High roads and low roads

Earthworks: any cutting, excavation, artificial dumping or banking of soil and rock.

Pore water pressure: the pressure of water in a saturated soil.

Piezometer: an instrument for measuring ground-water pressure head.

Inclinometer: an insvument for measuring ground movements in the horizontal plane.

Aphyric: volcanic rock texture with no phenocrysts.

Porphyritic: rock texture of large crystals set in finer grained crystal matrix.

Seismic prohle: result of seismic refraction method of ground investigation by geophysical means.

Closed circuit television: techmque of down-borehole inspection using small- diameter TV camera.

Grcywacke: lithological term for a dark, poorly graded argillaceous sandstone or fine grit.

Fig. 1. Simplified geological map with the three sites

OrdovicidSilllrian; 0 R S = Old Red Sandstone; Carb = Carboniferous; N R S = New Red Sandstone; G = granite; H B F = Highland Boundary Fault; S U F = Southern Upland Fault.

indicated. 01s =

PAUL CARTER & GUS WALLACE

One of the joys in working as an engineering geologist on Scottish road schemes is the variety of geology that has to be dealt with, from soft silts to hard metamorphics. The properties of these materials must be assessed, cuttings and embankments designed, and site work monitored carefully to ensure safe and economic construction.

“0 ye’ll tak’ the high road And I’ll tak’ the low road An’ I’ll be in Scotland afore ye But me and my true love Will never meet again On the bonnie bonnie banks 0’ Loch Lomond”

Geologists today are not only taking the high roads and the low roads to Scotland, they are also taking an increasingly important part in making them. At the time the ballad of Loch Lomond was written, over two centuries ago, Scotland’s roads were built for the marching feet and lumbering wagons of General Wade’s army. Wade’s roads were able to climb up the hills and plunge down into the glens on their way across the Highlands. Today’s roads have to carry fast traffic and heavy loads. As a result modern roads must be built to the flattest gradients and gent- lest curves that can be achieved.

In the hilly terrain of Scotland this means excavating large cuttings through the hills and

building high embankments across the valleys. Whilst these are not as environmentally unobtrusive as Wade’s roads, they do bring valuable opportunities for geologists. The con- struction of deep cuts in hard rock and high embankments on valley-floor deposits requires intensive geological investigation as a basis for a sound appreciation of the ground concerned and this provides employment for engineering geologists, of whom there are about one thousand in Britain today. At the same time, our store of geological knowledge is increased.

Road building has expanded ,dramatically in Scotland over recent years and in this article we describe the geology of three of these new roads (Fig. 1). These are the M90 motorway to Perth, the A75 trunk road to Stranraer and the A82 Loch Lomond road. All three have involved major earthworks and all have required con- siderable geological input.

To the ‘fair city’ If you drive north to admire the deeply incised glacial terrain of the Grampian mountains or the high granite plateau of the Cairngorms, you will almost certainly use the M90 motorway between Edinburgh and the ‘fair city’ of Prrth. After leaving behind those magnificent engineering achievements, the Forth road and rail bridges, you will cross the Kingdom of Fife, climb over the Ochils and suddenly the wide valley of Stratheam will appear spread out below you.

Site investigations for the motorway across Strathearn show the valley to be infilled with a thick sequence of soft late-glacial and post- glacial sediments (Fig. 2 ) which posed major problems for road construction. In late-glacial times the valley floor was 70 m lower than the present sea level. As the glaciers retreated to the Highland line some 13 000 years ago, the sea flooded in to about 30 m above present sea level. Rapid erosion of the recently bared ground led to extensive infilling of the new estuary with laminated clays, clayey silts and fine silty sands. Subsequent isostatic uplift left these soft sedi- ments open to erosion, and flood plain gravels were deposited in the newly formed river chan- nel. These in turn became inundated by the

%/GEOLOGY TODAY March-Apn’l I987

Flandrian transgression of 6 000 years ago. Soft organic silts were deposited which soon developed growths of peat. These were subse- quently capped with Recent (Holocene) river alluvium.

