Geotechnical properties of the Great Limestone in northern England

Engineering Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

G E O T E C H N I C A L P R O P E R T I E S OF T H E G R E A T L I M E S T O N E IN NORTHERN ENGLAND P. B. ATTEWELL

Engineering Geology Laboratories, University of Durham, Durham (Great Britain) (Received July 20, 1970)

ABSTRACT ATTEWELL, P. B., 1971. Geotechnical properties of the Great Limestone in northern England. Eng. Geol., 5(2): 89-116. This paper is concerned with the mechanical properties of the Great Limestone in northern England. After first outlining the geological structure of the limestone at East Layton in north Yorkshire, the problem of the basal dolomitization of the limestone is introduced and the geochemistry of the rock profile evaluated. It is shown that chemical alteration due to dolomitization is accompanied by a mechanical weakening of the rock and a reduction in the quality of the bond between a tar or bitumen coating medium and the surface of the stone. The two types of rock are compared via specific gravity, moisture absorption, unconfined compressive strength, aggregate impact and aggregate crushing tests. The results from a detailed series of shear box tests taken through from peak to residual shear strength are interpreted through the fundamental strength properties of calcite and dolomite to show that whereas in the size range > 200 B.S. sieve, dolomite is a little weaker than limestone due to internal weaknesses associated with volume change; the situation is reversed as the particles decrease in size to form an aggregate of single dolomite crystals which are inherently more resistant to shear.

INTRODUCTION

The numerous quarrying operations in the Great Limestone in northern England bear evidence as to its importance as an economic geological horizon. It is located in the Upper Carboniferous between the Underset (Four Fathom) and Little Limestone, the intervening measures being the usual intercalations of sandstones and shales. The base of the Great Limestone is now taken as the base of the Millstone Grit sequence, that is, the Namurian/Vis6an boundary and E1/P2c goniatite junction (JOHNSONet al., 1962). The thickness of the Great Limestone in the north of England varies from about 35 ft. in Northumberland to about 80 ft. in Weardale in the north Pennines. The present paper is specifically directed towards the Great Limestone in the East Layton area of north Yorkshire where a 50-ft. section of stone is quarried. East Layton is situated just to the north of the A66 road and 5 miles west of the A66/A1 junction (see 1 inch to 1 mi!e Geological Survey Sheet 32 and Ordnance Survey 6 inches to 1 mile Sheets NZ1 l, SE and SW; also refer to Fig.1 and 2). Geologically, the area lies on the northern limb of the Middleton-TyasEng. Geol., 5 (1971) 89-116

90

P.B. ATTEWELL

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Fig.1. Location of the East Layton area in the north of England. Sleightholme anticline. This anticline--a pre-Permian structure and probably of early Late Carboniferous a g e - - was initiated by north-south compressions and probably post-dates the sedimentational events in the Stainmore trough in Early Carboniferous times (WELLS, 1955, p.172). It is one of a number of folds in the Stainmore Gap across the Pennines between the Alston and Askrigg blocks. Structurally, the anticline takes the form of a gently easterly-pitching (around 1 o), somewhat asymmetrical fold, the axis of which trends slightly (10 °) south of east. The north flank of the anticline, dipping at about 4 °, passes without interruption

(9%100). , 91 Fig.2.A (pp.fUmIRIGeologicat detail of the area of interest. B (p.BJ) North-south sections across the East Layton area. Eng. Geol., 5 (1971) 89-116

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Eng. Geol., 5 (1971) 89-116

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P . B . AFTEWELL

into the southern limb of the Stainmore syncline. On a local basis, a number of small folds and faults widen and complicate the outcrops in the East Layton area. This faulting is responsible for the introduction into the limestone of such minerals as malachite, azurite, chalcopyrite (partially altered to goethite), bornite, covellite and the rare and highly soluble mineral chalcanthite (Wells, 1955). The easily precipitated copper minerals probably had their origin in subsequent Permian deposits which percolated along the fault zones in company with magnesiumbearing solutions. Two lithologies in the limestone are common: a fine-grained mud and a coarse crinoidal limestone with thin shale partings. However, all gradations between these two extremes can be found. In the area, there are two predominant joint sets at mean orientations of N 138 ° and N 329 o, the intersection of which produces limestone blocks of variable size but with a noticeably high density of side length of about 10 ft. by 6 ft. A feature of the Great Limestone is that it has been subjected to basal dolomitization, the vertical extent of this alteration being rather variable on an areal basis. There are two possible explanations to account for this phenomenon. Deposition of primary dolomite could have taken place in very shallow evaporating seas during periods of extensive coral (Chaetetes depressus) growth. A subsequent elevation of the sea level or a depression of the sea floor would encourage the deposition of calcareous algae and a reversion to a much purer calcite. Such a genetic interpretation is supported in some measure by the widespread areal extent of a dolomite horizon in the Great Limestone and by the fact that there are locations where the horizon is sandwiched between limestone (for example, in Allendale, northwest Northumberland). On the other hand, the restriction of the dolomitization to the basal unit of the Great Limestone and a reasonably clear-cut boundary between the "blue" and "brown" stone strongly suggest an original water table situation for dolomitization in contrast to a gradation in facies which would surely accompany the transition from a primary dolomite to a limestone. Yet again, the process of diagenetic dolomitization may also produce sharp lithological boundaries in the context of the crossing of geochemical boundaries. Evidence from the East Layton area of present interest strongly suggests that the dolomitization is directly associated with the Permian Magnesian Limestone which overlies Carboniferous rocks unconformably 6 miles to the east. Magnesian Limestone remnants have been found less than two miles to the north of the area at Caldwell at a present-day altitude of 400 ft. O.D. (ordnance datum) and it is possible that the Magnesian Limestone originally outcropped over the East Layton area. It is also possible that the area formed an island in the sea from which the Magnesian Limestone was deposited, since the overlying Little Limestone gives every indication of being free from dolomitization. Either situation would have encouraged the percolation of magnesium-bearing solutions necessary

