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Geology and Foundations
Chapter 2
Geology and Foundations
GENERAL
All engineering structures, however simple or sophisticated they may be are founded on geological materials. An appreciation of geology is therefore fundamental to the understanding of foundation-structure interactions. In many instances the foundation materials approach the engineering ideal of predictable uniformity, thereby making design and construction procedures more straightforward. The degree of difficulty of a foundation can, therefore, be a direct consequence of the degree of deviation of the geological realities from this engineering ideal but also may be the result of other factors — the scale, cost or sensitivity of the structure, publrc awareness or failure during investigation to anticipate fully the ground conditions. In this chapter it is the geological factors which can give rise to particular difficulties in foundation design which will be highlighted and in summary these include the presence of the following foundation conditions: Compressible, weak or variable materials. Deep overburden. Irregular rockhead. High permeability associated with high water table. ( subsidence Ground movements < seismicity ( mass movement Deleterious physical and chemical changes. Returning to the simple engineering ideals for the present it is clear that soil and rock can be simulated by essentially uniform materials. Taking clay as illustrative of the ideal engineering soil, then a mass of isotropic clay (Figure 2.1, A ) , being composed of finegrained particles is as close to homogeneity as can be achieved. However, in practical terms clays can never achieve such a degree of uniformity for with burial and consequential dissipation of gravitationallyinduced pore pressures the clay will increase in strength and decrease in deformability progressively with depth (Figure 2.1, C ) . During this same process fissuring will develop within the clay (Figure 2.1, C ) thereby creating discontinuities within the clay and increasing the deviation from uniformity. Sedimentary variations will also give rise to lateral and vertical changes in grain size and mineralogy thus contributing to internal variations in material properties (Figure 2.1, D ) . 116
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Similarly, the ideal rock mass can be simulated as a brick wall-like structure with individual joint-defined blocks of uniform materials stacked together in a regular pattern as illustrated in B, Figure 2.1. Three particular factors can contribute to deviations from this ideal — the occurrence of sedimentary layering and associated variations in material properties (Figure 2.1, E ) , less regular, consistent patterns of discontinuities (Figure 2.1, E) and the consequential effects of weathering and stress release (Figure 2.1, F ) . Combined SOIL
ROCK
A
C
B
E
D
F
G
Figure 2 . 1 Diagrammatic representation of the geological factors in soils and rocks which influence their foundation behaviour. A and B are idealized representations; C, D, E and F introduce the geological realities of variability; G combines soil as overburden in contact at rockhead with underlying bedrock
together (Figure 2.1, G ) these various factors introduce a potentially complex pattern of material and mass properties combined with the additional component of rockhead forming the interface between overburden and bedrock.
ENGINEERING
SOILS
Clays are possibly the most quoted example of potentially uniform foundation materials even though the geological history of clays as sediments can contribute
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to significant variations in their material properties. Clay of uniform classification properties deposited in water, and progressively buried, will develop a linear relationship between undrained shear strength and depth such materials being described as normally consolidated sediments (Figure 2.2). If the rate of sedimentation is rapid, for example in excess o f a few cm/yr in the case of marine UNDRAINED
SHEAR STRENGTH
>
\
\
DEPTH
\
\ NormalIv conso
1 \
\
/Weathering
\
idated \
A
Erosion Rapid
v
deposition
Underconsol idated idatec
V
Figure 2.2 Distribution of undrained shear strength with depth for clay sediments
clays, there will be inadequate dissipation of pore water pressures and the clay will not increase in strength with depth. Such underconsolidated materials occur in areas of rapid deposition such as deltas, or lagoons used for the disposal of materials such as fly ash or mine tailings. If the normally consolidated sediment is exposed to subaerial weathering, possibly as a consequence of a fall in sea level, there will almost certainly be a drop in the water table, partial desiccation of the sediment and a consequential increase in strength thereby forming a crust resting on top of weaker materials (Figure 2.2). If the overlying crust together with the underlying clay is removed by erosion then the clay is described as being in an overconsolidated state. Fresh, overconsolidated clay is likely to become softened as a consequence of weathering. Complex situations can develop where there are fluctuations in sea,
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or lake level giving rise to rhythmic sequences of both normally and overconsolidated clays interleaved with stiff fossil soils. One of the best documented examples of marine, normally consolidated clays are in the late-glacial and post-glacial deposits of southern Scandinavia. These sediments were deposited in the sea when the local Earth's surface had been depressed by the weight of glacial ice. With subsequent rebound of the Earth's surface, the sea retreated to the present position and around Oslofjord, for example, the vertical rise has been in excess of 220 m. As a consequence normally consolidated sediments occur both below the sea as well as covering parts of the surrounding land areas. These clays demonstrate a progressive increase in strength with depth, variations in the strength gradient being a consequence of grain size variations which can also be correlated with past climatic changes. There is a surface crust of 2 to 5 m thickness and this has been used as a structural foundation, thereby enabling loads to be spread over the underlying softer and more deformable clays. A factor contributing to soil instability has been the creation of 'quick' clays resulting from the leaching of the salt content from the pore water originating from the sea water in which the clay was originally deposited. A clay laid down in sea water develops a flocculated structure as a consequence of the electrolyte contained in the pore water. Removal of the electrolyte creates a metastable situation so that the clay particles can only achieve stability by collapse of the flocculated structure to a more compact, dispersed form together with the release of excess pore water. Piles driven into such clays thereby rework the clay causing a catastrophic reduction in shear strength. As a result end-bearing, rather than friction, piles founded on bedrock have been commonly used for deep foundations in Oslo. Even though the Oslo clays are relatively young and of low strength, consistent fissure patterns are present both within the soft clays and the overlying crust. The influence of pore water chemistry on clay particle orientation, as reflected by the deposition of clays in fresh or marine waters, continues to be reflected in the engineering behaviour of clays, and shales, at even relatively advanced states of nullification. Thus, in the case of the London Clay, which has been buried to depths of the order of 600—800 m, the stress-strain curves of samples orientated horizontally and vertically (as related to the flat-lying bedding) are markedly different. The vertically-orientated specimens are, in relative terms, more deformable and weaker, and this implies that a horizontal parallelism of clay particles has been impressed on the primary random flocculated structure. Original differences in the salinity of pore water can also contribute to differences in the primary clay fabric. In the case of the Leda Clay (a post-glacial marine clay) and the Seven Sisters Clay ( a glacial lake clay) from Canada the variations in fabric can be1demonstrated by changes in response during shrinkage and subsequent swelling . For example, the Seven Sisters Clay shrinks significantly more at right angles to the bedding (Figure 2.3) than parallel to the bedding; this is a consequence of the preferred clay particle orientation parallel to the bedding formed during deposition in fresh water. In contrast (Figure 2.3) the Leda Clay demonstrates the same difference, but far less markedly, thereby reflecting only a slight parallelism imposed on the original flocculated clay structure formed in sea water. Apart from such alterations in fabric resulting from gravitational compaction, the development of fissures can have important implications on the subsequent engineering behaviour of clay. Fissures can be
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Geology and Foundations 30
/
CHANGE
/ /
/v
IN LENGTH
/ /
0
7 0
MOISTURE
0
J/
CONTENT
80
%
Figure 2.3 Comparative shrinkage curves for Seven Sisters (left) and Leda Clays (right) for vertically and horizontally orientated samples
formed by alternative processes and these include imposed gravitational or tectonic stresses, release of load associated with erosion, and as pore water drainage channels generated during consolidation.
