Elsevier Encyclopedia of Geology

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expansive clay minerals as they absorb water. This

breakdown process has been referred to as slaking.

These expansive clay minerals form when basic vol-

canic glass, olivine, pyroxene, or plagioclase is sub-

jected to deuteric alteration brought about by hot

gases and fluids from a magmatic source migrating

through the rock. Disintegration can also be brought

about by the absorption of water by zeolites, minerals

that commonly develop in groundwater and are pre-

sent in gas bubbles (amygdales). However, the break-

down of basalt also depends on its texture, since

water must have access to the minerals that swell as

water is absorbed. The disintegration of some basalts

may take the form of crazing, an extensive microfrac-

turing that develops on exposure to the atmosphere

or to moisture (Figure 6). Such microfractures expand

with time, causing the basalt to disintegrate into

gravel-sized fragments. Some dolerites – intrusive

rocks with a similar mineral composition to basalt –

also undergo slaking.

Individual lava flows may be thin and transected by

a polygonal pattern of cooling joints. They may also

be vesicular or contain pipes, cavities, or even tunnels

(Figure 7).

Pyroclastics (see Pyroclastics) usually give rise to

extremely variable ground conditions owing to wide

variations in their strength, durability, and permea-

bility. Ashes tend to be weak and are often highly

permeable. Those that are metastable are likely to

undergo hydrocompaction on saturation. Moreover,

ashes deposited on slopes are frequently prone to

sliding, yet the irregular shapes of their constituent

Figure 4 Weathered granite grading into soil and showing a

large corestone; near Hillcrest, Natal, South Africa.

Figure 5 An inselberg north of Mbabane, Swaziland, showing

sheet joint up.

Figure 6 Crazing developed in basalt on exposure in the

Transfer Tunnel; Lesotho Highlands Water Scheme.

Figure 7 Amygdales and pipes in basalt, exposed in the Trans-

fer Tunnel; Lesotho Highlands Water Scheme.

546 ENGINEERING GEOLOGY/Problematic Rocks

particles, which can therefore interlock, may enable

very steep slopes to be excavated that can stand up, at

least in the short term. The strength and behaviour of

tuffs depend on their degree of induration. However,

the durability of some basaltic tuffs is poor or very

poor, and they may be susceptible to frost. An ignim-

brite is a pyroclastic rock consisting predominantly of

pumiceous material that shows evidence of having

been formed from a hot and concentrated pyroclastic

flow. Once deposited, induration may be brought

about by welding of viscous glassy fragments, by

devitrification of glassy material, by deposition of

material from escaping gases, and by compaction.

Accordingly, ignimbrites have a wide variety of geo-

technical characteristics, which are attributable to

their modes of eruption, transportation, and depos-

ition. At one extreme they are weak materials that

behave as soils in the engineering sense; at the other

extreme they are strong hard rocks in which extensive

sets of essentially vertical cooling joints are de-

veloped. In fact, non-durable, intermediate, and

highly durable ignimbrites have been recognized.

The non-durable ignimbrites are characterized by

low densities, high porosities, and low unconfined

compressive strengths (5 MPa or less). A hyaloclastite

consists of a mixture of rock and glass fragments.

Initially, hyaloclastites are loose deposits, but as a

result of weathering and diagenesis they usually

become harder, particularly due to palagonitization,

which is a solution–precipitation mechanism whereby

the glass is hydrated and ions are leached out of it.

The process slows down as the precipitation of authi-

genic minerals reduces the porosity and permeability

of the rock mass. An increase in strength due to

palagonitization can occur quickly; for example,

loose pyroclastic material can be transformed into

quite hard compact rocks within 20 years. Weaker

varieties are probably more frequent than stronger

types, often being hidden below the palagonized

hyaloclastites that form the uppermost altered beds.

Slates, phyllites, and schists are characterized by

textures that have a marked preferred orientation.

Anisotropic structures such as cleavage and schistos-

ity, attributable to such textures, not only adversely

affect the strengths of metamorphic rocks but also

make them much more susceptible to decay. Gener-

ally speaking, slates, phyllites, and schists weather

relatively slowly, but the areas of regional meta-

morphism in which they occur have been subject to

extensive folding so that, in places, such rocks may be

extensively fractured and highly deformed. The qual-

ity of schists, slates, and phyllites is generally suspect.

