English and Welsh colliery spoil heaps - mineralogical and mechanical interrelationships

Engineering Geology, 9(1975) 39--52 ©Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

ENGLISH AND WELSH COLLIERY SPOIL HEAPS AND MECHANICAL INTERRELATIONSHIPS

-

-

MINERALOGICAL

R. K. TAYLOR Engineering Geology Laboratories, University of Durham (Great Britain) (Received March 21, 1974; revised and accepted August 20, 1974)

ABSTRACT Taylor, R. K., 1975, English and Welsh colliery spoil heaps -- mineralogical and mechanical interrelationships. Eng. Geol., 9: 39--52. Seventy-four post-failure triaxial test specimens from fifteen colliery spoil heaps in England and Wales have been chemically and mineralogically analysed. Three mineral groupings emerged from statistical correlations: clay-minerals, coal and carbonates, with the coal group showing strong inverse relationships with the clay-minerals group. Correlation matrices of physical and mechanical properties versus mineralogical and chemical components have enabled the main mineralogical controls on the physical characteristics to be recognized. Coal is believed to be a major factor which influences not only peak shear strength, but fundamental properties such as natural moisture content, plastic limit and indirectly bulk density which correlates positively with the clay-minerals.

INTRODUCTION Part of the post-Aberfan research programme concerning the characteristics o f existing spoil h e a p s in Britain has b e e n d i r e c t e d t o w a r d s i d e n t i f y i n g possible m i n e r a l o g i c a l c o n t r o l s o n p h y s i c a l and m e c h a n i c a l p r o p e r t i e s , especially p e a k shear s t r e n g t h . F r o m an initial s p e c t r u m o f 1 0 6 p o s t - f a i l u r e triaxial s p e c i m e n s f r o m English and Welsh tips i n v e s t i g a t e d in this p r o j e c t , s o m e 74 s p e c i m e n s w e r e u n a m b i g u o u s l y f o u n d to b e m a i n l y u n b u r n t or slightly b u r n t colliery discard. P e r t i n e n t t o t h e s u b j e c t o f spoil h e a p investigations, t h e s p e c i m e n s w e r e f r o m t h e suite c o n s i d e r e d b y M c K e c h n i e T h o m s o n a n d R o d i n ( 1 9 7 2 ) . It can b e seen f r o m T a b l e I t h a t s p e c i m e n f r e q u e n c y reflects a bias t o w a r d s S o u t h Wales w h i c h is a n a t u r a l c o n s e q u e n c e o f the e m p h a s i s p l a c e d on spoil h e a p investigations in t h a t coalfield f o l l o w i n g t h e A b e r f a n disaster in 1 9 6 6 . CHEMISTRY AND MINERALOGY T h e m a j o r e l e m e n t g e o c h e m i s t r y ( T a b l e I A ) was d e t e r m i n e d b y X - r a y f l u o r e s c e n c e o n a c a r b o n , c a r b o n d i o x i d e and w a t e r free basis. Organic c a r b o n , C O s , H2 O ÷ and a d s o r b e d w a t e r w e r e d e t e r m i n e d b y w e l l - k n o w n w e t

40 TABLE I A: Average chemistry of 74 specimens considered (wt. %) Mean Total SiO, Al:O~ Fe:O, MnO MgO CaO Na,O K,O TiO~ S P~O~ CO, Corn H,(~* H~O

46.23 19.74 5.39 0.10 1.01 0.74 0.41 3.40 0.88 0.96 0.18 1.81 13.30 5.04 1.15

Standard deviation

Spoil heaps and specimen frequency

7.77 2.74 2.69 0.0l 0.32 0.71 0.19 0.55 0.10 0.72 0.07 1.60 8.16 1.77 0.27

S o u t h Wales

100.34

Britannia Bryn Navigation Cambrian Lady Windsor Mardy Myndd Brithweunydd Nantgwyn Western

3 11 10 4 1 10 7 3

East M i d l a n d s

Sherwood

8

Yorkshire

Askern Elsecar Grimethorpe

4 1 1

Lancashire

Bold

6

Durham

Crookhall Herrington

4 1

B: Comparison with non-weathered illite-rich shales and discards

(1) (2) (3) (4) (5) (6) (7) (8)

English and Welsh tips (74 specs.) Yorkshire Main Tip (13 specs.) Wales, Nr. Rotherham, marine shale (5 specs.) Little Smeaton, marine shale (6 specs.) Tinsley Park, marine shale (3 specs.) Brooch roof, Littleton Colliery Eight foot Floor, Littleton Colliery Park Floor, Littleton Colliery