ROCKS
In contrast to clay, sandstone can be adopted as a rock material which in the simplest terms can be composed of quartz grains bonded together. It is, however, apparent that even the simplest even-grained sandstone will be a multicomponent system being composed of quartz grains, matrix (which may not be quartz) and voids. More complex sandstones may involve an assemblage of different mineral grains (e.g. quartz, felspars, micas, lithic fragments) of varying grain size and with a range of alternative cements. This multi-component micro-structure sandstones can be reflected in their response to uniaxial loading. For example, Bunter Sandstone when loaded to failure has a sigmoidal stress-strain curve (Figure 2.4) with greater deformation at low stress levels thereby reflecting the closure of voids and the response of more deformable cementing materials. Repeated loading (Figure 2.4) results in the progressive closure of voids, the development of a permanent strain and a consequential decrease in deformability. Thus, even small scale rock specimens reflect, by their behaviour, the same responses to loading as do rock masses. In adopting an example of a sandstone foundation, the Hartlepool Power Station, in north-east England, has been selected as representing a site with essentially uniform bedrock conditions. The site is underlain by 25 m or more of recent and glacial sediments resting on top of flat-lying Bunter Sandstone bedrock. The upper part of the overburden is composed of soft post-glacial clays
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(Figure 2.5) resting on top of a complex o f glacial outwash sandy gravels. As a consequence, there were alternative choices in foundation selection with2 the glacial sequence and the bedrock forming logical possibilities. In the event , the lighter parts of the structure including the Turbine House were founded on driven cast in situ piles, whereas the Reactor Halls were founded on 2.2 m diameter concrete piles formed in shafts excavated from the ground surface into bedrock (Figure 2.6).
2 MN/m _
20
•
2 /
STRESS
MN/m
10 • 10
0.5 STRAIN
%
0.1
0.2
Figure 2.4 Larger graph is full uniaxial stress-strain curve to failure for Bunter Sandstone core; smaller graph is cyclic loading test for similar core
Figure 2.5 Longitudinal sketch cross-section through Hartlepool Nuclear Power Station site, (a) fill, ( b ) sand dunes, ( c ) marine sands and gravels, ( d ) soft marine silty clays, (e) sands and gravels, (f) stony clay (till), (g) laminated clays, (bs) Bunter Sandstone, (km) Keuper Marl
The investigations for the foundations in rock were carried out by drilling supplemented by cross-hole and down-hole seismic velocity measurements. In this situation the major problem which arises relates to the extent to which the precise settlement characteristics of the rock mass essential to the design of the structures can be anticipated and, particularly, the influence of weathering effects immediately below rockhead identified. Drilling yielded adequate
122
RH
TH
bs
50 m
Figure 2.6 Section through reactor hall 2 and turbine house at Hartlepool nuclear power station
VELOCITY m / s 2000 25p0
Crosshole
•
Downhole
j
Laboratory
I
DEPTH
Figure 2.7 In situ and laboratory longitudinal wave velocity measurements in Bunter Sandstone plotted with depth for Hartlepool nuclear power station site
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samples for conventional laboratory testing and this was supplemented by cyclical loading tests (Figure 2.4) intended to establish the extent to which nonrecoverable strains generated by such means could be equated to strains resulting from long-term creep. The in situ velocity measurements provided an indication of the extent to which the rock mass was fractured. It was of particular interest (Figure 2.7) to recognise that, in the main, the in situ velocity was greater than that measured on saturated specimens in the 2 laboratory. In these circumstances it was apparent that the confined rock mass at depth was not significantly fractured. Settlements recorded for the Reactor Halls were less than 1 cm during construction, well within that anticipated from the laboratory and related tests. It is apparent, therefore, that for an essentially homogenous, confined rock mass in which the effect o f fracturing is not significant programmes of laboratory-scale testing can lead to practical conclusions.
B
A
c• • f f f kn
Figure 2.8 Diagrammatic representation of influence of rock structure on alternative foundation situations
However, in the more general case discontinuities within the rock mass and variations in rock material properties can influence foundation behaviour. Four examples reflecting the most common situations which arise are set out in Figure 2.8: A . Essentially uniform foundations. B. Foundation based on materials with distinct properties giving rise to risks of differential settlement; possible seismic displacements on fault. C. Risk of differential settlement arising from dipping strata with significantly different mass/material properties. D. Instability arising from undercutting of dipping strata. Although these four situations identify those which arise most commonly in practice, other factors, such as the influence of scale, time or water, can be of considerable importance.
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Load transmitted through a rock mass is primarily carried in two ways, by shear transference across joints or by the spanning effect of joint-defined blocks. In consequence, the discontinuity pattern, and thus the slope of joint blocks, has a critical bearing on the manner in which a rock mass will respond as a foundation. Within recent years attempts have been made to relate joint sepa3 quantitative figures for ration, as a readily measurable geological parameter, to the deformability of rock mass. For example, Hobbs has established relationships between the rock mass factor (ratio of the deformability of the rock mass in situ to the deformability of intact rock) and fracture separation, lower rock mass factors being associated with more closely spaced fracturing.
1 Z'0.001
_ —-— ——'
;*•/ \t /*//
(v/v,)
2
----~z^aoi
f
0.5
—-~z5m z=0.5
40
30
20
n
10
FRACTURES /metre
N
(*) - — 2
2
Figure 2.9 Relationship between velocity ratio, ( v f / V j ) and fracture separation, (a and b) results of model experiments with different fracture spacings, (c) quartzitic sandstones at Farahnaz Pahlavi Dam, Iran, ( d ) chalk in south England
A similar approach can be adopted using the velocity ratio (ratio o f in situ to 4 laboratory longitudinal wave velocity squared) and this is illustrated by Figure 2.9; Malone established a series of theoretical curves from the formula:
\ViJ
(l+Nz)
where Af = number of fractures per metre; z = V[t' where t' is the time loss for each fracture. Where velocity measurements are used to assess foundation deformability it is important to bear in mind the effect of confining conditions. This is clearly
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illustrated by Figure 2.10 which is based on laboratory tests on intact rock and a simulated fractured rock with N = 15. It will be noted that the measured velocity is sensitive to the applied stress which results in a closure of discontinuities and thus a higher measured velocity. It is certain that this situation has contributed to the observations made at Hartlepool Power Station (Figure 2.7).
