TECHNICAL NOTE

"oI:a c eo oc ica~isI;ory: a root ea ) )roac i I:oIInc ersI;an> incsiI:e conc iI;ionsBy Peter Fookes, consultant engineering geologist, UK,

Frederick Baynes, consultant engineering geologist,

Australia, and John Hutchinson, Emeritus Professor,

Imperial College, London.

100I

gMOOmbO

LDmm9.g

«C

0Desk study Walkover,

mappmg and

tnal pits

Imtial site appraisal

Main ground

mvesbgabon

Supplementary

mvestigation

and

observations

dunng

construction

well done

less well done

eotechnical

quisition

Stage

Figure f:Estimated upper and lower bounds of geological infoima5on anticipatedduring the states of a successful site investigation.

ne of the principal uncertainties in geotechnical engineering isthe risk of encountering unexpected geological conditions. Thisis because geological materials are often irregularly arranged and

highly variable in their properties. Failure to anticipate ground condi-tions generally results from an inadequate geological understanding.This note forms a summary of a paper that presents an approach to siteevaluation designed to assist anticipation of all potential geologicalconditions, from desk study to project construction, that is based ondeveloping an understanding of the total geological history of the site.

This invited paper was presented at the GeoEng2000 conference heldin Melbourne, Australia, 19-24 November 2000. The full paper ispublished in Volume 1 of the Conference Proceedings, pp 370 to 460.

Ground conditions at any site are a product of its total geological andgeomorphological history (generally abbreviated to "total geologicalhistory") which includes the stratigraphy, the structure and the past andpresent geomorphological processes and climatic conditions.

The total geological history is responsible for the mass and materialcharacteristics of the ground. To understand this history, thedevelopment of a site specific geological model is required. based onconsideration of the regional and local geological and geo-morphological history and the current ground surface conditions(Fookes, 1997).The engineering performance of the site during and afterconstruction results from the influence of the engineering works on thetotal geological history.

Figure 2:Relationship ofthe InRtat modelsfor identifyingand building thesite checkgst atdesk study stage

Identifyrelevant global tectonic model(s)

Identifyrelevant initial site scale

geological + geomorphological model(s)

Preliminary site check Ikd(e) )

The object is to describe and evaluate how:~ Initial desk study knowledge of the engineering geology environmentcan help in the anticipation of geological and geomorphologicalconditions at a project site. Such anticipation is used to help constructthe preliminary geological model for the site, plan the investigation andassist geologists and engineers in the site investigation and design of theproject. This model develops progressively to be site specific as theunderstanding of the local geology improves during the project.~ Improved understanding of the site as observations are made duringinvestigation and construction also helps the anticipation anddefinition of ground conditions.

This paper is targeted at site investigators and financial decisionmakers who will probably never be aware of its existence.

The approachThe essence of the approach involves understanding the geology andgeomorphology to evaluate anticipated conditions. To do this and tounderstand the models, there must be some appreciation of thegeological history of the world.

The approach starts with a series of simple, related geological andgeomorphological models to generate questions about the site and toprovide the basis for a checklist. The models represent the end membersof a continuous range of possibilities. One or more of these initialmodels will be identified to represent the particular site, enabling theearliest planning to take account of the broadly anticipated geology andgeomorphology. The geological concepts embodied in the selected initialmodels and the specific checklists for the site are then investigatedthoroughly as the site specific engineering geology model developsduring the project studies.

To help anticipate the regional scale geology, 10 two-dimensionalinitial global scale tectonic models based on the concepts of platetectonics are considered. To help anticipate the local scale geology, 17three-dimensional initial site scale geological models are considered,each with an anticipation list, encompassing the rock-formingenvironments and tectonic and diagenetic modifications to theseenvironments. To help anticipate the local geomorphology, eight 3Dinitial geomorphological landform models are considered, each againwith an anticipation list, related to climate and geomorphologicalprocesses.

The approach allows the broad anticipation of the geology andgeomorphology from a desk study of the site location, where theknowledge of site conditions may be minimal. Using this approach, it isnecessary to:~ form an understanding of what is revealed by the desk study~ anticipate what geology conditions might be encountered at the site~ develop the understanding through subsequent stages of theinvestigation, design and construction.

