STRUCTURAL PROPEWrIES OF SOILS

ROLE OF MACROPORES IN TtIE SLUMPING OF LOESS

M. Minkov and K. S t o i l o v UDC 6~.lA31.328: 62,l,131.243

"Macroporcs"(as distinct from micropor~:D are. of course, pores which ate visible to the naked eye. having dimendons of 0.I to 2 ram. r Smaller cavities are called mlcropores, larger ones are called passages ot cavern,s [I].

In o~ invcstlg~dons the real volumes of maeropotes were found by the mcdtc~I of L~tloltov [~].

Using a bhmcul~r lens with = ndcromc~cr, w¢ found th~ mean oxen occupied by macroporrson all three palls of opposite faces of achbe with ~?cm ed~e~, A~ a control, thh operation was tepe~trd on ¢I~r¢~ cubes from tl~esame blocl~ of soll ~nd the =verage t~l~en. ~t the same time, using a hand t~s, w~ al.so determL~cd the ratio between the vo|umes ofstable and umtabl~ macropotcs.

Lartonov', method h to some extent arbitrary. He a~sumes that under a lo~d of 3 kg/cm t ,~It th~ unstable (aon- lnctmtcd) mactoporcs collapse, although they do not doso completely. This =s=~mptt0n somewhat ~,'ceesthuates the rol~, of m~cropores In the process of slumping ~d settlement, at~d must be remembered in S~b~¢q¢=¢nt cot~cht~fons,

In dctermlnln~ the qu~ntRattv¢ rol~ of mactopotcs In deformation w~ determined tl~¢ fol!owh~ h~dlc¢~,

(D the teM, volumes of the mac~opor¢~ nmt - the ratio of the volume of the ruacroporc~ to the volume of the sp¢clmrn, e×ptc=~¢d as a perccnt,~ge, found by L~rionov's meth~:

(li) the frcc volume of the mactoporcs nmf - the dlffcrenc¢ between nmr a~d the volume of stabl¢ macro- pc¢~=, e×prrsse,J ~, a pcrcertta~;

(lid the partial active pore volume naz ~ the diffcrct~ce bct~,'ce~ tl~c pore voh~me, bcfor~ ~ d Mter lo.~dit~ and w¢td~g of the ,p~ctmcn under ;t prc~nttc of 3 kg/cm: (for brevity we ~all rail lhi.~ th¢ ac¢ivc pore vohmt¢);

(iv) the. shttttp voh~t~t¢ of the pores Itig - the porosity which entre,pore], to the dccrca,c in vohtmc oft ,hm~p- lag ui~¢t a p re~ tc of 3 k j cm: , , expressed as a percentage;

(v) the settlement Ahsi - the deformation of the specimen at natttral mohtutc cotttettl at~d pressure 3 kg pet em t, expressed as a pcrceltt~tg¢ of the hctght of the specimen bcfotc loading;

(vi) the slump ~pr - the deformation of the spcctmrn after wetttt~g at 3 kg/cmt~

(vi~) the IotM d¢fomtation S = &h,l + Ahpr. expressed a~ a lv2teentage.

For compati,on, all the to,t, wet~ performed under a load of 3 k~/cm t on three to si,x parallel ~pcclmen, from th¢ same soil block; tt was a~umrd that all the utterable mactol:,orcs collapse under this load.

To find out how the disposition of the macroporcs affects the d~:fo~mation, we to,ted two scr~cs of specimens in a con~olldometcc u*~det nomtaJ mid lateral compre~slott. The tc~t re~ult, ore plotted in gigL 1 and 2. The~¢ figures dtsplay thre* Charoctcristic cases of pore disposition and dt~tributioit: (I) Mn~o,t cotnpletc absence L;I (so, Fig. 1)t (~) unffomi distribution lit the vertical direction L t (see Fig. 1); (3) unifo;m di,tdbutton and networked dh- position - the zone of ltffluenc¢ of buried ~olls in Figs. I and 2.

On comparing th¢ slump curves In all th~¢c cases, we can draw the following conclusions.

If there are a large number of macropores, distributed n|alttly vertically (zone LI) , settlement under normal loads 1, greater and slumping less than for lateral loads (curves 1, 2, 3, and 4). In the form.or case the difference be- tween scrtlemcnt and slump is greater (c~ves 7 and 8) than in the latter (curves 9 and 10)~

If there are many macropotes with mainly networked dhtrlbution (th¢ CaCO: zone in Fig. 1- and the whol¢ enos, ~ection In fig. 2), settlement u~tder normal load l= les~, and flump greater, than for lateral loads (curvet I. 2,

m

Bulgaria. 1066,

'rr~nslated f~om O~nowniya, Fundamenty I Mckh.',nlka Grunmv, No. I , pp. 10-12, ]anu;lry-February,

