Ground Models For Tunnelling Applications Tom Robinson Engineering Geologist London Bridge Associates Ltd

Warwick University M.Sc. Course 2014Geological Investigation & Ground Characterisation.

Ground Models For Tunnelling Applications

Tom Robinson Engineering Geologist

London Bridge Associates Ltd.


What is a ground model?A ground model is a visual summary of the ground conditions likely during tunnel construction. These can be general or very specialised:

• Geomorphological models – how do landforms affect tunnel route?

•Lithological models – what rock/soil materials are present?

•Hydrogeological models – how does water affect tunnel construction?

A ground model is more than just a cross-section. It should give a 3D picture of the underlying ground.


How to build a ground model – Desk Study 1

First consider what information you know from your design brief:

• Where is the proposed tunnel alignment?

•How many shafts will there be and where are they located? Do these locations seem sensible?

•What is the topography? Does the topography suggest obvious geotechnical hazards such as landslides?


What do we know from the start?

• Tunnel Alignment

•Shaft Locations

•Topography

??



Step 2: What do we know about the subsurface?

• Is there a geological map of the area? In the UK, there will be, but otherwise we may need to map the area ourselves...

• Does the geological map tell us about drift/superficial deposits? Not always...

• Are there any pre-existing boreholes? Maybe, if the area was/is populated...


Specialised geological maps can often be found in the literature – often with subtle differences to the BGS regional memoirs.

If published data exists, use it!

Reproduced from Whitworth et. al. 2000


Overlaying Digimap solid and superficial geology allows us to estimate horizons at depth.

Surface geological boundaries follows topography:

-So geological units should be flat lying.



Step 3 - Consult geological and engineering literature concerning site. If nothing exists, can help to ask the locals/client.

• What is the record of industrial/human activity at the site?

1. Look at historical maps. How has land use changed?

2. Are there deep water wells or piled foundations along tunnel alignment?

3. Have tunnellers had problems in the area before?


Formation Description Surface morphology

Marlstone Rock Formation

Strong brown closely jointed oolitic and fossiliferous LIMESTONE.

Cap rock to midslope lithological bench (170–180 m OD).

Dryham Formation

Moderately weak orange brown SANDSTONE and SILTSTONE with subordinate bands of SILT and CLAY.

Steep scarp faces below the lithological bench. Occasional large rotational landslides (Colliers Knap).

Charmouth Mudstone Formation

Dark grey CLAY with occasional bands of argillaceous limestone. Grades at depth into weak mudstone.

Foot slopes and valley base. Mantled by superficial deposits and solifluction deposits.

Engineering geological literature contains information about

strength, jointing, and geomorphology.

Most geological literature emphasises stratigraphy,

not lithology.

The two need not be the same thing.

Modified from Whitworth et. al. 2005


Overlaying an Ordnance Survey Map from the 1950’s shows little has changed in the area since.

But it marks the location of springs, giving some idea of the groundwater regime 60 years ago.

This can give you an idea about whether abstraction has occurred.


Overlaying an Ordnance Survey map from the 1890’s show still very little has changed.

This is a rather rare state of affairs in the UK, especially in urban areas!

Note Farncombe House existed back in the 1890’s:

-Did they know back then that they had built on the crown of a major landslide?

-If yes, would they have records of landslide activity?


Bear in mind the Victorians had a rather interesting way of describing borehole logs:


•Blue Clay? – Probably refers to the London Clay A3 or A2 horizons

•Rock Chalk? – Does this refer to the white chalk or something else?

•Green coated flints? – Probably refers to the Bullhead Beds.

I have had to interpret borehole logs with all these terms.

If only all borehole logs were this good!


How to build a ground model – Walkover Survey 1

So far, we have conducted what is called a desk study. We haven’t gathered any new data.

Now we need to visit the site, and confirm our findings from the desk study. This is known as a walkover survey, and is crucial to the ground model. Ground models always change with new evidence!

e.g. If the literature suggests deep seated rotational landslides with slip surfaces through tunnel alignment – do the gradients of geological strata in the field confirm this?