Care must be taken to detect weaker components; for

instance, talc, chlorite, and sericite schists are weak

rocks containing planes of schistosity only 1 mm or so

apart. Some schists become slippery upon weathering

and can therefore fail under a moderately light load.

Sandstones

Sandstones (see Sedimentary Rocks: Sandstones, Dia-

genesis and Porosity Evolution) exhibit a wide range

of strengths (from less than 5.0 MPa to over

150 MPa), depending on their porosity, the amount

and type of cement and/or matrix material, and the

composition and texture of the individual grains.

Higher cement or matrix content and lower porosity

are characteristic of the stronger sandstones. In add-

ition, their strength, like that of other rocks, is re-

duced by moisture content. The strength of saturated

sandstone may be half of what it is when dry. For

instance, the Kidderminster Sandstone (Triassic) has

an average dry unconfined compressive strength of

around 2.5 MPa, but when saturated this may be

reduced to as low as 0.5 MPa. Nevertheless, sand-

stones generally do not give rise to notable engineer-

ing problems. Indeed, sandstones usually have

sufficiently high coefficients of internal friction to

give them high shearing strength when restrained

under load. As a foundation rock, even poorly

cemented sandstone is not normally susceptible to

plastic deformation. Moreover, with the exceptions

of shaly sandstone and rocks where the cement is

readily soluble, clastic rocks are not subject to rapid

deterioration on exposure. Nonetheless, salt action

can give rise to honeycomb weathering in sandstone,

which can cause relatively rapid disfigurement and

deterioration when sandstone is used as a building

stone. The process involves the progressive develop-

ment of closely spaced cavities in the rock. Individual

cavities range from a few millimetres to several cen-

timetres in diameter, although larger cavernous

weathering features, termed tafoni, can develop in

sandstone exposures.

The presence of discontinuities can obviously ad-

versely affect the behaviour of sandstone, reducing

its mass strength. When inclined, discontinuities

may cause rock to slide into unprotected excavations.

Laminations impart a notable anisotropy to sand-

stone, reducing its strength to a significant degree

along the planes of lamination. Furthermore, in cer-

tain engineering situations additional problems can

develop. Friable sandstones, for example, can intro-

duce problems of scour within dam foundations.

Sandstones are also highly vulnerable to scouring

and plucking actions in the overflow from dams and

consequently have to be adequately protected by suit-

able hydraulic structures such as stilling basins.

Quartzose sandstone in a tunnel being excavated by

a tunnel-boring machine can prove highly abrasive to

ENGINEERING GEOLOGY/Problematic Rocks 547

the cutting head, and the cuttings produce an abrasive

slurry when mixed with water.

Sandstones are frequently interbedded with shale.

When such a formation is inclined, the layers of shale

may represent potential sliding surfaces. Sometimes

such interbedding accentuates the undesirable prop-

erties of the shale by concentrating water along the

sandstone–shale contacts, thereby further weakening

the shale.

Mudrocks

Mudrock is the commonest sedimentary rock, the

two principal types being shale and mudstone. Shale

is characterized by its lamination. Mudrock com-

posed of grains of a similar size range and compos-

ition that is not laminated is usually referred to as

mudstone. There is no sharp distinction between

shale and mudstone; one grades into the other.

Shale is frequently regarded as an undesirable

material to work with. Certainly, there are many

examples of failures of structures founded on slopes

in shales. Nonetheless, shales do vary in their engin-

eering behaviour, largely according to their degree of

compaction and cementation. Cemented shales are

invariably stronger and more durable than com-

pacted shales. The degree of packing – and hence

the porosity, void ratio, and density of shale –

depends on the mineral composition, grain-size dis-

tribution, mode of sedimentation, subsequent depth

of burial, tectonic history, and effects of diagenesis.

When the natural moisture content of shales exceeds

20%, they frequently become suspect and tend to

develop potentially high pore-water pressures. Gener-

ally, shales with a cohesion of less than 20 MPa and

an apparent angle of friction of less than 20



are likely

to present engineering problems.

The higher the degree of fissility possessed by a

shale, the greater the anisotropy with regard to

strength, deformation, and permeability. For instance,

the compressive strength and deformation modulus at

right angles to the laminations may be more than

twice those parallel to the laminations.