The motorway crosses Strathearn on embank- ments, and the presence of the soft silts and clays created two major problems for embank- ment construction: instability and settlement. As embankments are constructed, the increased ground pressure is initially carried by the water present in the soils, called pore water pressure. High pore water pressures force the soil struc- ture apart and can cause it to become unstable, thereby increasing the risk of dramatic slip fdure. Normally, the pressurised water gradu- ally dissipates from beneath the embankment, the pore water pressures reduce, the soil struc- ture compresses and strengthens, and the ground surface settles. The Earn embankments are particularly high, up to 13 m in places, and the underlying soils are particularly weak and saturated, up to 50% by volume of water. Increased pore water pressures were therefore of critical importance during construction.

Detailed site investigations were carried out in order to assess geological condtions beneath the embankments and predict stability and settle- ment during construction. The large number of boreholes included several which were con- tinuously cored through the soft sediments and logged by a geologist who noted every possible seepage plane for water. The clay and silts turned out to be well laminated with coarser laminae every few millimetres, a factor which would help to dissipate the excess pore water pressures more rapidly. Specialised laboratory testing was undertaken to determine the strength of the soils under increasing levels of pore water pressure.

Many instruments were installed in the sub- soils to monitor the effects of construction on the pore water pressures and soil movements. The readings were used to ensure stability during construction. Piezometers were used to measure pore water pressures, inclinometers to record sideways soil movements and settlement gauges to monitor vertical ground surface movements (Fig. 3). Pore water pressures rose rapidly dur- ing construction and embankment construction was limited to increases in height of only 0.5 m per month to ensure stability. In one area the piezometers revealed dangerously high pore water pressures and these were relieved by the rapid installation of vertical sand drains.

Sideways ground movements of up to 0.1 m occurred at the embankment toe, but at no time did movement threaten to escalate into major slip failure. Vertical settlement of over 1.0 m was experienced, mostly during construction (Fig. 4). Additional embankment material was used to make up the settlement and the road

I3 Stiff Crust

laminated grey silt

Soft brown lam. clav

Very soft 0 0 0 m

0

- South to Ochills

Settlement gauge

I

0 -Piezometers-

0

Northto 4

Moncrieffe Hill River

1 2 3 I I

kilometres -100m

surface has had to be raised since the opening of the motorway, but settlement is reducing rapidly with time.

The rockfill for the construction of the embankments came from the Craigend rock cut- ting which is the next feature to catch your eye as you drive north to Perth. At 55 m high this is Britain’s highest motorway cutting, blasted through Devonian lavas. In the cutting, the junction between the upper aphyric lava and the lower porphyritic lava forms a prominent fea- ture. The cutting excavation was one of the first times that the new ‘pre-split’ method of blasting was used. This method is employed to counter an unfortunate consequence of using modem high explosives for blasting. This is the damage caused to the rock behind the design cutting face. The railway engineers of 100 years ago used low charges of ‘black powder’ which left the unexcavated rock undamaged, and their cut- tings are still remarkably stable today. Pre-split blasting aims to create the same stability.

Fig. 2. Section through Saarhearn. 1 = Glacial Drift; 2 = brown laminated silty clay; 3 = grey laminated clayey silt; 4 = grey sandy silt; 5 = line grey silty sand; 6 = sand and gravel with silt lenses; 7 = very soft organic silt; 8 = stiff crust.

Rejuvenation: river undergoing a renewal of downward erosion resulting from an uplift of rhe land.

Pile: a deep foundation inserted in rhe ground to carry vertical loads andlor lateral loads.

Schist: foliated crystalline rock formed by regional me tamorphism .

In Fe-split blasting a line of closely spaced Stereogram: presentation of holes is drilled along the cutting face and is detonated milli-fractions of a second before the

3~d~ensiona~srructura1 information (dip and

main bulk blast. The fracture caused by tearing along the ‘dotted line’ of drill holes prevents gas $”:? proiection Of a pressures from the main blast damaging the rock behind the cutting face. The line of pre-split holes is left as a series of ‘half barrels’ running down the finished face. AS this method was still new at the time of construction, investigations

azi.murh of plane) on a 2-

Fig. 3. Typical instrumentation layout.

