By A . J. P E T E R S O N
T h e a u t h o r re v ie w s so m e of th e d ifficu lties s ta n d in g in th e w a y o f s ta n d a r d iz a tio n o f a llu v ia l sa m p lin g m e th o d s .
R e ce n t p ro n o u n ce m en ts on th e sam p lin g of allu v ial d ep o sits h a v e te n d e d to cre ate a feeling th a t an a tte m p t is being m a d e to say th e final w o rd on a su b je ct w hich long discussion h as p ro v ed to be still ex p e rim e n ta l a n d co n tro v ersial. T h ere are m a n y difficult p roblem s arisin g d u rin g th e te stin g of such d ep o sits w hich h av e heretofore e ith e r n o t been m e n tio n e d o r h av e been h a s tily d is
m issed a n d it is n o t surprising, th e re fo re, t h a t in som e cases th e pu rp o se of th e e x a m in a tio n is lo st b y to o close a tte n tio n to th e m ode of pro ced u re. Som e engineers seem to be of th e opinion t h a t m o st of th e field w ork— such as th e d irec tio n of th e m ain tra v e rs e an d th e o rie n ta tio n of b o re
hole sy m m etry — can be p la n n e d b efo reh a n d in th e office, w hen o ften a n e x a m in a tio n of th e site w ith m a p in h a n d will rev eal in su p e r
able difficulties in th e w ay of a n y such p re p a re d schem e. I n th e follow ing article th e a u th o r en d eav o u rs to show w h a t is th e m ain p urpose of allu v ia l te stin g a n d in w h a t m a n n e r cond itio n s m a y affect procedure, his rem a rk s being illu stra te d b y M alayan p ractice.
T he pu rp o se of th e e x a m in a tio n of an y d ep o sit is, of course, v a lu a tio n a n d th e u p p erm o st th o u g h t d u rin g th e co n sid eratio n of, say, a tin a re a su ita b le for d redging should su rely be how q u ic k ly it is possible to d e te r
m ine th e m in im u m y ard a g e co n jo in tly w ith v alue w hich w ill w a r ra n t th e in sta lla tio n of a d redge a n d also th e co n tin u a n c e of sam pling.
T he u su a l p ra c tic e is to su b d iv id e th e m a in a re a b y th re e o r fo u r row s of holes, th e resu lts of w hich q u ic k ly decide a b a n d o n m e n t or give en co u rag em en t for co n tin u an c e.
L e t us suppose it is th e la tte r a n d t h a t an a re a h as b een m a rk e d for sy ste m a tic sam pling.
I f ju d g m e n t h a s been used th e re is a lre a d y th e fram ew ork of a p re d e te rm in e d sy m m e try of th e sy stem , w hich m a y le ad even to th e
" 2 b y 2 ” ch a in sq u a re , co n sidered b y so m a n y engineers to be th e o n ly c riterio n of efficiency. Is n o t th is p ra c tic a l p ro ced u re as c e rta in as a n d m u c h less c o stly th a n a p ersis
te n t ex ten sio n b y eig h t-c h a in sp acin g to m a in ta in from th e b eginning to th e end a sq u a re sy m m e try ? M oreover, a lth o u g h it is n o t o ften recognized, th e re a re — o r soon w ill be to th e m a th e m a tic a lly m in d e d — rows a n d colum ns of re su lts offering th em selv es in a n u m b e r of w ays to a lte r n a te av e rag in g a n d co m p ariso n of divergences. W h y , it m a y be asked, choose a " d efinite a r e a ,”
w hich, from a m a th e m a tic a l ap p re c ia tio n of irre g u la rity , is no guide, w hen th e w hole are a to be sy ste m a tic a lly sam p led offers a m ean s of d e te rm in a tio n of re q u isite spacing a n d sufficiency of holes ? D oes it n o t follow t h a t if it is logical to ac ce p t th e decision of divergence of, say, p e rh a p s 2% o r 3%
from a lte r n a te a v e rag in g on co m p letio n t h a t th e pleasing in fo rm a tio n m a y be giv en t h a t th e sam p lin g h as been checked, or in o th e r w ords th a t th e sa fe ty fa c to r is a t le a st 2 ?
