Clay Minerals (1973) 10, 113.

T H E A L T E R A T I O N C H A R A C T E R I S T I C S OF L O W E R O X F O R D C L A Y

J. O. J A C K S O N

Faculty of Engineering, University of Lagos, Nigeria

T H E

(Received 24 July 1972; revised 4 February 1973)

A B S TRACT: Alternating 'hard' and 'soft' bands at close intervals are exhibited by the slope faces of brick-pits in the Lower Oxford Clay which have been subjected to prolonged exposure. This phenomenon was investigated to evaluate the geotechnical properties which were significant in its development. The results of laboratory tests indicate that the alteration or short-term weathering characteristics of clay-shales may be influenced by the higher degree of preferred orientation of the clay-mineral component, in inducing a stronger inter-particle bond in 'hard' samples which inhibits weathering.

I N T R O D U C T I O N

The Lower Oxford Clay- -a bituminous, marine clay-shale, is worked for brick-making at various sites along the Oxford Clay outcrop which extends from Dorset to Yorkshire. Apar t f rom the coast sections these brickpits provide the only suitable sites for sampling.

At Chickerell near Weymouth (Dorset) and Yaxley near Peterborough (Hunts) pit- faces which have been abandoned but left exposed for 7 years or more show alternating bands of 'hard ' and 'soft ' material (Plate 1). 'Hard ' bands are typified by blocky material extending from one natural fissure to another, whereas 'soft ' bands are more fissile and discontinuous.

The eroded overburden thicknesses at the Chickerell and Yaxley sites have been estimated from geological literature to be approximately 650 m and 130 m respectively, (Jackson, 1972).

Neither pre-consolidation pressures nor the associated temperatures due to geo- thermal gradients could have been significant factors in the formation of these alter- nating bands.

Field observations of freshly exposed faces have revealed no evidence of these bands; laboratory tests were therefore carried out to determine the factors which would have been important in their formation (and which may also be significant in

114 J. O. Jackson

the general weathering characteristics of clay-shales), as part of an investigation into the effects of depth of burial on the geotechnical properties of the Lower Oxford Clay. (Jackson & Fookes, in press).

S I T E D E S C R I P T I O N S

Chickerell

The Chickerell pit is situated at Crook Hill a few miles northwest of Weymouth, Dorset. The Oxford Clay and Kellaways Beds which underlie much of this area are poorly exposed and nowhere is there a continuous exposure of all the stratigraphical zones.

The brick-pit owned by thc Mitchell Brick Company, is situated on the southern limb of the Wcymouth anticline; a structure of Mioccne age. Although no faulting is evident at the pit, two minor faults occur about 400 m west and 800 m southeast of the pit. The Radipole fault, the nearest major fault, lies at about 1200 m north of the pit. The effect of tectonic activities in this locality may bc considerable though the accom- panying strcsses cannot be quantified.

The ground level here is + 120.00 ft O.D. with the general slope about 5 ~ south- wards. The beds exposed in this pit arc complete from the upper Athleta Zone of the Middle Oxford Clay down to the top of the K Jason sub-zone of the lower Oxford Clay (Smith, 1969).

Yaxley

Yaxley pit is situated a few miles southwest of Peterborough, Huntingdon (where the classic study in stratigraphical zoning was accomplished by Brinkmann (1929)). The brick-pit here is owned by the London Brick Company. Two small faults striking east/west are indicated on the map, 2�89 miles north and 5 miles southwest of Peter- borough. A third, striking northeast/southwest is found 1�89 miles south of Yaxley. No other major faults have been mapped in the area.

The ground is nearly level at approximately + 100.00 ft O.D. The Lower Oxford Clay is being mechanically worked and fresh faces are typically sloping at about 80 ~ .

