11.5.2 Slopes and cuts in rock masses

(1)P The stability of slopes and cuts in rock masses shall be checked against translational and rotational modes of failure involving isolated rock blocks or large portions of the rock mass, and also against rock falls. Particular attention shall be given to the pressure caused by blocked seepage water in joints and fissures.

(2)P Stability analyses shall be based on reliable knowledge of the pattern of discontinuities intersecting the rock mass and of the shear strength of the intact rock and of the discontinuities.

(3) Account should be taken of the fact that failure of slopes and cuts in hard rock masses, with a well defined pattern of discontinuities, will generally involve:

  • sliding of blocks or rock wedges;
  • toppling of blocks or slabs;
  • a combination of toppling and sliding.

depending on the orientation of the slope face in relation to that of the discontinuities.

(4) It should be considered that failure of slopes and cuts in highly fissured rock masses and in soft rocks and cemented soils may develop along circular or almost circular slip surfaces passing through portions of intact rock.

(5) Sliding of isolated blocks and wedges should usually be prevented by reducing the inclination of the slope by providing berms, and installing anchors, bolts and internal drainage. In cutting slopes, sliding should be prevented by selecting the direction and orientation of the slope face so that movements of isolated blocks are kinematically impossible.

(6) To prevent toppling failures, anchoring or bolting and internal drainage should normally be applied.

(7) When considering the long-term stability of slopes and cuts, the detrimental effects of vegetation and environmental or polluting agents on the shear strength of discontinuities and on the strength of the intact rock should be taken into account.

(8) In highly fractured rock masses in steep slopes and slopes susceptible to toppling, spalling, ravelling and slumping, the possibility of rock falls should always be analysed.

(9) In cases where reliable provisions to prevent rock falls are not feasible, rock falls should be allowed to occur with the provision of nets, barriers or other suitable provision to trap the falling rock.

(10) The design of provisions to trap rock blocks and debris falling down a rock slope should be based on a thorough investigation of the possible trajectories of the falling material.

11.5.3 Stability of excavations

(1)P The overall stability of the ground close to an excavation, including excavation spoil and existing structures, roads and services shall be checked (see Section 9).

(2)P The stability of the bottom of an excavation shall be checked in relation to the design pore-water pressure in the ground. For the analysis of hydraulic failure (see Section 10).

(3)P Heave of the bottom of deep excavations due to unloading shall be considered.

11.6 Serviceability limit state design

(1)P The design shall show that the deformation of the ground under characteristic actions will not cause a serviceability limit state in structures and infrastructure on or near the particular ground.

(2) Subsidence of the ground due to the following causes should be considered:

  • change in ground-water conditions and corresponding pore-water pressures;
  • long-term creep under drained conditions;
  • volume loss of deep soluble strata;
  • mining or similar works such as gas extraction.

(3) Since the analytical and numerical methods available at present do not usually provide reliable predictions of the deformation of a natural slope, the occurrence of serviceability limit states should be avoided by one of the following:

  • limiting the mobilised shear strength;
  • observing the movements and specifying actions to reduce or stop them, if necessary.

11.7 Monitoring

(1)P The ground shall be monitored using appropriate equipment if:

  • it is not possible to prove by calculation or by prescriptive measures that the occurrence of the limit states given in 11.2 is sufficiently unlikely;
  • the assumptions made in the calculations are not based on reliable data.

(2) Monitoring should be planned to provide knowledge of:

  • ground-water levels or pore-water pressures in the ground, so that effective stress analyses can be carried out or checked;
  • lateral and vertical ground movements, in order to predict further deformations;
  • the depth and shape of the moving surface in a developed slide, in order to derive the ground strength parameters for the design of remedial works;
  • rates of movement, in order to give warning of impending danger; in such cases a remote digital readout for the instruments or a remote alarm system may be appropriate.

Eurocode 7 Geotechnical design Part 1 : General rules