5000 {
VELOCITY
m/s 4000
0
5 APPUED
STRESS
2
MN/m
Figure 2.10 Influence of applied normal stress on longitudinal wave velocity for unfractured rock (spots) and rock with 15 fractures/m (crosses)
An alternative approach is that illustrated by the application o f classifications which provide a numerical rating to different geological characteristics which 5 can influence the engineering behaviour of rock masses. Of these, the Geomechanics Classification (see Chapter 1) allocates points according to the following scales: Points available for allocation Strength of intact rock RQD Joint spacing Condition of joints Groundwater
0-15 3-20 5 30 0-25 0 10 100 maximum
The ratings range from Very Good rock ( > 90) to Very Poor rock ( < 25). It is of interest to note that there is a broad relationship between the rock mass factor and the points awarded under the Geomechanics Classification (Figure 2.11). This provides a method for estimating the relative deformability of rock masses from essentially geological information. One of the major questions which arises in foundation evaluation occurs when the foundation spans rock materials of radically different properties, thereby inducing the possibilities of differential settlement. In the case of the
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Geology and Foundations
Farahnaz Pahlavi Dam, a 112 m buttress dam in northern Iran, the right bank buttresses founded on a steep hillside span a steep zone of shales sandwiched between quartzitic sandstones. In these circumstances there is a risk that shear stresses transmitted along the foot of the buttress webs will not be carried by the poorer quality rock but will be shed downstream, thereby risking the overstressing o f the dam. Passive thrust blocks were constructed downstream of the dam thus securing the buttress web from excessive displacement. I
*
A X X Q5
x X
x
A 0
x
X
X
X A 50 GEOMECHANICS
100"% CLASSIFICATION
X Sedimentary A Metamorphic
• Igneous
Figure 2.11 Relationship between ratings based on Geomechanics Classification for various sites and rock mass factor, (Ef/E,)
Undercutting of dipping strata can result in potential instability and may require special support measures particularly in those cases where the critical area will form a part of the foundations. During the construction of the Hallamshire Hospital in Sheffield during the mid 1960s a slope failure occurred during excavation of Coal Measures shales dipping at 16—18° in a downhill direction. Subsequent shear box tests on bedding planes yielded a peak angle of shearing resistance of about 17°. Although in this example the slip interfered only slightly with construction, as allowance had been made for anchored supports to 6 excavated faces, the event highlighted somewhat greater risks in the Bristol Royal Infirmary development. In the latter case , an alternating sequence o f quartzitic sandstones, siltstones and shales (including clay shales) dipped in a downhill direction at 27°. The excavation plans allowed for undercutting o f the dip, the undercut benches being designed to carry foundation loads. It was apparent that limited failure, at the minimum, was almost inevitable. For these reasons, the foundations of the Outpatients Building were excavated in trench and formed as a series of concrete load-bearing walls and retaining walls. Bulk excavation was then carried out, the rock strata being fully supported by the pre-existing structural walls. Small-scale slips occurred on bedding planes within the trench excavations confirming that the measures adopted had been fully realistic of the ground
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conditions. Similar problems occur in those situations where inclined loads from arch or other kinds of dams are transmitted onto unfavourably orientated discontinuities. This condition was a primary factor in the failure of the Malpasset Dam,in southern France, where the left bank abutted against a potentially unstable structural wedge, one component of which outcropped on the valley side downstream of the dam. SUBSIDENCE Major foundation problems do arise in those cases where there is a likelihood of subsidence and such circumstances arise, in particular, where there are natural or artificial cavities below the ground surface. Removal of solid materials associated with mining will create voids and these may be associated with displacement o f the overlying rocks. Imposition of load onto areas which have been undermined at shallow depth can give rise to large total and differential settlements. Upward propagation of mining-induced voids through rocks can be induced by ground disturbance, loading or modification to the groundwater flow pattern. The 7 effects of such mining activity and the consequences of foundation design are now generally recognised but there is less wide recognition of the effect of 8 undermining on the subsiding rock. 9 It has been argued that subsidence creates permanent void-openings which give rise to an enhanced secondary permeability. However, investigations in association with the design of the Liege Metro in Belgium have demonstrated that subsidence can give rise to a deterioration in the physical properties of both rock materials and rock mass. Samples of rock were collected from boreholes drilled in the Coal Measures o f the Liege area, these samples being tested for unconfined strength under uniform conditions, and also for quartz content. By separating out material from sites which have, and have not, been undermined it is apparent (Figure 2.12) that the rock material collected from undermined areas is significantly weaker than where there has been no mining. Possibly somewhat less surprising is the fact the pressuremeter tests carried out in the boreholes revealed that the mean deformability is consistently greater in those rocks which are in undermined areas as set out in Table 2.1. Table 2.1
Coal Silty Mudstone Sandy Mudstone Sandstone
2
2
Modulus of deformation MN/m Undermined Not undermined
Modulus of elasticity MN/m Undermined Not undermined
15 700 980 1050
720 2150 4700 4700
330 5200 5800 5000
900 8300 7000 10000
The moduli quoted are mean values for all results obtained, the modulus of deformation being based on a single cycle test whereas the modulus o f elasticity is derived after the non-elastic strain has been eliminated by cyclic loading. Although it has been recognised that subsidence fracturing can occur and may, in certain circumstances, be of massive proportions the only numerical evaluation of the consequences of undermining on foundations are those which can be derived from the Liege Metro studies.
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The solution of carbonates and evaporites will result in the generation of voids which can cause, or contribute to, subsidence. In the case of limestone cavities modification to the groundwater flow pattern by groundwater extraction could result in internal erosion of overlying materials into the cavities, thereby resulting in upward, and often catastrophic, transference of voids to the ground surface. If there is a depression in the rockhead associated with a thickening of the overburden differential settlement may occur as a result of a reduction in groundwater pressures and consequential increased consolidation of the thicker cover.