Conventional approaches to desk studies for the feasibility and earlyphases of site evaluation typically start using local maps and literaturewhich commonly exist for many developed areas around the world. TheTransport Research Laboratory Report (TRL 192), 1996, by Perry andWest, Sources of Information for Site Investigations in Britain (Revisionof TRL Report LR 403) is a good example from the UK of theseapproaches. Such works also generally describe various forms of mapsand remote sensing for engineering purposes.

In some locations little may be known, or much may be known already.At sites where geological maps do exist or there are good airphotographs or other imagery. the initial local geology can be quite wellanticipated by site specific models prior to any preliminary inspection(see Fookes 1997,page 348 and Figures 10,41a to 41e, and Table 5).

The site inspection, preliminary and full ground studies can be pro-

42 GI(GUND L'N( INHIIHING LIAR( H 2001

TECHNICAL NOTE

gressed quite quickly at locationswhere much is known, to give asdetailed a picture of the geologyas is considered sufficient or nec-essary. Nevertheless, this maystill result in some shortfall inthe final anticipation of the totalgeological picture if a fundamen-tal understanding of the regionaland local geology has not beenformed.

At sites where little is known,the procedures followed are thesame, but more work may berequired in the early stages.

Anticipating the total geologi-cal picture means that all the geo-logical and geomorphologicalcharacteristics of the site havebeen considered together withthe range of possible variation(sizes, locations, properties) ofthe characteristics identified.Ideally there should be no condi-tion that comes as a surprise dur-ing construction.

To illustrate the usefulness ofthe approach, Figure 1 is a crudeapproximation of how well-designed conventional site inves-tigation studies develop increas-ing geological and geotechnicalknowledge. Note that it is sug-gested that by using thisapproach geological knowledgerises more quickly than geotech-nical in the earlier stages of aproject: the gathering of geotech-nical data develops faster wheninsitu and laboratory testing andrigorous description is intro-duced during the latter stages ofinvestigations (Fookes, 1997).Relatively few publicationsdescribe investigation activityduring construction but the prin-ciples and techniques are essen-tially those used in pre-construc-tion investigations. For guidance,see Eddleston et al (1995).

Geological knowledgeEducationPersonal experienceAttitude

Ability

GeomorphologyGeologyHydrogeology

ProjectGeneral arrangementDesignsSchedulesCostsFreedom to make the model

Site unseenDesk studyRegional knowledgeGlobal tectonic modelsSite scale geological and geomorphological modelCheck list

Engineering knowledgeEducationPersonal experiencePrecedentStandard engineeringSoil mad+niceRock mechanics

1 r

Anticipation engine(l 1(

Anticipation

3

as(

information ~4 ~ Model buildingengine team

1(

Have the questionsbeen answered?

100%Cost benefit Subsurface

PitsBoreholesGeophysicsLaboratoryetc.

MappingStratigraphyStructureGeomorphologyProcessesDistribution

HistoryActivity

Quagty conbolMap

StraggraphyHistory

Exptanafion

Peer review

Understanding acceptabkrV

0000000

VmjTotal geology

I

Site investigationAssumptions sensitive to ground conditions identifiedInvestigation methods chosenInformation obtained to reduce exposure to risk

Check list

Systematic pressnta5on ofinformation and uncertainty

Design and plan constructionFeasibility demonstratedCosts and risks acceptable

Checking duringconstru(ftton

As constructed assessmentAs encountered conditions checked and if differentthen further questions and design changes

site conditions, answer checklist questions d' fan give in ormation to

ui d the real site model(s). It is not the intention here to elab t hdevelo ment of

oe a ora cont ep en o the specific site geological/geotechnical model(s) and

— gui ance is given inthe conduct of the ground investigations — ida

checklist shnumerous publications —but it is worth emphasising that th t

should continue to be systematically evaluated and developed,a e sic

using geological and geomorphological judgement and in t'ninves igation

It is also necessary to emphasise that the investigation should notconsist solel of bso e y o oreholes. Adequate engineering geology/geomorphology mapping should be carried out or be in place early in the

observation tinvestigations. Trial pits, trenches and other hi h ro servation techniques should figure largely. Engineering geological

observand geomorphological interpretation must be continuous d I

ation and deductions continuously reviewed, objectives definedand questions asked (see Stapledon, 1983 and 1996).This process mustcontinue into construction and the service life of the project.

