1975_12_84

The Presence and Influence of Fissures in the Boulder Clays of West Central Scotland

Department of Civil Engineering, University of Strathclyde, Glasgow G4 ONG, Scotlnnd Received May 21, 1974

Accepted September 6, 1974 The recent urban and rural developments in west central Scotland have involved the execution

of much more widespread, and often deeper, excavations than previously undertaken in the boulder clays of the area. A number of failures in deep trenches and slope excavations have occurred, whose character appeared to be influenced by the presence of macrofabric features. A detailed field study has confirmed the presence of fissures and showed that they exhibit definite preferred orientation patterns. The influence of these fissure patterns on undrained shear strength and consolidation properties have been assessed using test specimens ranging from 76 to 254 mm diameter and correlations between observed laboratory anisotropic behavior and behavior in excavations have been suggested.

Les rtcents developpements urbains et ruraux du centre ouest de I'Ecosse ont ntcessitt I'extcution d'excavations beaucoup plus Ctendues et souvent plus profondes qu'auparavant dans les argiles a blocaux de la rtgion. Un certain nombre de ruptures de tranchtes profondes et de talus d'excavations se sont produites, dont le caractere semblait influenct par des facteurs relies a la macro fabrique du mattriau. Une Ctude ditaillte sur le terrain a confirm6 la prtsence de fissures et a montrt que I'arrangement de ces fissures suivait une orientation prtftrentielle marqute. L'influence des ces rtseaux de fissures sur la rtsistance au cisaillement non drain6 et sur les proprietts en consolidation de I'argile a i t 6 ivalute partir d'essais sur des tchantillons de diamhtre variant entre 76 et 254 mm, et des corrtlations entre le comportement anisotrope observt en laboratoire et le comportement des excavations ont kt6 suggtrtes.

[Traduit par la Revue]

Introduction In recent years, some failures have occurred

in trenches and slopes cut into boulder clay in the west central valley of Scotland. This relatively recent build up of problems may be explained by the fact that in urban areas, such as the City of Glasgow, the earliest development was confined to the low lying areas around the River Clyde, then in the early 1800's expansion took place into the surrounding hills which are composed of glacial till and sometimes bedrock (Price 1971 ) . These developments were gradual, and generally in sympathy with the topography. Main roads, railways, and canals were all constructed around and between the many glacial drumlins which were then used to accommodate housing and commercial build- ing developments.

In the last 10 to 15 years the City has been subjected to major redevelopment on a vast scale. A new urban road system based on traffic flow patterns is being built, and widespread commercial and municipal office and housing regeneration and redeployment is well under- way. Geological considerations, however, in- Can. Geotech. J., I?., 84 (1975)

fluence only the engineering details of these developments, such as inclination of slopes, length of piles etc., but not the overall plan. The balance between topography and urban development has therefore been upset. Simi- larly in the rural areas, new major roadworks have been undertaken on the basis of linking or by-passing urban areas with maximum re- strictions on road grade, curvature, and the like. A collision between the topographic considerations and the traffic flow considerations has inevitably occurred in these areas also.

Thus as a result of the development works many excavations have been undertaken. In the deeper trench excavations in boulder clay failures have occurred on the long sides and could thus be reasonably attributed to insuf- ficient shoring. But there were also some failures noted in the ends of trenches where the sides remained reasonably stable. In a number of cut slopes in the same soils relatively shallow failures occurred and in some slopes which were opposite sides of an excavation at the same inclination, one failed, whilst the other remained stable. A study of aerial photographs

McGOWN AND RADWAN: FISSURES IN BOULDER CLAYS

of the area revealed such one sided failures in cut slopes to be relatively common in new cuttings and that they could also be found in old railway cuttings. A possible explanation for these was that the soils contained a macrofabric which influenced their behavior. As macrofabric might be influencing many properties, a detailed field and laboratory study was undertaken at Strathclyde University into the geological derivation and engineering prop- ertics of boulder clays in west central Scotland.

Geological Background of Glacial Deposits The central belt of Scotland is one of the

areas in the British Isles where a well developed drumlin field can be found. In fact the area is overlain by not one, but a number of drumlin sets, the result of the successive arrival of the same glaciation from different directions (George 1958). Thus the drumlin orientation in any relatively small area may be complex.