-’Om

e Surface Levels \ I

I 10m

Glacial sand 1 Inclinometers 1 and gravel

Rockhead . .:. .... . , . . . . . . . . ........ ,..... ~ ,...._.... L....... . ....... ....... ......, ...... . ......... .,.......... . . . . . . . . . , . . . . ( . ) , -). :( :): :y ;)! i( jJ! i,$ $

. . . . . . . . . . . . . . .

GEOLOGY TODAY March-Apn’l 1987155

Fig. 4. Stratheam embankments under

were carried out to assess its effectiveness. Seis- mic profiles over the face, horizontal drilling into the face and closed circuit television scan- ning of the drill holes all showed the same positive result. There was no blast damage to the rock behind the pre-split face, whereas damage up to 8 m behind the face affects other rock cuttings in Scotland excavated with normal bulk blasting.

To Galloway and Ireland If you drive west to examine the metamorphic aureoles of the Galloway granites or to take the ferry to Northern Ireland for a visit to the Giant’s Causeway in the Antrim basalts, you will travel the busy A75. During recent years a major effort has been made to improve thls road by building by-passes round the picturesque Gallo- way towns. One of these by-passes takes the heavy through traffic away from the attractive Gatehouse of Fleet. This bypass has been con- structed through very variable geology. On the

construction.

valley sides the road has been cut through Silu- rian greywackes; on the rolling uplands it has met drumlins of boulder clay and thick lenses of peat; and near the Water of Fleet it has been built on an embankment over a deep buried valley similar to that of Strathearn. The buried valley is at least 30 m deep (Fig. 5) and has been infilled with a sequence of gravels and silts. Glacial outwash gravel covers the rockhead and this is overlain by a soft late-glacial laminated clay. Rejuvenation of the Water of Fleet resulted in a flood-plain gravel being laid over the lamin- ated clay and this in turn was overlain by organic silt of Flandrian age. A thin blanket of recent alluvium occurs at ground surface.

The weak valley infill deposits posed a prob- lem for construction of the large Cardoness Bridge which carries the A75 over the Water of Fleet. This structure is a 65 m long 3-span bridge with a composite steel and concrete deck, and it weighs some 3 000 tomes. Such a large weight would cause failure of the weak soils, so it was necessary to carry the bridge on piles driven to bedrock. Steel H-section piles were used, a total of 130 piles being required (Fig. 6). A design consideration was the possibility that the construction of the road embankments would set up ground stresses which in turn could cause the weak soils to move sideways and disturb the piles. If this had been allowed to occur, the safety of the bridge could have been at risk. Pile design included an allowance for lat- eral loading of this kind based on estimates of sub-soil movements. In order to substantiate these predxtions, a number of instruments, such as inclinometers, strain gauges and settle- ment gauges, were installed on some of the piles. The readings from the instruments showed that the piles have not been subjected to s ighcant levels of pressure or movement. The instru- ments continue to be monitored.

Along the bonnie banks If you are heading for the rugged ice-carved peaks of the Cuillin ridge on Skye or the classic Moine Thrust of the north-western Highlands, you will take the A82 along the ‘bonnie banks of Loch Lomond’. Two-and-a-half centuries ago Sir John Cope (a contemporary of Wade) built a road on the hillside above the western shore. It was built after the 1715 Jacobite rebellion to

1 SO00 honzontal 1.700 vertical

56lGEOLOGY TODAY March-Apnll987

ensure fast northerly passage of Government Redcoat troops into the Highlands to put down any further rebellions. But when Bonnie Prince Charlie raised the Jacobite standard in 1745 he used this and other roads to help his dramatic armed dash for power which reached as far south as Derby.