The greatest variation in the engineering proper-

ties of mudrocks can be attributed to the effects of

weathering. Weathering reduces the amount of in-

duration or removes it completely, leading to an

increase in moisture content and a decrease in density.

Indeed, weathering ultimately returns mudrock to a

normally consolidated remoulded condition by des-

troying the bonds between the grains. Initially,

mudrocks degrade rapidly to form a dominantly

gravel-sized aggregate; this is facilitated by the pres-

ence of polygonal fracture patterns, joints, fissures,

and bedding.

Depending on the relative humidity, some shales

may slake almost immediately when exposed to the

atmosphere. Alternate wetting and drying causes the

rapid breakdown of compaction within the shale.

Low-grade compaction shales, in particular, com-

pletely disintegrate after just a few cycles of drying

and wetting. On the other hand, well-cemented shales

are fairly resistant to slaking. If mudrocks undergo

desiccation, air is drawn into the outer pores and

capillaries as high suction pressures develop. On sat-

uration, the entrapped air is pressurized as water is

drawn into the rock by capillarity. Slaking therefore

stresses the fabric of the rock. Disintegration conse-

quently takes place as a result of air breakage after a

sufficient number of wetting and drying cycles.

The swelling properties of certain mudrocks have

proved to be extremely detrimental to the integrity of

many civil engineering structures. Swelling, espe-

cially in clay shales, is attributable to the absorption

of free water by expansive clay minerals, notably

montmorillonite. Highly fissured overconsolidated

shales have a greater tendency to swell than poorly

fissured clayey shales, because the fissures provide

access for water. The failure of poorly cemented

mudrocks occurs during saturation, when the swell-

ing pressure or internal saturation swelling stress

developed by capillary suction pressures exceeds the

tensile strength.

Uplift is common in excavations in shales, and can

be attributed to swelling and heave. Rebound on

unloading of the shale during excavation is attributed

to heave due to the release of stored strain energy.

Shale relaxes towards the newly excavated face, and

sometimes this occurs as offsets along weaker seams

in the shale. The greatest amount of rebound occurs

in heavily overconsolidated compaction shales.

Settlement of shales can generally be managed

by reducing the unit bearing load, for instance, by

widening the base of a structure or by using spread

footings. In some cases, appreciable differential

settlement is provided for by designing an articulated

structure that is capable of accommodating differen-

tial movement of individual sections without damage.

Severe settlement may take place in low-grade com-

paction shales. However, compaction shales contain

fewer open joints and fissures, which can be com-

pressed beneath a heavy structure, than would the

equivalent cemented shales.

When a load is applied to an essentially saturated

shale foundation, the pore space in the shale decreases

and the pore water attempts to migrate to regions of

lesser load. Owing to the relative impermeability of

shale, water becomes trapped in the voids and can

migrate only slowly. As the load is increased, there

comes a point at which it is partly transferred to the

548 ENGINEERING GEOLOGY/Problematic Rocks

pore water, resulting in a build-up of pore-water pres-

sure. Depending on the permeability of the shale and

the rate of loading, the pore-water pressure can in-

crease, to the extent that it can equal the pressure

imposed by the load. This greatly reduces the shear

strength of the shale, and structures can fail under

such conditions. By contrast, problems with pore-

water pressure are generally less important in

cemented shales.

Sulphur compounds are frequently present in argil-

laceous rocks. An expansion in volume large enough

to cause structural damage can occur when sulphide

minerals, such as pyrite and marcasite, oxidize,

yielding products such as anhydrous and hydrous

sulphates. Significant heave can occur when sulphur

compounds resulting from the breakdown of pyrite

combine with calcium to form gypsum and jarosite.

Movements in excess of 100 mm have been recorded,

with heave rates of about 2 mm per month. The reac-

tions are exothermic, and the resulting increase in

temperature can also have adverse effects, especially

on the ventilation and insulation systems of basement

and underground structures. The decomposition of

sulphur compounds gives rise to aqueous solutions

of sulphate and sulphuric acid, which react with the

tricalcium aluminate in Portland cement to form cal-

cium sulpho-aluminate or ettringite. This reaction is

accompanied by significant expansion and leads to

the deterioration of concrete.

s0030Carbonate Rocks

Carbonate rocks are those that contain more than

50% carbonate minerals (such as calcite and dolo-

mite). The term limestone (see Sedimentary Rocks:

Limestones) is applied to those rocks in which the

carbonate fraction exceeds 50%, over half of which

is calcite or aragonite. If the carbonate material con-

sists chiefly of dolomite, the rock is named dolostone.