Eng. Geol., 5

(1971) 89-I 16

GEOTECHNICAL PROPERTIES OF THE GREAT LIMESTONE

93

to dolomitize not only part of the Great Limestone but also the Underset Limestone. The section of dolomitized rock is not uniform throughout the district, the variation being a result of the development of the Middleton-Tyas-Sleightholme anticline. If one pictures the dolomitization over a much wider area, one finds that the phenomenon is not present at Bowes (995134) ~ in the northwest of the anticline. On the other hand, at Spanham (010100) to the south, the full thickness of the Great Limestone is dolomitized near the Spanham fault. Dolomitization is also present, frequently to a height of 20 ft. above the base of the Great Limestone, along the southern limb of the anticline. These areas are at 1,000-1,500 ft. O.D. compared with 400 ft. O.D. at East Layton. TROTTER (1929, p.167) pointed out that the eroded northern Pennines had been subjected to a Late Tertiary eastward tilt along the outer Pennine fault. To accommodate this movement, WELLS (1955, plate III, p.172) using the technique of READING (1954), was able to construct a map of the pre-Tertiary structure of the anticline by drawing contours of the base of the Great Limestone. At that time, the Great Limestone was at 350 ft. O.D. at both Spanham and East Layton. The Great Limestone on Feldom M o o r (115050) is dolomitized at the base, and although its present-day altitude is 1,200 ft. O.D., its pre-Tertiary level was 700 ft. O.D. Since the central dome of the anticline has been eroded away, the highest level of dolomitization is not known. But if this reconstruction is acceptable it would appear that the Magnesian Limestone sea must have reached an altitude, with respect to the preTertiary structure, of 700 ft., the areas of limestone affected by dolomitization being at that time controlled by the pre-Permian landscape of the Carboniferous rocks. Within this landscape, where limestones or faults outcropped into the sea, ground waters rich in MgCO3 were able to percolate into the limestones. One of the most important uses of the limestone is as a macadam in road construction, and this paper describes the results of an investigation into the comparative properties of the undolomitized and dolomitized stone. Samples for analysis were taken from a working quarry at East Layton. Tests were conducted not only to assess the adhesional qualities of the two types of stone with respect to tar and bitumen binders, but also to study the strength of the rock via aggregate impact, aggregate crushing and unconfined compression tests on cores of the material. Water absorption tests are also important in the case of coated aggregates and direct shear tests on cohesionless aggregates give some idea of the contribution of the stone to the total strength of tar or bitumen macadam used as "black-top" wearing course, open-textured wearing courses or as base-courses on roads. Finally, in view of the visible evidence of chemical changes in the limestone, these mechanical tests were backed up by chemical and mineralogical analyses

1 Ordnance Survey grid reference.

Eng. GeoL, 5 (1971) 89-116

94

P. B. ATTEWELL

using X-ray fluorescence spectrographic a n d X-ray diffraction techniques. These latter will be considered first. CHEMICAL ANALYSISOF THE GREAT LIMESTONEAT EAST LAYTON D o l o m i t i z a t i o n of a limestone involves a partial replacement of calcium ions by m a g n e s i u m ions. This chemical r e a d j u s t m e n t is p r o b a b l y a c c o m p a n i e d by a 13 % v o l u m e decrease which, if the process is post-diagenetic, can lead to fracturing of the host rock. Resulting from the ingress of magnesium-rieh g r o u n d waters along pre-existing discontinuities in the form of joints a n d bedding planes, there is often likely to be a gradational decrease in the degree of dolomitization inwards from an exposed rock surface. In order to be able to attribute the prime difference between the blue and b r o w n limestone to the processes of dolomitization, a n u m b e r of samples were collected for chemical a n d mineralogical analysis by X-ray fluorescence spectrographic a n d X-ray diffraction methods. X . R . F . (X-ray fluorescence) analyses, TABLE I DESCRIPTION OF HAND SPECIMENS FOR PRELIMINARY ANALYSES

Sample no.

Description

Brown, weathered limestone taken from the outside of a block of gradationally dolomitized limestone Blue-grey limestone taken from the inside of the same source block as 1 Coarse-grained, blue crinoidal limestone Randomly chosen sample of limestone aggregate from a stock heap at a quarry Selected blue limestone aggregate Selected non-blue limestone aggregate

TABLE II CHEMICAL COMPOSITION BY X.R.F. ANALYSIS (WT.~oo) OF THE SAMPLES DESCRIBED IN TABLE I

Sample

Si02

CaO

MgO

AI20z Fe203 NaO

1(20

Ti02

Mn02 S

P20z

59.44 35.73 11.20 20.75 20.25 17.73

24.11 59.90 82.11 74.10 75.25 72.11

3.79 0.10 0.08 0.80 0.45 3.63

3.97 0.03 -

1.35 0.42 0.29 0.39 0.57 0.31

0.26 0.07 0.09 0.08 0.12 0.07

0.12 0.11 0.07 0.14 0.10 0.22

0.18 0.66 0.86 0.69 0.72 0.73

II0.

1 2 3 4 5 6

6.31 2.53 1.03 2.55 2.20 375

0.28 0.08 4.46 0.35 1.41

0.20 0.36 O.18 0.43 0.79 0.10

Eng. Geol., 5 (1971) 89-116

GEOTECHNICAL PROPERTIES OF THE GREAT LIMESTONE

95

TABLE II1 DESCRIPTION OF HAND SPECIMENS FOR DETAILED ANALYSIS

Sample designation

Description

AI/A2

brown, shelly fragmental dolomitized limestone from the west end of East Layton quarry patchy, fine-grained, grey-brown limestone with dolomitized fossils medium to coarse-grained grey, shelly fragmental limestone taken from pocket of blue limestone near the base of the Great Limestone, from the lowest part of the quarry floor brown limestone, adjacent to D I A and DIB brown, friable limestone near the base of the Great Limestone, in a zone of complete recrystallization fine-grained brown limestone blue-grey drill powder produced from open-hole percussive drilling brown drill powder produced from open-hole percussive drilling brown weathered cores taken from brown a piece of grey limestone showing a grey-brown ~ weathered brown gradation and weathered grey on the outside calcareous limestone between the Upper and Lower Little Limestones Upper Little Limestone

