Environmental & Engineering Geology Flashcards
What intrinsic factors affect tunnel stability?
- Rock is not a manufactured material
- Discontinuous
- Inhomogeneous
- Anisotropic
- Nonlinear elastic
- Rock type - intact strength
- Rock structure - fractures (orientation etc) / spacing
- Condition of discontinuities
- In-situ pore water pressure
What factors relating to tunnel setting affect tunnel stability?
- In-situ stress (depth, tectonic setting etc)
- Tunnel orientation wrt discontinuity sets
What factors relating to design and construction/support affect tunnel stability?
- Shape and size of tunnel opening etc
In the context of tunnelling, what are the rock mass characteristics that should be considered?
- Intact rock strength
- RQD (Rock Quality Designation)
- Joint spacing (of joint sets)
- Condition of joints
- Groundwater
Plus
- Fracture orientation
- State of in-situ stress
(note: each parameter is weighted differently according to its importance)
What is RMR?
The Rock Mass Rating (RMR) classification system takes 5 or more important characteristics (i.e. parameters) known to influence overall strength.
Each parameter is assigned a rating and these are summed to give an overall RMR score or Index out of 100.
Define RQD
Rock Quality Designation (RQD) is an index of the ratio between the sum of the lengths of core fragments longer than 100 mm and the total length of the Core run
Give the equation for Rock Quality Designation (RQD)
What are some criticisms of RQD?
- RQD is conveinient as it is obtained from core samples from a rotary drill, however artifically indiced breaks can be hard to distinguish.
- It can be argued that defining this indicator of fracture scarcity is arbitrary in using 100 mm; why not 200, or 50 mm?
- The RMR uses RQD and the average fracture spacing as a seperate parameter and so will tend to count the influence of fracture density twice!
What are the effects of making a hole underground?
i.e. what are the effects of excavation on stress and groundwater?
- Rock moves inwards - bc material is essentially elastic (creates a ‘squeeze’ issue on rock boring machines)
- Stress refraction: Normal & Shear stresses on the excavation wall are zero
- Excavation acts as a sink if it’s a porous, fractured rock mass - bc pressure in excavation is reduced to atmospheric, fluids flow into excavation under pressure gradient
What parameter is the most heavily weighted out of the RMR classification parameters?
Condition of discontinuities, with a rating of 30
What parameter is the least heavily weighted out of the RMR classification parameters?
Strength of intact rock material
or
Ground water,
both w/ rating of 15
The RMR system utilizes which six rock mass parameters?
- Uniaxial compressive strength of intact rock material
- Rock quality designation (RQD)
- Spacing of discontinuities
- Condition of discontinuities
- Groundwater conditions
- Orientation of discontinuities
The ‘Condition of discontinuities’ parameter of the RMR system is split into what sub-parameters?
- Length, persistence
- Separation
- Smoothness
- Infilling
- Alteration / weathering
The rating of each of the RMR parameters are summarized to give a value of RMR.
But how are the parameters measured?
All RMR parameters are measureable in the field and can also be obtained from borehole data.
How is the RMR classification applied to a tunnel build project?
- The rock mass along a tunnel route is divided into a number of structural regions, i.e. zones in which certain geological features are more or less uniform.
- The 6 RMR classification parameters are determined for each structural region from measurements in the field.
- Once the classification parameters are determined, the ratings are assigned to each parameter according to the RMR Table. In this respect, the typical/average/overall conditions, rather than the worst (or every detail of) conditions, are evaluated.
- F.m. it should be noted that the ratings, which are given for discontinuity spacings, apply to rock masses having three sets of discontinuities. Thus, when only two sets of discontinuities are present, a conservative assessment is obtained.
What does this figure show?
Stand-up time as function of unsupported span and RMR values
- An interpretation of time and span can be made from RMR class number due to experience of many projects and a lot of data
- Also: Continuum properties (cohesion of rock mass, friction angle of rock mass) can be approximated, taking the assumption that the whole rock mass behaves homogeneously (average/overall properties)
What support in rock tunnels is suggested for a rock mass class stated ‘very good rock’, with an RMR score of 81-100?
- Generally no support required except for occasional spot bolting
What support in rock tunnels is suggested for a rock mass class stated ‘good rock’, with an RMR score of 61-80?
- Locally bolts in crown, 3 m long, spaced 2.5 m with occasional wire mesh
- 50 mm shotcrete in crown where required
What support in rock tunnels is suggested for a rock mass class stated ‘fair rock’, with an RMR score of 41-60?