The initial models and anticipations are offered as aid -'

e as ai es-memoire foren geo ogists, geomorphologists and engineering geologists.

Desk study stageThe relationship of the initial models for identifying and building thesite checklist is shown in Figure 2. Wh th I'ne pre iminary siteengineering geology environment model has been developed, it is usedfor overall planning and for design of the preliminary groundinvestigation, or the full ground investigation should there be nopreliminary stage.

Ground investigation stage(s)A well designed ground investigation should now progressively identify

Anticipating site conditionsfrom tahe modelsThe global tectonic modelsprovide the setting for the othertwo groups of models. Only thesite scale geomorphological andgeological models haveannotations and key descriptions Figure 3:Total geology and development of the site mod .to form the basis for the checklists,

o es m el.

since it is these models which provide the initial basic conceptualpicture of the local potential conditions. Full understandin of theactual geolo andgy geomorphology of the site must come from the

san ing o t e

subsequent ground investigation.It is essential that an experienced engineering geologist or

geomorphologist is involved in important projects. Their tr'x

erience i np e is needed to interpret and develop the engineering geologyhistory and to contribute to the planning and the investigation design.

(II(o(INI) KN(iINKKI(IN(i MAI(('.H 2001 43

TECHNICAL NOTE

Initial models Engineering geological environments around the world,which began to be shaped long ago, can present abewildering array of conditions that can impact onprojects: a lifetime is not long enough to see them all. Themodels offer assistance to those breaking potentiallynew ground and therefore will need constant review andaddition. Some of the annotations and text on the modelscan, with little time or effort, be dismissed asinappropriate for a particular site: others may takeextensive and expensive investigation to prove ordisprove their presence and relevance.

Very little geology or geomorphology on a site will beunforeseen if the evaluation is done properly — allgeological conditions affecting the engineeringperformance of the ground should be reasonablyforeseen by a considered investigation. What can beunforeseeable is the detailed variation in their location,form and size or specific engineering characteristics(the potential range of which must be established) whichmight not be capable of evaluation within time andmoney constraints.

A simple example of this would be the presence of akarst ic cave system anticipated by the model and provedby drilling. The precise configuration of the cave systemwould require either underground mapping, whichmight not be feasible, or an enormously large number ofboreholes, which would be impracticable. The cavesystem has been foreseen and allowed for in the contractarrangement, but its detail is unforeseeable. In suchcircumstances, adoption of the observational methodwould overcome potential engineering problems.

Figure 3 summarises, as a flow path, the activitiesrequired during a large site investigation to complete atotal geology-based site investigation.

Figure 4 compares the total geology method (TM)with the conventional or "predetermined" method (PM)and the observational method (OM). The figure showswhere the emphasis comes in the three principal stages:the preliminary investigation, the main investigationand during construction. As can be seen, the TM is moreprominent in the relatively inexpensive preliminaryinvestigation stage, the PM has the greater prominencein the main investigation stage and the OM has thepotential to be the most prominent in construction stage.Arbitrary judgements on the quality of the performancerated excellent to poor have been added to the figure at

the end of the investigation stages andat the end of construction stage.

The full paper includes 31 selectedcase histories that are used to illus-trate how the geological model helped,or could have helped, to anticipateground conditions that proved to becritical to the project engineering.Thirty-eight initial models are alsopresented (Table 1).

