • the geometry of the tested specimens;
  • water content and degree of saturation at time of test;
  • test duration and stress rate;
  • method for determination of the Young's modulus and the axial stress level or levels at which it is determined. Shear strength of joints

(1)P In assessing the shear strength of joints of rock materials, the influence of the following features shall be considered:

  • orientation of the joint within the rock test in relation to the assumed direction of actions;
  • orientation of the shear test;
  • number of specimens tested;
  • dimensions of the sheared area;
  • pore-water pressure conditions;
  • possibility of progressive failure governing the behaviour of the rock in the ground.

(2) Planes of weakness in rock normally coincide with joints or planes of bedding, schistosity or cleavage, or with the interface between soil and rock or concrete and rock. Measurements of the shear strength on these planes should normally be used for the limit equilibrium analysis of rock masses.

3.3.9 Permeability and consolidation parameters of soil and rock Permeability and consolidation parameters of soil

(1)P In assessing permeability and consolidation parameters, the following factors shall be considered:

  • the effects of heterogeneity;
  • the effects of anisotropy;
  • the effects of fissures or faults;
  • the effects of stress changes under the proposed loading.

(2) Permeability measurements made on small laboratory samples may not be representative of the in-situ conditions. Whenever possible, in-situ tests, which measure average properties of a large ground volume should therefore be preferred. However, consideration should be given to possible changes in the permeability with increased effective stress above the in-situ value.

(3) Sometimes permeability should be evaluated on the basis of knowledge of the grain size distribution. Permeability parameters of rock

(1)P Since the permeability of rock masses depends mainly on the degree of jointing and the existence of other discontinuities such as fractures and fissures, it shall be measured by appropriate in situ tests or evaluated from local experience.

(2) In situ permeability may be determined by a system of pumping tests combined with flow logging, with due consideration of the spatial, hydrogeological flow conditions around the structure and the mapping of the patterns of joints and other discontinuities.

(3) Laboratory permeability tests should only be used to study the effect of discontinuities, for example, in terms of variable aperture.

Eurocode 7 Geotechnical design Part 1 : General rules