- Systematic rock bolts 4 m long, spaced 1.5-2 m in crown and walls with wire mesh in crown
- 50-100 mm shotcrete in crown, and 30 mm in sides
What support in rock tunnels is suggested for a rock mass class stated ‘poor rock’, with an RMR score of 21-40?
- Systematic rock bolts 4-5 m long, spaced 1-1.5 m in crown and walls with wire mesh
- 100-150 mm shotcrete in crown, and 100 mm in sides
- Light steel set ribs spaced 1.5 m where required
What support in rock tunnels is suggested for a rock mass class stated ‘very poor rock’, with an RMR score <21?
- Systematic rock bolts 5-6 m long, spaced 1-1.5 m in crown and walls with wire mesh. Bolt invert
- 150-200 mm shotcrete in crown, and 150 mm in sides, and 50 mm on face (cutting face)
- Medium to heavy steel set ribs spaced 0.75 m with steel lagging and forepoling if required. Close invert
Tunneling is normally done between 0-500 m depth.
What is the dominant stress at this depth?
Average horizontal stress is normally higher than vertical stress between 0 and 500 m i.e. normal Civ Eng Tunnel Depths
Discuss the importance of the prevalent stress regime in relation to tunnels?
- Must know the depth to which a particular tectonic stress regime acts
- Affects how support around the tunnel is going to be designed
- What’s the stress across the tunnel, + along the line of the tunnel
- Stress regime is going to change across the tunnel section (consider channel tunnel example)
What if a tunnel opening was rectangular and had corners - what affect would this have on the wall stress magnitudes?
- Firstly, the stress has to refract around the hole
- Therefore, even in a cylindrical tunnel, there is a high localized stress on the tunnel walls, partiularly at the crown on the tunnel, or in the wall opposite to σ3.
- Stress is amplified bc of the stress concentration in these areas.
- In a rectangular tunnel, stress concentration is near/just below the corners, and is greatly amplified - with a magnitude 6.8 x greater than the original horizontal stress.
What is shown in the image below?
The effect of an excavation intersecting a pervasive planar rock fabric, i.e., foliation, on the potential location of rock failure at the excavation periphery. This potential is a function of both the magnitude of the rock stress acting parallel to the excavation boundary and the orientation of the foliation relative to the excavation boundary, as indicated by the white arrows and the shading in the individual rock samples, respectively
What is the optimal orientation of tunnels to avoid damage caused by stress concentrations?
The optimal orientation of tunnels to avoid damage caused by stress concentrations is parallel to the maximum principal stress component, i.e., as in the right-hand sketch.
The stress concentration around the tunnel in the left-hand sketch will be higher than that in the right-hand sketch
What are the two main causes of failure in rock engineering?
How can these be mitigated?
Adversely high rock stresses and the movement of rock block
Key block theory (kinematic analysis) allows joint mapping to try to get good representation of discontinuities;
Here, you can map out most problematic part of structure from a point of view of sliding and falling blocks (w/ stereographic projections)
Re: discontinuity properties and assessment, how might one be objective about spacing?
scanline mapping
What’s the persistence, trace length etc?
When assessing discontinuities, what properties must be made note of?
- Length, persistence (how many sets etc)
- Seperation
- Roughness
- Infilling (gouge)
- Weathering (wall strength)
Also:
- Aperture
- Seepage rates
Software can be used to stereographically project discontinuity data. This can help with what?
- Deriving block distributions
-
Kinematic analyses in tunnels (just like for rock slope failure problems)
- Discontinuity sets (bedding and joints) can be plotted on a stereonet and considered in terms of kinematic stability and sliding into the tunnel
What tunnel orientation relative to the strata is more favourable?
More favourable for tunnel to run perpendicular to the strata than for the tunnel to run parallel to strike.
If tunnel runs parallel to strike, it can lead to problems.
What is the benefit of using RMR classifications, other than safety?
- More economic to know that there’s some good rock (e.g. dolomites) that doesn’t require expensive support
- Also get an idea of how difficult the rock is going to be to excavate.
Is RMR the only classification system that can be used for tunnel support design?
- No
- Q-Index
- Tunnel engineers tend to prefer the Q-index System for rock mass classification
What’s the difference between RMR and Q-Index?
- The Q-Index is obtained by multiplying together three ratios (not the sum of 5):
- Block size
- Shear strength between blocks
- Influence of state of stress
- The interpretation chart for Q-Index is more practical and offers suggested reinforcement categories
How are Numerical Models used in tunneling?