19

• ~uOPnl~UO~ J~4]mj o.~! ot sn ~p~Ol- ~n~ o~o~1 ,,lO ~|~l~ue ~V

'olq~o~l~O~ ~o~u s! spool |~a~l pu~ l~mmu aoj ~uoll~ojop 1~o~ ~1~ u~o.~q oau~a~j,llp ~q~ '~'o~Joaa~iu Jo ~qwnu

Olll JO ~mi~n:l~oJ:~|~u ~l|) "o'| 'lOlOll~ecl o~I jo ~l~lOeCl Tenure o~I~ ~ Inq 's~cloa~em ~ql ~q ~ou pos~clx~ ~I plno~ • ~cYoa~o;tu~ ~le~ ~ql ose~ slql ui '(I, pu~ '~ ';~ '~[ s~Jno) pVOl l~J~lel JoJ ueqi a~;~a~ d~UnlS o~p pue S~al Sl p~ol

Iem~ou Japun ~uo~ualn~S oq] '(~, ouoz) uop~qla~Ip Jf~q~ jo ~Salp;elaa "llemS Sl $~ocloa~etu jo ~l~unu oql I

"(I "a13 u~ ~ a~ues o W

I |I::I;IIFII-~

1

wa~ 1 •

.~t, tl

~i~ ¸ ~,

.~d ~- ~

-%+~i

So~l

Sandy loess Typical loe~s Th= s~nt¢ The same Clay loess The same Loess-type clay

"mr, '~ nat, q°

0,8 10.I 2.8 17.9 ~.0 20.1 6.~ 20.6 5.6 18.6 3.o 13.o 2.4 6.S

~ote~ The data refer to the uppctmo=t ncs~ in eight pits.

115Z,

4,3 6.6 8.5 8,4 1.5 4.7 1.4

loess hotizotl,

"mr, %

0,8

4.~

4,8 3.3 2.2

nat/nat

12,6 6.9 4.~ 4.0 3.0 3.9 3.0

amf/naz '° 100, q~

8,0 14,5 22.4 26,2 25.9 25.3 33,9

,ttz/n,,,f

$,4

1.9 1o6 1.6 1,4 0.6

nmf/n~z' I00, q~

18,8 37.0 41.~ 61,9 64.0 70,0

14~.8

traced from north to south; the F ar t given fOr'the whole thick-

L C~t~natc and elttvially calcined horizons with $t,~ble ma¢topor¢~, with a~$tegatc srructurc a~d basa! catb- oaa¢~ dl~!ribuffort, dc~pitc thc~,r high porosity ~nd mactoporoslty, are l ~ M l y st.~bili;:¢d zones In the complex.

2. For approximately rquM torn! d¢tormat{on~, the se¢tlemenrs u,~drr normM and transverse load= have a ratio ~h|ch vadc~ accotd~n~ ¢o the mtcr~tructutc,

The to|¢ of the mawoF~res i~ clearest when w¢ compare the fare volume of the macropor¢~ nmf with ¢}~e active volume n;~: ~nd (}~c slump volgr'nc a~z. The. ¢omp~tiso~t of nmf with n~.: t~ ,~rb}tr~rv, bccatt~c =~tl=f i= co=~tant, whct~s ate vaHc~ wlth d~c spcc~mc=t. Their ra{~o depends ca the mo|i~utc co,~tca¢ of the .~pcc{man before w~tth~. Th~ dr}~t the Ip~:~|mc(h fl~c l~rc~tcr h n~;~ ~nd tha less is the rcl,tt|ve f~actioa {;~b Even whc~t they ~c eqttal, it is difftc~l~ ~o bc ~t=r~ that sl~mp is due oitly ~o ttmf. It is rherrfor¢ more correct to compare nmf w{dt aa~ ~orrcspo,~d - ~t~g to die term dcfo~matlo~h trot dcp~=~ding ca the wetness, a~d const,~nz for a ~ivcn ~pccimC¢t t!~dcr co¢=~t,==~t load. Th|s colt+part+on i+ jt~+t+fiad by the facts that the t++acropore+ dcfornt both under dr+rap and under _~cttlcmcar, at~d that there it no exact crt~crlo~t to de,ermine their rol~ In ¢tthet c ~ ~cpJtatcly. "t'hc comi)~t~o~l d~ta ~how tidal the fret; martopor~ volume is le~s t l~ t the ~lcth.c pete volttme by a fetter of 3.6=74, l.e.; it i~ horn l.,3 ¢o :lO~of the l.~ttct;

q ' the free macto~otevolum¢ t~ la~ than the ~luml~ volume by a factor of 1.2-51LS, i.e., it is fto~a 3 to ~l , .~f the latter.