T he a u th o r does n o t propose to a t te m p t item ized criticism of b o rin g p ra c tic e as it
h a s been d escrib ed b y o th e rs, b u t he feels th a t m a n y e x p la n a tio n s h a v e been giv en th a t are c o n tra d ic to ry , m a n y s ta te m e n ts m a d e w hich n eed q u a lify in g , a n d t h a t m a n y c o n sid eratio n s of im p o rta n c e h a v e n o t been m e n tio n e d . T h e re has, for ex a m p le , been som e discussion of “ fa c to rin g .” I t m a y a t once be said t h a t “ f a c to r i n g ” h a s a n ill r e p u te in M alaya, w h ere th e use of a figure expressing th e re la tio n sh ip of sa m p lin g re su lt to th e e s tim a tio n of e x tra c tio n b y dred g in g has been w idely held to be a suspicious p rac tice . T h is a ttitu d e of th e ig n o ra n t is, p erh a p s, u n d e rsta n d a b le , b u t it is in c o m p re hensible t h a t m a n y engineers sh o u ld be b lin d to th e tr u e m e an in g of th e rela tio n sh ip , for a fa c to r w on from th e ex p erien ce of b o th sam p lin g a n d d redge e x tra c tio n is essen tial.
T o o b v ia te th e use of a n open ly -ex p ressed figure m e asu rin g boxes of special dim ensions, giving volum e o r ex p an sio n allow ances a u to m a tic a lly , a re w idely used. T hese allow ances are of th e o rd e r of 1 3 3 i% to
150% .
Several m e th o d s of bore-hole sam p lin g h av e been used in M alaya, each one of w hich u n d e r efficient su p erv isio n h as p ro v e d itself ca p ab le of affording a su rp risin g ly precise re su lt. T he resu lts from th e se v eral m e th o d s a re w idely different. A n engineer n o t know ing d red g in g o p era tio n w ould be a s to u n d e d to see resu lts o b ta in e d b y g re a t care of m a n ip u la tio n a n d c o m p u ta tio n a n d expressed o ften to th re e places of decim als m u ltip lie d b y high fac to rs of such la titu d e . I t sh o u ld be rem em b ered th a t a t b e st th e o p era tio n of d red g in g is crude. U n d u g g ro u n d , spillage from b u ck e ts, screen losses, a n d jig o r ta b le losses are all serious. N o r c a n th e to ta l loss be d e te rm in e d ex c ep t a fte r a p erio d of sev eral y ears. T he only a lte rn a tiv e is v e ry in te n siv e sam p lin g — b o th ah e a d an d , la te r, a s te rn of th e d re d g e — th e re su lt from w hich w ould o n ly a p p ly to th e v e ry sm all volum e ex a m in e d . T h is m a y ex p lain th e r a th e r ind efin ite fac to rs em ployed, for u n d er such circ u m sta n ce s of com parison a v e ry co n sid erab le m a rg in is necessary.
I t does n o t, how ever, ex p lain th e fa c t t h a t th e tin -d re d g in g in d u s try is alm o st u n iq u e in re q u irin g a fa c to r of m u ltip lic a tio n an d n o t of division in e s tim a tin g e x tra c tio n .
A com p ariso n of th e se v eral m e th o d s used in bore-hole sam p lin g w ould be a d v a n ta g eo u s.
I t m a y be a c c e p te d t h a t th e se agree in com ply in g w ith th e m a th e m a tic a l co n sid eratio n s in v o lv e d in co m p u tin g v o lu m e a n d average v alu e p er cubic y a r d from th e previously- d e te rm in e d volu m e v alu e of th e holes. I t
is in th is d e te rm in a tio n t h a t th e m e th o d s em p lo y ed a re v e ry d iffe ren t. S am ples o b ta in e d in b o rin g w ith a B a n k a d rill are in v a ria b ly w et a n d m o re o r less d isin te g ra te d . T he m a te ria ls co n sist of soil, clay, sa n d , and g ra v e l (an d p e rh a p s tin o xide), som etim es alone, b u t v e ry fre q u e n tly so m ix e d th a t th e p rev io u s re la tio n s h ip in s itu seems in d e te rm in a b le . T h e p h y sic a l ch a ra c te rs of th e se m a te ria ls are v e ry d iffe ren t and each re q u ire s sp ecial c o n sid e ra tio n .
Som e y e a rs ago th e p r o p o rtio n in g of ag g re g a te for th e p u rp o se of m a k in g concrete w as d e te rm in e d a lm o st e n tire ly in reference to v o id filling. V ery m a n y d e te rm in a tio n s of v o id s in s a n d a n d g ra v e l u n m ix e d and m ix e d artific ia lly a n d fro m b a n k s h a v e been p u b lish ed . A lth o u g h th e se figures n a tu ra lly refer m o stly to loose a n d d ry -p a c k e d m a teria l, th e re are a n u m b e r of d e te rm in a tio n s of void p erc e n ta g e s a fte r w et p a c k in g to be com pared.