- Continuum approach
- RMR numbers used to generate failure envelopes that can be put into software that can predict:
- Deformation
- Squeezing of tunnels
- What support is needed
- Etc.
What determines whether continuum or discontinuum approaches are used in numerical modelling in tunnelling?
- Size of domain controls how many fractures we’re going find (domain size defined in numerical model)
- Scale of interest tells what type of technique that can be used (continuum or discontinuum)
Give a summary of the empirical approach to tunnel construction.
- Perform Site Investigation for proposed tunnel of given cross section shape
- Characterize the geology and important rock mass properties
- Perform kinematic stability prediction from discontinuities - key blocks
- Evaluate zones of tunnel support from RMR or Q class (along tunnel sections)
- Consider tunnel excavation methods available and support
- Design portal and or shaft access
- Begin exploratory tunnel construction and adopt forward borehole survey
- Update RMR with continuous mapping, monitoring and data analysis
- Drill forward boreholes for grout injection if to seal and minimise water seepage
- Advance excavation-construction with support
- Adapt to changing geological conditions during advance
- Monitor
In recent years a number of high‑latitude permafrost regions have begun to thaw due to increased average air temperatures. As successful construction in these environments depends on preservation of the permafrost, it is likely that infrastructure in regions like Alaska will be adversely affected by such climatic change.
Sketch a typical North‑South cross section through a high‑latitude region showing the spatial relationships of relevant features. Include those geological features indicative of the freeze/thaw processes taking place in such an environment and explain breifly how they form.
- Permafrost currently covers 20% of Earth’s land surface; Active layer (1-3 m thick) thaws during summer; Impearmeable subsoil prevents drainage → waterlogging and flow.
- iw = ice wedge,
- il = ice lens
- Freezing of water → expansive forces → shattering of frozen material → ground heave.
- Pingos (h→50m; Ø→300m) have been unheaved by intrusions of ice; usually deeply fissured, may have crater in top.
In recent years a number of high‑latitude permafrost regions have begun to thaw due to increased average air temperatures. As successful construction in these environments depends on preservation of the permafrost, it is likely that infrastructure in regions like Alaska will be adversely affected by such climatic change.
The Dalton Highway in Alaska extends North from Fairbanks almost 800 km to Prudhoe Bay. Much of the highway crosses the Arctic Coastal Plain where polygonal ground is commonly encountered.
How does polygonal ground form, and what are its principal features both during and after formation?
- Polygonal ground is created by nets of ice wedges;
- Occurs where permafrost is over 60% ice.
- Typical polygons are 15-75 m across, bounded by almost straight ice wedges each 10-40 m long between 3 or 4 point junctions.
- A uniform distribution of ice wedges creates polygons that tend to hexagonal.
- Many polygons have no surface expression where they are stable or growing slowly in undisturbed frozen ground. Others are visible as slight ridges or vegetational contrasts over growing ice wedges.
- Polygons are most conspicuous where their ice wedges are thawing, to create meltwater troughts over them.
- Any slight melting of the permafrost creates a hollow that then fills with water.
- Summer water levels stand below the level of the frozen ground, and each ice wedge is melted back at that level, to form a cave, that then collapses to create a gully - revealing the network of ice wedges (Fig. 10).
Where else are you likely to see polygonal ice wedges, like those in the Arctic Costal Plain?
Polygonal nets characterize periglacial lowlands, and East Anglia has many that are relics from the Pleistocene.
The Dalton Highway in Alaska extends North from Fairbanks almost 800 km to Prudhoe Bay. Much of the highway crosses the Arctic Coastal Plain where polygonal ground is commonly encountered.
The presence of the Dalton Highway has dramatically affected the polygonal ground over which it passes. Describe this situation.
- Cut slopes were initially unstable and unsightly as the frozen ground thawed and collapsed, but after about five seasons the slopes had slumped to stable profiles with established vegetation, and the permafrost was preserved behind the new organic mat.
- Most modifications to the road have been modest drainage features to eliminate inter accumulations of ice.
- E.g. the road crosses a slope of frozen soliflucted till.
In areas of frozen ground and periglacial conditions, construction may cause a particular problem to the landscape.
Describe this kind of problem, paying attention to its cause and nature, and when and why they occur. How might a natural analogue of these conditions form?
- Solifluction is downslope movement of saturated debris as a viscous flow, 10 mm/a > v > 300 mm/a.
- Most significant form of mass movement in tundra.
- Solifluction deposits are poorly sorted, poorly bedded, display fold structures, t→3m.
- Tend to be found at the foot of slopes, are easily reactivated by drainage changes, erosion, construction.