As the solid geology of the central belt of Scotland is highly variable then so the tills, which are in the main locally derived, are found to be variable in their lithology. Color changes are quite common but two distinct colors of till are typically found: ( i ) red till derived from various old red sandstones and barren red coal measure sandstones and (ii) grey black till dcrivecl from carboniferous shales and sandstones and calciferous sandstones.

Engineering tests do not make a great deal of distinction between any of the tills in the region. The variability within any one till being as great as any diffcrencc between tills. Thus the tills in the area have been trcated as one soil for the purposes of indicating their general geotechnical properties.

General Geotechnical Properties A typical ~noisture content and plasticity

profile for the boulder clay is shown in Fig. 1. Generally a weathered zone exists in some 2 m depth but this can vary in some tills from 1 to 3 in. Bencath this the properties are much more uniform. The average moisture content of the unwcathercd till is 12.3% with a standard deviation of 2.2%. The moisture content of the weathered till is much more variable with a mean of 20.5% and standard deviation of 7.2%, indeed use of this material in engineering works can often prove to be problematic

Description of soil

Top Soi l

Weat hered Boulder

c lay

Dark Grey Boulder

Clay

Water

W = water conten W L = liquid l imit W P = plastic l imi t

FIG. 1 . Geotechnical properties o f boulder clay from North Hanover St., Glasgow.

(McGown and Iley 1973). The variation in bulk density of the tills is similar to that of the moisture content with a mean of 2.26 t/m" for the unweathered till and 2.04 t/m"or the weathered till. The plasticity characteristics, as shown in Fig. 2, are close to the limits suggested by Casagrande ( 1947) and Alderman (1959) and within those suggested by Bus- bridge (1968) and Anderson (1972). The plasticity characteristics of the weathered tills are usually much less than the unweathered tills.

The boulder clays are typically very well graded with particle sizes ranging from massive boulders down to clay sizes, Fig. 3.

Field Investigations of Macrofabric (i) Nature of Fissures

It must be emphasized at the outset that various types of macrofabric features are to be found in boulder clay. Sand layers or lenses are quite common and major shear planes have been noted in some boulder clays in the Glas-

CAN. GEOTECH. J . VOL. 12, 1975

60

50 ( A L D E R M A N )

Upper L i m i t of G lac i a l C lays 40 ( C A S A G R A N O E )

30 wer L i m i t of Glacial Clays . L i m i t s for GI

20 Lower L i m i t of Boulder Clay

L i m i t for Glasgow and West Scotland T i l l (ANDERSON)

10

0 0 1 0 20 30 4 0 50 60 7 0 80 90 100

L i q u i d L i m i t , WL%

FIG. 2. Plasticity chart s h o w i n g t h e ranges r e p o r t e d for glacial deposi ts.

I Loa Settl ing V e l o c i t y - Cm Per sec 1 BS Sieve Number Aper ture Size i n inches I

FIG. 3. Grading curves for samples from Glasgow.

c l a y 1 F i n e I ~ e d i u m l Coarse 1 F i ne 1 Medium 1 Coa rse 1 F i ne M e d i u m Coarse ' ~ r a c t i o n 1 Si ~t F rac t i on Sand F rac t i on 1 ~ r a v b l ~ r a c t i b n B ~ ~ ~ , J ~ ~ ~ ~ __L

McGOWN AND RAQWAN: FISSURES IN BOULDER CLAYS

gow area. However, the feature, or better, the set of features which are of particular interest in this study are the fine cracks and joints which on very close examination are apparent in the boulder clays.

There are basically two types of fissures to be seen in the soils so far studied. Firstly, near vertical fissures which are often very tight and need to be pulled apart, Fig. 4. The reason for this is that stones in the boulder clay often act as reinforcement bars linking the intact clay on either side of the fissure, Fig. 5. Secondly, there are near horizontal fissures, which may also be found inclined at low dip angles, which are generally coated by a thin layer of silt or silty clay, Fig. 6. The distinct difference between the near vertical and horizontal fissures is quite striking. Rarely are the vertical fissures coated in silty materials like the horizontal fissure, although they may be coated in weathering products. Having so recognized the presence of fissures in the boulder clay the need existed to quantify their orientation and distribution. This has been carried out in the manner described by Fookes and Denness (1969) with some modification to accommodate the particular nature of the very well graded till as described by McGown et al. (1974). (ii) Measurement of Orientation and

Distribzltion of Fissures The basic field measurements are simple,

consisting of digging several cavities into the till, each cavity having a total volume of 1 m3. The soil is excavated by picking out, very care- fully, small pieces of soil using a fine spatula

FIG. 4. Vertical fissures.