Some representative geotechnical properties of car-

bonate rocks are listed in Table 2. It can be seen that,

in general, the densities of these rocks increase with

age, whilst the porosities are reduced. Furthermore,

the porosity has a highly significant influence on the

unconfined compressive strength: as the porosity in-

creases, the strength declines. As both dolomitization

and dedolomitization can give rise to increased por-

osity, both can be responsible for lower compressive

strengths.

Both lithology and age frequently influence the

strength and deformation characteristics of carbonate

rocks. For instance, micritic (microcrystalline calcium

carbonate) limestones may have a higher strength

than sparitic (coarsely crystalline calcite) types.

Most Silurian, Devonian, and Carboniferous lime-

stones in Britain have compressive strengths of over

50 MPa, whereas Jurassic and Cretaceous lime-

stones are often moderately weak, having unconfined

compressive strengths of less than 12.5 MPa.

Table 2 Some geotechnical properties of British carbonate rocks

Property

Limestone

Carboniferous

(Derbyshire)

Magnesian

Limestone Permian

(South Yorkshire)

Great Oolite

Jurassic

(Wiltshire) Lower Chalk Upper Chalk

Dry density (Mg m

3

)

Range 2.55–2.61 2.46–2.58 1.91–2.21 1.85–2.13 1.35–1.61

Mean 2.58 2.51 1.98 2.08 1.44

Porosity (%)

Range 2.4–3.6 8.5–12.0 13.8–23.7 17.2–30.2 29.6–45.7

Mean 2.9 10.4 17.7 20.6 41.7

Dry unconfined compressive

strength (MPa)

Range 65.2–170.9 34.6–69.6 8.9–20.1 19.1–32.7 4.8–6.2

Mean 106.2 54.6 15.6 26.4 5.5

Saturated unconfined compressive

strength (MPa)

Range 56.1–131.6 25.6–49.4 7.8–10.4 8.6–16.2 1.4–2.2

Mean 83.9 36.6 9.3 13.7 1.7

Young’s modulus (GPa)

Range 53.9–79.7 22.3–53.0 9.7–27.8 7.5–18.4 4.2–4.6

Mean 68.9 41.3 16.1 12.7 4.4

Dry density: very low, less than 1.8 Mg m

1

; low, 1.8–2.2 Mg m

1

; moderate, 2.2–2.55 Mg m

1

; high 2.55–2.75, Mg m

1

Porosity: low, 1–5%; medium, 5–15%; high, 15–30%; very high, over 30%.

Unconfined compressive strength: moderately weak, 5–12.5 MPa; moderately strong, 12.5–50 MPa; strong, 50–100 MPa; very strong,

100–200 MPa.

Young’s modulus: very highly deformable, less than 5 GPa; highly deformable, 5–15 GPa; moderate, 15–30 GPa; low, 30–60 GPa; very

low, over 60 GPa.

ENGINEERING GEOLOGY/Problematic Rocks 549

Similarly, the oldest limestones tend to possess the

highest Young’s moduli.

As can be seen from Table 2, the unconfined com-

pressive strengths of the four limestones are reduced

by saturation. The smallest reduction, 21%, is ex-

hibited by the limestone of Carboniferous age,

which is also the strongest. The reductions in strength

of the Magnesian Limestone (Permian), Lincolnshire

Limestone (Jurassic), and Great Oolite (Jurassic) are

35%, 40%, and 42% respectively. In other words, the

strongest material undergoes the smallest reduction

in strength on saturation, and there is a progressive

increase in the average percentage reduction in

strength after saturation as the dry strength of the

limestone decreases; the weakest material shows the

greatest reduction in strength.

Thickly bedded horizontally lying limestones that

are relatively free from solution cavities afford excel-

lent foundations. On the other hand, thinly bedded,

highly folded, or cavernous limestones are likely to

present serious foundation problems. The possibility

of sliding may exist in highly bedded folded se-

quences. Similarly, when beds are separated by layers

of clay or shale, especially when they are inclined,

these may serve as sliding planes and result in failure.

Limestones are commonly transected by joints.