Tectonic elements of the worldSection showing major tectonic elements of the earth's crustSimple tectonic crustal collision mechanisms

Tectonic model- intraplate setting —cratonsTectonic model- intraplate setting —mobile bedsTectonic model —intraplate setting —platform sediments and basinsTectonic model —divergent plate boundary- continental riftsTectonic model- divergent plate boundary —oceanic riftsTectonic model- convergent plate boundary- accretionary prismsTectonic model- convergent plate boundary —fold and thrust beltsTectonic model- convergent plate boundary -magmatic arcsTectonic model- convergent plate boundary —collision complexesTectonic model-convergent plate boundary- foreland basins

Oioldrrftrldsnlogfeak~-contlnefrtal~giafeatureaQeamor'yhologkal ~-~elalibi&3reaGdomeryhokrgiealnifadel-tarayprate c5nfate featuresGeoraoryhofogkaaI mffdal-hrN4ry climate featuresGetameryho~~'f'-hot wet climate faratureeGeogaoryhokrghs)fI~l-eosaf3)l featuresGermforyhol~~l-aoiuble rockf~~dferm scale)

Tahle 1:The 38 lnltiai models presented in the full paper.

Key: E = Excellent F = FairG = Good P = Poor

Time Understanding Understanding

Main investigation(boreholes inslrumenlakon

Preliminary investigation

tdesk study site inspection

mapping shallow test pits)

In sum.a smail

fraction ofthe cost if

geoiogymisinterpreted

test ng back analysis)

ExpensiveInexpensiveConcluding remarksFrom the numerous case historiesexamined. the overwhelmingconclusion was that for well over acentury there has been a repetition ofcommon reasons which. individuallyor in association with the others, haveled to failure to anticipate geologicalconditions. This in turn has led tofailure in the project engineering.Little new was learnt in this respectand many authors have reachedsimilar conclusions before.

There was no statistical evaluationof causes, nor of features leading tocauses of the failures, since the casehistories differ widely in degree ofdetail available, and in the numbers ofvarious types of case. However, somebroad principal conclusions on theconduct of a project investigationwere reached;

Predeterminedmethod

(PM)

Observations may or may not~

be made during constructtong~ P

"Standard"ite investigation

P

Emphasis on observationsand predetermined coursesof action for deviationsfrom the expected

Observationalmethod

(OM)

EtoF

Reduced site

Pinvestigation

Totalmethod

(TM)

Investigation based on"TOTAL" approach less

expensive than "STANDARD"

Total geology/geomorphology

approach

Observations made duringconstruction, but chances of

major deviations almost ehminated

QuantitatneQuahtative

Figure rh Comparison of the three approaches to site investigation.

Geological model —igneous-basic volcanicsGeological model —igneous- acid volcanicsGeological model —igneous- plutonic extrusionsGeological model —sedimentary —continental fluvial, colluvial and lacustrineGeological model- sedimentary -continental deltaic and coal measuresGeological model —sedimentary- shelf carbonates and evaporatesGeological model- sedimentary- deep marine and continental slopeGeological model- structural- normal faultsGeological model —structural- strike slip faultsGeological model —structural —thrust faultsGeological model —structural-joints in undeformed sedimentsGeological model —structural- open folds and jointsGeological model —structural- plastic folds with cleavageGeological model- structural- multiple folds and shearsGeological model- structural- tectonised melangeGeological model —metamorphic —schists and phyllitesGeological model- metamorphic —gneisses and migmatites

tlaotiNI) IIN(IINIgKIIIN(I Ivt.srltt'H 2001

TECHNICAL NOTE

Worked exampleUse of the approach can be illustrated by acoastal exposure in north Cornwailofstrongly folded and faulted Carboniferousage thick sandstones, thin turbiditesandstones and shales (Figure 5).

The starting point is a map and section ofthe world, subdivided into the majorgeological components, forming a globalsettingof thesite(see fullpaper). Theregional tectonic setting of the site duringthe tropical shallow/deep marinedeposition and subsequent deformation ofthe original sandy beds is that of aconvergent plate boundary with fold andthrust belts (Figure 6).

These rocks were subsequently affectedby subtropical weathering during theTertiary when the plate on which Britainwas moving passed north of the Equator, onthe way to its present position. This wasfollowed by periglacial weathering whichstrongly affected the ground in theQuaternary ice ages. The differentprocesses can be depicted separately infour site scale geological andgeomorphological models (Figures 7-10overleaf), but in geological time havesuccessively combined and result in thecomplex and variable ground conditionsshown in Figure S.

la il

Rgure 5:strongly hdded and faulted coasbd exposure In north Cornwag.