FIG. 5. Stone across vertical fissures.

FIG. 6. Near horizontal fissures.

and a pointed knife. Where a fissure is en- countered the strike of the fissure is established, the orientation of the strike and the magnitude of the dip is measured. From this data alone it is possible to establish the spatial orientation of fissures.

Several types of plots for presenting orientation data are available but the type used is the equal angle lower hemisphere plot. This is considered the most suitable as it gives true angular relationships and, with the large number of fissure readings, 200 to 500 in each cavity, it does not become inaccurate nor im- precise like other types of diagrams (Denness 1969). The data are analyzed and drawn out by computer and a typical data output is as shown in Fig. 7a. From this a very much sim- plified plot may be drawn, Fig. 7b, to indicate approximate relative orientations of dominant

CAN. GEOTECH. J. VOL. 12, 1975

FIG. 7. (a) . Contoured polar diagram represent- ing fissures from North Hanover St., Glasgow. (b) Stereogram of main fissure sets, North Hanover St., Glasgow.

fissure sets using typical fissures for each set. To establish spatial distribution it is neces-

sary to relate the position of the fissures to one another. The method adopted is to set out reference axes at the corner of the cavity dug into the slope and to measure the size and distance of the fissures to these axes. The data so obtained can then be processed and spacing, in the reference directions, can be obtained, Fig. 8.

To comdete the data from field measure-

Fig. 9, in terms of their asperity (roughness) and texture nature of coating, weathered, silkensided etc., after McGown et al. (1974). (iii) Geotechnical Extent of Fissure Data

Data has been obtained from boulder clay drumlins over a wide area in west central scotland. In the various areas, drumlins in close proximity to one another have been investigated to establish local variations in the nature and characteristics of the fissures in boulder clav.

J

In all of these investigations fissures were found and their spatial distribution and orientation were generally very similar. The two types of fissures; near vertical, clean or weathered. fissures and near horizontal. sedi- ment covered, fissures were in evidence in all cases. The near horizontal fissures comprised one set, whilst the vertical fissures comprised two or sometimes four sets, in pairs lying conjugately on either side of the direction of glacial movement. The fissure pattern in the boulder clay was not therefore random but possessed definite prcferred orientations. Also ihe intensity of fis&ring was found to increase towards the surface with an approximate average spacing 148 mm at depth and 64 mm near the surface.

Appreciative of the relatively small geographical extent of the study area, a survey of the literature has been undertaken and reports of fissuring in other geographical areas have been found, Table 1. Although fissures appear to have been identified in these other boulder clays, the only quantitative data on fissure orientating and distribution are for Scottish boulder clay (McGown et al. 1974; McGown and Radwan 1974).

Laboratory Investigation of Influence of Macrofabric on Engineering Properties

(i) Site Investigation and Testing Techniques In connection with the redevelopment works,

a large number of site investigations have been carried out using open drive U4 (102 mm nominal diameter) sampling. Tests have been performed on 38 mm diameter specimens in triaxial and unconfined compression strength tests and on 76 mm diameter consolidation test specimens. These investigation techniques are generally inappropriate and very widely scat-

ments the &face of the fissures are described, - tered results are obtained.


bept hl C A V I T Y C O - O R D I N A T E S

X- Axis ml

Spacing- mm

Y- Axis

Soacing -mm

Ave S P. = l84mm I

4verage Vol . of Minintum

Element Block

3 280 000

c u . m m

5 820 000

C U . m m

FIG. 8. Fissure spacing in boulder clay from North Hanover St., Glasgow.