These have generally been subjected to various

degrees of dissolution, so much so that some may

gape (Figure 8). Rain water is usually weakly acidic,

and further acids may be taken into solution from

carbon dioxide or from organic or mineral matter in

the soil. The aggressiveness of the water to a limestone

can be assessed on the basis of the relationship be-

tween the dissolved carbonate content, the pH, and

the temperature of the water. At any given pH,

the cooler the water, the more aggressive it is. If dis-

solution continues, its rate slackens, and it eventually

ceases when the water becomes saturated. Hence,

solution is greatest when the carbonate concentration

is low. This occurs when water is circulating such

that fresh supplies with low lime saturation are con-

tinually available. Freshwater can dissolve up to

400 mg l

1

of calcium carbonate. The solution of lime-

stone, however, is a very slow process. For example, the

mean rates of surface lowering of limestone areas in

Britain tend to range from 0.04 mm year

1

to 0.1 mm

year

1

. Nevertheless, dissolution may be accelerated

by manmade changes in the groundwater conditions

or by a change in the character of the surface water that

drains into the limestone.

An important effect of dissolution within limestone

is the enlargement of the pores, which enhances water

circulation, thereby encouraging further dissolution.

This increases the stress within the remaining rock

fabric, which reduces the strength of the rock mass

and leads to increasing stress corrosion. On loading,

the volume of the voids is reduced by fracture of the

weakened cement between the particles and by the

reorientation of intact aggregates of rock that have

been separated by the loss of bonding. Most of the

resultant settlement takes place rapidly, within a few

days of the application of the load.

The progressive opening of discontinuities by dis-

solution leads to an increase in mass permeability.

Sometimes dissolution produces a highly irregular

pinnacled surface, which may become exposed as a

limestone pavement following soil erosion (Figure 8).

The size, form, abundance, and downward extent of

the aforementioned features depend on the geological

structure and the presence of interbedded impervious

layers. Solution cavities present numerous problems

for the construction of large foundations.

Sinkholes may develop where opened joints inter-

sect, and these may lead to an interlocking system of

subterranean galleries and caverns (Figure 9A). Such

features are associated with karstic landscapes (see

Sedimentary Processes: Karst and Palaeokarst). The

latter are characteristic of thick massive carbonate

rocks. Sinkholes may be classified on the basis of

origin into dissolution, collapse, caprock, dropout,

suffusion, and buried sinkholes. Dissolution sink-

holes form by slow dissolutional lowering of the

outcrop or rockhead when surface water drains

into carbonate rocks. Collapse sinkholes are formed

by the collapse of cavern roofs that have become

unstable owing to removal of support. Rapid collapse

of a roof is very unusual; progressive collapse of

a heavily fissured zone is more common. Caprock

sinkholes are formed when an insoluble rock above

a cavity in a carbonate rock collapses. Dropout

sinkholes are formed by subsidence of a cohesive

soil into cavities beneath. For example, clay soil

Figure 8 Gaping joints due to dissolution, forming grykes in

a limestone pavement above Malham Cove, North Yorkshire,

England.

550 ENGINEERING GEOLOGY/Problematic Rocks

may be capable of bridging a void developed in an

underlying limestone. As the void is enlarged, the clay

cover ravels until it eventually fails to form a dropout

sinkhole (Figure 9B). Ravelling (breaking away) fail-

ures are the most widespread and probably the most

dangerous of all the subsidence phenomena associ-

ated with karstic carbonate rocks. Rapid changes in

moisture content lead to aggravated slabbing in clays.

In particular, lowering of the water table increases the

downward seepage gradient and accelerates down-

ward erosion. It also reduces capillary attraction

and increases the instability of flow through narrow

openings, and gives rise to shrinkage cracks in highly

plastic clays, which weaken the mass in dry weather

and produce concentrated seepage during rains. In-

creased infiltration often initiates failure, especially

when it follows a period when the water table has

been lowered. Suffusion sinkholes develop in non-

cohesive soils where percolating water washes the

soil into a cavity in the underlying limestone. Buried

sinkholes occur in limestone rockhead and are filled

or buried with sediment. They may be developed by

subsurface solution or as normal subaerial sinkholes

that are later filled with sediment.

Unfortunately, the appearance of sinkholes at the

surface is commonly influenced by human activity,

especially groundwater withdrawal or an increased

flow of water into the ground from a point source.