F ro m th e se it w ould seem t h a t w e t-p a ck ed g ra v e l fro m a b a n k c o n ta in s on a n average a b o u t 4 0% vo id s. T h is figure w a s te ste d m a n y tim e s b y th e w rite r in M a lay a in d e te rm in in g v o id p e rc e n ta g e in w ash and re su lts show ed a close a g re e m e n t. Sand also, of a n irre g u la r size a n d w et packed, gives a p p ro x im a te ly th e sa m e p ercentage.
S an d of a u n ifo rm size c o n ta in s , of course, a low er p e rc e n ta g e . C lays also h a v e voids.
In th e n a tu ra l s ta te of c o n so lid a tio n th e void p e rc e n ta g e is, ro u g h ly , 3 5% to 4 0 % . W hen w e tte d th e re is co n sid e ra b le sw elling owing to th e increase in size of th e w a te r films se p a ra tin g p artic le s. W h e n p u d d le d and se ttle d th e v o id p e rc e n ta g e in c re ase s to 60% to 8 0 % . T h is re p re s e n ts a n increase in v o lu m e of a t le a st 5 0 % .
T he fo rm a tio n of a n a llu v ia l d e p o s it is well u n d e rsto o d . A b ed of g ra v e l is fo rm e d b y the a c c u m u la tio n of p eb b les a n d ro ck fragm ents w hich h a v e been ro lle d fo rw a rd b y m o v in g ’ w ate r. S e ttle m e n t is d u e to o b sta cles—
o th e r p eb b les— b lo c k in g a d v a n c e r a th e r than to a selectiv e choice of p e r m a n e n t seats a n d to th e low ering of v e lo c ity of th e w ater c a rrie r. S a n d m o re o r less c o n c u rre n tly se ttle s w ith th e g ra v e l. I t likew ise is carried along b v m o v in g w a te r a n d a ch a n g e of w ater v e lo c ity w ill, p e rh a p s, be su fficien t to cause it to sink to n e a r th e b o tto m , w h ere velocity is still less, a n d it se ttle s in to crevices or in te rsp a c e s in th e g ra v e l. L a te r, when co n d itio n s h a v e m u c h ch a n g ed , th e area m a y , p e rh a p s, be a b a c k w a te r and alm o st q u ie sc e n t— a slow d e p o sitio n of clav occurs, g ra d u a lly filling th e v o id s of the
F E B R U A R Y , 1933 87
p revious d ep o sit. T h is filling m a y c o n tin u e slow ly long a fte r th e d ep o sitio n of th e clay h as ceased. R a re ly does th e filling p e rsist to th e b o tto m of th e grav el. A lm ost in v a ria b ly in bo rin g an in ru sh of w a te r occurs soon a fte r p e n e tra tio n of w ash. I t is obvious th a t voids can n ev e r be m ore th a n filled a n d c o n seq u e n tly g rav e l, o r sa n d if g ravel be a b s e n t, d eterm in e s th e volum e.
C om pactness of g ro u n d in a r a th e r in d e te r m in ab le w a y increases w ith d e p th a n d p ressure w hich, a t least for th e d e p th s u n d er co n sid eratio n , is insufficient to cause crushing a n d f ra c tu re of p article s.
F ro m th e foregoing co n sid eratio n s it m a y be ex p ected t h a t n o t one p a rtic u la r m a n n e r of m easuring volum e w ill sa tisfa c to rily m eet all th e co n d itio n s of sam pling, y e t th is is a tte m p te d — such as b y p ipe-volum e m e a su re m e n t, b ox m e a su re m e n t, w a te r-d isp la c e m e n t m e asu rem en t, d ry w eighing, w et w eighing, etc. T he pipe-volum e m e a su re m e n t w ould seem th e sim plest, b u t in B a n k a -d rill p ra c tic e it h as only b een p a r tly successful.
I n te r m itte n t o p era tio n a n d irre g u la rity (in g ravel th e p u m p m u st be forced ah e a d of th e casing) ' ren d e r th e v olum es w on m ost in c o n sta n t in rela tio n to d e p th bored. O nly for clay or a clay m ix tu re (sand a n d g ravel p erc en ta g e low) can a n y reliable rela tio n sh ip be estab lish ed . T he w a te r-d isp lac em e n t m e asu re m e n t of sa n d a n d g rav e l d eterm in e s th e volum es of solids, leav in g th e voids un m e asu red . T he erro r m a y be as high as 40% . Clay d ra in e d o v ern ig h t h as sw ollen, p erh a p s, 20% or 30% before m e asu rem en t n e x t m orning.
