Elsevier Encyclopedia of Geology

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Site Classification

A W Hatheway, Rolla, MO and Big Arm, MT, USA

Introduction

Site characterization may be defined as the

...three-dimensional engineering geologic description of

the surface and subsurface of the location for intended

construction of engineered works, for habitation, com-

merce, resource development, mitigation of natural

hazards or conduct of groundwater protection, waste

management or environmental remediation.

Site characterization provides the design informa-

tion necessary for project planning. Site character-

ization is frequently broken and discontinuous in

nature, reflecting the naturally flawed ground that is

being evaluated (Figure 1). The broad elements of

site characterization are listed in Table 1. No such

table can be wholly inclusive, but it serves to estab-

lish a format for discovering and predicting the elem-

ents that should be included within the scope of

construction work.

The broad goals of site characterization include:

identification of sites likely to experience ground

displacement from fault movement;

identification of the stability of a site‘s configur-

ation;

establishment of the need to improve the engineer-

ing characteristics of the ground so as to resist

damaging deformation;

establishment of the need to control groundwater

to prevent excess pore pressure and subsequent

failure under the involved structural loads; and

identification of the potential for creating green

space not only for social gain but also to incorporate

active faults and existing geohazards.

Purpose and Scope of Site

Characterization

Site characterization is a major component of site

investigation, the purpose of which is to determine

ground conditions at a proposed construction site.

The current terminology signifies a broadening of the

purpose and scope to encompass the determination of

relevant design needs for a wide variety of construc-

tion, resource development, hazard mitigation, and

environmental goals.

Site investigation matured from the end of World

War II through the 1970s, and then expanded in

the 1980s, providing broad scope for the work of

engineering geologists.

There is some published guidance on site character-

ization, but it is the responsibility of the engineering

geologist in charge of each project to define the scope

of the characterization in order to meet the design

needs of the planner and engineer. These needs fall

into seven basic categories:

a three-dimensional description of the project site

to the depth of influence (i.e. where the maximum

applied (live and dead) static load from the

intended use of the site has dropped to 10%);

a description of the length and breadth of the pro-

posed site and of the area beyond the property

boundaries to such a distance as will encompass

any anticipated effect of the activities contemplated

at the site (this distance will be sufficient to cover

the origins of geological constraints that may im-

pinge on the contemplated site, including slope

instability, groundwater inundation, and loss of

ground support);

the identification of ground that may be affected

by displacement, volumetric change (shrinkage or

swelling), or other forms of loss of physical sup-

port or chemical integrity relating to the intended

construction;

the discovery or prediction of ground instability

above, below, on, or adjacent to the site;

the detection and measurement of groundwater

conditions (including perched and transient condi-

tions) that may affect the construction, operation,

or maintenance of the proposed works (including

the estimation of annual variations and chemical

composition, and in particular the potential to

harm human health or the environment);

the discovery or prediction of conditions that may

adversely affect the rate, progress, or method of

construction of the project; and

the detection or prediction of ground conditions

that may be unsafe or endanger the health of per-

sons engaged in construction, or in work, visitation,

or occupancy of the developed site.

Typical Site Characterization

The owner has the primary and ultimate responsibil-

ity for ordering and funding a site characterization

and for ensuring that any constraints identified by the

ENGINEERING GEOLOGY/Site Classification 1

site characterization are addressed in the design, con-

struction, operation, and maintenance of the engin-

eered works. Table 2 assesses the typical goals and

components of the site characterization of a variety of

engineered works.

Areas Subjected to Site

Characterization

In reality, no two parties engaged in planning and

conducting a site characterization will produce iden-

tical pieces of work. The quality and applicability of

every site characterization reflects the experience and

competence of its leadership. Strictly speaking, the

basic (minimal) characterization must assess the site

within the property boundaries. There are many cir-

cumstances, however, in which additional areas of

ground should be considered in order fully to address

the client‘s needs. Table 3 presents some of these

considerations.

Pitfalls of Site Characterization

There are no hard and fast rules or standards that can

be applied to all site characterizations. It is incumbent

on the engineering geologist to inform the owner of

those elements of the work that may best meet the

needs of the design engineers and the owner‘s goals

for the project.