S A M P L I N G

The slope faces of brick-pits at the two sites which have been left exposed for more than 7 years show prominent alternating 'hard' and 'soft' bands which are not noticeable on freshly exposed faces, (Plate 1). The terms 'hard' and 'soft' are used here to reflect the relative resistance of these bands to the effects of exposure. The 'hard' bands stand out more prominently from the pit-face than the 'soft' bands. The degree of weathering of both bands can be described as faint. (Geol. Soc. Engng Grp Work- ing Party Report, 1970).

The 'hard' bands which have a minimum thickness of 5-8 cm, stand out in blocks

PLATE 1

(a) General view of Chickerell site. (b) Close-up of 'hard' and 'soft' bands (ChickereU).

(Facing p. 114)

PLATE 2

Electron micrographs. (x 2500) Chickerell site. (a) 'Soft' sample No. 2. (b) 'Hard' sample No. 1.

Alteration of Oxford clay 115 of about 15 cm width. On dose examination (Plate 1) the blocks are seen to consist of surface shrinkage cracks spaced at about 5 mm extending from one natural fissure to the adjacent fissure. At Chickerell, the natural fissures are yellowish and/or reddish in colour, due to the presence ofjarosite and iron oxides respectively.

The 'soft' bands are more intensely fractured producing fragments which are some- times only 1 mm thick. (Plate 1). These bands are inset into the face indicating greater weatherability.

The weathered bands were excavated until fresh material was exposed before sampl- ing was carried out. Field samples were stored in polythene bags and sealed for trans-

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116 J. O. Jackson

port to the laboratory. Figure 1 shows an elevation of the sampling faces at Chickerell and Yaxley, and the sampling positions relative to the stratigraphical zones of Cal- lomon (1968) and Smith (1969).

G E O T E C H N I C A L P R O P E R T I E S OF SHALES

The geotechnical factors which influence the general engineering characteristics of clay- shales and shales have been reviewed by Underwood (1967), Kenney (1967), Taylor & Spears (1970). These factors can be grouped under the following headings.

(i) Preconsolidation pressure (e.g. Meade, 1936). (ii) Cementation effects (e.g. Meade, 1936; Underwood, 1967). (iii) Particle size (e.g. Underwood, 1967). (iv) Mineralogy--whole rock mineralogy in terms of oxidizable pyrite or soluble

calcite, and mineralogy and associated adsorbed layer complex: (e.g. Taylor & Spears, 1970; Nakano, 1967; Badger, Cummings & Whitmore, 1956; Bjerrum, 1967).

(v) Other factors such as fissility, and the pattern of discontinuities are recognized as important in the field behaviour of these materials (e.g. Ingrain, 1953).

However, the role of fabric so far reported has been limited to the effect it has on the development of fissility, e.g. Ingram (1953) and the development of anisotropy in the engineering properties such as strength and permeability. A further significance of fabric not previously investigated is the direct increase in inter-particle forces due to increased effective area of contact between the clay minerals with increasing degree of preferred orientation.

Laboratory tests were carried out on the 'hard' and 'soft' samples to evaluate the influence of the above factors and especially to verify this role of fabric.

L A B O R A T O R Y TESTS

Although a number of tests required undisturbed samples, e.g. slake durability, moisture content etc., other tests viz. chemical, mineralogical analyses etc. required sieved material which had to be truly representative. Details of the procedure adopted to ensure this are given in Jackson (1972).

Chemical, mineralogical and c.e.c, analyses were carried out on material passing the B.S. No. 200 test sieve, and Atterberg Limit tests were run on material passing the B.S. No. 36 test sieve.

Chemical analyses

The results of chemical analyses are given in Table 1 together with Weaver's average composition for shales (Weaver, 1958).

The results indicate that 'hard' and 'soft' samples cannot be distinguished on the basis of chemical composition.

The Ca0 content of Chickerell samples is low, but the Yaxley samples have a sufficiently high average CaO content for them to be considered as calcareous, with the

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Alteration of Oxford clay 119

average values for 'hard ' and 'soft ' samples very close at 13.1 ~ and 13.4~o. These figures are total Ca0 contents which include fossil remains and gypsum in addition to the true calcareous cement values. Mineralogical analyses, however, show that gypsum is not an important cementing agent in these clay-shales.