Rgureg: Tectonhmodel- convergent pbrte boundary, fold and guust bene

Uthologles:dominated by deep water marine sedlnunds,flysch and greywackoswnh some pyrochwgcs and vtdcanodasecs le vtdcanlc asstudagons(formerlycalled eugoosytudlnal fac4e) In deeper parts of the original basin, and wgh shelfdeposns of Nmestones, sandsbmes and mudstones wghno volcanicassoclagons(formerly mlogeosyncgnal fades) In shalhw parts of the orlglnalbasin. Epltomlsedby "slate beNs" where thick monotonous sequences affinegrained sediments have accumulated In deep benches.

Overthrustmetamorphic

complex

Tens to hundreds of kilometres

Foreland

Structures:towards the centre of an orogenhbeltgwre Is generally a fold andthrust belt dominated by deep water marine sedhneats wNh volcanic '~

Late sta e Basal decollement

associations wbkdl transNons Iatsragy to sbalknu water sbeN sodlmonh. Tbo Basement plutonic intrusionortgbmlbaslns In whkh the deep water marine sediments were deposNed maybee forearc or a hackers basln wNh gw shalbw water sheN sediments deposged on adJacent stable congnental crust There Is chars terlsgcagya low angle stwar thatseparates the rocks of the fold and thrust belt from the underlying basement The deep water sedhnenh tend to be Ideas to the orogenic core and are usuallymetanunidiosed to groenschlst fades and above wgh phtsgc folding that can be Incgned or even recumbent and can fmmnappes. Slaty cleavage and fogagon Is welldeveloped andmuNple folding Is common. Towards the shallow margins mulgple thrusts wgh duphx structures are common and bedding parallel faults form In

Incompetent layers. Shallower thrusts tend to be younger resuNng In progradlng delormagon.

Relatedmodels: deep marine and continental slope (Figure 7);muNplo folds and shears (Figure 5).Examples: slate belt of North Wales, southern Appalachians, USA,

Canadian Rocldes.

GVERLEAF: Geological and geonunphokaglcalmodeh

~ There must be a specific and determined endeavour to understand theengineering geology and geomorphology environment of the site and toincorporate that understanding into the project design.~ Around the world it is often the problems related to the Quaternarygeology and geomorphology events that dominate.~ The engineering geologist must be competent, well trained, experi-enced and a good geologist if they are to be either the lead engineeringgeologist in the team, or the only engineering geologist in the team.~ The engineering geologist must have a good knowledge ofgeomorphology and on occasion will need to work with ageomorphoiogist. Appropriate experience is most important in theprofessional development of qualified geologists and geomorphologists.~ Each site evaluation, however small, should ideally have at least oneengineering geologist involved in the work. This does not always occurin practice.~ The preliminary stage is when the engineering geologist probably canhave the most significant influence on the project by indicating potentialhazards and their consequence on the economy of design, matters ofconstruction and expected performance of the works.

ReferencesFookes PG (1992).The First Glossop Lecture. Geology for engineers: the geologica) model.prediction and performance. Quarterly Journal of Engineering Geology. 30.293-431.Edd)eaton M. Murfin RF. and Walthall S (1995).The role of the engineering geologist mconstruction. In Eddlcston M. Walthall S. Cripps JC and Culshaw MG (Eds). Engineeringgeology of construction, Geological Society Engineering Geology Special Publication No.10,389.401.Emery D and Myers KJ(eds)(199i). Sequence stratigraphy, Blackwell Science, Oxford.Moores EM and'f wiss RJ (1995).Tectonics, WH Freeman and Company, USA.Perry J and West G (1996).Sources of information for site investigations in Britain. TRLReport 192 (Revision of TRL Report LR403). The Transport Research Laboratory.Crowthorne.Ritter DF (1986).Process geomorphology (2nd Edition). Wm C Broivn Publishers. USA.Stapledon DH ()983).Keynote address: Towards successful waterivorks. Proceedings of thesymposium for engineering for dams and canals. Institution of Professional Engineers.New Zealand. 15)-1.15.Staple<Ion DH (1996). Keeping the "geo": why and hove Proceedings of the ith AN/conference on geomechamcs, Adelaide, Institution of Engineers. Australia 3-18.Thomas WF (1994).Geomorphology in the tropics. a study of weathering and demidationin low latitudes, John Wiley and Sons, England.