If the owner chooses to underfund the site charac-

terization or selects characterization activities on a

price-sensitive basis, there are numerous ways in

which the end product may fail to give adequate

design-related information. Where price has not

been a determining factor in the scope of the site

Figure 1 Engineering geologists generally work with already flawed earth materials. This weak rock of Pennsylvanian age at Little

Rock, Arkansas, needs to be ripped on excavation, yet it is wholly discontinuous and platy.

2 ENGINEERING GEOLOGY/Site Classification

characterization, there are still categories of charac-

terization flaws that, if not heeded, could endanger

the achievement of design goals. Some of these are

presented in Table 4.

Conclusion

The majority of the project technical responsibilities

accorded to the engineering geologist fall under the

remit of site characterization relating to construction

and waste disposal. The aim is to ensure that properly

funded exploration and evaluation can establish

the geological information required by the design

engineers. This information is located by spatial co-

ordinates and is usually managed and displayed by

computer-assisted drawing and geographical infor-

mation systems. The geologist thereby develops a

database of usable, relevant information in the form

of colour-coded perspective drawings that are easily

assimilated by an array of (non-geoscience) pro-

fessionals who can subsequently manipulate the

data for parametric analysis or for their own better

understanding, and can do so largely without fear of

distorting the relevant facts.

Table 1 General elements of site characterization

Element Purpose Important considerations

Stratigraphy Identify and describe geological formational

units expected in design and construction

Individual engineering geological units

Groundwater regime Define character of groundwater Perched water

Vadose zone

Peizometric surface

Potentiometric surface (if present)

12 months of observation

Top of rock Define elevation or surface below which

excavation is difficult

Little’s grades of weathering will be

helpful here

Rock-mass

characterization

Delimit observable or likely subsurface

bounds of each detectable hard-rock unit

Contacts

Lithological character and variations

Geological structure represented by equal

area stereographic projection; to include

bedding or foliation attitude, discontinuities,

faults, fault zones, shear planes,

metamorphic foliation

Zones of bad ground, by virtue of alteration,

weathering, or shearing

Application of one of the standard rock-mass

qualification concepts

Presence of weak rock Basis of definition, including why the rock is

determined to be weak

Recommendations as to how and why such

weak rock may pose problems in design and/

or construction, operation, and maintenance

Potential problems related to

sedimentological, structural,

or geomorphological

conditions

Portions of surface or subsurface that may be

affected by otherwise unanticipated

features

Buried valleys

Stratigraphical ‘holidays’

Facies changes

Fault-displaced blocks of ground

Dissolution cavities (karst features)

Glacial erratics (boulders)

Irregular features of glacial and periglacial

origin

Potential instability Hillsides Unstable slope masses

River banks Subsurface mined or dissolution caverns

Ground expanse of site

Likely excavation characteristics Relative ease or difficulty of moving earth

media

Outstanding troublesome departures include

glacial, bedrock knobs at top of rock, and

some types of hard igneous intrusions such

as dykes and sills

Geological constraints Observed or postulated geological conditions

that may be known or felt to place the

project concept in jeopardy; may require

additional funds for resolution

Otherwise known as ‘geological hazards’ but

more properly known as ‘geological

constraints’ after the life work of Robert

F Legget

ENGINEERING GEOLOGY/Site Classification 3

Table 2 Elements of site characterization

Construction works Design citeria Characterization

Construction

Foundations Performance of foundation Optimal use of foundation geology to install

minimum-cost foundationStability of adjacent buildings

Stability of excavations Basements and parking spaces Basements are an economic imperative in

cities owing to the cost of landLateral cuts for building space

Recognition of groundwater

Stability of adjacent structures

Drainage and dewatering Keep excavation free of water Construction control of water versus long-

term maintenance of groundwater and

earthworks pore water

Floodworks integrity

Materials of construction Use on the project Dealing with public opposition

Export to urban areas Preservation of water quality

Land takings Provide value to owner Possible mineral resource value

Transportation

Highways and rapid transit Stable roadway Sufficient to move traffic from A to B with