The average organic carbon content of 'hard ' and 'soft ' samples are 3-9 ~ and 3.6 Yo for Yaxley samples; 8.5 ~ and 4.5 ~ for Chickerell samples. These values are high enough for the deposit to be considered as carbonaceous (Underwood, 1967), but at both sites some 'soft ' samples have higher organic carbon content than 'hard ' samples.

The distribution of organic matter in these samples was studied by optical micro- scopy techniques, but showed no consistent variation f rom 'hard ' to 'soft ' samples.

Mineralogical analysis

A Philips X-ray diffractometer P.W. 1050 was used in these tests, using Ni-filtered CuK~ X-rays generated at 40 kV and 20 mA. Tests were run at 1 ~ a minute and beam characteristics were l~ �9 1 ram-1 ~ Scale factors of 8 and 16, were employed for 'whole rock ' and clay fraction tests respectively, with a time constant of 2 and multiplier of 1.

The results summarized in Tables 2 and 3 show no consistent variation in mineral- ogical composition for 'hard ' and 'soft ' samples, a fact which is supported by the e.e.c, values which range f rom 7 to 19 m Eq/100 g, but show no consistent variation (see Table 1).

TABLE 3. Clay-sized fraction ( - 2 tzm) mineralogical composition (~)

Sample nos 1

Chickerell Yaxley

Hard Soft Hard Soft

5 7 2 4 6 9 8

Feldspar (4"03 A) 2 0 2 1 1 0 0 2 Quartz (4'26 A) 1 2 2 1 t 1 1 2 Kaolin (7-1 A) 18 29 19 22 21 20 23 23 Illite (10'0 A) 80 64 77 76 78 78 73 69 Chlorite (14-2 A) 0 5 0 t 0 0 2 3

Fabr~

The parameter used to characterize fabric in this work was the degree of preferred orientation of the major clay mineral components (kaolinite and illite), measured by X-ray and optical methods. These analyses were supplemented with electron micro- scopy studies.

Details of the sample preparation procedure are given in Jackson (1972). The

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samples were not carbowax impregnated but were cut on a special machine. Grinding and subsequent 'peeling', mounting and 'shielding' were carried out by a method similar to that employed by Tchalenko, Burnett & Hung (1971).

X-ray method

Fe-filtered CoKa radiation was used. Chamber evacuation was found to produce a

Alteration of Oxford clay 121

30 % increase in peak amplitudes and a smoother background. The degree of preferred orientation was expressed by an orientation index defined by the ratios:

(001)V (002) V and

(001) V+ (001)H (002) V+ (002)H

where V and H represent peak amplitudes or areas of specimens cut perpendicular and parallel to bedding respectively, for planes whose Miller indices are shown in brackets. High degrees of preferred orientation are characterized by low values of orientation indexes. Figure 2 shows typical diffractogram traces. The results which are given in Table 4 show that the 'hard' samples were better oriented than the 'soft' samples.

The organic matter however, showed no influence on the orientation indexes as had been reported by other authors, e.g. Ingrain (1953), Odom (1967).

The kaolin component was also slightly better oriented than the illite component. A similar result was obtained for London Clay and Upper Lias Clay by Tchalenko et al. (1971). This tendency together with the reported decrease in degree of preferred orientation with increase in montmorillonite content (Odom, 1967), suggests that the type of clay mineral may be an important factor in re-orientation of clay minerals in a multi component natural sediment under increasing overburden pressure.

Optical method

The specimens cut perpendicular to bedding previously used for X-ray fabric analysis were ground down to make thin sections and were subsequently analysed to obtain birefringence ratios by the method adopted by Mitchell (1956), Tchalenko (1968) etc.

Readings ,~ere recorded using an Electro-selenium Ltd micro-photometer at 30 and lC0 magnifications with a x 10 objective; tee fields of view were 4 mm and 2 mm in diameter respectively.