The~c estt~mc valt~c~ ~e ~ot d~nlftc:mt. It it more coi lclu~lve thai, tit 13 otlt of ~0 tc~t~ (~;8% of the cancel, the frc¢ =tt;icro|)orc volume h Ic~_~ tll;~=} I0% of the active iota vol.uttt¢, i.e., .~ctth~nlcn.t dtl~ to nl,~cropotc~ Im =mot 0.1 of the total sk, aci{~ic~t dcformattoo. "rile positiott is similar in the ca~e of the ~lt, mp volume: ttt ~O~,of the ca~c{ nmf is lass titan !.5%of nsz, I.e., the ~cttlemqnt duc to maCtol~orc~ (even if wc ~ u m c that titcy arc an@~tla¢cd ottly during the slump preach) is .:le;~s th~'~ t||o flun~p by a factor of 7.

The mel-~tivo d~at¢ of mac~oporo~ i{ most Impott~t~t ht d~o xo.~o of tnfl~¢,,~cc of tho sot{m, t.¢., la the {cct|ons wRI~ ma×h~mm development of nct,.~orked macropo~o~Ity, in tier ~,.one of {nflurnc~ of n,z~," .~oii, ~mf is ~3.9-27.q% of nat and ,53.3-,~6.1='~of nsz, while ia buried ~ol[{ the figures arc 17.2-3.~.~_5'.,of nat and ~{-71..|~Lof n=z. In the re- manning parts of th¢ cro~s ~ecdott the rclativ~ shale of nmf h =mat,at, and in the latct,ncdtate parts, especially h= tone L,, it I$ neatly down to zero. Scrtlcmcn¢ at:d slutup arc mai,,ly duc to tl~c ~tacroporc volume. It follow~ that $¢~lentcnt and sh~mp of teals !m~ay a}xo occur without the prc~cncc of m,tctop0reL and somctint,~{ the total dcforma- tio~t in s¢ctJon~ deprived of mactoporc$ maydlsplay local maxlrna relative to the surroundh~g mncropotou~ loc~s, The role of mactopo~es is cleart:r when w¢ t~acc the cotm¢ctions between nat, nsz. at~d nmf ia v,=rlou~ var~ctle~ of loess (see table). The tnbl~ ~hows the foIlowlng:

I, In sandy loe~s with practically no macroporc~, nmf Is ~ of naz, or lSq~ of nsz, We can therefore reckon that saltier, tacit and slump are duc o~ly to changes in the normal pores.

2, In = typical loess, nmf is on average I0-2.5%of naz and 25-60%of nsz, i.e., the macropores play a con- ddet~bl¢ part in slum{) and settlement, but their tote h a subordinate on~.

$. In clay loess, which Is highly macroporous, nmf is on average 25-26%of nat and 60-70~=of n=z. Hanec, mmcmpores play a subotdln,~t¢ tolc in total d~forma¢lon, but in slump they have the same influence as the micro- pores, It '#as established that, ryes in the stabler toil s~ructurrL the cotlapsc of ftc¢ macropo~cs is rno~t complete In clay loess,

2~

4, In loess-type clays nmf ts 0,|~, of nat and up to 1.13~'.. of ns:. The whole ot the s~ump dcfom~ation is com- pcasat©d by o~,ly half of tl~c volume of the t'tee mac,opores. This h the typical case of the p~cscnec of t~ee macro- po~es in ~ 1table structure. They could collapse, but only under loads greater than 3 kg/cm 1,

A ~cessary condition for the onset of slump is the presence of frec pore volume, while a sufficient conditfot~ (or cause) is wetting ~nd loading. According to Abelev [2}, the necessary condition is macroporostty, whereas, ac- cording to Dehisce [3}, tt is mtcropo~ost~y of the loess. Our {nvcsHgatio~ have established the following.

1. Settlement and slump In loess can occur in st~uclu:es with practically no macmpores. This ~cfcrs to a wide- {l~ead facies variety of loess in ~htch the relative slump is up to G-'/%

¢, In all other va~tedes or loess-except loess-type clays and buried seth, slump is due mainly to h~gh m|cro- porosity.

~ In loess-type clays and buried soils only, slump is almost entirety duo to macropores (which arc al~o no~ wholly annihilated), wt~hou~ appreciable reduction {n the micropor¢ vo}umc.

Thus, In w:Ictles consttt~ting the loess subgrot~p,slurnp never occu:s g, ttimut app~cciabie decrease In the mt¢~oporo volun~c, thc n~aczopo~cs playing little p~t. A ~s¢cess.~ty condition of l{,~btlit~,, to sl~mp is the presence of hlgh s~rue~u~l rnicroporostty.

S.

LITERATURE CITED

¥u, M, ^belev. ~Inciplos of Pl~nnin~ and ~u{Idh!g on Macropo~ou~ Ground. Moscow (19{8). k. }C, Larlo~ov, Loess Kock~ ~zsd Their S~rn~zur~! P~op¢:tfc~. Go~g~oltehh{;:da~ (t050). H. Y~. Denhov, S~ructural Pxopct~{cs of Loc~s and Loess-Type l,oam~ (2{~d edition, rcvhcd and nit,men:cO), Gosstrotizdat (1%~).