2
MN/m
200
UNCONFINED STRENGTH
100
0
QUARTZ a o
50
CONTENT
100%
No mining Undermined
Figure 2.12 Relationship between uniaxial strength and quartz content for Coal Measure rocks from Liege Metro investigations, a distinction being drawn between those from sites which have, and have not, been undermined by coal extraction
10 Diversity in subsidence processes can be illustrated by the collapses occurring on the northern flanks o f the Western Rand where the underlying dolomitic rocks are dewatered from below by the underlying gold mining operations. Both catastrophic subsidences, associated with no surface evidence of underlying instability, and areas of settlement, possibly related to deeper areas of overburden have been recognised. Mineral extraction of fluids (e.g. water, brine, oil) 1 1 the ground will, similarly, give rise to potentially uncontrolled and gas from 12 which has occurred in Britain has settlement . Possibly the most classic situation been the wild brine pumping in Cheshire and Staffordshire resulting in extensive total and differential settlements. In the case of the Baldwin Hills reservoir in California, the failure of the asphaltic lining to the reservoir appears to have arisen as a consequence of ground displacements around the Inglewood oilfield associated with extraction
13
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129
of hydrocarbons . As subsidence took place the ground above the oilfield subsided with the ground surface being put into compression. Surrounding this zone the ground surface was in tension and this was reflected by ground ruptures on \
\
Reservoir i§; Ground r u p t u r e on fault \
Margin of tension and c o m p r e s s i o n " zones
OILFIELD Figure 2.13 Sketch map of Baldwin Hills reservoir
faults (Figure 2.13). T w o of these faults passed below the reservoir and eventually differential movements cracked the lining causing loss of water and partial washing out of the surrounding embankment.
OTHER
FACTORS
The influence of groundwater on foundation conditions can be significant. The control of groundwater flows and pressures may be an important factor requiring careful evaluation and monitoring during design and construction. However, groundwater can also influence, to a major degree, the engineering properties of certain soils, and some rocks, particularly in the cases where metastable materials or soils subject to internal erosion are present. Granular soils laid down under partially saturated or dry conditions are liable to be deposited in a porous state and this can give rise to uncontrolled collapse when saturated or subject to vibration. The removal of fines, by groundwater flow, from coarser grained soils can result in increases in permeability and, in the case of gravels, the development of an openwork texture. This situation has been identified in High Wycombe, in southern England, where permeability increases in alluvial gravels appear to have been induced by heavy groundwater extraction during trench excavation in the last century. Fines were apparently washed out of the gravels at this stage, subsequently giving rise to high groundwater velocities and consequential erosion damage to cast in situ piles installed several decades afterwards. A variety of physical and chemical changes can contribute to deleterious
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Geology and Foundations
effects on foundation materials. Unstable or reactive minerals such as sulphate or pyrites, or the presence of swelling clay minerals, can result in a deterioration in foundation properties and, in appropriate circumstances, aggressive responses in concretes. Mechanical breakdown of rock materials can occur particularly where the rock is subject to conventional impact loadings. For example chalk is readily re-worked into a soft putty chalk as pore water is released during mechanical breakdown. Similarly driven steel piles founded on weak slates have been found to lose their set rapidly suggesting that softening of the fragmented rock material has taken place. Finally, reference needs to be made to the importance that rockhead can have on foundation problems. Commonly foundations have to be constructed on, or just below, rockhead and, in consequence, the detailed structure of the overburden-bedrock contact may have importance on both design and construction procedures. Figure 2.14 identifies a range of rockhead situations each of
cl
Al
Et
Dl
F
G Figure 2.14 Differing rockhead situations. ( A ) till on rock with glaciallyinduced shears containing till infilling, ( B ) bulged periglacially disturbed rockhead, ( C ) weathered rock below rockhead, ( D ) solution-affected limestone, ( E ) buried channel with alluvial fill, ( F ) buried wind-eroded and undercut rock face, ( G ) buried channel with slope failures on flanks
which illustrates a different origin to the rockhead surface and, by implication, the superincumbent overlying soils. Each of these cases poses different technical questions as presented by the variation in rockhead geometry, the preferential concentration of specific soil types above rockhead, the degree of penetration of the overburden into the rock mass as a whole and the extent to which the upper
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part of the rock mass has been subject to deterioration in engineering properties. In conclusion, therefore, a consideration of the engineering aspects o f rockhead presents a microcosm of the potential influence o f geology on foundations.
References 1. Warkentin, B. and Bozozuk, M., 'Shrinking and swelling of two Canadian clays,' Proc. 5th Int. Conf. SoilMech. and Found. Eng., 1, 851 (1961). 2. Coates, F.W. and Taylor, R.S., 'Hartlepool Power Station: major civil engineering features,'Proc. ICE., 60, 95 (1976). 3. Hobbs, N.B., 'Factors affecting the prediction of settlement of structures on rocks with particular reference to Chalk and Trias,' article in Settlement of Structures, British Geotechnical Society, Pentech Press, 579 (1975). 4. Malone, A.W., 'Elastic wave measurement in rock engineering,' PhD Thesis, University of London. 5. Bieniawski, Z.T., 'Engineering classification of jointed rock masses,' Trans. S. Afr. Inst. Civ.Engrs., 15,355 (1973). 6. Phillips, L. and Knill, J.L., 'Foundation construction at the Bristol Royal Infirmary Phase 1,' Quart. J. Engg. Geol, 6, 207 (1973). 7. Bell, F.G. (Editor), 'Ground conditions in mining areas', article in Methods of treatment of unstable ground, Newnes-Butterworths, London, 112 (1975). 8. Knill, J.L., 'Rock conditions in the Tyne Tunnels, north eastern England', Bull. Ass. Eng. Geol, 10, 1 (1973). 9. Polo-Chiapolini, C.I. Characteristiques geomecaniques des roches du bassin houiller de Liege (Belgique),'Mem. CERES new series, No. 47 (1974). 10. Jennings, J.E., Brinto, A.B.A., Louw, A . and Gowan, G.D., 'Sinkholes and subsidences in the Transvaal Dolomite of South Africa, Proc. 6th Int. Conf. SoilMech. Found. Eng, Montreal, 1,51-54 (1965). 11.Poland, J.F. and Davis, G.H., 'Land subsidence due to withdrawal of fluids,' Reviews in Engineering Geology, Am. Geol. Soc, 190 -269 (1963). 12.Bell, F.G., 'Salt and subsidence in Cheshire, England', Engng Geol, 9, 237-247 (1975). 13.Kresse, F.C., Baldwin Hills Reservoir Failure, article in Engineering Geology in Southern California, Ed by Lung, R. and Proctor, R., Spec publ Ass. Eng. Geol, (1968).