Na~ OIAI: 0:~

K~ O/AI~ O~

0.02 0.03 0.01 0.03 0.02 0.03 0.02 0.03

0.17 0.17 0.18 0.19 0.16 0.19 0.17 0.17

chemical t e c h n i q u e s , used previously for investigations o f spoil heap materials (Taylor, 1973). As in past investigations, q u a n t i t a t i v e mineralogical analyses were carried o u t by X-ray d i f f r a c t i o n using b o e h m i t e as an internal s t a n d a r d ; the mineral standards and the resulting calibration curves were specifically a d o p t e d f o r w o r k on spoil-heap materials. E x p a n d a b l e m i x e d - l a y e r clay was e s t i m a t e d b y g l y c o l a t i o n o f d i s c - m o u n t e d specimens ( T a y l o r and Spears, 1 9 7 0 ) and e x c h a n g e a b l e cations (CEC) were d e t e r m i n e d b y the a m m o n i u m acetate technique (Chapman, 1965). Precision is high f o r chemical analyses, whereas past e x p e r i e n c e has s h o w n t h a t variations in q u a n t i t a t i v e clay m i n e r a l o g y (Table II) can involve an overall error o f up to 10%. In t e r m s o f mineral c o m p o n e n t s (Table II) the m e a n s s h o w t h a t 1 0 £ m i c a c e o u s clay-minerals (illite and m i x e d - l a y e r clay) are d o m i n a n t , with kaolinite being a relatively m i n o r c o m p o n e n t . The D u r h a m Coalfield samples

41 TABLE II Average mineralogy of 74 specimens considered (wt. %) Illite Mixed-layer clay Kaolinite Chlorite Quartz Carbonates* Organic carbon (dominantly coal)

31.5 26.0 10.5 0.5 17.5 1.0 13.0 100.0

Trace sulphates, pyrite, feldspar, rutile, phosphate (apatite) less than 2%. *Dominantly siderite (FeCO~).

however, were richer in kaolinite than any others, which agrees with previous findings related to fine discards (tailings) from the northern coalfields (Taylor and Spears, 1970). Comparison of means by Student's t test implies that at the 95% confidence level the total clay-mineral content of the English spoil samples (bar Durham*) is higher than in the South Wales suite, whilst at the same confidence level quartz is higher in the Welsh materials. Previous investigations (Spears et al., 1971; Taylor and Spears, 1972), together with more recent work on spoils from Littleton Colliery, Staffordshire, imply that there is very little evidence of chemical degradation or weathering once unburnt spoil is deeply buried within a tip. Potassium and sodium ions can largely be attributed to clay-minerals (10-A micaceous or illitic minerals in particular), since other K ÷- and Na+-bearing minerals like sulphates and feldspars, are on average present in very small quantities indeed (Table I). Similarly, alumina (A1203) is almost entirely a clay-minerals c o m p o n e n t and does not change appreciably during weathering (Krauskopf, 1967). Hence, high K20/A1203 and Na20/A1203 ratios are symptomatic of shales rich in illite (10A minerals), whilst low ratios imply that other clay-minerals like kaolinite are predominant. Since K* and Na ÷ can both be leached from the clay-mineral lattices however, a low ratio may also be indicative of either a low 10-£ content, or weathering conditions. Although the samples in the present study were from depths ranging from near-surface to just over 30 m (mean = 7.25 m), we k n o w that the average day-mineral types are 10-£ species (Table II). Comparing the average K2 O/A1203 and Na2 O/A1203 ratios of the 74 specimens under discussion with those of other u n w e a t h e r e d Coal Measures shales in which 10-A minerals are the principal types (Table IB) leads to the conclusion that the ratios are all generally similar. In other words, this wider spectrum of spoil samples suggests that chemical weathering is unlikely to be of much significance in ancient unb u r n t tips. The bulk of the detritus (Table II) comprises minerals which are *The significantly higher organic carbon content of the Co. Durham samples (22.59%, Durham c.f. 10.78% other English spoils) tends to suppress the clay mineral content, so these samples were excluded from t-test considerations.

42 TABLE 1II Exchangeable cations ( m equiv/100g) Na For 74 specimens: Mean Maximum Minimum Askern Tip, Yorkshire: (two highest Na values)

K

Ca

Mg

2.14 9.64 0.32

1.48 3.10 0.48

7.32 54.02 0.50

4.78 9.19 0.84

9.64 8.81

1.87 1.70

2.44 2.49

3.08 3.10

6.1 1.0 1.8

2.5 2.2 2.4

Argillaceous Coal Measures rocks which break down readily in water: Ryder* 15.0 5.0 Rushy Park* 5.5 4.9 Stafford tonstein (contains 70% mica-montmorillonite) 14.2 4.9 Park seatearth, Littleton Colliery 12.0 1.8 Brooch seatearth, Littleton Colliery 10.1 2.3