As an example, more than 4000 sinkholes have been

catalogued in Alabama as being caused by human ac-

tivities, with the great majority developing since 1950.

The largest is called the ‘December Giant’ because it

developed suddenly in December 1972. It measures

102 m in diameter and is 26 m deep. Sinkholes are

particularly dangerous when they form instantan-

eously by collapse, and they often occur in significant

numbers within a short time-span. They have resulted

in costly damage to a variety of structures and are a

major local source of groundwater pollution.

Areas underlain by highly cavernous carbonate

rocks possess the most sinkholes; hence, sinkhole

density has proven to be a useful indicator of poten-

tial subsidence. Solution voids preferentially develop

along zones of high secondary permeability because

these concentrate groundwater flow. Data on fracture

orientation and density, fracture intersection density,

and the total length of fractures have been used to

model the presence of solution cavities in limestone.

Accordingly, the locations of areas at high risk of

cavity collapse have been estimated from the intersec-

tions of lineaments formed by fracture traces and

lineated depressions. Aerial photographs have proved

particularly useful in this context. Subsidence-suscep-

tibility maps can be developed using a geographical

information system that incorporates the relevant

data in a spatial context.

The chalk in southern England exhibits a wide

range of dry density values, ranging from as low as

1.25 Mg m

3

up to 2.5 Mg m

3

. Generally, chalk in

northern England (i.e. Yorkshire) is denser than that

in the south-east (Kent). The average porosity of

chalk ranges between about 25% and 40%. The dry

unconfined compressive strength of chalk varies from

moderately weak to moderately strong; the chalk in

Yorkshire tends to be appreciably stronger than that

in Kent. When saturated, chalk undergoes a notable

reduction in compressive strength, frequently by over

50%. The deformation properties of chalk in situ

depend on its strength and on the spacing, tightness,

and orientation of its discontinuities. In addition, the

values of Young’s modulus are influenced by the

amount of weathering that the chalk has undergone.

Six grades of weathering have been recognized, vary-

ing from completely unweathered material to a struc-

tureless me

lange consisting of unweathered and

partially weathered fragments of chalk in a matrix

of deeply weathered chalk. Fresh chalk has a low

compressibility and compresses elastically up to a

critical pressure, which has been termed the apparent

preconsolidation pressure (the yield stress). Marked

breakdown and substantial consolidation occur at

Figure 9 (A) A sinkhole developed as a result of groundwater

lowering at a miner’s recreation centre at Venterspoort, South

Africa. (B) Mechanisms of ravelling.

ENGINEERING GEOLOGY/Problematic Rocks 551

high pressures. The yield stress of intact chalk may

mark the point of pore collapse in high-porosity

chalks (where the porosity exceeds 30%) as the

cement bonds break down. Some interparticle slip

and local grain crushing may also occur. With initial

values of Young’s modulus of 100 MPa and over,

settlement in chalk is not normally a problem

provided that the yield stress is not exceeded.

Chalk, being a relatively pure form of limestone, is

subject to dissolution along discontinuities in the

same way as the other carbonate rocks. However,

subterranean solution features are less common in

chalk, possibly because it is usually weaker than lime-

stone and so collapses as solution occurs. Neverthe-

less, solution pipes and swallow holes are known to

occur in chalk, commonly being found near the con-

tact of Cretaceous chalk with the overlying Tertiary

and Quaternary sediments. High concentrations of

water, including run-off from roads, can lead to the

reactivation of swallow holes and the formation of

small pipes within a few years. Moreover, voids can

gradually migrate upwards through chalk due to ma-

terial collapse. Dissolution can lower the chalk sur-

face beneath the overlying deposits, disturbing the

latter and lowering their degree of packing. Hence,

the chalk surface may be extremely irregular in

places, causing problems of differential settlement to

the foundations of buildings.

Evaporite Rocks

Evaporite deposits are formed by precipitation from

saline waters in arid areas. They include anhydrite

(CaSO

), gypsum (CaSO

 nH

O), rock salt or halite

(NaCl), and sylvite (KCl).