(The illustrations in this section are the copyright of the author)
Geology and Foundations
GENERAL
All engineering structures, however simple or sophisticated they may be are founded on geological materials. An appreciation of geology is therefore fundamental to the understanding of foundation-structure interactions. In many instances the foundation materials approach the engineering ideal of predictable uniformity, thereby making design and construction procedures more straightforward. The degree of difficulty of a foundation can, therefore, be a direct consequence of the degree of deviation of the geological realities from this engineering ideal but also may be the result of other factors — the scale, cost or sensitivity of the structure, publrc awareness or failure during investigation to anticipate fully the ground conditions. In this chapter it is the geological factors which can give rise to particular difficulties in foundation design which will be highlighted and in summary these include the presence of the following foundation conditions: Compressible, weak or variable materials. Deep overburden. Irregular rockhead. High permeability associated with high water table. ( subsidence Ground movements < seismicity ( mass movement Deleterious physical and chemical changes. Returning to the simple engineering ideals for the present it is clear that soil and rock can be simulated by essentially uniform materials. Taking clay as illustrative of the ideal engineering soil, then a mass of isotropic clay (Figure 2.1, A ) , being composed of finegrained particles is as close to homogeneity as can be achieved. However, in practical terms clays can never achieve such a degree of uniformity for with burial and consequential dissipation of gravitationallyinduced pore pressures the clay will increase in strength and decrease in deformability progressively with depth (Figure 2.1, C ) . During this same process fissuring will develop within the clay (Figure 2.1, C ) thereby creating discontinuities within the clay and increasing the deviation from uniformity. Sedimentary variations will also give rise to lateral and vertical changes in grain size and mineralogy thus contributing to internal variations in material properties (Figure 2.1, D ) . 116
Geology and Foundations
111
Similarly, the ideal rock mass can be simulated as a brick wall-like structure with individual joint-defined blocks of uniform materials stacked together in a regular pattern as illustrated in B, Figure 2.1. Three particular factors can contribute to deviations from this ideal — the occurrence of sedimentary layering and associated variations in material properties (Figure 2.1, E ) , less regular, consistent patterns of discontinuities (Figure 2.1, E) and the consequential effects of weathering and stress release (Figure 2.1, F ) . Combined SOIL
ROCK
A
C
B
E
D
F
G
Figure 2 . 1 Diagrammatic representation of the geological factors in soils and rocks which influence their foundation behaviour. A and B are idealized representations; C, D, E and F introduce the geological realities of variability; G combines soil as overburden in contact at rockhead with underlying bedrock
together (Figure 2.1, G ) these various factors introduce a potentially complex pattern of material and mass properties combined with the additional component of rockhead forming the interface between overburden and bedrock.
ENGINEERING
SOILS
Clays are possibly the most quoted example of potentially uniform foundation materials even though the geological history of clays as sediments can contribute
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Geology and Foundations
to significant variations in their material properties. Clay of uniform classification properties deposited in water, and progressively buried, will develop a linear relationship between undrained shear strength and depth such materials being described as normally consolidated sediments (Figure 2.2). If the rate of sedimentation is rapid, for example in excess o f a few cm/yr in the case of marine UNDRAINED
SHEAR STRENGTH
>
\
\
DEPTH
\
\ NormalIv conso
1 \
\
/Weathering
\
idated \
A
Erosion Rapid
v
deposition
Underconsol idated idatec
V
Figure 2.2 Distribution of undrained shear strength with depth for clay sediments
clays, there will be inadequate dissipation of pore water pressures and the clay will not increase in strength with depth. Such underconsolidated materials occur in areas of rapid deposition such as deltas, or lagoons used for the disposal of materials such as fly ash or mine tailings. If the normally consolidated sediment is exposed to subaerial weathering, possibly as a consequence of a fall in sea level, there will almost certainly be a drop in the water table, partial desiccation of the sediment and a consequential increase in strength thereby forming a crust resting on top of weaker materials (Figure 2.2). If the overlying crust together with the underlying clay is removed by erosion then the clay is described as being in an overconsolidated state. Fresh, overconsolidated clay is likely to become softened as a consequence of weathering. Complex situations can develop where there are fluctuations in sea,
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or lake level giving rise to rhythmic sequences of both normally and overconsolidated clays interleaved with stiff fossil soils. One of the best documented examples of marine, normally consolidated clays are in the late-glacial and post-glacial deposits of southern Scandinavia. These sediments were deposited in the sea when the local Earth's surface had been depressed by the weight of glacial ice. With subsequent rebound of the Earth's surface, the sea retreated to the present position and around Oslofjord, for example, the vertical rise has been in excess of 220 m. As a consequence normally consolidated sediments occur both below the sea as well as covering parts of the surrounding land areas. These clays demonstrate a progressive increase in strength with depth, variations in the strength gradient being a consequence of grain size variations which can also be correlated with past climatic changes. There is a surface crust of 2 to 5 m thickness and this has been used as a structural foundation, thereby enabling loads to be spread over the underlying softer and more deformable clays. A factor contributing to soil instability has been the creation of 'quick' clays resulting from the leaching of the salt content from the pore water originating from the sea water in which the clay was originally deposited. A clay laid down in sea water develops a flocculated structure as a consequence of the electrolyte contained in the pore water. Removal of the electrolyte creates a metastable situation so that the clay particles can only achieve stability by collapse of the flocculated structure to a more compact, dispersed form together with the release of excess pore water. Piles driven into such clays thereby rework the clay causing a catastrophic reduction in shear strength. As a result end-bearing, rather than friction, piles founded on bedrock have been commonly used for deep foundations in Oslo. Even though the Oslo clays are relatively young and of low strength, consistent fissure patterns are present both within the soft clays and the overlying crust. The influence of pore water chemistry on clay particle orientation, as reflected by the deposition of clays in fresh or marine waters, continues to be reflected in the engineering behaviour of clays, and shales, at even relatively advanced states of nullification. Thus, in the case of the London Clay, which has been buried to depths of the order of 600—800 m, the stress-strain curves of samples orientated horizontally and vertically (as related to the flat-lying bedding) are markedly different. The vertically-orientated specimens are, in relative terms, more deformable and weaker, and this implies that a horizontal parallelism of clay particles has been impressed on the primary random flocculated structure. Original differences in the salinity of pore water can also contribute to differences in the primary clay fabric. In the case of the Leda Clay (a post-glacial marine clay) and the Seven Sisters Clay ( a glacial lake clay) from Canada the variations in fabric can be1demonstrated by changes in response during shrinkage and subsequent swelling . For example, the Seven Sisters Clay shrinks significantly more at right angles to the bedding (Figure 2.3) than parallel to the bedding; this is a consequence of the preferred clay particle orientation parallel to the bedding formed during deposition in fresh water. In contrast (Figure 2.3) the Leda Clay demonstrates the same difference, but far less markedly, thereby reflecting only a slight parallelism imposed on the original flocculated clay structure formed in sea water. Apart from such alterations in fabric resulting from gravitational compaction, the development of fissures can have important implications on the subsequent engineering behaviour of clay. Fissures can be
120
Geology and Foundations 30
/
CHANGE
/ /
/v
IN LENGTH
/ /
0
7 0
MOISTURE
0
J/
CONTENT
80
%
Figure 2.3 Comparative shrinkage curves for Seven Sisters (left) and Leda Clays (right) for vertically and horizontally orientated samples
formed by alternative processes and these include imposed gravitational or tectonic stresses, release of load associated with erosion, and as pore water drainage channels generated during consolidation.