No consistent trend for differentiating between 'hard' and 'soft' samples can be deduced from these tests on fabric. The sensitivity of this test is believed to have been considerably affected by high organic matter content and the high non-clay mineral content of the samples.

Electron microscopy studies

The sections cut perpendicular to bedding and previously used for X-ray fabric studies were examined in the electron microscope.

The main disadvantage of this method is that even at the relatively low magnifica- tions ( x 2500) used to observe fabric, the area of view was only about 40 tzm square and the representability of the micrographs (Plate 2) is uncertain. The studies confirmed the results of better orientation of the 'hard' samples obtained by the X-ray analysis.

Element integrated counts on the electron microscope showed no relationship between the proportion of Mg, A1, Si, K, and Fe, and alteration characteristics.

J. O. Jackson

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Alteration of Oxford clay 123

Clay-sized fraction

This was determined by the pipette method (B.S.I., 1967), on the fraction passing the B.S. test sieve No. 200. The difference between 'hard' and 'soft' samples is slight (Table 5) but it is tentatively suggested that the lower clay-sized contents of the 'hard' samples suggests that the function of this fraction may be one of reactivity, i.e. higher clay-sized content results in higher swelling and shrinkage characteristics, rather than one of cementation.

TABLE 5. Classification tests results (~)

Chickerell Yaxley

Hard Soft Hard Soft

Sample nos 1 5 7 2 4 6 Clay-sized content 47"2 50"0 47"0 64"8 66'0 55"6 65"8 62.7 Slake durability index 98 96 98 79 72 93 73 70

Plasticity and activity

The results of Atterburg Limit tests performed on the fraction passing the B.S. test sieve No. 200 are summarized in Fig. 3. The samples were mixed and allowed to mature for 72 hr before testing. Samples from the two sites plot parallel to the A-Line. Chickerell samples plot below the A-Line on account of their high organic matter content.

'Hard' samples plot below 'soft' samples at both sites. Figure 3 shows that the 'activity' values (Skempton, 1953) for both sets of samples are very similar at each site, confirming the trend of siltier 'hard' samples and the reactive function of the clay- sized fraction in these clay-shales.

Slake durability

The slake durability test for assessing the degree of weathering in rocks has been described in detail by Fookes, Dearman & Franklin (1971) and has been used in slightly modified forms by several authors e.g. Badger et al. (1956), Taylor & Spears (1970) etc. The procedure of Fookes et al. (1971) was generally adopted except that samples were allowed to cool to 50~ prior to immersion in water to reduce excessive flaking. The test results (Table 5) show that the 'hard' samples can be distinguished by higher slake durability index values.

Moisture content

Suitable samples for moisture content determinations were obtainable from the Chickerell site only. The results (Table 1) show no trend with 'hardness' or 'softness' of

124

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the sample. Porosity, and density are related to moisture content and were therefore not measured. In addition, stress history and the effect of cementation would be expected to reduce the dependence of alteration characteristics on these parameters. Consolidation tests on samples from other sites, Calvert, Stewartby, etc. show that the Lower Oxford Clay which is highly fissured, is also highly anisotropic (Jackson & Fookes, in press); consequently permeability tests were not performed.

Alteration of Oxford clay 125

C O N C L U S I O N S

The results indicate that preconsolidation pressure, cementation, mineralogical composition and particle size are not sufficient to explain the difference between 'hard ' and 'soft ' bands. The fabric of the clay mineral component is an important factor influencing the weathering properties of the clay-shale. 'Hard ' samples are better oriented than 'soft ' samples. The mechanism may be envisaged as higher 'diagenetic' or inter-particle bonding between clay mineral particle due to increased effective areas of inter-particle contact resulting from a higher degree of orientation. The variation in degree of preferred orientation of the clay-shale at the two sites where the previous overburden pressures are high, reflects the influence of the conditions of deposition, possibly the rate of deposition. O'Brien (1971), has postulated that rapid rates of deposition induce better orientation of the clay mineral particles through the early destruction of flocculating bonds.