minimal impact on existing infrastructureBorrow for above-ground works

Balanced cut and fill

Airfields Runway space Huge land surfaces subject to incidental

wastes from operationFuel storage

Runoff and waste management Anti-terrorist considerations

Tunnels and underground Optimal excavation method

caverns Promote ground stability Consider spoil handling and disposal, and

stability of overlying build environmentGround control

Management of groundwater

Ports and harbours Wave protection Associated dredging may disturb historic

contaminated sedimentOperational wastes

Navigable rivers Floodworks and flood control Associated dredging may disturb historic

contaminated sedimentLocks and dammed pools

Energy and resource

Dams Foundation integrity

Seismic-withstand behaviour Locate an inherently stable dam-axis

positionSlope stability

Emergency spillways Provide nearby construction materials

Pipelines Route versus geological costs Objective of minimal excavation, siphonage,

and above-ground support Anti-terrorist

considerations

Erosion protection

Seismic stability

Stability of adjacent ground

Power transmission Mainly tower stability Minimize grubbing impacts

Forest resources Slope stability Harvest the resource without diminishing

the condition of the remaining landHaul road location

Potential hydraulic damage

Mineral resources Minimize exploration damage Minimize exploration damage

Opencast Manage acid mine drainage

Underground workings Spoil management

Protection of water and soil values

Mineral beneficiation Concentrate ore value Manage soil impacts

Wastewater effluent

Reservoirs Reduce bottom leakage Requires a considered evaluation of

geomorphology and the primary and

secondary porosities of geological units in

contact with stored water

Sedimentation protection

Shoreline protection

Water quality

Groundwater recharge Use of storm waters Introduce flood and grey waters to the

groundwater regime before their loss to

oceans

Use of treated wastewater

Environmental

Waste disposal facility siting Minimize adverse impacts Control of leachate

Optimize geological conditions Protection of water quality

Locate sufficient daily cover

Water-quality protection Suspended solids Cation exchange leads to degraded water

quality (pollution), which can lead to

health effects (contamination)

Dissolved solids

Discharge into governed water

4 ENGINEERING GEOLOGY/Site Classification

Military impacts Manoeuvre and exercise grounds Vehicular disturbance of ground leads to

sedimentation

Munitions impact zones

Battle damage Spilled fuels and discharged munitions lead

to environmental degradation

Derelict land Waste dumps Ground receives contamination and surface

waters and groundwaters transport this to

human and natural-life receptors

Industrial brownfields

Abandoned mined lands

Battle damage

Environmental remediation Uncontrolled hazardous waste Characterization of site and waste is

essential to formulation of remedial

actions

Without this actual risks become

unacceptable

Saline intrusion Control freshwater withdrawal A mature art requiring only the application

of sufficient fundingSubsurface barriers

Sedimentation Soil particles Results from disturbed ground

Cation exchange Diminishes reservoir capacity when inflow

ceasesRiver and stream transport

Damages wildlife habitat by turbidity

Hazards

Flood hazards Flood channels Depends on geomorphological evidence of

past flooding, integrated with runoff

character of the related watershed

No-build zones in floodplains

Rock falls Gravitational dislodgment of rock and rock

masses above inhabited or frequented

areas

Delimit rock or masses of rock above

endangered populations or transportation

routes

Retain or devise protection from

dislodgment or trajectory impact

Snow avalanches Recurrence intervals, and conditions and

timing of recurrence

A sideline use of geological expertise in

geological-image interpretation

Seasonal advice on mitigation by

explosive intervention or shed

deflectors over railroad and

highway routes

Active faults Ground trace Dealing with vertical splaying of related fault

splays in unconsolidated units

Sense of displacement

Recurrence interval

Likely nature of strong motion Blind thrust faults as unknowns

Unstable ground Collapsing soils (water inflow) Soil properties and character make them

susceptible to damage

Expansive soils (water inflow)

Liquefaction (earthquake strong motion in

the presence of near-surface

groundwater)

