THANH PHAN KET CAU CUA DAT
Bang 2-8. Khoang vat set dang liroi - lop
So lap 2 khoi 8 mat hoac lop gipsit 3 khoi 8 mat hoac lap bruxit
Caolinit Secpentin
2 Dickit Crizotin
Nacrit
Profilit Tan
Muscovit Biotit
3
Monmorilonit Clorit
Ilit (hydromica) Vecmiculit
Khong gian Chieu dung Hinh 2-2. Don vi cau true cua khoang set
a) Khoi 4 mat; b) Khoi S mat
Cac khoang vat co cau true krai 2 lap thi co 1 lap 4 mat don ket hgp vai 1 lap 8 mat dande hinh thanh cau true krai 1:1. Caolinit la khoang vat co cau true lirai 2 lap dien hinl. Trong cac khoang vat co cau true krai 3 lop: co 1 lop 8 mat dan kep giira 2 lop 4 mai dan tao thanh cau true lirai 2:1. Do la cau true mang tinh the cua Hit tuy nhien, mot so iguyen tir Si dirge thay the b^ng A l+3, cac ion K + lai co mat a giira cac lop 4 mat cua cac tinh the k& ben. Do la nguyen nhan dan den sir khac nhau v6 mat do dien tfch tren be matciia cac khoang set. Cau true mang tinh the ciia Monmorilonit tirong tir nhu' Caolinit nlung, khac nhau a cho: trong tap mang tinh the cac lop khoi 4 mat co cac oxit silic d&u lien k6 nhau va moi lien ket giira cac mat mang (phong) ciia tinh the’ yeu hon Caolinit.
Cautnic nay lam cho phong ciia mang tinh the Monmorilonit linh dong (xem hinh 2-3), nendd hap phu niroc va co tfnh tnrong no manh hon Caolinit. Mot vai dac diem va tfnh cha ciia cac nhom nhu' sau:
m
Ki hieu
c)
AL(OH)
C O
Ki teu
b)
Hai khoi tam mat
Ba khoi tam mat
d)
\
( :
, r /
j o 1 6 I
/ \
Hai khoi tam mat
e)
Ba khoi tam mat
Hinh 2-3. Set do cau true mang tinh the cua khoang vat set a) Ldp silic; h) Ldji gipxit; c) Lap Bruxit; cl) Ludi 2 ldp; e) Ludi 3 ld\i
1. Nhom Caolinit gdtn cue khoang vat: Caolinit, Nacrit, Dikit va Halozit. Dac diem quai trong cua Caolinit la mang tinh the khong linh dong do moi lien ket chac giua silic va tVp gipsit tao ra mot tang trong mang tinh the co cau true lap khong doi xung.
Khcang vat co cau tao dang bong dien hinh do su sip xep chong len nhau cua khoang lOOlop. Khong co hien tirong trao doi ion trong mang, khoang vat dang tam, dang vay, dug thanh tao trong moi truang axit (PH = 6 -7). Dung lugng trao doi phu thuoc vao do phai tan (kfch thuoc hat) va thuong la 3 - 15mg.dl/100g dat. Cac ion co tfnh trao doi chi
co d mat ca sa cua mang, tfnh chat hoa ly thay doi yeu theo didu kien bien doi cua inoi trudng. Khi dung nude eat de tron vdi caolinit thi W ch = 60%, W d = 35% va Id = 25%. Khi bi udt, khoang each cac ldp khong thay doi cho nen do truong nd thap, do truong no cua Ca-caolinit la R H = 60 - 100% va Na-caolinit la R H < 150%. Ti le dat: nirdc dd tao ra dung dich keo: Ca-caolinit la 1:1; Na-caolinit la 1:2. He so nen lun a, 5_2 = 0,07cnr/kG va lie so tham dudi cap ap lire khong doi l,5kG/cnr trong ngay la k = n .l0 4cm/s. Tuy nhien, kha nang tham nirdc cua dat caolinit ldn han nhidu so vdi dat chua Monmorilonit (dac biet la cac loai khoang Bentonit). Caolinit ton tai pho bien d mien trung du Viet nam trong tang tan tfch, sudn tfch do feral it bi phong hoa hoa hoe.