Collier’s Knapp – flat ground at landslide crown

Strong brown closely jointed, horizontally bedded LIMESTONE acting as lithological bench

Moderately weak SANDSTONE and SILTSTONE

Limestone is dipping at 20° to east:- Good evidence for deep seated rotational landslide

Curved landslide scarp and toe: - Suggests deep seated rotational landslide

Schematic Model of the Collier’s Knapp Landslide

New road runs through toe:- Is excavating the toe a good idea?



• Do you notice anything on site that could cause a serious risk to tunnelling:

1. Is there a severe flood risk to the site? Is the surface or groundwater highly acidic, for example due to mining?

2. Have ground conditions changed since literature published? Has site since been built upon – is there made ground?

3. Can you physically get heavy plant to the site? Is there space to assemble TBM or for SCL works? Does the site constrain how you remove spoil from site?


Overlaying a modern Ordnance Survey map tells you about road access to site even before the walkover survey.

Do you think bringing heavy SI plant to this site e.g. a rotary drill rig, will be a problem?

Are groundwater springs in the same place as they were 60 years ago?



Can we take any basic measurements in the field, that do not require plant or special equipment to be brought to site?

1. For both hard and soft ground, describe the rocks/soils you see onsite (using standard geological descriptions)

2. For rocky ground, this description includes measurement of discontinuities (dip/dip direction, persistence, aperture, etc.)

3. Your description may be different than the desk study. If it is different, there is every possibility you are right, as you have described the rock/soil in that location.



What we can learn from a walkover survey can depend on the conditions on the day, so we may need to prioritise:

1. Make sure you record/measure any in-situ structure (faults/folds, bedding/foliations etc) in the field. You can interpret these measurements using stereonets later.

2. Take samples and photographs of the outcrop. You can describe the material afterwards if need be.

3. But don’t go too far! Conditions permitting, it is far better to both take measurements and describe the rock in-situ.


How to build a ground model – Site Investigation

How can we use SI data in the ground model?

• After the walkover survey, subsurface horizons are either extrapolated or interpolated. SI boreholes constrain what each lithological horizon is doing at depth.

• The walkover survey can give the locations of groundwater springs, but with the SI boreholes you can determine the position of the water table.

• Is all this data enough for the model? Do we need to go back and do more SI?


When is the ground model complete?

The simple answer is never! throughout construction you will be getting more data about the ground, especially if you probe during tunnel advance.

But after the final SI, we need to make sure there is enough information for the contractor to build the tunnel. Don’t exclude data without very good reasons!

Keep a record of the ground models you make, and sign and date them. This is why I often still draw my ground models by hand. In court, it’s far harder to argue with original models you made before construction...


Reproduced from Fookes (1997)

Some lessons from a master of ground modelling...

Some of the ground variability you might see in the humid tropics.

Here highly weathered rock is very important at tunnelling depths...


But is the master infallible?

Professor Fookes’ ground models are undoubtedly works of art and contain a great deal of valuable information for the tunneller and foundation engineer:

• But there isn’t much quantitative data e.g. UCS/Cerchar Abrasivity.

• e.g. if we have borehole data recording rising piezometric levels, we can estimate the water ingress rate into the tunnel.

•What about the direction of groundwater flow? Can we tell this from the presence of sand lenses, and buried channels or eskers?


A ground model I made for a hypothetical tunnel through a previously glaciated system and misfit river valley.

The processes that form the landscape, while fascinating, are often of little use to the tunnel designer or contractor, but data and numbers are...

A ground model for a glacial system


A short summary so far

Ground models provide 3D data about the ground to the tunnelling engineer.

Start off with a desk study, using all the available data from literature and maps.

The walkover survey should test the initial ground model, and inform the SI. The SI provides borehole and other evidence for our previous assumptions about subsurface geology.

A ground model is never truly completed, even after construction (monitoring etc.). It is your best guess at the time.


Scenarios to think about...

Scenario 1:

• A shallow pipe-jack tunnel is to built, with a minimum ground clearance of 10m and an internal diameter of 3m.