BI/B2 C DIA/DIB D2 E F G H II 12 13 J1 J3 L1 L2

on a water-free basis, were carried out on a Philips PW 1212 automatic X-ray fluorescence spectrograph using an iterative computer technique based on wetchemically analysed standards already available in the Geology Department of Durham University. The data processing was by the university IBM 360/67 computer. X.R.D. (X-ray diffraction) analyses were conducted on a Philips PW 1010 1KW diffractometer. Six preliminary X.R.F. analyses were performed on powdered material taken from the six hand specimens described in Table I. The results of this analysis are given in Table II. These first results suggest an overall high silica content. From the higher sodium content, sample 3 probably also contains some detrital feldspar. The brown colouration is clearly associated with an increase in MgO content and a concomitant decrease in CaO. Sample 1 results show that the effect of weathering has been to remove CaO rather more easily than MgO, but since the silica content is not affected, it contributes a higher proportion to the remaining material. The results from the random sample 4 show that there is virtually no difference in chemical quality between the selected blue stone aggregate and the material stockpiled ready for the coating plant at the quarry. The main, more refined analysis was on 19 samples as described in Table III. Eng. Geol., 5 (1971) 89-116

7~

5.71 5.74 5.30 3.85 5.39 2.45 2.53 3.25 2.63 6.19 3.32 3.51 14.13 15.80 15.34 14.04 14.70 64.181 9.69

A1 A2 B1 B2 C D1A D1B D2 E F G H I1 I2 13 J1 J3 L1 L2

0.12 0.11 0,07 0.19 0.32 0.03 0.04 0.30 0.04 0.28 0.18 0.21 1.19 1.39 1.33 1.23 0.98 3.00 0.51

2.55 2.54 0.88 3.971 0.33 3.971 3.921 3.05 3.51 1.48 1.10 2.33 1.39 1.15 1.07 1.36 1.08 1.85 0.46

A120oo Fe203

0.88

13.32 14.32 19.70 11.53 2.32 12.65 12,88 16.24 4.22 17.75 4.32 12.68 3.48 3.09 3.11 3.51 2.51 1.02

MgO

34.86 34.14 29.211 37.32 49.61 37.11 36,55 32.86 46.76 30.761 48.13 37.41 41.30 41.37 41.56 41.99 43.02 16.981 47.9•

CaO

0.04 0.05 0.03 0.04 0.07 0.03 0.03 0.06 0.03 0.07 0.05 0.05 0.28 0.24 0.21 0.24 0.18 0.31 0,08

KzO

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.13 0.02 0.03 0.03 0.03 0.03 0.03 0.03 0.15 0,01

Ti02

0.29 0.37 0.26 0.42 0.25 0.55 0.63 0.35 1.21 0.77 0.23 0.46 1.00 0.38 0.44 0.58 0.57 0.32 0.38

S03

42.50 42,44 44.46 41.92 41.51 42.97 42.99 43.54 41.36 43.55 42.53 43.24 36.25 35.88 36.05 36.37 36.54 14.401 38.64

COz

Dolomite

63.34 64.94 89.49 52.83 10.56 57.49 58.02 73.41 19.15 80.04 19.75 58.20 15.61 13.88 13.98 15.78 11.37 4.67 3.96

Calcite

27.83 25.93 3.90 37.93 82.88 35.25 34.20 19.23 73.14 10.88 75.21 35.13 65.44 66.46 66.86 66.54 70.70 27.66 83.41

1 Indicates that the quoted value is outside the upper or lower limit of the standards used, and is therefore unreliable.

Si02

Sample description

CHEMICAL C O M P O S I T I O N BY X . R . F . ANALYSIS (W'I.~oo) OF T H E SAMPLES D E S C R I B E D IN T A B L E III

TABLE IV

MgC03

28.92 29.95 42.21 24.12 4,86 26.47 26.95 33.96 8.83 36.67 9.03 26.53 7.29 6.46 6.52 7.34 5.25 2.13 1,83

CaCOz

62.25 60.97 52.17 66.64 88.58 66.27 65.27 58.68 83.46 54.25 85.93 66.80 73.76 73.88 74.36 74.98 76.82 30.20 85.54

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GEOTECHNICAL PROPERTIES OF THE GREAT LIMESTONE

101

To the results of the X.R.F. analyses were added the CO 2 values determined wet-chemically by the classical carbon train technique (GROVES, 1951 ). The results were then expressed in terms of the calcium and magnesium carbonate percentages, and these in turn were transposed into free calcite and dolomite percentages by assuming that all the magnesium carbonate occurs in the dolomite. There is 55 ~o calcium carbonate and 45 ~o magnesium carbonate by weight in dolomite. The results of this exercise are given in Table IV. Examination of Table IV confirms the purity of the blue-grey limestone through sample C, but the results from sample D show that an isolated pocket of apparently undolomitized blue limestone in a dolomitized host rock will still contain a high percentage of dolomite. The result for sample E is seemingly anomalous and would be expected to have been more in line with the results from sample F which are consistent with the visible evidence. Silica contents of the A to H samples vary from about 2.5 ~o to 6 ~ . Samples I and J have higher silica contents (14-15 ~o) than the other Great Limestone samples analysed. These samples were taken from blocks which showed a colouration gradation from blue-grey at the centre to brown at the outside. However, there is only a small increase in the dolomite content and a very small increase in the iron oxide content towards the outside of the block. This suggests that visible impressions may not give a true indication of physical and chemical changes in the rock composition. On the other hand, such a progressive gradation between blue and brown stone in an intact block is not typical of the run of rock in a quarry: the interface between the two varieties is usually more pronounced. Although the X.R.D. analyses confirmed the previous trend of calcite and quartz variation throughout the range of samples, the individual X.R.D. percentages were in general a little lower than the corresponding X.R.F. values. The high silica content of sample L1 was, however, confirmed. Any fine-grained, nearamorphous quartz fraction would not of course respond as a series of unique X.R.D. peaks, and the presence of AI20 3 suggests that some of the silica in the samples will be required to satisfy the clay minerals that are present. There is no visible evidence of quartz in hand or thin section and one concludes that it must be finely disseminated throughout the rock as a variable depositional constituent, but with an insufficient concentration for the formation of chert nodules. WELLS (1955) quotes some chemical analyses made on samples taken from a quarry at East Layton. The samples were taken at different heights above the base of the Great Limestone but excluding the lowest 8 ft.--the dolomitized zone. Those results, which are plotted in Fig.3, suggest a free calcite content of greater than 80~o and a dolomite content varying up to 19~. From the diagram there would appear to be a definite control exerted on the MgCO3 content of the rock by the major horizontal joints. We can conclude from these analyses that the economic usefulness of the two types of stone--blue and brown unweathered--as a coated aggregate is Eng. Geol., 5 (1971) 89-116