*From Badger et al. (1956)--using dilute HCI, all other determinations by 1N NH~ OAc at pH7. t h e m s e l v e s t h e r m o d y n a m i c a l l y relatively stable u n d e r a m b i e n t c o n d i t i o n s in Britain a n d w o r k to d a t e suggests t h a t even n o n - d e t r i t a l t y p e s are s u b j e c t to little d e g r a d a t i o n o n c e t h e y are w i t h i n t h e b o d y o f a spoil h e a p . E x p a n d a b l e m i x e d - l a y e r clay is believed to be o n e o f t h e f a c t o r s p r o m o t i n g b r e a k d o w n o f freshly e x c a v a t e d argillaceous rocks, p a r t i c u l a r l y if Na ÷ is a p r o m i n e n t i n t e r l a y e r c a t i o n ( T a y l o r a n d Spears, 1 9 7 0 ; Gillott, 1 9 6 8 ) . Mixedl a y e r clay is q u a n t i t a t i v e l y high in m a n y o f t h e s a m p l e s u n d e r discussion (Table I) and it is t h e r e f o r e p e r t i n e n t t o e x a m i n e t h e a p p o r t i o n m e n t of exc h a n g e a b l e c a t i o n s ( T a b l e III - - c a t i o n e x c h a n g e capacities, CEC). In t h e case o f e x c h a n g e a b l e c a l c i u m a n d m a g n e s i u m it is fairly e v i d e n t t h a t t h e values are s t r o n g l y i n f l u e n c e d b y t h e soluble s u l p h a t e and c a r b o n a t e c o n t e n t . Drever {1971) c o m m e n t s on t h e e n h a n c e m e n t o f e x c h a n g e a b l e Ca 2÷ in s e d i m e n t s b e c a u s e o f t h e p r e s e n c e o f c a r b o n a t e s , a n d it is o f i n t e r e s t t h a t in m a n y w e a k l y - b o n d e d shales {e.g. O x f o r d Clay - - Parry, 1 9 7 2 ) , high g y p s u m or calcite c o n t e n t s u n d o u b t e d l y give rise to the p r e d o m i n e n c e o f c a l c i u m c a t i o n s in the p o r e fluids. On T a b l e I I I are also s h o w n CEC values f o r certain British Coal Measures r o c k s w h i c h d i s i n t e g r a t e r a p i d l y in water. O n l y t w o o f the p r e s e n t values ( A s k e r n Colliery, Y o r k s h i r e , T a b l e III) a p p r o a c h t h e relatively high e x c h a n g e a b l e s o d i u m figures f o r t h e u n s t a b l e r o c k s s h o w n . A l t o g e t h e r nine s a m p l e s h a d s o d i u m CEC values g r e a t e r t h a n 5 m e q u i v / 1 0 0 g a n d f o u r o f t h e s e w e r e f r o m S h e r w o o d Colliery in t h e East Midlands coalfield. P e r h a p s n o t u n e x p e c t e d l y , t h e s e latter samples, like A s k e r n , w e r e generally fine-grained, a n d e x h i b i t e d a r a t h e r low p e a k q~' value o f 31 ° (c' = 0, least squares r e g r e s s i o n ) * * . **¢/ = angle of shearing resistance, c' = cohesion on an effective shear strength basis.

43

Although the Askern spoil heap has a relatively low peak ~' value C34°) in the low normal stress range (National Coal Board, 1972), the two individual samples with the high exchangeable Na ÷ cation levels show even lower peak ~' values of 28 ° and 32 °, respectively. The present consensus of evidence (Table III) would suggest, however, that colliery spoils with high exchangeable Na ÷ levels are restricted. Dilution by more "inert" rock types is possibly the most c o m m o n reason for the generally low levels. Within a spoil heap the Na ÷ in the interlayer structural site will itself be subject to exchange by divalent cations such as Ca 2÷ and Mg 2÷ from sulphates and carbonates and intraparticle expansion (and hence disintegration) will logically be more restricted. In any case it seems reasonable to suppose that after passing through a coal preparation plant and then being exposed in the near-surface layer of a heap prior to burial, disintegration p r o m o t e d by all factors, including sedimentary rock structures and desiccation/saturation cycles (air breakage -- Badger et al., 1956; Taylor and Spears, 1970) will have been largely achieved. Limited evidence presented by McKechnie Thomson and Rodin (1972}, supports this view and further research is currently in progress. M I N E R A L O G I C A L C O N T R O L S ON F U N D A M E N T A L P R O P E R T I E S O F D I S C A R D