•The area is mixed former industrial buildings and housing, and 200m of the route is under a derelict gasworks.

• The tunnel is to be used for a drinking water ring main.

What additional factors should we consider?


Scenarios to think about...Scenario 2:

• A highway is to built through a high mountain range, consisting of a series of 12m ID tunnels and interconnecting viaducts.

•The area has had little historical settlement, due to the steep topography and high active seismicity of the area.

• The tunnelled sections cross a combination of metamorphosed sedimentary rocks and andesite volcanics. At least one of the tunnel portals is located in a former quarry for gneiss and schist.

What additional factors should we consider?


Scenarios to think about...

Scenario 3:

• A small (2id) microtunnel is to built to connect a sewage treatment plant to an existing combined sewage outflow.

• The final 400m of the tunnel are on flat ground, with the first 200m on steep forested hillside.

• The area is humid tropical and flat ground consists of deep rockhead with overlying latasols hydraulically connected to a river.

What additional factors should we consider for our desk study and SI?


Scenarios to think about...Scenario 4:

•A 3km long, 7.5m ID railway tunnel with a max. invert depth of 30m is to built through a densely populated urban area in the north of England.

•The area has a 500 year history of underground coal mining. Surface industries including coking plants for nearby steelworks and other heavy industry.

•The bedrock geology is coal measures sandstone for 2/3 of the route, faulted against Jurassic Kimmeridge Clay for the remainder of the route.

What additional factors should we consider for our desk study and SI?


Thank you for listening

Any further questions?


Optional Site Investigation Slides


How to build a ground model – Site Investigation 1

The site investigation (SI) should be done after the walkover survey, and should be a test for our ground model.

The walkover survey should have determined what types of SI (intrusive/non-intrusive) are possible, necessary and appropriate.

E.g. Can you think of any cases where intrusive site investigation may be inappropriate? What alternatives exist?


How to build a ground model – Site Investigation 2

The vast majority of site investigations for tunnelling are intrusive, but always bear in mind what properties you are trying to investigate:

• For rock tunnels, you are interested in the properties of the rock mass, rather than the rock material. Groundwater flow will normally be along discontinuities.

• For soft ground tunnels, you are normally interested in the material properties of the engineering soil. You are also interested in the effects of pore water pressure and the effective stress in the ground.

• Design your SI accordingly!


How to build a ground model – Site Investigation 3a

The SI for almost every tunnel will specify borehole drilling, but care is needed when specifying the drilling technique:

• Cable Percussive – Good if you do not expect to penetrate rockhead. Allows use of SPT and CPT with the same rig. Cheapest method of coring.

Reproduced from http://www.driftgi.co.uk/services 21/10/2011


Rotary Drilling is more expensive but allows you to drill through rock. Often continued from CP in same borehole.

Rock Probing is similar to rotary drilling but destroys sample. It is a good technique to use at the tunnel face.

Be very wary if you get asked (e.g. In a GBR) to core or SPT at the tunnel face. Why?

Reproduced from WG Groundwater website on 21/11/2012 http://www.wjgl.com/node/134

How to build a ground model – Site Investigation 3b


How to build a ground model – Site Investigation 3c

What about non-intrusive site investigation involving geophysics? When is it useful?

• If you cannot physically get plant to site, e.g. the entrance to your tunnel is on a steep slope.

•If your tunnel runs through somewhere it is difficult to drill, e.g. under a cemetery.

• If we really need continuous data along the alignment, e.g. If we suspect boulders or sand channels.


How to build a ground model – Site Investigation 3d

Geophysics (e.g. resistivity on left) can give us good continuous data in 1D and 2D...

But care is needed. GPR (right) gives very detailed images, but is best at spotting planar features, i.e. Not underground services and shallow tunnels! Reproduced from

http://www.quality-geophysics.net/archaeology.asp on 23/10/2011

Reproduced from http://www.subsurfaceevaluations.com/eri on 23/10/2011

Documents

Ground Models For Tunnelling Applications Tom Robinson Engineering Geologist London Bridge Associates Ltd