Simple rapid tests such as the Atterberg Limit tests and slake durability tests can be used as a guide to distinguish between the 'hard ' and soft" samples.

The clay-sized fraction even when present in quantities of up to 50 % does not always act as a cementing agent in clay-shales. The type of clay mineral may influence the degree of preferred orientation induced in a multi-mineral component consolidated under pressure.

The organic matter content of the Lower Oxford Clay has no influence on the degree of preferred orientation of the clay mineral component of this highly oriented clay-shale.

ACKNOWLEDGMENTS

The author acknowledges with thanks the assistance in the field given by Mr J. Horrell of the London Brick Company and Mr R. T. Smith of Swansea University; and the Geochemistry Department of Imperial College for the chemical analyses.

The author acknowledges the continual encouragement given him by Dr P. G. Fookes throughout the course of this work and the helpful suggestions of Mr D. J. Shearman, and Dr J. S. Tchalenko, all of Imperial College, London.

The author finally acknowledges the financial assistance from the British Government and the University of Lagos which made this research possible.

REFERENCES

BADGER C.W., CUMMINGS A.D. & WHITMORE R.L. (1956) d. Inst. Fuel 29, 417. BJERRUM L. (1967) J. Soil Mech. Fdns. Dio. Am. Soc. cir. Engrs. 93, 3. BRINKMANN R. (1929) Nachr. Ges. Wiss. Grttingen, 3, 1. BRITISH STANDARDS INSTITUTION (1967) Methods of soil testing for civil engineering purposes. British

Standards 1377. Bust-I P.R. (1970) Chem. Geol. 6, 59. CALLOMON J.H. (1968) The Geology of the East Midlands (Ed. by P. C. Sylvester-Bradley and T. D.

Ford). Leicester University Press. FOOKES P.G., DEARMAN W.R. & FRANKLIN J.A. (1971) Q. Jl Engng Geol. 4, 139. GEOLOGICAL SOCIETY ENGINEERING GROUP WORKING PARTY REPORT (1970) Q. Jl Engng Geol. 3, 1. INGRAM R.L. (1953) Bull. geoL Soe. Am. 94, 869.

126 J. O. Jackson JACKSON J.O. (1972) Geotechnical properties of the lower Oxford clay related to deep burial. Ph.D.

Thesis, University of London. JACKSON J.O. & FOOKF.S P.G.Q. J1 Engng Geol. (In press). KENNEY J.C. (1967)Proc. Geotech. Conf. Oslo. 1, 65. MEADE W.J. (1936) Trans. 2nd Int. Congr. Large Dams. 4, 183. MITCHELL J.K. (1956) Proc. Higher Res. Bd. 35, 693. NAKANO R. (1967) Soil Foundation, 7, 1. NEVlNS M.J. & WEINTRITT D.J. (1967) Bull. Am. Ceram. Soc. 46, 587. O'BRIEN N.R. (1971) Sedimentology, 15, 229. ODOM I.E. (1967) J. sedim. PetroL 37, 610. SKEMPTON A.W. (1953) Proc. 3rd Int. Conf. Soil Mech. Found. Eng. 1, 57. Svnln R.T. 0969) Int. Field Syrup. on the British Jurassic. Excursion 1. Guide for Dorset and South

Somerset. Keele University PubL A41. TAYLOR R.K. & SeEARS D.A. (1970) lnt. J. Rock Mech. Min. ScL 7, 481. TCHALENKO J.S. (1968) Q. Jl Engng Geol. 1, 155. TCHALENKO J.S., BURNETT A.D. & HUNG J.J. (1971) Clay Miner. 9, 47. UNDERWOOD L.B. (1967) d. Soil Mech. Fdns Div. Am. Soc. cir. Engrs, 93, 97. WEAVER C.E. (1958) Bull. Am. Ass. Petrol GeoL 42, 254.