Avon Gorge A4 Realignment – Guidance Note

There are a number of essential items that should be include within any preliminary engineering geological assessment of rock cut slopes and a hard-rock tunnel. I have attempted to list these in this guidance note. This is guidance and as such, I would not expect every aspect to be included in your coursework for this assessment, however I have included a full coverage for completeness. Some of the concepts, such as developing a rock mass strength, will need some extra reading on your part and require the use of some Rocscience software. I will cover all the basic elements in the lecture series but you will need to read around the subject to consolidate your learning – I have recommended key texts to read throughout the Module, these link to the lectures and this coursework assignment.

Rock Cut Slopes

The objective is to establish the orientation and safe slope inclination for the tunnel portal slope face and the rock cut slope that will bound the road through the Great Quarry. Space is always an issue and the main design criteria is to develop as steep a slope as is safe. This reduces the amount of rock that has to be excavated and the amount of land take.
The development of a geological model for the slopes is essential and you have been provided with a full rock mass description with supplementary data such as discontinuity survey, roughness profiles of the discontinuities, Schmidt Hammer rebound values for the discontinuity surfaces, photographs, and a virtual fieldtrip to help you visualise the regional and site geology.
Hydrogeological model for the site – there is no provided groundwater data for the slope site apart from observations made during the virtual field trip.
A topographic section through each of the slope faces is a good start to help visualise what different cut angles would actually mean in terms of cut volume and land take at the crest (top) of the slope. You can create a section using Google Earth or by hand using a topographic map. You may be able to find LiDAR data which would also help in developing your sections.
A discontinuity survey has been carried out at the site and you have the data. You also have a detailed description of each of the main discontinuity sets. Using DIPS software, you will need to first identify the main discontinuity sets from the survey data – use the contouring application in DIPS program from the Rocscience Suite on AppsAnywhere to do this.
Once you have decided on cut slope orientation then a kinematic stability analysis is required to identify the potential for plane, wedge or toppling failure. To do this you will need to assign a friction value for the analysis. You can evaluate this from the data you have been given on discontinuity roughness and compressive strength from the field. You can do this by hand using stereonet projection or use the DIPS program from the Rocscience Suite on AppsAnywhere. During this process, you can evaluate what the effect on stability is by changing the slope inclination and orientation.
Potential failure modes then need to be analysed fully using the techniques developed in the lectures – limit equilibrium approach for instance. You can use the Rocscience programs: RocPlane, SWedge and RocTopple to help you analyse these failure mechanisms and to help you decide on how to stabilise any critical failure modes. You will need to develop discontinuity strength criteria for the critical discontinuity sets from the data given to you as input parameters together with slope geometry and critical surfaces.
This is only a preliminary design so at this stage the outputs would be: cut slope orientation and inclination (best shown on a figure or drawing); the identification of the critical failure modes; analysis of critical failure mechanisms and possible stabilisation solutions.

B. Rock Tunnel

1. Geological model – geological map and cross-section along the tunnel route;

2. Some attempt to consider the expected groundwater conditions, especially important as there is no information provided at this stage;

3. Engineering geological model – a cross-section that recognises that there maybe a difference in engineering behaviour along the tunnel route (zones), based primarily on a rock mass assessment. This model can be taken further to include preliminary rock mass material properties (eg. based on RMR) and preliminary support requirements. It is possible to indicate what excavation methods might be appropriate in different zones;

4. A key output from the report should an evaluation of intact and rock mass engineering characteristics, starting with geotechnical properties. These can be defined at intact and rock mass scale, drawing heavily on the work of Hoek & Brown, and covered in the lecture course. A table of geotechnical properties for the different zones should be provided. A difficult aspect to quantify is the discontinuity strength. Much of the required analysis can be done quickly in the RocScience programme RocData;

5. In situ and induced stresses must be evaluated. There are simple methods to estimate the pre-existing state of stress (vertical and horizontal), and these have been discussed in the lectures. The rock mass modulus will be required as will the Poisson’s ratio. Induced stress analysis starts with the Kirsch equations – there is a spreadsheet on Moodle that could be used for this, but there are simple charts that can also be used. The RocScience programme Examine 2D can also be used – this is based on the boundary element method, and is used to calculate stresses using elasticity theory (same basis as the Kirsch equations – so the ‘answers’ should be similar!). It is possible to take the analysis further by invoking a failure criterion (perfectly plastic Mohr-Coulomb, for instance), for which you need strength properties – intact or mass? This analysis is usually conducted to establish whether stress is likely to be an issue for the structure – i.e. does the predicted stress exceed the available strength in a particular location around the excavation and in a particular engineering zone;

6. A kinematic analysis is essential. This can be done in the RocScience programme Unwedge. The software can be used to consider a range of excavation shapes, and will indicate areas of potential instability, assuming sliding or falling failure mechanisms controlled by geological structures (orientation and discontinuity strength). Stress conditions can also be incorporated. Importantly, the analysis can be extended to consider support and reinforcement methods for those blocks/wedges that might fail – a starting point might be to prescribe the generic recommendations from no.3 above, and modify if required;