TABLE 1. References on fissuring in boulder clays

Author Features studied Area of study

Chamberlin, T. C. 1895 Sub-horizontal joints/deposition Richter, K. 1929 Divisional planeslshear layers/compaction Wentworth, C. K., and Delo, D. M. 1931 Sub-horizontal joints/deposition Biilow, K. V. 1939 Latent and visible shear layerslground moraine Harrison, P. W. 1957 Fabriclstone orientation joints Elson, J. A. 1960 Deposition/structure/fissility/joints Penny, L. F., and Catt, J. A. 1967 Jointslstone orientation/fold Westgate, J. A. 1969 Fracture/colurnnar structure Kaczynski, R., and Wysokinski, L. 1970 Strength/discontinuities Krajewska-Pininska, J. 1970 Strengthlwater contentlfissures Boulton, G. S. 1970 Deposition/fracture/tectonic shear Flint, R. F. 1971 Joints/structure/fabric Rowe, P. W. 1972 Fissures/permeability Kazi, A., and Knill, J. L. 1973 Fissures/orientation/stress conditions Pusch, R. 1973 Macrofa bric/microfabric/fissures/permeability McGown, A. et al. 1974 Macrofabric/fissures/strength anisotropy1

orientation McGown, A., and Radwan, A. M. 1974 Slope stability/fissuring

Greenland Germany United States Germany United States Canada England Canada Poland Poland Norway United States England England Sweden

Scotland Scotland

As a first attempt to overcome the spread of shear strength results, 102 mm diameter test specimens from U4's were used with lubricated end platens to check the influence of specimen size. Little increase in strength was found although the scatter was reduced in most cases investigated. This was attributed to the more representative size of the 102 mm diameter

specimens in relation to the size of particles contained in the boulder clays.

For consolidation testing, difficulty of pre- paring 76 mm diameter specimens was usually experienced and a great deal of selection of test specimens was involved. Larger diameter testing thus made easier the task of specimen preparation and certainly made the test speci-

CAN. GEOTECH. J. VOL. 12. 1975

B F i s s u r e S e t F 1 e ear H o r i z o n t a l Em IE I,

, , ::]Near V e r t i c a l

A s p e r i t y - m m

C o a t i n g S t a i n s F issures

N a t u r e of Fissure Sur face

FIG. 9. Surface properties of fissures from North Hanover St., Glasgow.

mens more representative in size in terms of size of particles in the till.

Thus the need to use larger diameter test specimens was appreciated and put into practise in the Strathclyde University test programme. In fact, tests were carried out on 102, 152, 255 mm diameter strength test specimens and 152, 254 mm diameter consolidation test specimens. These specimens were taken from 102, 154, 225, and 254 mm diameter samples taken in boreholes or test pits and from blocks.

It must be emphasized that the intention of this approach was to take more representative samples and to test them in a more representative manner from the point of view of test specimen size in relation to the size of the

constituent particles. The influence of the presence of a macrofabric (fissures) was not fully analyzed at this point, and in any case, taking larger samples was considered to cover this, assuming previous investigations into the rela- tionship of the presence of fissures to engineering properties were acceptable and applicable to boulder clay (Bishop and Little 1967; Marsland 197 1 ) .

(ii) Consolidation Properties If the fissures lie in definite directions and if

the fissures have different permeabilities then anisotropy (directional dependency) of all properties controlled by permeability will result. Measurement of this is a major task and the first step is to determine the size of representative specimens for testing. This is in fact the stage reached at Strathclyde. Various sizes of vertical samples (102, 152, 254 mm diameter samples) have been taken and tested at various specimen sizes using the standard 76 mm oedometer and 76, 154, and 254 mm Rowe cells.

From the e-log p curves of the same soil tested at different specimen sizes, Fig. 10, it is possible to conclude that the compression behavior is not directly influenced by macrofabric, hence by specimen size, and providing careful sampling is carried out, sample size is not significant either. The influence of different size specimens on the rate on consolidation has, however, not been found to be a constant. In some cases 152 and 254 mm diameter specimens are significantly different whilst in others they are not, or indeed, they can be in juxta- position, Fig. 11. Nevertheless, in general the 152 and 254 mm diameter specimens give much higher coefficients of consolidation than the standard 76 mm specimen and this is particularly so at the lower consolidation pressures. It appears, therefore, that 254 mm diameter specimens, or at least 152 mm diameter specimens, should be tested for consolidation data and that a reasonable number of samples will still be required to account for the variability between specimens which is evidenced in the consolidation data so far measured. Also, the importance of sample size and so by inference the importance of the macrofabric would appear to become greater at consolidation pressures at or below the overburden