A correlation matrix was prescribed under the following categories for all the elements, element oxides and mineralogical components relating to 74 samples: (a) data relevant to all samples, (b) data relevant to S. Wales*, and (c) data relevant to English spoil samples only. Product m o m e n t correlation coefficients, r (see Fisher and Yates, 1948) were c o m p u t e d for all variables. Statistically viable correlations were then checked by a computerized print-out of each pair of variables in order to ensure that the positive and negative correlations were not being artificially biased by one or two samples. Any such correlations were ignored. From the considerable knowledge of element/mineral associations in Coal Measures rocks {e.g. Nicholls and Loring, 1960; Spears, 1964) and to a lesser extent, from more recent work on colliery spoils, the resulting positive and negative associations can be explained rationally, {Fig.l). The presentation in Venn diagram form (Keeping, 1962) is convenient for present purposes since the fundamental physical properties of a more limited number of discard samples can be advantageously superimposed on the mineral groupings. By considering, the data under t w o headings (South-Wales tips and English tips, respectively -- Fig.l) the validity of the physical and mineralogical correlations can be further verified. In the more important aspects b o t h groups show marked consistency, particularly when it is remembered that the English spoil samples are from four different coalfields. The positive mineralogical/element associations shown on Fig.1 are of course complicated b y the presence of certain elements in more than one *Students's t testing of means showed no significant difference between the high-rank " a n t h r a c i t e " spoils of West Wales a n d t h e l o w e r - r a n k " b i t u m i n o u s " spoils o f E a s t Wales.

44

s0uT, ~ E 5

\o~%

~'~L ~Aes

e~LpSH S ~ L

/

/

\\\

He*~

I,

Fig. 1. Venn diagrams showing positive correlations of elements, element oxides and minerals for 74 specimens from English and Welsh spoil heaps. From a correlation matrix, the resulting associations between variables were found to fall into three "m i n er al " groupings. Physical properties for a smaller number of samples were available (see key on diagram); their positive associations with the respective chemical and mineralogical components are superimposed on the Venn diagrams.

mineral group. First of all however, the dominant apportionments will be briefly outlined. For example, total silica (SiO2) comprises free-silica (quartz)* and that combined in the clay minerals. Alumina (A1203), K2 O and Na2 O are attributed mainly to clay-minerals as previously mentioned, as is a proportion of the total iron content (expressed here as Fe203 ), calcium (CaO), magnesium (MgO) and manganese (MnO). The 1 0 - J minerals on Fig.1 comprise all micaceous types -- illite and mixed-layer clay. Titania (TiO~) is found as rutile inclusions in clay-minerals so its grouping with clay minerals on Fig.1 is not unexpected. In the spoil samples, however, the small content of carbonate minerals, such as siderite (FeCO3) and ankerite (a mixed Ca--Mg-Fe--Mn carbonate) tend to dominate the association of Fe, Mn and Ca in the English materials -- only MgO is primarily influenced by the clay-minerals (Fig.l). The position of CaO appears at first sight to be ambiguous, b u t this is not in fact so. CaO may also be associated with phosphate (P2Os) in the form of various phosphatic minerals which themselves may be found as minor constituents in coal, or in the r o o f and floor rocks associated with coal. The South Wales correlations (Fig.l) highlight another association, mamely CaO and sulphur. Coal itself is represented by organic carbon in Fig.1 and it can readily be perceived that in the South Wales samples gypsum (CaO and S) together with calcium phosphate, would appear to have closer affinities with coal than with any other mineralogical grouping. Organic carbon (coal) and sulphur show strong positive correlations in the English and Welsh tips. This again is not unusual since part of the small sulphur percentage (Table II) is attributed to the mineral pyrite found c o m m o n l y in coal, and to a lesser extent, organic sulphur. *Quartz is therefore shown with the clay-minerals group on Fig.1.

45 Separating the spoil samples into two "coalfield" groupings has not invalidated the expected chemical/mineralogical positive associations. The Venn diagram method of presentation however, does not readily categorize any dominant negative statistical associations. The correlation matrices however, showed clearly that the coal group elements, element oxides and minerals always correlated negatively with those of the clay-minerals group. On the other hand carbonate group components invariably showed positive associations with the coal group components. Coal (organic carbon), is invariably the main diluent in colliery spoils (e.g. Brancepeth tip -- Taylor, 1973). PHYSICAL PROPERTIES AND MINERALOGY The method of presentation enables physical (fundamental) properties of the discard samples to be readily superimposed on the three main mineral groupings (Fig.l). Fundamental properties --natural moisture content, bulk density, consistency limits and grain-size parameters -- were only available for a more limited number of samples. Furthermore, values for the same group of properties did not necessarily exist for each given spoil sample. Because the detailed chemistry and mineralogy had been determined for all samples it was still possible to set up correlation matrices for each individual set of fundamental properties versus their respective chemistry and mineralogy. Here again, at the 95% confidence level the resulting correlations for both Welsh and English samples show quite marked uniformity (Fig.l). The main associations are as follows: (a) Natural moisture content in both spoil heap groupings correlates positively with organic carbon (coal). It is a well known fact that the lower rank coals have a higher moisture content than high rank types (National Coal Board, 1970). For the Welsh samples themselves natural moisture content was the only statistically significant difference which we could detect between "ranked" samples -- samples with low rank coal affinities having higher natural moisture contents than "anthracitic" spoils. Adsorbed moisture (H20-) which in both cases correlates positively with the 10-A minerals, also correlates positively with natural moisture content. This is not at all surprising since the adsorbed moisture comprises a minor part of the natural moisture content of a soil or rock. The relationship with lattice water (H20 ÷) in the English samples (Fig.l) may be an indirect clay-mineral association, but the correlation with CaO is obscure. (b) Bulk density is clearly controlled by the clay-minerals and possibly to a very small extent by minor carbonates in the English spoils. A strong negative correlation with organic carbon was noted in all cases, i.e., density decreases as coal-content increases. (c) Plasticity (plasticity index) shows a positive association with the clayminerals group. That plasticity index should increase with increasing clayminerals content is again to be expected. The liquid and plastic limits tend to show differing associations in English and Welsh spoils, but this is probably