fg II~~.. www.uwaac.uk/geocal/totalgeology

( Rot)ND ENGINFERING KIARGH 2001 45

TECHNICAL NOTE

Coalescing piedmontalluvial fans

Figure 7:Geological model-sedimentary-deep marine andcon5nenhd slope (based on Emery and Myers t Egg).

Mountain front with large sourceof coarse debris

Talus cones

Slide backscar

Submarine canyon

Deposited by mass movement(sliding and slumping)

Coalescing hummocky lobes

Basin plain

Mixed sand and mud

filled levees

Depositional lobes

wID

ID

EOIIIo

D

An5cfpataUtholegles: Interbeddedmuds and sands, andmud/sandmhdures wNhmuds rtch systems dominating, some coarserdeposNs If the mountain front Is dosa Transport bygnwlty Now

(turtddNy curronts,fluldlsed flow, grain flows and coheshreNews) andmass movenwnt Induced by seismic evenh andstorm loading. Turhld5o beds charactertsedby Immature

sediments, scours and sole marks,dewaterlng sbuctures,comfolulo bedding, Inverse bedding,thin, iatoragy con5nuous,repelNhe beds, generally coarsening upwards and thhdumlng

upwards within a sequenca Thick sequences of turbldNes

referred to as flysch.Set5ng: on the continental tdopes or just off the congnenlalsheN, wNhln trenches at plate margins, Inrlfted basins atpassive marglns,forearc and backare basins.Generab environment currengy being explored for og and gas,rates of acgve sediment ransport, mass mmrement anddeposlgon depending upon system equglbrlum wNhhlgh

sediment Input rates behsg assotdated wNh acgve massnwvenumt of many cuMc kgomehus of sediment UblquNous

mass movemmtt and gravity Now represmrls a slgnigcanthazanl to og and gas devolopnumt Inhushuchua Tsunamls

caused by mass movenumt

ExamplesGrand Banks of Newhmndland; Norwegian continental margin

(Qorogga); Kidnappers sgde, New Zealand; West Fiortda tdopeIn Gulf of Mexico; ANangc continental margin of norlheu USA.

Deposited by gravityflow as turbidites

Late stage (D3) folding (F3)to produce crenulation

cleavage (S3)

Dip and strike ofcrenulation

Figure 8:Ge(dogical model-structural-mulgpiohdds and~hears.

Antkd pateUthohgles: muigplefohgng and faulting can occur In anyINhology but wglbe most deartyobservedlnbedded orfolhded INhologles. Generally Nw folding and faul5ng occursublquNously, and not separately as In the bhck dhtgram

(right), consequengy complex rock mass condions develop.The Interfering fold and fault pattwns occur at scales from

small outcrops to extensive regional seings and can lead tocomplex outcrop pa5mus wgh dome and basin structures.This level of tectonlsm M usually, but not always,assotdatedwith some sGght hnrel of metamorphism(ie gruonschlst fadesand higher).Suing:fohlbeNs, or(qpmic belts,toctonh orrldors or widefault zones subject to repeatadmevenwnls In dlffmunt

dhocgons, major porgons of most of tho world's foldmountain begs.Generakpredicgoa ofrockmass condigons dNNcuN due tocomplex structures. Several more phases of folding may ofhm

be discerned bydetaged slructural analysls but thhhwel ofdetag may not be Imporlant for project engineering,whkhlaoften controgedby the later stage brNtle structures.

Faan

caim

fractures in theform ofloints

Early isoclinally

folded strata with

foliation (Sl) parallel tooriginal layering (SO)

Post tectonic dykecross cutting all

phases of folding

(D4)

ExamplesPalaeozoics of north Cornwag In UK;Archaen greenstone beNs

of western Australia; Palaeozoics from Soruy In northerNorway; Palaemdcs from Pansboro In Nova Scega, Canada.