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CHEMICAL COMPOSITION, C U M U L A T I V E ' I °

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BY

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WEIGHT

Fig.3. Chemical composition of" the Great Limestone at Easl hayton (plotted after the information quoted by WELLS, 1955, p.Ill).

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103

GEOTECHNICAL PROPERTIES OF THE GREAT LIMESTONE

unlikely to be influenced unduly by differences in gross chemical composition. If any problems do arise they will probably be more attributable to possible mechanical weaknesses of discrete portions of the aggregate. SPECIFIC GRAVITIES, POROSITIES AND SATURATED MOISTURE CONTENTS OF THE GREAT LIMESTONE

The costs of coating an aggregate increase with increasing porosity of the stone simply because of the greater absorption facility. Furthermore, the drying costs can be high in the case of a high-porosity saturated or partially saturated aggregate, and if the moisture is not removed, when the stone is laid cycles of freezing and thawing of the pore water can lead to disintegration of the stone and breakup of the road surface. Of the methods described in British Standard, 812: BRITISH STANDARDS INSTITUTION, 1967) for the determination of saturated moisture content of an aggregate the pycnometer method using < 3/8th inch aggregate was adopted and, from the results, the apparent specific gravity and water absorption were calculated. Similar tests were conducted on a number of cores (30 "blue" and 25 " b r o w n " ) of limestone but in this case the cores were immersed in distilled water under vacuum for 24 hours. Each core was then weighed first in a saturated but surface-dry condition, then suspended in distilled water, and then in an oven(105°C) dry condition. The average results from all these tests are tabulated in Table V. Finally, samples of the two types of limestone were crushed < 200 B.S. sieve and the powder used to find the true specific gravities of the material. These values were then used to re-calculate the absolute porosities of the limestone cores using the dry weights, volumes and appropriate specific gravities. These results TABLE V SPECIFIC GRAVITY AND WATER ABSORPTION OF BLUE AND BROWN LIMESTONE

Material form

Parameter

Aggregate

apparent s.g. water absorption ( ~ ) apparent s.g. water absorption (~o) specific gravity

Cores

Powder

average s.g.

Type of limestone "'blue"

dolomitized

2.727 1.38 2.702 0.69 2.742 2.758 2.747 2.747

2.820 3.42 2.783 2.04 2.862 2.856 2.848 2.856

Eng. Geol., 5 (1971) 89-116

104

P.B. ATTEWELL

2<, 2.2 Z

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and the saturated moisture contents of the cores are conveniently presented as scatter diagrams in Fig.4. The "blue" and " b r o w n " populations in Fig.4 are sufficiently discrete to be able to draw the firm conclusion that, due to its higher porosity and moisture absorption capacity, the brown limestone is less favourable than the blue as a constructional aggregate material or as a facing stone. The increased porosity associated with dolomitization is due not only to the inevitable volume decrease but probably also to post-dolomitization leaching. The blue limestone comprises fine- and coarse-grained lithologies but the core sampling system was such as to include cores combining both types so that a representative continuous spread of porosities was recorded. Saturated moisture contents for both types of stone were all below 3 ~ - sufficiently low for freeze/thaw cycles not to have a deleterious effect. This was checked by oven-drying four cores, two blue and two brown, cooling them in a desiccator and accurately recording their dimensions by means of a micrometer gauge. The cores were then saturated for 24 hours in a vacuum and re-measured. There had been no dimensional change. The saturated cores were then frozen for 24 hours and measured while still frozen. Again, no change in dimension was recorded. The cores were then subjected to 50 cycles of freezing while saturated Eng. Geol., 5 (1971) 89-116

105

GEOTECHNICALPROPERTIESOF THE GREAT LIMESTONE TABLE VI FREEZING AND T H A W I N G TESTS ON BLUE AND BROWN LIMESTONE

Blue no.1 Blue no.2 Brown no. 1 Brown no.2

Oven-dry (105°C) dimensions (inches)

Dimensions (inches) after 50 freeze~thaw cycles

length

diameter

length

diameter

1.9663 2.0530 2.0738 1.9570

0.9980 0.9981 1.0015 0.9961

1.9687 2.0550 2.0752 1.9570

0.9989 0.9985 1.0017 0.9965

and heating to 105 °C. No apparent structural breakdown of the cores had occurred, the largest recorded dimensional change being +0.0024 inch. (See Table VI.) A.I.V, AND A.C.V. FOR THE GREAT LIMESTONE These tests are of fundamental importance for assessing the suitability of the material as a roadstone aggregate. The value of test results on different aggregates lies more in their comparative rather than their absolute nature. The impact test simulates the aggregate breakdown facility under pavement shock loading, while the crushing test represents the potential degradation of the aggregate under gradually applied loading conditions such as would be the case on a road carrying slowly moving traffic, or under a road roller during road construction. Aggregate impact (BRITISH STANDARDSINSTITUTION, 1967) In addition to the standard size of aggregate (passing {-inch B.S. sieve and retained on 3/8th-inch B.S. sieve) two other low standard sizes (as described in British Standard, 812) of blue and brown aggregate were used. The tests were then performed using various proportions of blue and brown in order to determine what percentage of brown limestone would be acceptable in the mixture before the A.I.V. (aggregate impact value) of the aggregate began to suffer. An average result for each aggregate size was calculated from the results of three tests using the same weight of aggregate each time. To supplement the aggregate impact values, which express the fines produced as a percentage of the original aggregate weight (and which, as it happens, are influenced by the stiffness of the floor upon which the apparatus is mounted), RAMSAY (1965) suggests recording the percentage of material retaining its original size on completion of the test (A.I.V.R.). In dealing with igneous, metamorphic and fluvio-glacial gravel material, he concluded that the A.I.V. was much less affected Eng. Geol., 5 (1971) 89-116