These issues are itemised as follows:

Need to establish geology along the line of the tunnel

longitudinal section based on geological map (England & Wales Sheet 106 BANGOR) and informed by associated memoir
need to comment on geological structures (regional) and the engineering significance of these:
- evidence of faulting (between units?)
- shear zones associated with faulting?
extract from geological sheet showing tunnel location is essential
some consideration of superficial materials – important at the portal areas

Groundwater conditions and inflow

see additional groundwater note, but if assuming a Darcy’s equations:
- ‘A’ must be defined in terms of the area of the excavated face (as tunnelling progresses) and the perimeter of the tunnel (unit length)
- permeability (k) must be estimated from literature for each unit – can use relationships between RQD and k
- hydraulic gradient assumes worst case of head difference equal to overburden thickness (topography?)
students should give some consideration to:
- recharge potential
- influence of fault zones on assumed permeability
- general influence of discontinuities (for instance, published k values are generally for intact rock)
- old mine workings (proximity to tunnel; hydraulic connectivity (existing and potential, due to excavation?)

Rock mass assessment using RMR and Q and the available data (assumptions may need to be made)

calculations must be presented (suggest in an appendix) with results tabulated and presented in main report body
engineering geological cross-section showing variation in rock mass quality (based on RMR/Q values) along tunnel section
- can different zones be defined along the tunnel alignment based on this?
- within the ‘model’ additional data can be incorporated, eg:
  - generic rock mass strength/stiffness
  - support/reinforcement requirements/categories
- groundwater conditions and level of risk? Drainage or waterproofing requirements?

Material characterisation at intact and rock mass scale (this entire process is done in the rock mass characterisation lectures, so there are slides and workshops that show this process from first principles)

intact rock data supplied, but incomplete:
- possible to complete this data-set using RocData (RocScience suite) to define σ_ci, c,  and σ_ti; c and  based on appropriate (and likely, approximate) m_i values and the σ_ci value (take s = 1 and a = 0.5 for intact rock, if using H-B failure criterion);
- GSI can be related to RMA (RMR/Q) though some literature search maybe required (it is discussed on slides in lecture course);
- key to completing the analysis is generating ‘synthetic’ triaxial test data
rock mass characterisation – strength and stiffness needed:
- use RocData to determine Mohr-Coulomb and Hoek-Brown parameters for intact rock (as above);
- if GSI and m_i are known, then H-B parameters for the rock mass can be calculated (m_b, s, a and σ_cm);
- to determine Mohr-Coulomb parameters then synthetic triaxial test data will need to be generated (really not difficult to do!) for the rock mass – this defines a relationship between σ₁ and σ₃ based on the H-B failure criterion using σ_cm;
- there are design charts that allow students to define M-C rock mass parameters using m_i and GSI, for comparison (lecture slides)
rock mass stiffness can be estimated from any number of a published relationships – most common ones use RMR – again, see slides on Moodle
discontinuity strength is difficult to quantify with any confidence. No data is supplied other than that available for the rock mass assessment. Plenty of data in the literature for shear strength of discontinuities in a range of different rock types. This property may be important as it is used to evaluate structurally controlled instability

In-situ and induced stress conditions

the pre-existing stress state can be readily estimated, and has been discussed in the lectures. The rock mass modulus will be needed for the Sheorey approach.
induced stresses can be evaluated based on elasticity theory, so students should use the Kirsch equations as a starting point
this can be extended to consider the excavation shape by using Examine 2D (RocScience)
in the lectures, the induced stresses could be directly compared with the material strength (tension, compression?) to predict whether stress induced failure might occur in the excavation boundary – within Examine 2D this can be done easily.
if stress induced failure is indicated, then students should recommend further action or investigation
also worth looking at Moji’s brittle/ductile transition – this gives an indication of whether material response will be brittle or ductile depending on the ratio of major to minor principal stress

Stability controlled by discontinuities must be evaluated and this may lead to different support or reinforcement mechanisms
- in lectures, this analysis has been conducted using stereonets (the ubiquitous method), but Unwedge can be used to provide a more comprehensive analysis by considering:
  - shape of excavation
  - stress regime
  - discontinuity strength
  - location (crown/invert or sidewalls – can be done with stereonets, but tedious)
- the software allows different support/reinforcement to be specified so that factors of safety, for instance, can be re-calculated. A useful starting point here is the RMA and the preliminary support requirements recommended by these.
The change in stresses caused by excavation may induce boundary deformations, in the form of strain. It is possible (see Hoek & Marinos, for example) to estimate boundary strains in different zones to establish whether excessive deformations are likely along the tunnel route.

the output can be presented as a graph along the line of the tunnel (strain v chainage?) as presented in the lecture slides

Dr Nick Koor

Reader in Geological Engineering