Strike and dip of axial

plane of first isoclinal phase(Dl) of folding (Fl) thatimparted a foliation (S1)

Strike and dip ofaxial plane of second phase (D2)

of folding (F2)

GRO(iNI) RN('INRRRIN(i xIAROH 2001

TECHNICAL NOTE

Okl d

tru

De

Deep weprof

half oran

Gullying and erosion orcolluvial and taluvial slopes

Deep weathering profilewith corestones developedin massive rocks and deepsaprolite in less massive rocksbeneath old land surfaces. Depth ofweathering profile controlled in part

by distnbut(on of discontinuities

Terracedeposits

Floodplain

Quaternary terrace,possibly with

duricrusts developed

Deep gullying in colluvial

pediment/alluvial terraces

Dambo fchannelless valleyor catchment depression)

Figure &geomorphologlcal model-hot wet cgmate features (based onThomas1$ 54).

AngclpateSuperfldal deposits: existing or former cover of thhk rein forest, deepweagwrlng progles, depth of weathering rehted to dlmale,geonunpludoghalhlstoryand geohghalmck type and struchuo but can bemany hms of metres. Durhrusts where groundwatercmtconhates solubleweagwrlng produch,cogapslng sogs whew there are porous extnnnelyweatheredrocks, relht dlscongnuiges In weathering pregle controlstabglty, prof gee range from older, leached, kaognNe and glbbsNedominated solh,to poorly drained,younger, monhnorglonHe (smecgte) rhhsogs. Deeply weathered coguvtal sogs oHen dltgcult to dlsgngulsh fromInsih weathering preghs. Penneabglty onlrasts othe conhul shpestabglty wHhhlghly permeable zones at bedrock/weathering pregleInterface and In the zone of piping fagure where sgty sogs form fromexhwnely weathered rock below redduel solh. Volcanh ashestend todevelop moWure susceptible andosols. Doe weagwrlng grades anddurlcrust terminology and avohl "laterHu progle" tenne because manydifferent inhntuehgons.Processes: acgve weathering somelhnes proceeding atshrghg rahswhere reck typos am suscepgble, mass movenumtassodakd wgh Indslonof deep weak weathering proghs Indudes gugy erosion, landelkh,mudgows etc. Systemmay also be responding to recent hnd ckwrtng byman and xtenslve cog erosion may resulL CharachrMc Intense ralntagleads to saturatkm of surface layers andinslaldgty, aho movement of highpressure water along soll pipes. Silty solh characterlsgc of dmtp weatheringprof gee suscepgbleto Internal eroshn and piping fagunL

ExamplesHong Kong central Malay Peninsula; Ddzungwa scarp In cenlraITanzanla;seuth~ coastalranges of western Austrega.

NoteThese are bulb an exlsgng acgve assembhge of landfonnsIn present day perlglachl environments and a commonlyoccmrlng assemblage of fossghndfonns enctmnteredinvast areas that were subject to perlglachl envlronmenhduring Nw Pleistocene, but which today are sub)ect totemperate dlmatea

Accumulation of scree dueto active slope processes

Boulder fields

Loess deposits blanketingthe landscape and forming

d

Figure 10:geonwrphologhalmodel- perlghclal features(hosed on Nltter1585).

Angd pateSupergchl deposHs: algphnathn twraces, bhckfhkh, rockgladers, ~gnmnd,frost mounds, plngos,thermokarsl, loess, talus. sPrucesseafrost wedglng, nhathm,fredtwndng/creep/sorgng,degocculatkm,cryoturhagon,geggucgon/solNlucgon, permafrost, he heave, massmovemonL p

Ma)or engineering probhms are assochted wNh pennafresL

Examphs:Acthre-hhska, Slberh. Fossg features-northernEurope,DK, northern DSL Tal

Taliks beneath rivers and small ponds

Active layer -~ ~A'rseasonally frozen n

Cryoturbation or originallyPermafrost -~.

horizontal sedimentspermanently

frozen ground

GRO(lNI) ENGINEERING MARCH 2001 47