46 d u e t o t h e fact t h a t the n u m b e r o f specimens for which data existed were limited. One i m p o r t a n t f e a t u r e o f interest in b o t h t h e S o u t h Wales and English spoils is the fact t h a t coal c o n t e n t (organic c a r b o n , F i g . l ) w o u l d a p p e a r to p a r t l y c o n t r o l the plastic limit -- the plastic limit increasing with increase in coal c o n t e n t {positive association o f plastic limit with the coal g r o u p in b o t h coalfield groupings). It should also be r e c o r d e d t h a t plasticity index and liquid limit for all available samples e x h i b i t e d a highly significant positive c o r r e l a t i o n (r = 0 . 8 7 1 8 , 44 specimens). SHEAR STRENGTH RELATIONSHIPS F o r t h e purposes o f investigating the stability of spoil heaps it is conventional p r a c t i c e t o c o n d u c t c o n s o l i d a t e d - d r a i n e d triaxial tests on 100 m m d i a m e t e r samples. These samples m a y well be t a k e n f r o m d i f f e r e n t levels in a n u m b e r o f b o r e h o l e s within a spoil heap. In this m a n n e r shear-strength p a r a m e t e r s m a y be o b t a i n e d for the h e a p f r o m a p l o t o f the " c o m p o s i t e " M o h r circles. M c K e c h n i e T h o m s o n and R o d i n ( 1 9 7 2 ) c o n c l u d e t h a t the c o h e s i o n i n t e r c e p t (c') is zero and t h a t in s o m e cases the M o h r envelope m a y be distinctly c u r v e d (see N a t i o n a l Coal Board, 1972 f o r details). An a r b i t r a r y equivalent angle o f shearing resistance, ¢'e was t h e r e f o r e i n t r o d u c e d * since this was t h o u g h t to be a c o n v e n i e n t m e t h o d o f respresenting the shear strength over the n o r m a l stress range applicable to high tips. In this w o r k h o w e v e r , the m i n e r a l o g y and c h e m i s t r y has been d e t e r m i n e d f o r each i n d i v i d u a l specimen. It is t h e r e f o r e logical to consider the shear s t r e n g t h o f each s p e c i m e n o n its o w n merits. Triaxial test data were available f o r 68 specimens and these have been p l o t t e d using the Kf-line ( M o h r circle " t o p p o i n t " ) c o n s t r u c t i o n (Fig.2). If the c o h e s i o n i n t e r c e p t is a s s u m e d to be zero t h e n the effective intergranular friction, ¢', can be d e t e r m i n e d since: sin 4' = tan a, where a is the slope o f the line f r o m the origin to the p o i n t in question. St'eF,l ~

Am~Ikd4 ~

d a ~ kw samNes WmLYrdL¢

ml;~ is w ~ B ; • ~ ~ J 6 ~ r o ~ r s ~LES~

-

iLsecAm I~OmKS~H~

•

agX'.V,,

~'

,o . . ~ ,

W~

~, ~, ~, ~, ~, ~ ~ ~ 700 ~, ~ ~0 , " ~ 2 ~QO ~ ,

300 6 ~

~"

kN/~ 2

,o ;o~ .o ~ .'o.o..o 6 ,~~ ~,~o,~,.o,.,o,,,.o 'o' 2

Fig.2. Peak shear strength data for 68 specimens from English and Welsh spoil heaps. *0'e is taken as the 0' angle drawn to the origin from the point on the Mohr envelope at which the effective normal stress = 350 kN/m ~.