Result damages foundations

Shore protection Wave-energy impact analysis Evaluation requires geomorphological

assessment in terms of the structural

character of the shore rock

Susceptibility of geological units to erosion

and toppling

Wave mitigation usually only transfers wave

energy to new targets

Volcanic Lava flows Termination or preclusion not yet possible

Ash falls Geological characterization can delimit

sources, routes, and probable areas

of impact

Pyroclastic bombs

Glowing avalanches

Mudflows (lahars) Mitigation involves avoidance or

channelization

Table 2 Continued

Construction works Design citeria Characterization

Continued

ENGINEERING GEOLOGY/Site Classification 5

Defence

Military posts and installations Characterization of ground for optimal use

in training and staging, while striving for

optimal environmental protection of

military bases

World economic situation currently favours

diminishing grounds allotted to military

forces, while increasing environmental

controls over lands used for military

training

Use of terrain Defence Trafficability

Offense Obstacles and fields of fire

River crossings

Potable water Exploration and development Alternative to use of water in the field of

operations or to importation in

competition with high-priority food

and munitions

Replacement of damaged wells and

reservoirs

Supplies for indigenous peoples and

prisoners of war

Anti-terrorism actions Defence from attack Use or exclusion of use of such features as

caves, excavated tunnels, and unstable

hillsides

Actions against terrorists

In all cases, the characterization should reflect the owner’s objectives, available funding, the normal and standing considerations for

public health, welfare and safety, and applicable and relevant regulations and codes, and should be conducted in compliance with or

with reflection on relevant codes and standards, and be completed by or under the direct supervision of a registered (chartered)

professional geologist. All specialization categories should consider the presence of geological constraints and natural hazards that

may affect the design, construction, operation, and maintenance of the project in a manner that meets the client’s objectives and

applicable national, regional, and local laws and regulations.

Table 3 Key considerations of site characterization

Construction works Key considerations Other considerations

Construction

Foundations To property boundary Adjacent ground that may be affected by the project or

have conditions that may affect the project

Stability of

excavations

Host ground to include likely active mass of

wall or face subject to possible failure

Possible adversely orientated planes or wedges of

geological weakness

Drainage and

dewatering

Maximum radius of cones of depression Search for possible hidden (geomorphological) pathways

of high hydraulic transmissibility

Materials of

construction

Secure projected quantities of acceptable

material

Locate superior materials at somewhat greater distance

Land takings Natural resource value Impact on groundwater resources

Transportation

Highways and

rapid transit

Right-of-way plus additional width affecting

existing infrastructure

Minimum of one boring or geophysical traverse for each

suspected adverse geological possibility

Minimize groundwater impact

Airfields Footprints of load-bearing areas Toxic-waste hotspots in areas of runway expansion

Tunnels and

underground

caverns

Route characterization for estimated

ground conditions

Geormorphologic and structural hazards such as

intercepted bad ground or groundwater flow pathways

Excavation

Ground support

Groundwater management

Spoil management

Ports and harbours Toxic dredge spoils Locations of toxic hotspots

Navigable rivers Lock and dam foundations Stability of caving river banks

Energy and resource

Dams Stability of embankment Consider adjacent ground hazards

Reservoirs Minimize bottom leakage Minimize incoming sediment

Groundwater

recharge

Maximize storativity Use to protect groundwater regime

Pipelines Assess geological excavation cost versus

alternative routes

Minimize environmental impact on the traversed ground

Table 2 Continued

Construction works Design citeria Characterization

6 ENGINEERING GEOLOGY/Site Classification

Power

transmission

Cost-effective tower locations Minimize impacts from unstable ground on or adjacent to

route

Forest resources Minimize hydraulic damage Minimize sediment produced

Mineral resources Minimize exploration and development

impacts

Devise safe spoil-disposal strategies and minimal cation-

exchange leaching

Mineral beneficiation Proper management of spoil Plans for plant closure

Environmental

Waste-disposal

facility siting

Isolate from groundwater Provide adequate cover sources

Water-quality

protection

Remove toxic hotspots Control entry of freshwater

Military impacts Remove explosive relics Grade land for minimal impact

Derelict land Remove toxic hotspots Control entry of freshwater

Environmental

remediation

Remove toxic hotspots Control entry of freshwater

Saline intrusion Control groundwater pumping Inject flood or treated waters

Sedimentation Reduce land disturbance Define interception features

Hazards

Flood hazards Establish likely inundation Evidence of catastrophic flooding

Rock falls Unstable rock masses Impact zones on adjacent ground

Snow avalanches Photogeological evidence of pathways Foundations and anchorage of structures to deflect snow