- Called head in the UK;
- Usually weak, compressible, permeable, if derived from chalk, can be weakly cemented with CaCO3.
What features of the polygonal ground formed in Southern England during the Pleistocene, should geotechnical engineers be alerted to, in order to determine whether or not it suffered solifluction?
Solifluction lobes
(e.g. Dartmor)
What effects can a glaciation have on the landscape?
- Frost splitting d→30 m in chalk of SE England.
- Hill creep/solifluction influenced by valley sides in frozen material being over-steepened.
- Valley bulging caused by removal of weight of ice.
- Rockhead may be unrelated to modern topography; depth to r/h may be irregular.
- Buried channels may lie beneath or be unrelated to modern valleys; irregular and variable depth/width.
Draw a suggested mechanism for Valley Bugle situation.
What is a periglacial environment?
cold climate; non-glacial with ground freezing
What is permafrost?
Ground at or below 0°C for > 2 yrs:
Can occur from cold air and winter snowfall.
What is the active layer?
Sees annual freezing and thawing.
Thickness of AL is ~cm scale (near pole) to ~10 m in doscount permafrost regions.
What is the ‘transition zone’ in periglacial regions?
Uppermost permafrost - much segregated ice in lenses.
Much evidence in Kent.
What is ground ice?
Will be in the form of pore, segregated, wedge and lense.
Wedge ice leaves vertical ice laminae where single thermal contraction crack occurs.
What weathering processes take place in periglacial landscapes?
- From features seen in Kent:
- Freezing and thawing - in situ expansion on freezing, thermal suction and segregation leads to fracturing - brecciation.
- Type 1: Angular fragments with infilled fractures having matched sides. By ice wedging, fewer F-T cycles.
- Type 2: Subangular to rounded lumps - lithorelics in a chalky matrix lacking stratification. See infilled dray valleys at Pegwell Bay. Often grades downwards into..
- ..Type 3, by many F-T cycles and possibly some ground movement; e.g. seen folded flint bands.
What peri-glacial processes formed the landscape we see in Kent today?
- Weathering processes
- Cold-climate aeolian processes and deposits
- Deformation processes and structures in soil and rock
- Slope processes and deposits
- Cambering strata and widened vertical joints (gulls)
What peri-glacial cold-climate aeolian processes and deposits do we see example of in Kent?
- Loess - wind blown mainly of coarse silt size, trapped by vegetated cover.
- Often find calcareous tubes marking positions of roots, clays help bind.
- BGS call it ‘brickearth’ or ‘head brickearth’.
- Several m thick.
- Hydroconsolidation is possible.
What peri-glacial deformation processes and structures in soil and rock do we see example of in Kent?
- Periglacial Involutions or Cryoturbations are structures caused by repeated frost action processes deforming the unconsolidated soils or breccias.
- Flame-like structures caused by soft sediment during active layer deepening.
What peri-glacial slope processes and deposits do we see example of in Kent?
- Mass wasting, in the form of solifluction (soil flow) of ‘head’ deposits - deposits that are periglacially derived from underlying weathered bedrock moving on slopes.
- Porewater pressure increases immediately after ice melts often enough to initiate sliding.
- Gravelly sediments may accumulate near bottoms of valleys.
How are the cambering strata and widened vertical joints (gulls) that we see in Kent formed?
- Large-scale flexing and stretching of competent caprocks over the upper parts of valley side slopes lead to beds dipping towards valley floor and blocks tilting.
- Probable mechanism is incompetent mudrock beneath harder beds flowing towards valley.
- In valley itself where river has cut through competent layer to reveal mudrocks and clays, greatest deformation is in lowest point of valley.
- Gulls are widened by tilting of blocks in strata next to deeply incised valleys unsupported on downslope sides, e.g. High Weald (Ardingly Sst.)
How might one decide what types of geomaterials are appropriate for use in the chosen construction?
Think about loads, stability, performance
Identifying the functional requirements of your construction project will lead you into thinking about what geomaterials are needed
How would one figure out how much geomaterial is needed for a given project?
- Detailed and average cross-sections and plans lead to the bulk volumes required.
- A range of different packing densities will affect quantities
What principles are applied when designing a particular project?
The consulting engineering company (structural, geotechnical and coastal expertise as a given) applies soil mechanics, rock mechanics, coastal hydrodynamics, codes of practice etc. to arrive at an optimal design that is dimensioned and with the geomaterials types specified.
i.e. from design you get sense of shape (width, height, slope of sides, etc) and t.f. cross-sectional area - then define length = volume
** from CSs and plans you can calculate volumes of geomaterials needed
What would be the functional requirements of this project?