47

A statistical "best fit" for all 68 specimens gives an average 4' of 35 ° (ignoring any curvatures), whereas the highest 4' for an individual specimen is 49.5 ° and the lowest value, 26 ° . Published test results from all the site investigations in England and Wales (National Coal Board, 1972) imply that irrespective of coalfield; the overall shear strength ranges are of the same order, with a lower limit of 4' = 25.5 ° and an upper limit of over 40 °. In the c o n t e x t of mineral components the significant correlations with 4' for the 68 samples in the current study are shown in Table IV. The striking feature of these correlations is that the element oxides of the "clay-minerals group" of Fig.1 -- SiO2, A1203, MgO, KzO and Na20 -- correlate negatively with 4'; this is further exemplified by the high level of negative correlation of the dominant micaceous 10-A minerals, and to a lesser extent by kaolinite, which correlates negatively at a lower confidence level (Table IV). Alumina (A1203) has already been designated as the oxide which is almost exclusively confined to the clay-minerals. Whether a reduced major-axis regression line, which does not invoke the concept of a dependent and independent variable T A B L E IV C o r r e l a t i o n o f s h e a r s t r e n g t h ( e x p r e s s e d as t a n a ) w i t h e l e m e n t s , e l e m e n t o x i d e s a n d mineral components (68 specimens) tan a Total Silica,

tan ~

tan a

Titanium oxide, TiO:

n.s.c,

Lattice water, H: O+

--95

Manganese oxide, MnO

n.s.c,

Adsorbed water, H2 O-

n.s.c.

Phosphorus pentoxide, P20s

n.s.c,

10-A illite a n d m i x e d - l a y e r clay

--99

--99

Sulphur,

+ 95

Kaolinite

--95

MgO

S

Chlorite

n.s.c.

Calcium oxide, CaO

Quartz

n.s.c.

n.s.c,

Organic carbon, C

+ 99

Sodium oxide, Na2 O

--99

Carbon dioxide, CO2

n.s.c.

Potassium oxide, K~O

--99

SiO:

--99

Alumina, AI: 03

--99

Ferric oxide, Fe2 03 Magnesium oxide,

Note: tan a -+ 9 9 + 95 n.s.c.

n.s.c.

= sin ~' = ( a , - - % ) l ( a ' l +0'3). = greater t h a n 99% c o n f i d e n c e level (r = 0 . 3 1 0 9 ; 6 8 s p e c i m e n s ) . = greater t h a n 95% c o n f i d e n c e level (r -- 0 . 2 2 4 4 ; 69 s p e c i m e n s ) . --- n o significant c o r r e l a t i o n ( b e l o w 95% level).

48

is used or the least-squares (x on y) linear regression line, the relationship between ~' and alumina shown on Fig.3 infers that ~' decreases with increasing clay-minerals content. A general trend represented by a decrease in ~'ewith the increase in material passing a No.200 B.S. sieve (McKechnie Thomson and Rodin, 1972; National Coal Board, 1972) is probably an indirect confirmation of this proposition. The "fines" contain both silt and clay-size materials, a high proportion having attained fundamental (clay-mineral) size. We have already seen that the "coal group" elements (organic carbon and sulphur) have a very marked negative association with the "clay-minerals group" (Fig.l). Here again, this antipathetic relationship (Table IV) is strongly reflected in terms of shear strength -- ~' apparently increases with increasing organic carbon, i.e., increasing coal content. It has previously been suspected that coal, which is generally stronger than the associated shale, could have the effect of enhancing ¢'. Tailings, and in particular slurries, which are reject wet fines usually of very high coal content, have been shown to have generally higher ¢' values than equivalent coarse discards from the same colliery (see for example McKechnie Thomson and Rodin, 1973, fig.63). If the elevated shear strengths of fine-grained discards are influenced by coal content then it is very reasonable that it will also play an important part in enhancing the shear strength of coarse discards as well. Some idea of the high ¢' values of coals of extreme ranks can be gained from Table V. Both of these samples were fabricated to the same grain size distributions as the discard from the respective coal preparation plants. The consolidated-drained triaxial tests were carried o u t under a back-pressure of 280 kN/m 2 at effective confining pressures of between 65 and 270 kN/m 2 . Both coals have ¢' values close to the upper shear strength boundary of Fig.2. It is also clear that in comparison with the full range of discards, these results imply that coals have ¢' values in excess of the 40 degrees or so determined from the complete Mohr envelopes for British spoils (e.g., National Coal Board, 1972). Natural moisture content gave no correlation with shear strength, which

Er,ll!~sh ~

~1~

SPoil H m * ~

-

., o

t,

~Z cJw~s "~ p . O 00~ * 3.$fSt j 1,18Ny ~ f , ~ = w v

.< 2s

|-

o~O~

~

2~

*

i

~

r

,

,

,

,

,

L

,

i

i

i

L

J

'

'

g' d*~ws

Fig.3. Relationship between alumina and peak shear strength, ~'. T w o possible regression lines (least squares x on y and reduced m a j o r axis), infer that 9' decreases with increasing clay-minerals c o n t e n t .