Dry densities and porosities for anhydrite and

gypsum are given in Table 3, from which it can be

seen that the ranges are relatively low. Anhydrite,

according to its unconfined compressive strength,

tends to be very strong, whereas gypsum tends to be

only moderately strong. It appears that the impurity

content of gypsum affects its strength and that mater-

ial containing more impurities is stronger. This may

be because impurities within calcium sulphate rocks

tend to reduce the crystal size and consequently in-

crease the strength. The Young’s moduli are generally

significantly higher for anhydrite than for gypsum. In

other words, the deformability of anhydrite is either

very low or low, whereas that of gypsum varies from

low to high. In both rock types the amount of creep

usually increases with increasing levels of constant

The unconfined compressive strength of rock salt

(halite) is generally moderately weak, whereas that of

sylvite is moderately strong. These two rocks are

either very highly or highly deformable (Table 3). In

rock salt the yield strength may be as little as one-

tenth of the ultimate compressive strength, whereas

in sylvite plastic deformation is generally initiated at

somewhere between 20% and 50% of the load at

failure. Creep may account for anything between

20% and 60% of the strain at failure in these two

evaporitic rock types.

Gypsum is more readily soluble than calcite. Sink-

holes and caverns can therefore develop in thick

beds of gypsum more rapidly than they would in

Table 3 Some geotechnical properties of British evaporite rocks

Property

Anhydrite

Cumbria

Gypsum

North Yorkshire Halite Cheshire

Sylvite

Cleveland

Dry density (Mg m

3

)

Range 2.77–2.82 2.16–2.32 1.92–2.09 1.86–1.99

Mean 2.79 2.21 2.03 1.94

Porosity (%)

Range 3.1–3.7 3.4–9.1 2.7–7.9 3.2–8.7

Mean 3.3 5.1 4.8 5.4

Unconfined compressive strength (MPa)

Range 77.9–126.8 19.0–40.8 9.4–14.9 18.5–31.8

Mean 102.9 27.5 11.7 25.8

Young’s modulus (GPa) (E

t50

)

Range 57.0–86.4 15.6–36.0 1.9–6.3 3.5–11.5

Mean 78.7 24.8 3.8 7.9

Dry density: very low, less than 1.8 Mg m

1

; low, 1.8 to 2.2 Mg m

1

; moderate, 2.2 to 2.55 Mg m

1

; high 2.55 to 2.75, Mg m

1

; very high,

over 2.75 Mg m

1

Porosity: low, 1 to 5%; medium, 5 to 15%. Porosity derived by air porosimeter method.

Unconfined compressive strength: moderately weak, 5 to 12.5 MPa; moderately strong, 12.5 to 50 MPa; strong, 50 to 100 MPa; very

strong, over 100 MPa.

Young’s modulus: very highly deformable, less than 5 GPa; highly deformable, 5 to 15 GPa; moderately deformable, 15 to 30 GPa; low

deformability, 30 to 60 GPa; very low deformability, over 60 GPa.

552 ENGINEERING GEOLOGY/Problematic Rocks

limestone. Indeed, in the USA, such features have

been known to form within a few years, for instance

where beds of gypsum are located beneath a dam.

Extensive surface cracking and subsidence may be

associated with the collapse of cavernous gypsum.

The problem is accentuated by the fact that gypsum

is weaker than limestone and therefore collapses

more readily. Where beds of gypsum approach the

surface, their presence is often indicated by collapse

sinkholes. Such sinkholes can take only a matter of

minutes to appear at the surface. However, where

gypsum is effectively sealed from the ingress of water

by overlying impermeable strata, such as mudstone,

dissolution does not occur.

Massive deposits of gypsum are usually less danger-

ous than those of anhydrite, because gypsum tends to

dissolve steadily, forming caverns or causing progres-

sive settlement. In fact, the solution of massive

gypsum is not likely to give rise to an accelerating

deterioration beneath a foundation if precautions,

such as grouting, are taken to keep seepage velocities

low. On the other hand, massive anhydrite can be

dissolved, leading to runaway situations in which

seepage flow rates increase in a rapidly accelerating

manner. Even small fissures in massive anhydrite can

prove to be dangerous.