E f f e c t i v e St ress , k~'' ( l o g scale) ( A f t e r ~nderson1973)

FIG. 10. Void ratio - log effective stress curves for different size specimens from Grass- yards Road, Kilmarnock.

stress. Indirect measurements of the coefficient of permeability at different effective pressures clearly illustrates this, Fig. 12, and may be explained by the fissures opening up at pressures less than overburden and closing up at stresses in excess of this pressure. At pressures higher than overburden, vertically oriented boulder clay specimens therefore act very much like a homogeneous intact clay.

For directions other than vertical, behavior is likely to have the same general character as previously described, but will vary in actual

values of permeability etc., principally accord- ing to the intensity of fissures in the particular direction tested, the nature of the fissures surfaces and lastly with the confining pressure.

(iii) Shear Strength Under Undrained Conditions

The test procedure adopted for this aspect of the investigation was established by McGown et al. (1974) using triaxial testing with lubricated end platens and height to diameter ratios of 1: 1. The strain rates used were

CAN. GEOTECH. J. VOL. 12, 1975

1 1

E f f e c t i v e Stress, k d m 2 (Af ter Anderson 1973)

FIG. 11. Variation of the coefficient of consolidation with applied effective stress for different size specimens from Craufurdland, Kilmarnock.

selected to give equalization of pore water pressure in the test specimens.

As with consolidation measurements it is essential to establish the size of specimen necessary for testing in shear strength deter- minations. Previous investigations such as Bishop and Little (1967), Lo (1970), and Marsland (1971 ) have established the general form of sample size against measured strength. In Scottish boulder clays similar investigations have been carried out and as is shown in Fig. 13, a very similar trend to previously reported results is obtained. This investigatory technique has, however, been extended by testing specimens in different directions and the range of these measurements is also given in Fig. 13.

As can be seen, the minimum strengths mea- surable from this in any direction at any size were very much lower than the vertical strength measurements and close to the fissure strength. The explanation for this is that during testing in particular directions formation of the shear plane was occurring along a plane or set of planes of weakness, that is shearing along fissures which have definite preferred orientations.

Fig. 14a and b are lower hemisphere plots of the strengths measured in different directions using 102 and 225 mm diameter test specimens, respectively.

The average vertical strength has been taken as unity and the relative strengths only are given for the various directions. From this, the


Sample Set t lement pore water pressure

6 - 1 A --0.- -0-

6 - 2 A - - a - - It 6 - 2 B - - 0 - - -

-C

8 I I I I 50 100 150 200 250 300 350 400

E f f e c t i v e P r e s s u r e (P') - k ~ / m ' FIG. 12. Variation of the coefficient of permeability with applied effective stress for speci-

mens from North Hanover St., Glasgow.

directions in which strengths are less or greater than vertical strength are to be seen and it is to be noted that for a large number of horizontal directions the strength is close to the vertical strength hence strength anisotropy, vertical to horizontal alone, is not great, but there are particular orientations which are much weaker than the vertical (0.6 x vertical) and other directions which are much stronger (1.4 X vertical).

This particular form of directional strength

dependency is very much controlled by the orientation and the nature of the sets of the fissures in the boulder clay. This complex as- swiation of fissures orientation and directional strength is presently being investigated at Strathclyde.

Behavior of Slopes Slope failure may occur due to deficiency of

one or more of the strength components within the soil mass but the type generally found in

94 CAN. GEOTECH. J. VOL. 12. 1975

1.2

0

1.0 C * m w .

j; -, 0.8 m *. c -

0 .d

C ; 0.6 C ? z u !! 2 0.4 * .-

y.