49 TABLE V Peak s h e a r s t r e n g t h s o f t w o coals o f e x t r e m e r a n k , statically c o m p a c t e d t o B.S. low s t a n d a r d level Washery

Rank

0' (c' = 0)

0'e

Cynheidre (West Wales area)

101

46°15 ~

47030 '

Birch C o p p i c e ( S o u t h M i d l a n d s area)

902

43012 ,

43000 '

was the conclusion drawn from the wider suite of data (National Coal Board, 1972). It is of course clear from Fig. 1 that natural moisture content does have affinity for both antipathetic groups (coal and clay-minerals) and in any case the shear strength parameters are on an effective strength basis. For the 68 specimens concerned the lattice water (H2 O ÷) follows the clay minerals at a lower confidence level (Table IV), whilst the adsorbed water (which also showed a negative correlation) was just below the acceptable confidence level. The liquid limits and plasticity indices were available for 44 out of the 68 samples and as might be expected they showed a strong negative correlation with shear strength. These results should be treated with caution however, since consideration of all the physical tests did n o t reveal any conclusive trend (National Coal Board, 1972). A further trend that was established from 41 specimens for which bulk density information existed was a negative correlation with shear strength at the 95% level. This is itself confirmation of the general positive association between bulk density and the clay minerals, and thus the negative relationship with coal (Fig.l), i.e., although the shear strength increases with increasing coal content, the density of the spoil itself will tend to decrease. CONCLUSIONS

The average analysis of mainly unburnt materials from English and Welsh colliery spoil heaps shows that 10-£ minerals (illite and mixed-layer clay) are the major components. Kaolinite, which is a c o m m o n constituent in the Durham and Northumberland discards averages only 10.5% of the total constituents. Although quartz is quantitatively in excess of organic carbon (coal), the latter c o m p o n e n t is statistically important since it behaves antipathetically with respect to the clay-minerals. In other words, coal is the principal diluent. Sulphates, feldspars, pyrite, rutile and phosphates average less than 2%, whilst carbonates and chlorite average only 1% and 0.5% respectively. From previous research it has been concluded that expandable mixed-layer clay is one of the controls promoting the disintegration of shale-type rocks, particularly if Na ÷ is an exchangeable interlayer cation. In the spoil heaps investigated Na ÷ is not a particularly important exchangeable cation and

50 there is reason to believe that it may in any case be replaced subsequently by Ca 2÷ and Mg 2÷ cations, originating from the sulphates and carbonates in the waste materials. Limited evidence of low shear strengths being associated with fine-grained spoils with more elevated exchangeable Na ÷ values can, nonetheless, be found in a small number of English spoil heaps. Conclusions drawn from all spoil heap investigations to date suggest that physical disintegration and chemical weathering processes are virtually d o r m a n t once discards are deeply buried within an unburnt spoil heap. The average chemistry of the present samples tentatively supports the view that any further chemical leaching is of minor import. Statistical correlations have shown that in toto coal is directly or indirectly the major mineralogical control on a number of physical parameters. Hence, natural moisture content and plastic limit correlate directly with coal content, whilst bulk density correlates negatively with coal content and hence positively with the "clay-minerals group" of minerals and element oxides. Plasticity is directly related to the clay-minerals but the direct and indirect relationships between effective shear strength ¢', and the mineralogical variables imply that the shear strength increases with increasing coal content, and therefore with decreasing clay-minerals content; what is more the trends suggest that when ~' increases bulk density decreases. No correlation has been detected between natural moisture content and effective shear strength. The current energy situation has once more focused attention on the exploitation of the world's coal reserves. Experience in Britain has shown that modern mechanized mining methods result in a substantial increase in the coarse discard to be disposed of from 7 million tonnes in 1930, 25 million tonnes in 1960, to around 55 million tonnes today. Over the same period coal production has fallen by about 30%. Although about 6--7 million tonnes per annum of the National Coal Board's mining waste* has been used for a number of civil engineering and commercial purposes (Tanfield, 1971), spoil-heap management is a vital issue. In Britain all new heaps and lagoon embankments are now constructed in layers, with a rigorous background of site investigation, design and operational precautions (National Coal Board Technical Handbook, 1970; National Coal Board 1972). Similarly, these publications give some idea of the precautionary and remedial work which is applicable to any old tips which were deemed to be potentially unstable from the exhaustive site investigations and analyses carried out during the past 7 years. The general application of the results considered in this paper concerns primarily the identification of mineralogical and chemical components in coalmine waste, which may accentuate degradation processes -- clay-mineral types and elevated exchangeable Na ÷ values in particular. At the design stage of a new spoil heap it is important to know the short-term behaviour of the material with regards to the possible effects of excessive breakdown during *The National Coal Board alone owns around 2,000 spoil heaps.