ROCKS
In contrast to clay, sandstone can be adopted as a rock material which in the simplest terms can be composed of quartz grains bonded together. It is, however, apparent that even the simplest even-grained sandstone will be a multicomponent system being composed of quartz grains, matrix (which may not be quartz) and voids. More complex sandstones may involve an assemblage of different mineral grains (e.g. quartz, felspars, micas, lithic fragments) of varying grain size and with a range of alternative cements. This multi-component micro-structure sandstones can be reflected in their response to uniaxial loading. For example, Bunter Sandstone when loaded to failure has a sigmoidal stress-strain curve (Figure 2.4) with greater deformation at low stress levels thereby reflecting the closure of voids and the response of more deformable cementing materials. Repeated loading (Figure 2.4) results in the progressive closure of voids, the development of a permanent strain and a consequential decrease in deformability. Thus, even small scale rock specimens reflect, by their behaviour, the same responses to loading as do rock masses. In adopting an example of a sandstone foundation, the Hartlepool Power Station, in north-east England, has been selected as representing a site with essentially uniform bedrock conditions. The site is underlain by 25 m or more of recent and glacial sediments resting on top of flat-lying Bunter Sandstone bedrock. The upper part of the overburden is composed of soft post-glacial clays
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121
(Figure 2.5) resting on top of a complex o f glacial outwash sandy gravels. As a consequence, there were alternative choices in foundation selection with2 the glacial sequence and the bedrock forming logical possibilities. In the event , the lighter parts of the structure including the Turbine House were founded on driven cast in situ piles, whereas the Reactor Halls were founded on 2.2 m diameter concrete piles formed in shafts excavated from the ground surface into bedrock (Figure 2.6).
2 MN/m _
20
•
2 /
STRESS
MN/m
10 • 10
0.5 STRAIN
%
0.1
0.2
Figure 2.4 Larger graph is full uniaxial stress-strain curve to failure for Bunter Sandstone core; smaller graph is cyclic loading test for similar core
Figure 2.5 Longitudinal sketch cross-section through Hartlepool Nuclear Power Station site, (a) fill, ( b ) sand dunes, ( c ) marine sands and gravels, ( d ) soft marine silty clays, (e) sands and gravels, (f) stony clay (till), (g) laminated clays, (bs) Bunter Sandstone, (km) Keuper Marl
The investigations for the foundations in rock were carried out by drilling supplemented by cross-hole and down-hole seismic velocity measurements. In this situation the major problem which arises relates to the extent to which the precise settlement characteristics of the rock mass essential to the design of the structures can be anticipated and, particularly, the influence of weathering effects immediately below rockhead identified. Drilling yielded adequate
122
RH
TH
bs
50 m
Figure 2.6 Section through reactor hall 2 and turbine house at Hartlepool nuclear power station
VELOCITY m / s 2000 25p0
Crosshole
•
Downhole
j
Laboratory
I
DEPTH
Figure 2.7 In situ and laboratory longitudinal wave velocity measurements in Bunter Sandstone plotted with depth for Hartlepool nuclear power station site
Geology and Foundations
123
samples for conventional laboratory testing and this was supplemented by cyclical loading tests (Figure 2.4) intended to establish the extent to which nonrecoverable strains generated by such means could be equated to strains resulting from long-term creep. The in situ velocity measurements provided an indication of the extent to which the rock mass was fractured. It was of particular interest (Figure 2.7) to recognise that, in the main, the in situ velocity was greater than that measured on saturated specimens in the 2 laboratory. In these circumstances it was apparent that the confined rock mass at depth was not significantly fractured. Settlements recorded for the Reactor Halls were less than 1 cm during construction, well within that anticipated from the laboratory and related tests. It is apparent, therefore, that for an essentially homogenous, confined rock mass in which the effect o f fracturing is not significant programmes of laboratory-scale testing can lead to practical conclusions.
B
A
c• • f f f kn
Figure 2.8 Diagrammatic representation of influence of rock structure on alternative foundation situations
However, in the more general case discontinuities within the rock mass and variations in rock material properties can influence foundation behaviour. Four examples reflecting the most common situations which arise are set out in Figure 2.8: A . Essentially uniform foundations. B. Foundation based on materials with distinct properties giving rise to risks of differential settlement; possible seismic displacements on fault. C. Risk of differential settlement arising from dipping strata with significantly different mass/material properties. D. Instability arising from undercutting of dipping strata. Although these four situations identify those which arise most commonly in practice, other factors, such as the influence of scale, time or water, can be of considerable importance.
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Load transmitted through a rock mass is primarily carried in two ways, by shear transference across joints or by the spanning effect of joint-defined blocks. In consequence, the discontinuity pattern, and thus the slope of joint blocks, has a critical bearing on the manner in which a rock mass will respond as a foundation. Within recent years attempts have been made to relate joint sepa3 quantitative figures for ration, as a readily measurable geological parameter, to the deformability of rock mass. For example, Hobbs has established relationships between the rock mass factor (ratio of the deformability of the rock mass in situ to the deformability of intact rock) and fracture separation, lower rock mass factors being associated with more closely spaced fracturing.
1 Z'0.001
_ —-— ——'
;*•/ \t /*//
(v/v,)
2
----~z^aoi
f
0.5
—-~z5m z=0.5
40
30
20
n
10
FRACTURES /metre
N
(*) - — 2
2
Figure 2.9 Relationship between velocity ratio, ( v f / V j ) and fracture separation, (a and b) results of model experiments with different fracture spacings, (c) quartzitic sandstones at Farahnaz Pahlavi Dam, Iran, ( d ) chalk in south England
A similar approach can be adopted using the velocity ratio (ratio o f in situ to 4 laboratory longitudinal wave velocity squared) and this is illustrated by Figure 2.9; Malone established a series of theoretical curves from the formula:
\ViJ
(l+Nz)
where Af = number of fractures per metre; z = V[t' where t' is the time loss for each fracture. Where velocity measurements are used to assess foundation deformability it is important to bear in mind the effect of confining conditions. This is clearly
Geology and Foundations
125
illustrated by Figure 2.10 which is based on laboratory tests on intact rock and a simulated fractured rock with N = 15. It will be noted that the measured velocity is sensitive to the applied stress which results in a closure of discontinuities and thus a higher measured velocity. It is certain that this situation has contributed to the observations made at Hartlepool Power Station (Figure 2.7).