Active faults Establish likely rupture location Susceptibility to lateral spread or liquefaction

Unstable ground Mainly related to regional physiography Avoidance first; mitigation second

Shore protection Delimit susceptible ground Other shoreline influences, such as groundwater

seepage

Volcanic Pathway of impact Methods to deflect impact

Defence

Military posts and

installations

Footprints of structures Areas of intense use by vehicles

Use of terrain Obstacles Interdiction of enemy movement

Trafficability Neutralization of underground space

Potable water Exploration for wells Water supply basin

Anti-terrorism actions Features for approaches Unstable surrounding ground

Wellfield susceptibility to sabotage

In all cases, the characterization should reflect the owner’s objectives, the level of available funding for exploration and

characterization, and the normal geotechnical, hydrological, and environmental practices employed in the region.

Table 3 Continued

Construction works Key considerations Other considerations

Table 4 Some pitfalls of site characterization

Potential pitfall Nature

Failure to consider owner’s goals and

design conditions

Represent the stated truths of project impact on the geological environment. Owner

should be informed of the consequences of these considerations, though it is the

owner’s responsibility to act on such information

Failure to separate factual from interpreted

geological information

Discrimination should be made in order to represent the owner’s rights and position,

in which the construction contractor bears the normal risks associated with

construction under the conditions reported in the design documents and

construction specifications. Contractors are entitled to additional payments for

geological conditions not adequately defined in the contract documents. Separation

of factual from interpreted information more clearly defines the information

available for consideration by the contractor in bid formulation

Failure to apply site conceptual geological

model

Essentially a schematic drawing (cartoon) of known and possible geological

conditions at the site, superimposed on one or two (perpendicular) sketch

geotechnical profiles, from ground surface to maximum depth of expected impact

on site design

‘Geological possibilities’ It is proper to consider the possibility of certain sedimentological, stratigraphical,

structural, and geomorphological anomalies at the site, resulting from its general

regional physiographical affinity. These possibilities should be assessed in terms

of the site-development plan

Continued

ENGINEERING GEOLOGY/Site Classification 7

Further Reading

Anon (1970) Logging of cores for engineering purposes.

Working Party Report. Quarterly Journal of Engineering

Geology 3: 1–24.

Bell FG (1975) Site Investigations in Areas of Mining Sub-

sidence. London: Newnes-Butterworths.

Bell FG (1992) Description and classification of rock

masses. In: Bell FG (ed.) Engineering in Rock Masses,

pp. 54–77. London: Butterworths–Heinemann.

BellFG, Culshaw MG, Cripps JC, and CoffeyJR (eds.) (1990)

Field Testing in Engineering Geology. Engineering Geol-

ogySpecial PublicationNo. 6. London: Geological Society.

British Standards Institution (1999) Code of Practice on

Site Investigations, BS 5930. London: British Standards

Institution.

Cabrera JG and Hatheway AW (1991) Investigation of

preferred sites for selection and design. In: Kiersch GA

(ed.) Heritage of Engineering Geology – The First Hun-

dred Years, pp. 395–428. Centennial Special Volume 3.

Colorado: Geological Society of America.

Clayton CRI, Matthews MC, and Simons NE (1996) Site

Investigation.2ndedn.Oxford:BlackwellScienceLimited.

Craig C (ed.) (1996) Advances in Site Investigation Prac-

tice. London: Thomas Telford Press.

Culshaw MG, Bell FG, Cripps JC, and O‘Hara M (eds.)

(1987) Planning in Engineering Geology, pp. 151–154.