- West Bay (Dorset) being eroded
- Create harbour protection (to let boats in and out safely during rough seas; make calm water behind breakwater), trap beach materials (prevent erosion), provide amenity beach etc
- Design plan shape, profile shape, to resist loads from wave and storm action (e.g. breakwater needs to be strong enough so…)
- Calculate volumes needed, define construction materials requirements and source them -> Engineering Geology
While it is the consulting engineering company that is responsible for the design of a civil engineering construction project, it is the pieces of rock that make up the construction materials that eventually deliver the functional requirements of the construction project to the client.
Draw a labelled flow diagram that explains the geomaterials properties to be controlled and tested at the subseqent different stages in the life of a construction project.
A conceptual understanding of properties and functions of any geomaterial used in construction (such as gabion stone, recycled materials, concrete units, grouted stone, and the many and varied applications of aggregates and armourstone) is given by the scheme, illustrated here for armourstone.
In relating properties to functions, it is useful to distinguish the three types of property that relate to different phases in the life cycle of armourstone, for instance.
What are the three types of property?
- Intrinsic
- Production-induced
- Construction-induced
- (System response under loads)
Undertaking these steps is important to end up with the right rocky material for the functional requirements of your project!
In relating properties to functions, it is useful to distinguish the three types of property that relate to different phases in the life cycle of armourstone, for instance.
Describe what is meant by the ‘intrinstic’ properties of a geomaterial.
- Relate to the rock source properties, its geological history or the industrial process involving thermal or other modification.
- They account for engineering geology considerations such as:
- mineral fabric characteristics (mineralogy, density, intact strength, Young’s modulus etc)
- discontinuity sets (spacing etc)
- weathering grade (thickness of joints, colour staining etc)
- Colour
- tectonic context of the quarry
- Choose by fitness-for-purpose (by above) and transport cost as well.
In relating properties to functions, it is useful to distinguish the three types of property that relate to different phases in the life cycle of armourstone, for instance.
Describe what is meant by the ‘production-induced’ properties of a geomaterial.
- Properties of geomaterial following excavation
- Relate to the armourstone as an individual piece or as a granular material composed of individual pieces.
- They are affected by the intrinsic properties and controlled by the production technique/affected by excavation process such as:
- blast design,
- selection,
- handling and sorting techniques or devices
- Controls block integrity, shape (expressed as ratio; slab-shapes break more easily than cuboid-shaped rocks), size, size distribution (grading)
- Size distribution determined after stockpiling
In relating properties to functions, it is useful to distinguish the three types of property that relate to different phases in the life cycle of armourstone, for instance.
Describe what is meant by the ‘construction-induced’ properties of a geomaterial.
- Such as layer thickness or layer porosity (packing density), permeability, are controlled by the construction of the armourstone as a granular material and are..
- Heavily influenced by the placement technique, the shape and the conditions of execution, for example above or below water.
- e.g. Shear strength as a granular material depends on layout (porosity, interlocking?), size etc - e.g. low porosity with slab-like shape, w/ gaps only 20% of whole vol. (other shapes nearer to 40%).
- Important for wave-energy dissipation.
- Each property is also susceptible to change with time as a function of loadings from the physical, chemical and biological environment.
- These are considered further in terms of durability.
The road network of 345,000 km in the UK is at least 95% constructed with at least one layer of material consisting of bituminous mixes (asphalt and/or macadam).
What are the characteristics of asphalt?
- Dense mortar providing strength and stiffness
- High bitumen content
- High filler/fines content
- Low coarse aggregate content
- Load transmitted through mortar
The road network of 345,000 km in the UK is at least 95% constructed with at least one layer of material consisting of bituminous mixes (asphalt and/or macadam).
What are the characteristics of macadam?
- Well graded aggregate giving dense stable aggregate structure
- Low bitumen content
- Load transmitted through aggregate structure
With aid of a diagram, describe the cross section of a motor way road.
For granular geomaterials, define void ratio, giving the equation.
e = VV / VS
e = n/(1-n)
(Volume of void over volume of solids within an aggregate body).
Gives indication of the porosity (and permeability).
In the image, void = liquid + gas (non-solid).
For granular geomaterials, define porosity, giving the equation.
Porosity is governed by what?
n = VV / Vt
n = e/(1+e)
where e = void ratio
Volume of void over total volume of aggregate body.
Porosity (and permeability) is governed mainly by particle geometry i.e. sizes and shapes of particles.