P.B. AI'I'EWELL

106

by the aggregate shape than was the residual value. A.I.V. is a function of rock type whereas A.|.V.R. is more a function of grain shape, the latter determining the amount of the original sieve size material remaining to support further impact loading. The A.1.V.R. determined from a sequence of representative samples taken from an aggregate batch should be virtually invariate and this property could be used as a form of quality control on, for example, a flaky and elongate aggregate caused by crushing excessively large blocks of rock. The A.I.V. and A.I.V.R. results on different size aggregates (Fig.5) show

50 - -

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B Fig.5. Aggregate impact values for the two types of limestone. A. As a function of grain size; B. As a function of the percentage of blue and brown stone in a mixture of the two.

Eng. Geol., 5 (1971) 89-116

107

GEOTECHNICAL PROPERTIES OF THE GREAT LIMESTONE

that the smaller size ranges have better impact values and that the dolomitized limestone is less resistant to impact than the blue. A.I.V. improvement with reduced grain size arises because not only have the smaller particles a reduced density of intrinsic flaws available for exploitation, but also the number of grain contacts per unit volume are higher and this in effect depresses the compressive stress amplitude per contact. As would be expected, the A.I.V. and A.I.V.R. curves are divergent with respect to decreasing grain size as the residual fraction percentage improves inversely as the grading of the test fraction. The results from impact tests on various proportions of 3/8th inch blue/ brown aggregate suggest that the A.I.V. is increased with an aggregate containing more than 20 ~o of brown limestone. With more than 40 ~ of brown, the A.I.V.R. is considerably affected (see Fig.5). Based on these results, a 20 ~o blue-80 ~o brown limestone mixture appears to be least acceptable. Impact values in excess of 30 are not very meaningful. Values of around 10 are taken to be good and the average run of values fall in the region of 18-20 RAMSAY (1965) derived A.I.V.'s between 9 and 22 for typical unsorted aggregates from twenty different localities. The lowest values were for fine-grained volcanic rocks and the 22 was from a granite. It follows that the 19 and 21 for the blue limestone and dolomitized limestone respectively are fairly average values but the implication is that if a mixture of the two is used for base or wearing course construction then the brown fraction may under excessive shock loading conditions crush and powder out before the blue. The difference in impact resistance between the two stones is however minimal, and this latter point is of little practical significance.

Aggregate crushing (MARWICK and SHERGOLD, 1945; BRITISH STANDARDS INSTITUTION, 1967) In addition to the standard test on aggregate passing a ½-inch sieve and retained on a 3/8th-inch sieve, other non-standard aggregate sizes were tested TABLE VII AGGREGATE CRUSHING VALUES

Test conditions

Aggregate size

Dolomitized brown limestone aggregate A.C.V.

Blue limestone aggregate A.C.V.

6-inch diameter cylinder 40 tons load 3-inch diameter cylinder 10 tons load

1/g inch_Z/sth inch

25

20

t/2 inch-3/sth inch 1/4 inch-3/16th inch 1/sth inch-no.7 sieve

29 24 22

23 19 16

Eng. Geol., 5 (1971) 89-116

108

P. B. AT]'EWELL

using a smaller cylinder. Non standard-size test results should not be subjected to direct comparison with the standard test results. The overall interpretation of the results given in Table VII is, as before, that the brown limestone is a little less resistant to crushing than the blue limestone. UNCONFINED COMPRESSIVESTRENGTHTESTSON THE GREATLIMESTONE A number of unconfined compression tests at loading rates of 1 ton/min were conducted on cylindrical cores, 2 inches long and 1 inch diameter and the results adjusted to the equivalent compressive strength of a 1 inch long, 1 inch diameter core after OBERT and DUVALL (1967, p.332), viz., Sc Sc° -

0.2 0.8 + - LID

where, Sco is the compressive strength of a 1 inch long core, Sc is the compressive strength of the core used, L is its length and D is its diameter. Due to the variable lithologies of the limestone, the results in Fig.6 are scattered. Nevertheless, the evidence again confirms that in a mixed aggregate of blue and brown limestone, the brown is more likely to break up under stress than is the blue.

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I O

I

8

"6

6

X) B r o w n

/

/

//

i /

//

9"

/

I

/ I

10 ,

i/

/

',

12

/

/

I#., s "

4

2

6

I

I

I

1

2

3

I

1

I

I

4 5 6 7 8 POROSITY n %

I

I

9

10

Fig.6. Unconfined compressive strengths of the blue and brown limestone. Eng. Geol., 5 (1971) 89-116

109

GEOTECHNICAL PROPERTIES OF THE GREAT LIMESTONE

SHEAR STRENGTHS OF GREAT LIMESTONE AGGREGATES

An extensive and detailed series of direct shear tests was conducted on various sizes of blue limestone aggregate using small standard (36 cm 2) and a large (144 inch 2) shear box. The results and their theoretical basis are to be reported more completely elsewhere. For the small (reversing) shear box tests, a constant 153 g (0.3351 lb) weight of dry aggregate was compacted to a thickness of 2.7 cm ( l ~ inch) to give a constant sample bulk density of 1,569 kg/m 3 (98 lb/ft.a). Each size fraction of the material was displaced in shear under normal pressures of 10,20 and 30

6000

Normal

_O__.__._._o_-a---o~ o ~ ° "

4.000

'~ .,w, ,-

Normal ~o-

3000 2000

U3

~-

I A

LO00

"~ 3000 t.O~ •

o

Normal stress = 10[b/in 2 o o

I

I I 3/8" 3/z~' Grain Size ( Log. Scale)

o-- o -o----o

I

I

I

I

100 72 52 36 1/* B.S. Sieve No.