Regionally important geological material Failure to recognize that the subject earth material in the physiographical province

has special or unique characteristics that affect or its geotechnical behaviour

Geomorphological and structural

geological features

Failure to consider the potential for anomalous subsurface zones, pockets, or buried

valleys of materials that very significantly from those of the general host ground

Neglect to consider discontinuities Failure to recognize that discontinuities represent preformed surfaces of weakness

and therefore have the potential for displacement and sometimes groundwater or

contaminant migration

Assessment of ‘zones’ ‘Zones’ should be invoked whenever evidence suggests the existence of pockets,

bands, or other bounded bodies of earth material that could affect the design or

performance of the project. ‘Zones’ are labelled and delimited by the characterizing

engineering geologist

Deleterious minerals Ground that has been geochemically altered should be expected to potentially

produce

in situ conditions or construction material that may not perform adequately

in terms of project design. Often this occurs along the borders of intrusive bodies

Misassessment of karst Karst terrain typically shows a concentration of dissolution along major joints and at

their intersections. Much of the intervening ground may be devoid of dissolution

features of sufficient magnitude to affect the project. Use of geophysical techniques

is recommended.

Minimal time-sensitive hydrogeological

data

Only minimally accurate assessments of the character and seasonal behaviour of the

groundwater can be made with a full 12 month record of hydrogeological conditions

at and around the site

Inapplicable hydrological borehole testing A prime example is the conduct of downhole packed hydraulic conductivity testing in a

string of end-to-end positions, in which the conductivity is then averaged over the

packed interval, and sometimes over the entire tested interval, whereas the true

impact on project design and performance is more properly assigned to individual

layers or zones, which themselves should be subjected to packed testing. On

occasion this pitfall is used by litigation opposition in order to impart a condition

more favourable to the client’s interests

Belief in the ‘pristine’ brownfield

(derelict) site

All brownfield sites should be considered contaminated until shown otherwise;

brownfield sites cannot be adequately characterized for their toxic wastes without

knowledge of site technical and operational history and an understanding of the

waste-generation nature of the industrial flow path most likely to have been present

during operation

Notion of ‘non-aqueous’ phase liquids Most are toxic to people and the environment. ‘Non-aqueous’ is a chemist’s term;

these compounds have the potential to dissolve in surface water and groundwater

in concentrations inimical to health and the environment

Significance & VOCs and SVOCs Volatile organic compounds (VOCs) migrate generally and in the hydrogeological

system; semi-volatile organic compounds (SVOCs) more commonly reside at their

location of origin or dumping unless transported in sediment; SVOCs are

characteristically recalcitrant to natural or induced degradation

Table 4 Continued

Potential pitfall Nature

8 ENGINEERING GEOLOGY/Site Classification

Engineering Geology Special Publication No 4. London:

The Geological Society.

Department of the Environment (1976) Reclamation of

Derelict Land: Procedure for Locating Abandoned

Mine Shafts. London: Department of the Environment.

Griffiths JS (ed.) (2001) Land Surface Evaluation for En-

gineering Practice. Engineering Geology Special Publica-

tion No. 18. London: Geological Society.

Hatheway AW (2002) Geoenvironmental protocol for site

and waste characterization of former manufactured gas

plants; worldwide remediation challenge in semi-volatile

organic wastes. Engineering Geology 64: 317–338.

Hawkins AB (ed.) (1986) Site Investigation Practice: As-

sessing BS 5930. Engineering Geology Special Publica-

tion No. 2. London: The Geological Society.

Hempen GL and Hatheway AW (1992) Geophysical

Methods for Hazardous Waste Site Characterization.

Special Publication number 3. Lakewood CO: Associ-

ation of Engineering Geologists.

Hudson JA, Brown ET, Fairhurst C, and Hoek E (eds.)

(1993) Comprehensive Rock Engineering: Principles,

Practice and Projects. Oxford: Pergamon Press.

International Society for Testing and Materials. (1995 to

present) Various Site Characterization Standards De-

veloped by Committee D-18; Soil and Rock and Com-

mittee E-50: Environmental Assessment. West

Conshohocken, PA: International Society for Testing

and Materials

International Society for Testing and Materials (1995)

Guide to Site Characterization for Environmental Pur-

poses with Emphasis on Soil, Rock, the Vadose Zone and

Ground Water. Standard D 5730. West Conshohocken

PA: The International Society for Testing and Materials.