B ox m e asu rem en t p ro v id es n e a rly an ex a ct m e asu rem en t (th ere is, p erh a p s, even m ore slippage of p ebbles th a n in th e original se ttle m en t) of clear sa n d a n d g rav el a n d also of a san d-gravel-clay m ix tu re a fte r w ashing, p ro v id ed th e clay is infilling only a n d n o t all or p a r t e x tra n eo u s— t h a t is, from above or below th e lay er of w ash. D ry w eighing a n d w et w eighing are n o t used a g re a t deal a n d v ery little in fo rm atio n is available.
In m ore th a n one w ay each a p p e a rs u n s a tis fac to ry . H ow ever, it is a sav in g fea tu re th a t tin oxide is g enerally d ep o site d along w ith th e sa n d a n d g rav el a n d n o t w ith th e clay. W ith th is a p p re c ia tio n it seem s wise to ac ce p t th e m easu rin g box as th e best m e an s of m easu rin g sa n d a n d g rav e l volum es a n d th is can be e x te n d e d to include m e a su re m e n t of sand- g rav el-clay m ix tu re , w h en it is e v id e n t t h a t th e clay re p re se n ts v o id filling. A lth o u g h in good bo rin g p ra c tic e sam ples a re b ro k en
im m e d ia te ly change in th e c h a ra c te r of m a te ria l is recognized, sam ples fre q u e n tly c o n ta in e x tra n e o u s clay from ab o v e o r below, or if a th in b ed of w ash from b o th abo v e a n d below. In deciding th e u p p e r a n d low er lim its of a b ed of w ash a n em p irica l sa n d p erc en ta g e b ased on v oids a n d clay con
sisten cy is som etim es in d ic a tiv e or useful, or, b e tte r, a close ex a m in a tio n of successive tin w eighings a n d sa n d p erc en ta g es m ad e la te r a n d co m p ariso n w ith th o se of n eig h b o u rin g holes w ill help in deciding lines of d e m a rc a tio n . T h e low er lim it is p a rtic u la rly im p o r
ta n t. S an d a n d g rav el an d , of course, tin oxide are fre q u e n tly c a rrie d dow n several feet b y th e ju m p before com plete ac ce p ta n ce.
T h o u g h th e d e p th s m a y be fixed th e sa n d a n d grav el v olum es to be a tta c h e d are n o t q u ite tr u ly re p re se n ta tiv e . H ow ever, th e e rro r in tro d u c e d is c e rta in ly v e ry m u ch less th a n in a d e te rm in a tio n d ep en d in g on box m e asu rem en t of clay.
In sp e ak in g of m e asu re m e n ts th e a u th o r recalls th a t th re e o r fo u r y ea rs ago m a n y of th e co n sid eratio n s th a t h av e been o u tlin e d here w ere discussed w ith Mr. L . T . W illan in M alaya. M r. W illan h a d a t th a t tim e m a d e considerable ad v a n ce in pip e volum e m e asu re
m e n t a n d from his collection of d a ta — box m e asu rem en ts, g ra n u la r an aly ses, clay d en sities, e tc .— h a d been able to derive form ulae, w hich he w as using in d eterm in in g volum e valu es re la tiv e to th e a c tu a l occu r
rence of m a te ria ls as co n sidered in situ.
If th is review succeeds in show ing som e of th e m a n y com plex problem s arisin g in allu v ial v a lu a tio n w ork th e a u th o r will feel th a t it h as served a good p urpose.