I

7

Normal -

-o

stress

30

Ib/in 2

-O~O~O~O

I~

N o r m a l stress 20 I b / i n 2 "O

'- O - - ' O ~ o ~ O

u

Normal stress = 101blin2

t-

iooo

m

n..tt )

0

B

"-~

20 l b l i n 2 o

w

-6 ~ 2000 "O

stress

1000

200

,-,,

=30(blin2 o

._._.---- O ~

% 5000 '.-"

stress

-O

I

200

omO--o--o

I

I

I

O-

I

100 72 52 36 B.S. Sieve

I

No,

1/.

50" ~.....o~°~^

3o' C

~ Residu,~olt O~O ~ O - - - ' O - ' - - -

4) - - ' ' - ~ O

~)

I 200

I I I I 100 72 52 36 B.S. Sieve

I 1/. No.

I

I

3/8"

31£'

Fig.7. Peak and residual shear strengths of blue limestone aggregate as a function of aggregate size,

Eng. Geol., 5 (1971) 89-116

110

P. I~. AT[[-~WELL

lb/inch 2 to a residual state at a rate of strain of 0.00212 inch/min (0.09(~, strain/ rain). This was sufficiently low to inhibit any pore pressure build-up in the material which might otherwise take place even in a negligibly cohesive medium ~tt the minus 150 B.S. sieve level, The two largest sizes of aggregate to be tested--material retained on a ~-inch sieve but passing a I J-inch sieve and material retained on a 3/8th-inch sieve but passing a J-inch sieve--were used in the large shear box. Compaction densities of 80 lb/ft. 3 (1,275 kg/m 3) and 84 lb/ft. 3 (1,339 kg/m 3) were achieved with the coarser and less coarse fraction respectively, and shearing proceeded (dry, equivalent to perfectly drained) under normal stresses of ½, 1, 2 and 4 ton/ft. 2 in the case of the former fraction and under normal stresses of ½, 1 and l½ ton/ft. 2 in the case of the latter. Peak values were recorded on the large shear box but these two coarsest aggregates were not sheared to residual. From the results, which are summarized in Fig.7, the peak shear strengths for particular normal stresses are slightly sensitive to aggregate size but the residuals are minimally so. Over the wide spectrum of aggregate sizes, q~ and q~i vary from 40 ° to 49 ° and from 3 4 ' to 37 ° respectively. No comparative shear tests in this instance were performed on dolomitized aggregate, but figures are available from the Engineering Geology Laboratories at Durham University for the peak and residual angles of internal friction for dolomite slurry (WILD, 1969) and Magnesian Limestone crushed aggregate (TURNER, 1967). Direct shear tests oil the former (90~'~ passing 300 B.S. sieve) have produced a ~ , of 44 ° and a q'~ of 36 c'. Similar tests on the latter (100~o passing B.S. no.7 and 8 0 ~ retained on B.S. no.200) resulted in a ~b~,o f 4 6 c' and a q~ of 36::. It would therefore appear that in the comparative size range of B.S. 200 and below, dolomite aggregate shows an enhanced strength over and above that of limestone. A probable explanation for this arises from the relative strengths of dolomite and calcite single crystals, a state which is more closely approached as the aggregate grain size decreases. At room temperature, the calcite crystal is more ductile than the dolomite crystal. Whereas in calcite the shear stresses are readily accommodated by crystallographic twin gliding on e {0112) (a critical resolved shear stress of only about 30 kg/cm 2 required) or by translation gliding on r {10TI) or./I0221~, dolomite can deform only by translating on the basal planes at a critical resolved shear stress of around 1,500 kg/cm 2, the same order of stress necessary for r translation in calcite (HIGGS and HANDIN, 1959). Within either of the aggregates there should be little tendency for crystallographic preferred orientation with respect to the plane of direct shear but the multiplicity of e planes in calcite and their rotational facility contrast with the deformational restrictions inherent in the dolomite crystallography. Thus, one can conclude that, whereas in the size range greater than 200 mesh dolomite is a little weaker than limestone probably due to the nucleation of inter-

Eng. Geol., 5 (1971) 89-116

GEOTECHNICAL PROPERTIES OF THE GREAT LIMESTONE

1 11

nal weaknesses associated with volume changes, tbe situation is reversed as the particles decrease in size to form more nearly an aggregate of fundamental dolomite crystals. LIMESTONE AGGREGATE-BINDER ADHESION

For a coated aggregate wearing course to perform its function satisfactorily and maintain its condition for a long period of time while being continuously subjected to mechanical stress in the presence of a variable temperature and moisture environment, good adhesion between binder and aggregate is required. Such adhesion is restricted by the presence of water on the surface or in the interstices of the stone (because of their relative viscosities the aggregate is more easily "wetted" by water than it is by binder) or by surface dust on the aggregate. The high internal contact angles between bitumen binder and stone can, of course, be reduced by heating both components before mixing but even after satisfactorily coating and cooling, stripping in the presence of water can occur particularly when low-viscosity binders are used. Two basic forms of binder detachment can occur (HUGHES et al., 1960). One known as spontaneous detachment is a surface tension phenomenon and involves retraction of binder from the aggregate surface and its formation into discrete spheres. The other "detachment" takes the form of a binder-aggregate separation by a thin water film while retaining an external impression of overall competence. Indeed, in the detached condition, the binder-aggregate bond is almost as strong in tension as the dry binder-aggregate bond but there is a distinct weakness in shear along the water interface. Thus, when placed in situ, although the binder might be in effect in a detached state, stripping might be a problem only in those areas subjected to above-average shear stresses, such as at corners, road junctions, traffic lights and bus-stops. Although under dry conditions adhesion problems between stone and binder in bitumen macadams are minimal, spontaneous retraction can occur with soft binders or cut-back binders 1 in freshly laid macadam if the viscosity of the binder and binder-aggregate bond are together insufficiently strong to resist the retraction of the binder into a spherical configuration. There is evidence that binders are selectively detachable with respect to the type of component minerals in the aggregate. DOUGLAS (1946a), for example, demonstrated that tar adhered rather more firmly to olivine and augite than to such minerals as feldspar, hornblende and quartz, the inference being that there is an enhanced tar affinity for basic rocks over acidic rocks. This was confirmed by HUGHES et al., (1960), using aggregates of granite, gritstone, basalt and gabbro,

1 Addition of kerosine or fuel oil to reduce the low-temperature viscosity of, say, a 200 or 3(30 pen. bitumen. T h e viscosity of a cut-back b i t u m e n is defined by the time in seconds taken by 50 ml of the cut-back b i t u m e n to flow t h r o u g h a s t a n d a r d orifice at a certain t e m p e r a t u r e (B.S. 3235).