During the last century the present A82 trunk road was built along the lochside. It must count as Scotland’s most winding and slowest trunk road, throttling traffic between Glasgow and the Highlands. So now a completely new road is being built. If you had travelled north during the past year, you would have seen rigs perched up on the hillsides drilling blastholes, heavy dumper trucks shifting massive volumes of rock, and barges carrying rockfdl down the placid waters of the loch to create new lochside land on which to build the new road.

The rock cuttings are being excavated through Dalradian schists and grits which form the steep mountainsides north of the Highland Boundary Fault. Whilst the rock material is often very strong, it is also penetrated by a number of breaks or discontinuities including several prominent joint sets and cleavage. Faults and fracture zones are also present. The stability of the rock in the cuttings is very dependent on the orientation of discontinuities as compared with the orientation of the cutting face. Unfavourable orientations can cause various kinds of failure such as sliding, wedge failure and toppling (Fig. 7).

In order to assess the risks and to design safe slopes, a comprehensive study of the rock was made. Large numbers of discontinuities were recorded on rock outcrops including measure- ments of joint strike, dip, width of opening and persistence. Structural geology techniques were used and the information was plotted onto a series of stereograms. Failure conditions were also plotted on the stereograms and in this way it was possible to make rapid on-site assessments of potential failures (Fig. 8). This approach has now been computerised and a computer has been installed in the site office to check slope stability as construction progresses. Discon- tinuity strength was also assessed by laboratory

testing and this information was used in the slope design. The largest rock cutting on the Loch Lomond road and the highest on Britain’s highways is the 60 m high cutting through the Rubha Mhor promontory. Cutting faces are being excavated using the pre-splitting method described earlier for the M90 motorway.

It is critical to maintain accurate drill orien- tation for the success of this technique, and so a drill-orientation device is used to set out each drill hole. After excavation of the cutting, the face is examined by a geologist using moun- taineering skills to climb over the rock and examine it for potential unstable areas where slip failure might occur. These areas are being restrained by rock anchors, which are steel tendons in a protective sheath, grouted into 8 m deep horizontal drill holes and tensioned up to give additional strength to potential slip surfaces.

About 0.5 million tonnes of rock is being taken from the Rubha Mhor cutting as part of the 1.5 million tonnes of material being excavated from the 4.4 km section of road cur- rently under construction north of Inverbeg. This rock is being used to build large lochside embankments for a further section of road to be constructed between Luss and Inverbeg.

Fig. 6. Piling at Cardoness Bridge.

Direction of movement

_ _ _ _ - Dmcmfinuity trace

( 0 ) PLANE FAILURE MODEL (b) WEDGE FALLURE MODEL

Direction of movement

7 Osen fracture or void

( c ) TOPPLING FAILURE MODEL

Fig. 7. Typical modes of failure along disconcinuities (reproduced with permission from TRRL Laboratory Repom No. 1039).

GEOLOGY TODAY March-April 1987157

Fig. 8. Stereograms for fdure assessment: (a) Discontinuity data; (b) Potential failure modes.

MAGNETIC N O R T H

UACWE TIC NORTH

Bottom-dumping barges are being used to trans- port the rock down the lochside to where it is needed (Fig. 9). In this way further congestion of an already busy road by large dump trucks is avoided.

A large amount of geological investigation was required where the embankments are being con- structed. This included boreholes drilled from over-water platforms, sophisticated laboratory testing and a full-scale trial embankment. The embankment was monitored with soil instru- ments including piezometers, inclinometers and settlement gauges to assess the reaction of the soft loch bed sediments to the imposition of a large load of rockfill.

The loch bed sedunents reflect the complex post-glacial history of Loch Lomond and include very soft p a t s and organic silts as well as gravels and scree deposits, underlain in places by glacial till. The potential for movement of the softer loch bed settlements under the embank- ment loading, and consequent distress to the embankment, had to be assessed. The outcome of this assessment indicated that the embank-

Fig. 9. General view of Rubha Mhor and the A82 road cuts on the west side of Loch Lomond. At 60 m, this is the highest cut on Britain’s highways.