T h e v a lu a tio n of a n allu v ia l a re a is g enerally a m u ch m ore difficult u n d e rta k in g th a n th e v a lu a tio n of a lode m ine. In th e la tte r th e a c tu a l re la tio n sh ip of m a te ria ls can alw ay s be seen a n d definite a n d a c c u ra te m e asu re
m e n ts can be ta k e n . In a d d itio n th e re are sh a fts an d w inzes, levels a n d cross-cuts open for insp ectio n a n d sam pling. I t sho u ld be realized th a t to re p o rt on e ith e r req u ires special q u alitie s a n d special know ledge. T he a u th o r finds it d istu rb in g t h a t th e I n s titu tio n of M ining a n d M etallu rg y proposes, in th e form of rec o m m en d a tio n s, t h a t re p o rts of v a lu a tio n of a llu v ia l are as sho u ld be p re se n te d in a specified m a n n e r as in d ic a tin g a p resc rib ed p ro ced u re. P re se rv a tio n of th e s ta tu s of th e allu v ia l m em b ers of th e profession a n d th e possible g re a t h a rm to th e in v e stin g p u b lic is p le ad e d as ju stific a tio n for th is a c tio n . I t is felt th a t n e ith e r m e ch a n ica l m e th o d s n o r
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m e ch a n ica l o b se rv a n ce in p re s e n ta tio n , w hich th e le ast efficient m a y p erfo rm w ith th e a p p e a ra n c e of th e h ig h ly skilled, can e n h a n ce a n d m a in ta in p re stig e a n d it sh o u ld be rea lize d t h a t fu n d a m e n ta l scientific p rin cip les m a y be gro ssly o u tra g e d ev en in co m p ly in g w ith su c h a schem e. A m u c h -q u o te d sa y in g
h a s b een h a n d e d dow n : “ S ty le is th e m a n h im se lf.” M ay it n o t be sa id w ith th e sam e t r u t h a n d w ith ev en m o re force t h a t th e re p o rt is th e m a n him self, h is a b ility , his know ledge, a n d his h o n o u r. I t is to be hoped t h a t th e p ro p o sal will receiv e f u rth e r con
sid e ra tio n .
The I.M .M . B en e v o len t Fund T h e follow ing f u rth e r su b sc rip tio n s to th e B e n e v o le n t F u n d o f th e I n s titu t io n h a v e b ee n rec e iv e d d u rin g th e p a s t m o n th :—
N u n d y d ro o g M ines, L td . £ 10
s.
10 d.
0
A. J. B ensusan 5 5 0
J. D. H asleden 5 5 0
C. M. P. W rig h t . 5 5 0
J . G. L aw n 5 0 0
H . G. S co tt 5 0 0
C. R. P in d e r 4 4 0
T. P ry o r 3 3 0
G. R igg 3 3 0
G. W . E . T u rn e r . 3 3 0
V. L. A spland 2 2 0
S ir H aro ld C arp en ter 2 2 0
A. F . D ick-C leland 2 2 0
L. E . D jin g h eu zian 2 2 0
F . W . H a rb o rd 2 2 0
R . K a te le y . 2 2 0
F . H . L a th b u ry . 2 2 0
S. H . Steels 2 2 0
E . E . W atso n 2 2 0
J. M. B ell . 2 0 0
R . E . C om m ans . 2 0 0
K. E . B en su san . 1 1 0
P. C. C. C ayley . 1 1 0
F . G. C ornish 1 1 0
A. W. H . D ean 1 1 0
A. J. D ish m an 1 1 0
R. H . Goodw in 1 1 0
N. G. H a ck n e y 1 1 0
R. M. H a rlan d 1 1 0
C. H . Jo n e s. 1 1 0
B. G. Luff . 1 1 0
T. G. M adgw ick . 1 1 0
J. V. Moore 1 1 0
R. J. M organ 1 1 0
A. R. P hillips 1 1 0
J. R o b erts . 1 1 0
H . L. S arg en t 1 1 0
H . K. S co tt 1 1 0
H . Sim on 1 1 0
N. L. S m ith 1 1 0
W . G. W ag n er . 1 1 0
W . T. H . W h ite . 1 1 0
L. M. W in n . 1 1 0
H . C. W oolm er . 1 1 0
W . J. C. S c ru tto n 1 0 0
J. H . E llis . C. P en g illy . C. B ra ck e n b u ry
P rev io u sly acknow ledged . 1,275 17 17 10 10
0 0 6 9
T o ta l • ¿1,375 14 3
L E T T E R
T O T H EEDITOR
“ G old M ining in R u ssia ”
S i r ,— Since y o u h a v e su m m a riz e d in y o u r issue of O c to b e r la s t an a rtic le b y Mr. L.
S ch lo u n d e, p u b lis h e d in th e F a r Eastern R eview for J u ly , allow m e to s ta te th e re are a n u m b e r of in a cc u ra cie s th e re in . A s th e re su lt of a so m e w h at le n g th y e x p e rien c e in R u ssia I h a v e no h e s ita tio n in sa y in g th e article w as full of geological, g eo g ra p h ic a l, and s ta tis tic a l m is s ta te m e n ts , th e e n u m e ra tio n of w h ich w ould o cc u p y m u c h g r e a te r space th a n y o u w o u ld b e w illing to a c co rd m e.
G . T . Ev e.
L o n d o n , E .C . J a n u a r y 11.