Eng. Geol.. 5 (1971) 89 116

112

e. B. ATTEWELL

for they showed that in the case of the gritstone, bitumen binder was more readily detached from the feldspars and quartz than from the ferromagnesian minerals. Most of the tests for determining the quality of binder adhesion involve coating the surface of the stone with a particular binder, immersing the mixtures in temperature-controlled water which can contain added chemicals, and then estimating visually the percentage of stone surface stripped clean of binder in a given time. A modified Riedel and Weber chemical immersion test described by DOUCLAS (1946b) expresses the strippability in terms of the lowest normality of sodium carbonate solution necessary to initiate stripping with 24 hours of the beginning of the test, and goes some way towards quantifying the adhesion with respect to different aggregate types. Such a chemical test was used to compare the bonding properties of four different bituminous materials with the blue and the dolomitized limestones. CHEMICAL TESTS FOR BINDER-STONE ADHESION QUALITY

Samples of both blue and brown limestones passing a 3/8th-inch B.S. sieve and retained on a 3/16th-inch B.S. sieve were washed and air dried and then each retained fraction was partitioned into four approximately 100-g samples, each of these being pre-heated. A 50 e.v.t, tar was heated to 100°C and mixed thoroughly with a 100-g sample of blue and brown aggregate, the excess tar being drained off while heating the sample gently with a bunsen flame. Each coated aggregate was then cooled for two to three hours before being divided into seven portions, each of six portions being subjected to different strength solutions of sodium carbonate and one portion to distilled water. Samples of bitumen-coated aggregate using a 100 pen. bitumen were similarly prepared. The condition of all 28 samples was noted at 24 h and 48 h after immersion at 20°C, and then after boiling for one minute. The results of this exercise are shown in Table VII1. Two 100-g samples of both blue and brown limestone were then immersed in anionic and cationic bitumen emulsions I and left for several days to promote evaporation and coagulation. However, even before they could be divided for the chemical tests it was clear that there was no adhesion between either of the limestones and the bitumen from the cationic emulsion and only minimal adhesion i These emulsifiers serve to keep the bitumen droplets dispersed. The anionic component of an anionic emulsifier dissolves into the bitumen so that each droplet is surrounded by a negativelycharged layer, making the droplets mutually repulsive. Cationic emulsifiers react the opposite way, the cation part dissolving into the bitumen to cause each droplet to bear a positive charge. The quantity of emulsifier in, say, a 50-60~ bitumen-water emulsion controls the bitumen droplet coagulation rate via evaporation of water in the emulsion. Although there seems to be little selective aggregate reaction with anionic emulsions, flinty (acidic) aggregate is known to carry a small negative surface charge, so that if a cationic emulsion is used there is a pre-disposition to,yards bonding between the bitumen and the stone. This continues until the aggregate surface charge has been neutralized, whereafter evaporation promotes coagulation as before.

Eng. GeoL, 5 (1971) 89-116

113

GEOTECHNICAL PROPERTIES OF THE GREAT LIMESTONE T A B L E VIII BINDER STRIPPABILITY TESTS

Limestone

Binder

Sodium carbonate solution

Coating test results whether stripping occurred after: 24 h 48 h boiling for 1 rain

Blue

tar 54 e.v.t.

N1 N4 N 16 N64 N256 N1024 distilled water

no no no no no no no

no no no no no no no

yes yes yes yes yes no no

Dolomitized

tar 54 e.v.t.

N1 N4 N16 N64 N256 N1024 distilled water

no no no no no no no

no no no no no no no

yes yes yes no no no no

Blue

bitumen 100 pen.

N1 N4 N16 N64 N256 N1024 distilled water

no no no no no no no

slight slight slight slight no no no

yes yes yes yes yes yes yes

Dolomitized

bitumen 100 pen.

N1 N4 N16 N64 N256 N1024 distilled water

no no no no no no no

slight slight slight slight no no no

yes yes yes yes yes yes yes

between

the two aggregates

Emulsification limestone

treatment

tar binder.

and the bitumen

from the anionic emulsion.

processes are clearly out of the question is c o n c e r n e d

For the blue limestone

the most tenacious a tar binder

would

adhesion

but as far as brown is o b t a i n e d

be preferred

with a

to a bitumen.

MECHANICAL TESTS FOR BINDER-STONE ADHESION QUALITY When binder

the macadam

pavement

has

from stone can occur. An assessment

been

laid, mechanical

of the tensile strength

detachment

of

of the binder-

Eng. Geol., 5 (1971) 89-116

114

P.