5000 {
VELOCITY
m/s 4000
0
5 APPUED
STRESS
2
MN/m
Figure 2.10 Influence of applied normal stress on longitudinal wave velocity for unfractured rock (spots) and rock with 15 fractures/m (crosses)
An alternative approach is that illustrated by the application o f classifications which provide a numerical rating to different geological characteristics which 5 can influence the engineering behaviour of rock masses. Of these, the Geomechanics Classification (see Chapter 1) allocates points according to the following scales: Points available for allocation Strength of intact rock RQD Joint spacing Condition of joints Groundwater
0-15 3-20 5 30 0-25 0 10 100 maximum
The ratings range from Very Good rock ( > 90) to Very Poor rock ( < 25). It is of interest to note that there is a broad relationship between the rock mass factor and the points awarded under the Geomechanics Classification (Figure 2.11). This provides a method for estimating the relative deformability of rock masses from essentially geological information. One of the major questions which arises in foundation evaluation occurs when the foundation spans rock materials of radically different properties, thereby inducing the possibilities of differential settlement. In the case of the
126
Geology and Foundations
Farahnaz Pahlavi Dam, a 112 m buttress dam in northern Iran, the right bank buttresses founded on a steep hillside span a steep zone of shales sandwiched between quartzitic sandstones. In these circumstances there is a risk that shear stresses transmitted along the foot of the buttress webs will not be carried by the poorer quality rock but will be shed downstream, thereby risking the overstressing o f the dam. Passive thrust blocks were constructed downstream of the dam thus securing the buttress web from excessive displacement. I
*
A X X Q5
x X
x
A 0
x
X
X
X A 50 GEOMECHANICS
100"% CLASSIFICATION
X Sedimentary A Metamorphic
• Igneous
Figure 2.11 Relationship between ratings based on Geomechanics Classification for various sites and rock mass factor, (Ef/E,)
Undercutting of dipping strata can result in potential instability and may require special support measures particularly in those cases where the critical area will form a part of the foundations. During the construction of the Hallamshire Hospital in Sheffield during the mid 1960s a slope failure occurred during excavation of Coal Measures shales dipping at 16—18° in a downhill direction. Subsequent shear box tests on bedding planes yielded a peak angle of shearing resistance of about 17°. Although in this example the slip interfered only slightly with construction, as allowance had been made for anchored supports to 6 excavated faces, the event highlighted somewhat greater risks in the Bristol Royal Infirmary development. In the latter case , an alternating sequence o f quartzitic sandstones, siltstones and shales (including clay shales) dipped in a downhill direction at 27°. The excavation plans allowed for undercutting o f the dip, the undercut benches being designed to carry foundation loads. It was apparent that limited failure, at the minimum, was almost inevitable. For these reasons, the foundations of the Outpatients Building were excavated in trench and formed as a series of concrete load-bearing walls and retaining walls. Bulk excavation was then carried out, the rock strata being fully supported by the pre-existing structural walls. Small-scale slips occurred on bedding planes within the trench excavations confirming that the measures adopted had been fully realistic of the ground
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111
conditions. Similar problems occur in those situations where inclined loads from arch or other kinds of dams are transmitted onto unfavourably orientated discontinuities. This condition was a primary factor in the failure of the Malpasset Dam,in southern France, where the left bank abutted against a potentially unstable structural wedge, one component of which outcropped on the valley side downstream of the dam. SUBSIDENCE Major foundation problems do arise in those cases where there is a likelihood of subsidence and such circumstances arise, in particular, where there are natural or artificial cavities below the ground surface. Removal of solid materials associated with mining will create voids and these may be associated with displacement o f the overlying rocks. Imposition of load onto areas which have been undermined at shallow depth can give rise to large total and differential settlements. Upward propagation of mining-induced voids through rocks can be induced by ground disturbance, loading or modification to the groundwater flow pattern. The 7 effects of such mining activity and the consequences of foundation design are now generally recognised but there is less wide recognition of the effect of 8 undermining on the subsiding rock. 9 It has been argued that subsidence creates permanent void-openings which give rise to an enhanced secondary permeability. However, investigations in association with the design of the Liege Metro in Belgium have demonstrated that subsidence can give rise to a deterioration in the physical properties of both rock materials and rock mass. Samples of rock were collected from boreholes drilled in the Coal Measures o f the Liege area, these samples being tested for unconfined strength under uniform conditions, and also for quartz content. By separating out material from sites which have, and have not, been undermined it is apparent (Figure 2.12) that the rock material collected from undermined areas is significantly weaker than where there has been no mining. Possibly somewhat less surprising is the fact the pressuremeter tests carried out in the boreholes revealed that the mean deformability is consistently greater in those rocks which are in undermined areas as set out in Table 2.1. Table 2.1
Coal Silty Mudstone Sandy Mudstone Sandstone
2
2
Modulus of deformation MN/m Undermined Not undermined
Modulus of elasticity MN/m Undermined Not undermined
15 700 980 1050
720 2150 4700 4700
330 5200 5800 5000
900 8300 7000 10000
The moduli quoted are mean values for all results obtained, the modulus of deformation being based on a single cycle test whereas the modulus o f elasticity is derived after the non-elastic strain has been eliminated by cyclic loading. Although it has been recognised that subsidence fracturing can occur and may, in certain circumstances, be of massive proportions the only numerical evaluation of the consequences of undermining on foundations are those which can be derived from the Liege Metro studies.
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128
The solution of carbonates and evaporites will result in the generation of voids which can cause, or contribute to, subsidence. In the case of limestone cavities modification to the groundwater flow pattern by groundwater extraction could result in internal erosion of overlying materials into the cavities, thereby resulting in upward, and often catastrophic, transference of voids to the ground surface. If there is a depression in the rockhead associated with a thickening of the overburden differential settlement may occur as a result of a reduction in groundwater pressures and consequential increased consolidation of the thicker cover.