Lee LT, Davios WM, Goodson RA, Powell JF, and Register

BA (1994) Site characterization and analysis penetrom-

eter system (SCAPS) field investigation at the Sierra Army

Depot, Herlong, California. In: Report WES/TR/GL-94-

4, U.D. NTIS order number AD-A277887. Vicksburg,

MS: US Army Engineering and Waterways Experimental

Station.

Little AL (1969) The engineering classification of residual

tropical soils. In: Moh Z-C (ed.) Proceedings of the 7th

International Conference on Soil Mechanics and Foun-

dation Engineering: Specialty Session on Engineering

Properties of Lateritic Soils 28–29 August, 1969, Mexico

City, DF, Mexico. pp. 1–10. Bangkok, Thailand: Asian

Institute of Technology.

McCann DM, Eddleston M, Fenning PJ, and Reeves GM

(eds.) (1998) Modern Geophysics in Engineering Geol-

ogy. Engineering Geology Special Publication No. 12.

London: Geological Society.

McDowell PW, Barker RD, Butcher AP, et al. (2002) Geo-

physics in Engineering Investigations. CIRIA C562.

London: Construction Industry Research and Informa-

tion Association.

Olson O (1992) Site Characterization and Validation –

Final Report. Report STRIPA-TR-92-22, (US NTIS

order number DE93-603603). Stockholm: Swedish

Nuclear Fuel Supply Company (SKB; SvenskKaernbraen-

slefoersoerjning AB).

Sara MN (1993) Standard Handbook for Solid and Haz-

ardous Waste Facility Assessments, 2nd edn. Boca Raton,

FL: Lewis Publishers.

Sara MN (2003) Site Assessment and Remediation Hand-

book, 2nd edn. Boca Raton, FL: Lewis Publishers, CRC

Press.

Simons NE, Menzies B, and Matthews M (2002) A Short

Course in Geotechnical Site Investigation. London:

Thomas Telford Press.

Weltman AJ and Head JM (1983) Site Investigation

Manual. CIRIA Special Publication 25. London: Con-

struction Industry Research and Information Association.

Subsidence

A B Hawkins, Charlotte House, Bristol, UK

Introduction

Ground subsidence that results in settlement or col-

lapse of the ground surface is grouped into four main

categories: (1) subsidence due to man-made voids and

natural voids relatively close to the surface (e.g., due to

coal mining, stone mining, or karstic features), (2) sub-

sidence due to the removal of fluids from depth (e.g.,

water/oil extraction) and the consequential change in

effective stress conditions, (3) subsidence due to the

removal of soluble minerals (salt, gypsum) in ground-

water, and (4) subsidence due to the removal of fines in

suspension (piping). In addition, subsidence may occur

related to alluvial deposits, shrink/swell, volcanism,

and thermokarst.

Major Voids – Man-Made and Natural

Mining

The extraction of stone for building construction, and

coal for energy, has taken place since time imme-

morial. Early stone mining invariably extended from

adits excavated into a quarry face, either when moist

stone was required for carving and/or the overbur-

den was prohibitively thick. On the other hand, coal

mining generally extends from shafts, except where

coal seams outcrop on valley sides in areas such as

ENGINEERING GEOLOGY/Subsidence 9

South Wales. There are three main methods of mining,

long wall, pillar and room, and linear. In long-wall

mining, ground subsidence is expected, whereas in

pillar-and-room/stall mining, long-term subsidence is

considered possible. Subsidence is not anticipated in

linear mineral mining.