13. A-I'IEWELL

stone bond serves as a useful indication of detachment facility in the absence of a simulated or full-scale road test. Two-inch diameter cores of both types of limestone were each prepared with one end polished (800 grade carborundum) flat and normal to the core axis and the other end rough-finished. These latter ends were then fastened into platen-cups using an epoxy resin adhesive, and after allowing sufficient time for the resin to cure and then heating up the cores, two polished ends were then joined together with hot binder and left to cool for three hours. Of the potential weak points (natural discontinuities in the rock, binder layer, rock-binder boundary) in this rock-binder system when subjected to tensile stress normal to the plane of the rock-binder bond, tests at room temperature showed the binder itself to be the weakest link. However, when the samples were chilled to a temperature of - 10 ~C over a period of 40 minutes and then tested in a tensometer while still cold, the effective strength of the now brittle binder was increased to such an extent as to render the rock-binder contact the weakest plane, so weak in fact that the layer of binder could be removed quite easily by scraping with a metal edge. Because of its previously established inferior characteristics, sixteen directpull tension tests were performed on the dolomitized limestone compared with only three on the blue limestone. Also, since tar appears to compare favourably with bitumen as a limestone binder, a 54 e.v.t, tar was used to form the planar bonds. The average rock-binder tensile strengths for the brown and blue limestone were respectively 17 kg/cm 2 and 65 kg/cm 2 (242 lb./inch 2 and 924 lb./inch2). SUMMARY AND CONCLUSIONS This paper has been concerned with an area of economic importance in terms of the limestone available for exploitation. Partial dolomitization of the limestone has resulted, not only in a chemical alteration, but also--on the basis of the present series of laboratory tests--in a mechanical weakening of the dolomitized fraction and a reduction in the quality of the bond between a coating medium and the surface of the stone. More specifically, it has been shown that, as might be expected, dolomitization is accompanied by an increase in both specific gravity and the facility of the stone for absorbing moisture. Solely on this latter point, aggregate drying and coating costs might be expected to increase slightly as the percentage of dolomitized stone in the aggregate increases. Although there is a definite increase in porosity and saturated moisture content with dolomitization the level of the latter is insufficient to cause fragmentation of the rock when subjected to freeze-thaw cycles. Aggregate impact and aggregate crushing tests all demonstrate the slight strength reduction of the dolomitized stone and confirm its slightly greater tendency to crush out when contained in a coated admixture with the more Eng. Geol., 5 (1971) 89 116

GEOTECHNICAL PROPERTIES OF THE GREAT LIMESTONE

1 15

competent blue stone. These results have been qualitatively confirmed by the uniaxial compressive strength values derived from tests on cores taken from the two types of limestone. Ideally, one would conduct full-scale, or prototype tests on rolled macadam (see, for example, BROOMEand PLEASE, 1958) in order to study its overall mechanical competence and its resistance to fragmentation and wear. Probably the most useful representative-specimen-type tests aimed at studying the material properties of the coated aggregate are those which make use of the triaxial cell for the determination of the c, q~ shear strength parameters (see, for example, LEES, 1969). However, the present series of tests are probably quite adequate for comparing the response of the two types of stone: in some instances the isolated test technique can produce added dividends, as for example the interpretation of the enhanced (with respect to limestone) strength of dolomite with reduction in grain size in terms of the crystallographic parameters of the two minerals. The chemical and mechanical tests directed towards assessing the coating affinities of the two versions of the stone suggest that while there is little significant difference in the ease with which a 54 e.v.t, tar binder strips from undolomitized and dolomitized limestone aggregates when immersed in sodium carbonate solutions of different normalities, the average mechanical bond strength in tension with the dolomitized stone is only about one quarter that with the original limestone. ACKNOWLEDGEMENTS

The author is grateful to Tilling Construction Services Ltd. for facilities granted and for permission to publish information upon which this work was based. He also acknowledges the assistance given within the Geology Department of Durham University by Mr. R. L. Edwards and Mr. M. J. Reeves. REFERENCES BRITISH STANDARDSINSTITUTION,1967. Methods for sampling and testing of mineral aggregates, sands and fillers. Brit. Std., 821:104 pp. BROOME, D. C. and PLEASE, A., 1958. The use of mechanical tests in the design of bituminous road surfacing mixtures, II. Stability tests on rolled asphalt. J. Appl. Chem., 8: 121-135. DOUGLAS, J. F., 1946a. Adhesion between binders and individual rock forming minerals, J. Soc. Chem. Ind., 65: 377-379. DOUGLAS, J. F., 1946b. Adhesion between binders and aggregates. J. Inst. Civil. Engrs., 27: 292-315. GROVES, A. W., 1951. S:licate Analysis. Allen and Unwin London, 2nd ed., 334 pp. HtGGS, D. V. and HANDIN, J., 1959. Experimental deformation of dolomite single crystals, Bull. Geol. Soc. Am., 70: 245-278. HUGHES, R. I., LAMB, D. R., and PORDES, O., 1960. Adhesion in bitumen macadam. J. Appl. Chem., 10: 433-444.

Eng. Geol., 5 (1971) 89-116

1 16

P.B. ATTEWELL

JOHNSON, G. A. L., HODGE,B. L. and FAIRBA1RN,R. A., 1962. The base of the Namurian and of the Millstone Grit in north-eastern England. Proc. Yorks. Geol. Soc., 33: 341-362. LEES, G., 1969. The influence of binder and aggregates in bituminous mixtures. J. Inst. Highway Engrs., 16: 7--19. MARKW1CK,A. H. D. and S~ER~OLD, F. A., 1945. The aggregate crushing test for evaluating the mechanical strength of coarse aggregates, J. Inst. Cir. Engrs, 24: 125-133. OBERT, L. and DUVALL, W. 1., 1967. Rock Mechanics and the Design of Structures in Rock. Wiley, New York, N.Y., 650 pp. RAMSAY,D. M., 1965. Factors influencing aggregate impact values in rock aggregate, Quarry ManagersJ., 49:129 134. READING,H, G., 1954. The Stratigraphy and Structure of the Syncline of Stainmore. Ph.D. Thesis, University of Durham, Durham, 214 pp. TROTTER, F. M., 1929. The Tertiary uplift and resultant drainage of the Alston Block and adjacent areas. Proc. Yorks. Geol. Soc., 21: 161-180. TURNER, M. J., 1967. The Influence of Geological Structure on Rock Slope Stability. M.Sc. dissertation, University of Durham, Durham, 86 pp. WELLS, A. J., 1955. The Stratigraphy and Structure of the Middleton-Tyas-Sleightholme Anticline. Ph.D. Thesis, University of Durham, Durham, 270 pp. WILD, B. T., 1969. Pulverised Magnesian Limestone: Its Properties and Behaviour. M.Sc. dissertation, University of Durham, Durham, 74 pp.

Eng. GeoL, 5 (1971) 89-116