- 0

0 .-

.# 0.2 K

0 2 4 5 6 7

10 10 10 10 10

A r e a of P o t e n t i a l F a i l u r e P lane - rnm 2

FIG. 13. Strength size relation of blue London clay and boulder clay from Strathclyde University drumli;, Glalasgow.

boulder clay is structure type failure (Hager- man 1966). In structure type failure, 'blocks' of soil fail as units and the failure is generally quite shallow initially but can progress to greater depth and extend with time. The controlling factor is the orientation of the cut slope to the basal surface of the block. In the case of boulder clays these basal surfaces are the near horizontal fissures with the sides of the block bounded by the near vertical fissures. Depending on relative orientation of fissures and slopes then a critical situation may arise. Figure 15 shows a cross section of a cavity cut into a slope and the possibility of slip out of the cut surface is obvious. Figure 16 shows the fissure orientation at Hurlford, Ayrshire with the controlling basal fissure set shown as F1 and the slope shown as S. There is a measure of sympathy between the two, hence a partial failure occurred, as shown in Fig. 17. At Hanover Street in central Glasgow the F1 fissure was never more in sympathy than at right angles to the orientation of the slope surface hence no failure was evident, Fig. 7. But at a site to the north of Glasgow, the condi-

tion was much more critical. As can be seen in Fig. 18 the F1 set of fissures and the slope (S) were sympathetically oriented and failure occurred in this cutting in several places although on the one side only, Fig. 19. The opposite slope was stable with basal fissure set F1 inclined into the slopes.

All failures of this nature happen very quickly after cutting and stress relief. The complex total and effective stress analysis of these failures is still under consideration.

Conclusions Some instabilities of cuttings in the boulder

clays in west central Scotland have been shown to be directly attributable to the presence of fissures in the soil. These fissures have from field investigations been shown to exhibit definite preferred orientations and to be of two distinct types. The first are near horizontal fissures, often coated in silt or silty clay, and the second are near vertical fissures either clean or weathered and held tight by the action of stones cutting across them. The deviation of the coated fissures from the horizontal


( a E p e c i m . Diam

S

FIG. 16. Stereogram of main fissure sets from Hurlford, Ayrshire.

( A f t e r MCGown et a l 1974)

FIG. 14. Stereographic projection showing directional dependency of undrained strength, Hurlford, Ayrshire.

FIG. 17. Partial slope failure, Hurlford, Ayrshire.

FIG. IS. Cross section in cavity.

greatly influences the stability of cutting slopes, the stability being at its minimum when the slope and fissures are sympathetically oriented.

The laboratory studies carried out in parallel

with the field investigations have proven that the compressibility characteristics of the boulder clays are not greatly influenced by the presence of fissures, but that the permeability and related properties are, particularly at stresses below present overburden. Thus in conditions of stress relief, such as in trench or slope excavations, the mass permeability of those boulder clays can be expected to significantly increase. In Scottish meteorological conditions this will very quickly lead to moisture content increase along the fissures and probably soften- ing with consequent strength reductions.

Laboratory undrained strength determina- tions have shown that the boulder clays have quite definite anisotropic strength properties,

CAN. GEOTECH. J . VOL. 12, 1975

influence of fissures on the directional perme-

FIG. 18. Stereogram of main fissure sets from North of Glasgow.

FIG. 19. Extensive slope failure, North of Glas- gow.

with a range of strength of 0.6-1.4 x vertical strength. The directions of maximum and minimum strengths are determined by the fissure set orientations and closely related to the strength along the fissures. Thus previous shear strength test specimens are now seen to be in- adequate as are any strength anisotropy investigations based only on vertical and horizontal specimens in these boulder clays. The possibility of moisture content increase along fissures on stress relief is also now seen to be a critical factor in overall strength anisotropy in these soils.

Further study will be required before the disposition of fissures in tills is fully under-

abilitv of stress relieved soils around excavations 6as yet to be considered. everth he less, the present study has revealed the presence and nature of the fissures in the Scottish boulder clays and inferred some of the engineering implications of these. As many other investi- gators have previously observed the presence of fissures in boulder clays from other widespread geographical locations, it is suggested that the findings of this study could be applicable to many boulder clays in areas other than west central Scotland.

Acknowledgments The authors wish to acknowledge the per-

mission to publish data given by Mr. J. F. Clark, Mr. D. Hossain. Acknowledgment is also made of the financial assistance given to the project and personnel by the Natural En- vironment Research Council and the University of Assiut, Egypt.

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McGOWN AND RADWAN: FISSURES IN BOULDER CLAYS 97

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