/ FAILURF

menis required to be constructed with large toe sections and that some of the softer loch bed sediments would be displaced by the rockfill on being placed in a particular sequence of oper- ations (Fig. lo). Construction of the embank- ments is now nearly complete.

In Scotland afore ye The three sites on the M90, A75 and A82 roads illustrate some of the varied problems that face engineering geologists and demonstrate how they have been solved, using many lfferent techniques. So as you speed along the high roads and the low roads of Scotland on your geological field visits, please give at least a passing glance to the major earthworks that have made your jour- ney possible, and a passing thought for the engineering geologists who have ‘been there afore ye’. Suggestions for further reading Kinder, G. & Carter, P. 1986. Rock cuttings -

British experience. Proceedings of Sinc-British Highways G, Urban Traffic Conference, Insti-

-

58/GEOLOGY TODAY March-April 1987

NEW A82 -

MINIMUM EARTHWORKS OUTLINE SAND GRAVEL

KEY Rockfill placed by barge

[Rockfill end tipped by lorry

tution of Highways and Transportation. Whittow, J.B. 1979. Geoloby andscenery inScot-

land. Pelican Books. Matheson, G.D. 1986. Design and excavation of

stable slopes in hard rock, with particular reference to pre-split blasting. Proceedings of Conference on Rock Engineering and Exca- vation in an Urban Environmrnt. Institution of Mining & Metallurgy.

BS 603 1 : 198 1. Code of Practice for Earthworks.

Paul Carter and Gus Wallace both work with Babtir Ceotechnical, which specialises in the fields of engineering geology, hydrogeologv and soil and rock mechanics. The authors thank the Chief Road Enpneer, Scottish Dez>elopment Department, for permission to publish this article.

British Standards Institution. -. - ~~~ ~~ ~

~~ ~ - ~~~~~~ . . . .

Collecting fossil fishes in cowboy country DAVID M. MARTILL

The Eocene Green River Formation fishes of southwestern Wyoming are among the best preserved and best known fossil fishes yet discovered. Collecting them is hard work and a very technical and time-consuming process, but is made worthwhile by the fantastic setting in which the fossils occur. This is an account of a collecting trip under the leadership of Dr Lance Grande to expand the already spectacular collection of Green Riverfishes in the Field Museum of Natural History, Chicago.

w h e n the American West was being won and the railways were blasting their way through the Rocky Mountains of Wyoming and Utah, rail- way engineers discovered one of the world’s most exciting fossil deposits. They found it in the southwest corner of Wyoming where the railway was cutting through big country - high flat-topped buttes, aspen-covered ridges and rainbow-coloured badlands. This spectacular scenery was formed in sediments of the Eocene Green River Formation, in a small, almost iso- lated basin now known to geologists as Fossil Lake (Fig. 1). This was one of several larger intermontane basins in the Colorado, Utah and Wyoming region that were formed by large-scale thrust tectonics, which produced a ‘basin and high’ topography from formerly flat-lying Meso- zoic sediments.

Fig. 10.Secrion through lochside embankrnenrs.

With the coming of the railways, a small town developed at the foot of one of the larger buttes, now called Fossil Butte. This town had two industries. Its main occupation was to provide coal and water for the steam trains; its second industry was selling fossils to the passengers of the trains. This town became known as Fossil and used to boast several bars and a school. Now it is the home of the Lewis Ranch and the site of a small museum belonging to a fossil collector. The decline of Fossil was a direct result of a conversion of the railways to diesel fuel. The hope that Fossil was to become the big oil centre of Wyoming never materialised. Almost the only people who visit Fossil now are keen amateur palaeontologists who stop by at the fossil gallery after a visit to the nearby Fossil Butte National Monument. The small cottage industry which

Ichthyologist: a person who srudiesfish.

~ ~ ~ ~ ~ ~ h ~ ~ ~ ~ ~ t , ~ ~ ~ ~ ~ - like fossils.

for preparing

GEOLOGY TODAY March-April 1987159