51

compaction, for example. Although mixing with "inert" spoil from other locations is not usually commercially viable, it is a point worth considering in the development of new collieries or in the extension of existing coalfields. In a similar way the characteristics of the spoil in old tips are important from a regrading or restoration point of view; the possibility of renewed short-term degradation after many years of dormancy within the b o d y of a heap is a factor that one can a t t e m p t to establish. The incidence of potentially weak spoils in British coalfields due to intrinsic mineralogical and chemical factors would appear to be very restricted, but this is not necessarily the case in other countries. Moreover, the apparent dormancy of further degradation within the b o d y of an unburnt spoil heap may also not apply to countries with more extreme climatic regimes than Britain. It has been demonstrated that the presence of coal and coaly materials enhances the peak shear strength which is an additional bonus with respect to stability. The risk of ignition (spontaneous or induced) must be stressed with respect to this t y p e of spoil, and British practice is to construct in 0.3-m layers. Compaction reduces the liability to spontaneous ignition, but it is important to ensure that the exposed edges of the layers are well compacted, otherwise the added expense of quelling this major source of pollution may have to be resorted to. The residual strength of colliery discards is outside the scope of this paper, but the work to date implies that the composition and subsequent geochemical history of the material are u n d o u b t e d l y important controls on this parameter (Taylor, 1973). ACKNOWLEDGEMENTS

The writer is indebted to the National Coal Board for grants in aid of this research (Project 17). The views expressed are those of the writer and not necessarily those of the Board. To Mr. G. McKechnie Thomson (formerly Chief Civil Engineer, N.C.B.) and Mr. D. E. Eccles (formerly Senior Soils Engineer, N.C.B.) warm thanks are given for encouragement and help during the project. Mr. S. Rodin (Director) and Mr. S. B. Webb and Mr. J. T. Finey of Wimpey Laboratories Ltd., kindly ensured that material was made available and readily provided advice, during numerous discussions. Most of the physical and mechanical data were provided by Wimpey Laboratories. The weight of the mineralogical analyses were carried out by Mr. R. G. Hardy (Research Assistant) and the writer is greatly appreciative of his assistance. Similarly, Mrs. Anne Furness and Mr. Simon Tasker provided considerable assistance in data processing. Mr. D. E. Hope kindly assisted with exchangeable cations.

52 REFERENCES Chapman, H. D., 1965. Cation exchange capacity. In: C. A. Black (Editor), Methods of Soil Analysis, 2. American Society of Agronomy, 1572 pp. Badger, C. W., Cummings, A. D. and Whitmore, R. L., 1956. The disintegration of shales in water. J. Inst. Fuel, 29: 417--423. Drever, J. I., 1971. Early diagenesis of clay minerals, Rio Ameca Basin, Mexico. J. Sed. Pet., 41: 982--994. Fisher, R. A. and Yates, F., 1948. Statistical Tables for Biological, Agricultural and Medical Research. Oliver and Boyd, London, 112 pp. Gillott, J. E., 1968. Clay in Engineering Geology. Elsevier, Amsterdam, 296 pp. Keeping, E. S., 1962. Introduction to Statistical Inference. Van Nostrand, Princeton, N.J., 451 pp. Krauskopf, K. B. Introduction to Geochemistry. McGraw-Hill, New York, N.Y., 721 pp. McKechnie Thompson, G. and Rodin, S., 1972. Colliery spoil tips -- after Aberfan. Institution of Civil Engineers, London, Pap. 7 5 2 2 : 5 9 pp. McKechnie Thompson, G and Rodin, S., 1973. Colliery spoil tips -- After Aberfan. Proc. Inst. Cir. Eng., 55: 677--712. National Coal Board, 1970. Spoil Heaps and Lagoons. National Coal Board, London, 2nd draft, 233 pp. (Technical handbook). National Coal Board, 1972. Review of Research on Properties of Spoil Tip Materials. National Coal Board, London, 100 pp. Nicholls, G. D. and Loring, D. H., 1960. Some chemical data on British Carboniferous sediments and their relationship to the clay mineralogy of these rocks. Clay Miner. Bull., 4: 196--207. Parry, R. H. G., 1972. Some properties of heavily overconsolidated Oxford Clay at a site near Bedford. Geotechnique, 22: 485-507. Spears, D. A., 1964. The major element geochemistry of the Mansfield Marine Band in the Westphalien of Yorkshire. Geochim. Cosmochim. Acta, 28: 1679--1996. Spears, D. A., Taylor, R. K. and Till, R., 1971. A mineralogical investigation of a spoil heap at Yorkshire Main Colliery. Q. J. Eng. Geol., 3: 239--252. Tanfield, D. A., 1971. Construction uses for colliery spoil. Contract Journal, 14th and 21st January. Taylor, R. K., 1973. Composition and geotechnical characteristics of a 100-year-old colliery spoil heap. Trans. Inst. Min. Metall., Sect. A (Mining Industry), 82 (A1--14): A145--147. Taylor, R. K. and Spears, D. A., 1970, The breakdown of British Coal Measure rocks. Int. J. Rock Mech. Min Sci.. 7: 481--501.