Long-wall mining is a relatively recent develop-

ment. Up to 80% of the horizon may be removed,

generally using machinery to win material from a

long face. During the extraction process, the roof is

supported; behind the advancing face, the roof is al-

lowed to collapse. Although some bulking will occur

as the material collapses, the depth of settlement at

the surface is commonly one-third to two-thirds the

thickness of the worked seam. Settlement is likely to

extend over a larger area than that mined because the

failure of the ground is controlled by the angle of

draw, commonly 25–45



from the vertical, depending

on the nature of the strata. This method of mining is

applicable only when the material won can be frag-

mented, and hence in Britain has been used mainly for

coal extraction and the working of the Fuller’s Earth

Bed at Bath.

p0020 Pillar-and-room/stall mining has been carried out

for many hundreds of years. It is the main method

of stone extraction and an early method of winning

coal and salt. The size and shape of the support pillars

depend very much on the nature of the material being

won and the depth of the resource. In stone mines,

the pillars may be narrower because the material is

stronger, but because it is also more brittle, failure

may occur by surface spalling. In weak materials such

as coal, salt, and gypsum, creep within the material

would be anticipated and hence the pillars are invari-

ably bigger, the height/width ratio being an important

consideration. As a consequence of the mining, the

ground above the rooms/stalls is de-stressed and spal-

ling of the roof material may occur, resulting in void

migration. Important considerations are the dimen-

sions of the rooms/stalls and the nature of the over-

lying strata. Mudrocks may experience stress-release

fissuring and spall or may act as a competent mater-

ial. Over shallow mines, despite bulking, the voids

may extend through to the surface to form a crown

hole collapse. Alternatively, the upwards migration of

the void may be intercepted by a sandstone horizon

with fewer discontinuities, with the result that the

original height/width ratio of the pillar may be greatly

changed. In this situation, the sandstone acts as a

bridge, but if overstressing of the elongated pillars

results in the collapse of one or more supports, the

adjacent columns must take the additional weight,

creating a scenario whereby larger areas may collapse

as a result of the domino effect. Typical examples

are in the Coal Measures at Bathgate, south-west of

Edinburgh, and the Heidegroeve area in the Nether-

lands, where the roof of a calcarenite mine failed in

June 1988. On both occasions, the sudden collapse of

the ground produced ‘earthquake’ shocks.

p9000In addition to the failure of the roof over pillar-and-

room workings, collapses frequently occur close to

adits and/or mine shafts where the ground experi-

ences more intense climatic change than it does deep

in the mine. Ground failure also commonly occurs

when shafts or bell pits have been inadequately filled

and/or capped.

Linear mineral mining is generally related to the

extraction of lodes of metalliferous minerals. Such

narrow workings rarely cause ground subsidence be-

cause the nature of the rock does not facilitate void

migration and the rocks are sufficiently competent

to arch over the void. However, subsidence may

occur related to old shafts, unless they have been

appropriately filled or capped.

Subsidence manifested through a particular rock

horizon may be created by failure of mining under-

taken in lower strata. Such a phenomenon is common

in the West Midlands, where the Silurian limestones

have been mined as a flux for the iron and steel indus-

try but the subsidence is seen in the overlying Coal

Measure rocks.

Mining frequently involves the necessity to drain

the strata. This may result in consolidation of the

overlying sediments and/or dissolution as ground-

water passes down to much greater depths than

would otherwise occur. The consolidation resulting

from a modification of the groundwater regime in the

area may result in structures above and adjacent to

the workings experiencing settlement. If dissolution

of cements occurs, the percolating groundwaters may

carry a fine fraction in suspension, which can again

result in settlement.

Increasingly, tunnels and other areas of under-

ground space are now being developed. During the

driving of a tunnel, the ground above the work first

experiences extension, and then, as some settlement

occurs, the stress regime changes to one of compres-

sion. Even when every attempt is made to reduce sub-

sidence and its effect on transport routes and property

above a tunnel, including the use of compensation

grouting, a settlement trough is a common feature.

Karst

Calcareous rocks are prone to dissolution by slightly

acidic water, whether the acidity is carbonic acid from

the atmosphere, humic acids from the near-surface or-

ganic-rich horizons, or sulphuric acids associated with

the weathering of iron sulphides. Although the older

limestones (such as the Carboniferous limestones),

10 ENGINEERING GEOLOGY/Subsidence

Elsevier Encyclopedia of Geology - vol I A-E

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