If gypsum or anhydrite occur in particulate form in

the ground, their subsequent removal by dissolution

can give rise to significant settlement. In such situ-

ations, the width of the solution zone and its rate of

progress are obviously important as far as the loca-

tion of hydraulic structures is concerned. Anhydrite is

less likely than gypsum to undergo catastrophic solu-

tion in a fragmented or particulate form. Another

point that should be borne in mind, and this particu-

larly applies to conglomerates or breccias cemented

with such soluble material, is that when this material

is removed by solution the rock is greatly reduced in

strength. In addition, when anhydrite comes into con-

tact with water it may become hydrated to form

gypsum. In so doing, there is a volume increase of

between 30% and 58%, which exerts swelling pres-

sures that are commonly between 1 MPa and 8 MPa

and on rare occasions exceed 12 MPa. No great length

of time is required to bring about such hydration.

Rock salt is even more soluble than gypsum, and

the evidence of slumping, brecciation, and collapse

structures in rocks that overlie saliferous strata bear

witness to the fact that rock salt has gone into solu-

tion in past geological times. It is generally believed,

however, that, in humid and semi-arid areas under-

lain by saliferous beds, measurable surface subsidence

is unlikely to occur, except where salt is being ex-

tracted. Perhaps this is because an equilibrium has

been attained between the supply of unsaturated

groundwater and the salt available for solution. Ex-

ceptional cases of rapid subsidence have been

recorded, such as the Meade salt sink in Kansas.

Karstic features, particularly sinkholes, may develop

in salt formations in arid areas. Salt extraction

by some types of solution mining can give rise to

serious or even catastrophic subsidence of the ground

surface.

Organic Rocks: Coal

Coal is an organic deposit composed of different

types of macerated plant tissue, which occurs in asso-

ciation with other sedimentary rocks such as shales,

mudstones, and sandstones. Many coal seams have a

composite character. At the bottom the coal is fre-

quently softer, with bright coal in the centre and dull

coal predominating in the upper part of the seam,

reflecting changes in the type of plant material that

accumulated and in the drainage conditions. Coal

seams may be split, wholly or partially, by washouts.

Coal usually breaks into small blocks that have three

pairs of faces approximately parallel to each other.

These surfaces are referred to as cleat. The cleat dir-

ection is fairly constant and is best developed in

bright coal. Cleat may be coated with films of mineral

matter, commonly calcite, ankerite, and iron pyrite.

The breakdown of iron pyrite frequently gives rise to

acid mine drainage, which is associated with coal

mining activity and has caused problems in mining

areas around the world. The heavy metals in, and the

acidity and sulphate and iron contents of, acid mine

drainage may pollute groundwater and surface water,

and contaminate soils and sediments.

The unconfined compressive strength of coal

varies, but generally it is less then 20 MPa. Exception-

ally, the unconfined compressive strengths of some

coals, such as the Barnsley Hard Coal, may exceed

50 MPa and their Young’s moduli may be greater that

25 GPa. Consequently, the presence of coal seams in

foundations does not usually present a problem.

However, when coal has been mined at shallow

depth, the presence of abandoned bell-pit or pillar-

and-stall workings frequently cause foundation prob-

lems. Longwall mining of coal, which involves the

total extraction of panels of coal, especially when it

occurs at shallow depth, can give rise to notable

subsidence at the ground surface, which, in turn,

may damage buildings and structures or lead to

flooding of agricultural land.

Another problem associated with coal is that it

can spontaneously combust on exposure to air. This

can be regarded as an atmospheric oxidation pro-

cess in which self-heating occurs (i.e. an exothermic

reaction emitting between 5 kCal and 10 kCal per

ENGINEERING GEOLOGY/Problematic Rocks 553

gram of coal). What is more, coal may be oxidized in

the presence of air at temperatures below its ignition

point. If heat is lost to the atmosphere, then the igni-

tion temperature for coal is between 420



C and

480



C. However, where the heat of reaction is

retained, the ignition point falls appreciably to be-

tween 35



C and 140



C. If the heat generated cannot

be dissipated, the temperature rises, which increases

the rate of oxidation so that the reaction becomes

self-sustaining if there is a continuous supply of

oxygen. Some coals ignite more easily than others;

for example, high-rank coals are less prone to spon-

taneous combustion than are low-rank coals. It is not

just coal seams that may spontaneously combust

when exposed at or below the surface: colliery spoil

heaps are also subject to spontaneous combustion.

Obviously, the spontaneous combustion of coal, es-

pecially when it is being worked by opencast mining,

and of spoil heaps when being reclaimed present

appreciable problems.

Elsevier Encyclopedia of Geology - vol I A-E

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