2
MN/m
200
UNCONFINED STRENGTH
100
0
QUARTZ a o
50
CONTENT
100%
No mining Undermined
Figure 2.12 Relationship between uniaxial strength and quartz content for Coal Measure rocks from Liege Metro investigations, a distinction being drawn between those from sites which have, and have not, been undermined by coal extraction
10 Diversity in subsidence processes can be illustrated by the collapses occurring on the northern flanks o f the Western Rand where the underlying dolomitic rocks are dewatered from below by the underlying gold mining operations. Both catastrophic subsidences, associated with no surface evidence of underlying instability, and areas of settlement, possibly related to deeper areas of overburden have been recognised. Mineral extraction of fluids (e.g. water, brine, oil) 1 1 the ground will, similarly, give rise to potentially uncontrolled and gas from 12 which has occurred in Britain has settlement . Possibly the most classic situation been the wild brine pumping in Cheshire and Staffordshire resulting in extensive total and differential settlements. In the case of the Baldwin Hills reservoir in California, the failure of the asphaltic lining to the reservoir appears to have arisen as a consequence of ground displacements around the Inglewood oilfield associated with extraction
13
Geology and Foundations
129
of hydrocarbons . As subsidence took place the ground above the oilfield subsided with the ground surface being put into compression. Surrounding this zone the ground surface was in tension and this was reflected by ground ruptures on \
\
Reservoir i§; Ground r u p t u r e on fault \
Margin of tension and c o m p r e s s i o n " zones
OILFIELD Figure 2.13 Sketch map of Baldwin Hills reservoir
faults (Figure 2.13). T w o of these faults passed below the reservoir and eventually differential movements cracked the lining causing loss of water and partial washing out of the surrounding embankment.
OTHER
FACTORS
The influence of groundwater on foundation conditions can be significant. The control of groundwater flows and pressures may be an important factor requiring careful evaluation and monitoring during design and construction. However, groundwater can also influence, to a major degree, the engineering properties of certain soils, and some rocks, particularly in the cases where metastable materials or soils subject to internal erosion are present. Granular soils laid down under partially saturated or dry conditions are liable to be deposited in a porous state and this can give rise to uncontrolled collapse when saturated or subject to vibration. The removal of fines, by groundwater flow, from coarser grained soils can result in increases in permeability and, in the case of gravels, the development of an openwork texture. This situation has been identified in High Wycombe, in southern England, where permeability increases in alluvial gravels appear to have been induced by heavy groundwater extraction during trench excavation in the last century. Fines were apparently washed out of the gravels at this stage, subsequently giving rise to high groundwater velocities and consequential erosion damage to cast in situ piles installed several decades afterwards. A variety of physical and chemical changes can contribute to deleterious
130
Geology and Foundations
effects on foundation materials. Unstable or reactive minerals such as sulphate or pyrites, or the presence of swelling clay minerals, can result in a deterioration in foundation properties and, in appropriate circumstances, aggressive responses in concretes. Mechanical breakdown of rock materials can occur particularly where the rock is subject to conventional impact loadings. For example chalk is readily re-worked into a soft putty chalk as pore water is released during mechanical breakdown. Similarly driven steel piles founded on weak slates have been found to lose their set rapidly suggesting that softening of the fragmented rock material has taken place. Finally, reference needs to be made to the importance that rockhead can have on foundation problems. Commonly foundations have to be constructed on, or just below, rockhead and, in consequence, the detailed structure of the overburden-bedrock contact may have importance on both design and construction procedures. Figure 2.14 identifies a range of rockhead situations each of
cl
Al
Et
Dl
F
G Figure 2.14 Differing rockhead situations. ( A ) till on rock with glaciallyinduced shears containing till infilling, ( B ) bulged periglacially disturbed rockhead, ( C ) weathered rock below rockhead, ( D ) solution-affected limestone, ( E ) buried channel with alluvial fill, ( F ) buried wind-eroded and undercut rock face, ( G ) buried channel with slope failures on flanks
which illustrates a different origin to the rockhead surface and, by implication, the superincumbent overlying soils. Each of these cases poses different technical questions as presented by the variation in rockhead geometry, the preferential concentration of specific soil types above rockhead, the degree of penetration of the overburden into the rock mass as a whole and the extent to which the upper
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131
part of the rock mass has been subject to deterioration in engineering properties. In conclusion, therefore, a consideration of the engineering aspects o f rockhead presents a microcosm of the potential influence o f geology on foundations.
References 1. Warkentin, B. and Bozozuk, M., 'Shrinking and swelling of two Canadian clays,' Proc. 5th Int. Conf. SoilMech. and Found. Eng., 1, 851 (1961). 2. Coates, F.W. and Taylor, R.S., 'Hartlepool Power Station: major civil engineering features,'Proc. ICE., 60, 95 (1976). 3. Hobbs, N.B., 'Factors affecting the prediction of settlement of structures on rocks with particular reference to Chalk and Trias,' article in Settlement of Structures, British Geotechnical Society, Pentech Press, 579 (1975). 4. Malone, A.W., 'Elastic wave measurement in rock engineering,' PhD Thesis, University of London. 5. Bieniawski, Z.T., 'Engineering classification of jointed rock masses,' Trans. S. Afr. Inst. Civ.Engrs., 15,355 (1973). 6. Phillips, L. and Knill, J.L., 'Foundation construction at the Bristol Royal Infirmary Phase 1,' Quart. J. Engg. Geol, 6, 207 (1973). 7. Bell, F.G. (Editor), 'Ground conditions in mining areas', article in Methods of treatment of unstable ground, Newnes-Butterworths, London, 112 (1975). 8. Knill, J.L., 'Rock conditions in the Tyne Tunnels, north eastern England', Bull. Ass. Eng. Geol, 10, 1 (1973). 9. Polo-Chiapolini, C.I. Characteristiques geomecaniques des roches du bassin houiller de Liege (Belgique),'Mem. CERES new series, No. 47 (1974). 10. Jennings, J.E., Brinto, A.B.A., Louw, A . and Gowan, G.D., 'Sinkholes and subsidences in the Transvaal Dolomite of South Africa, Proc. 6th Int. Conf. SoilMech. Found. Eng, Montreal, 1,51-54 (1965). 11.Poland, J.F. and Davis, G.H., 'Land subsidence due to withdrawal of fluids,' Reviews in Engineering Geology, Am. Geol. Soc, 190 -269 (1963). 12.Bell, F.G., 'Salt and subsidence in Cheshire, England', Engng Geol, 9, 237-247 (1975). 13.Kresse, F.C., Baldwin Hills Reservoir Failure, article in Engineering Geology in Southern California, Ed by Lung, R. and Proctor, R., Spec publ Ass. Eng. Geol, (1968).
(The illustrations in this section are the copyright of the author)