Trong khoang vat Halozit cac ldp bi cac phan tu nirdc chia tach ndn, cac hat co dang c|ue, dang ong, trai ngugc vdi hau het cac khoang set khac va no bi khir nude d nhiet do 60°C.
2. Nhom Monmorilonit gom co: Monmorilonit, Safonit, Beydenit, Caolinit man luc, chung la san pham phong hoa cua biotit va clorit hoac hidromica bi bien doi trong moi trudng kidm vdi do PH = 7 — 8,5. Mang tinh the cua chung rat linh dong, co kha nang trao doi rat manh vdi cac cation hoa tri thap han d moi trudng xung quanh nhu:
Mg+:thay the cho Fe+' hay A P 3 thay the cho Si+4 lam cho mang tinh the du thua hoa tri, lam tang kha nang hap phu cua khoang vat. Mang tinh the cua chung co the xay ra sir trao doi cation d phong cua mang (tdi 80%) va ca d mat co sd (tdi 20% ) vdi cac cation moi trudng. Dung luong trao doi phu thuoc vao do phan tan cua hat khoang va thudng bien doi tir 80 ~ lOOmg.dl/lOOg dat. Tfnh chat hoa ly thay doi manh theo sir bien doi dieu kien (nhu thanh phan ion trao doi, nong do dung dich) cua moi trudng, dac biet khi dung dich bao hoa Na+. Khi dung nirdc cat tron vdi chung thi set Ca-Bentonit co W ch = 160%, W d = 90% va Id = 70%; set Na-Bentonit co W ch = 500%, W d = 100%, Id = 400%.
Khi bi udt, khoang cach cac ldp thay doi cho ndn do truong nd cao, do truong nd cua Ca-Bentonit la R H = 200 — 300%; Na-Bentonit co R H = 600 — 700%. Ti le dat : nirdc de tao ra dung dich keo: ddi vdi Ca-Bentonit la 1:3; Na-Bentonit tu 1:10-1: 2. He sd nen lun a, s.20 = l,0cm2/kG va he sd tham dudi cap ap lire khong doil,5kG/cnr trong ngay, ddi vdi Ca-Bentonit k = n.10'5; Na-Bemtonit k = n. 10 cm/s. Tfnh hap thu nude cua chung lai dac biet ldn.
3. Nhom Hidromica (Ilit) gom co: Hidromuscovit, glauconit, ilit, xeladonit, hidrobiotit va Xerixit, chiing la cac khoang vat n&m trung gian giua nhom Caolinit va nhom Monmorilonit, do khoang vat nguyen sinh (cac mica bi bien doi d giai doan kiem dau lien hoac dat nhieu thanh phan bi bien doi trong moi trudng kidm, PH tdi 9,5) co nhidu kali, ham luong kali trong khoang hidromica co the dat 5% trd len. Tin rong gap hidromica d cac tirong khac nhau (luc dia, bien co nhidu kali). Khac vdi mica, hidromica co chua nhieu nude va ft kali; so vdi monmorilonit thi no chua nhieu kali hon, kha nang truong nd cac ldp cua chiing khong co hoac the hien yeu khi co tac dung cua nude hay cac chat hCiu co. Cau
true mang tinh the’ cua hidromica giong nhir monmorilonit. Tuy vay, trong mang tinh the cua hidromica cac K + phan bd d giira cac tap lop lam cho cac tap nay chac hon, tao cho no co do bin Ion va khong co tinh linh dong nhir mang tinh the cua monmorilonit. Co tinh deo khi am udt va tinh tham nirac cua no co tri sd trung binh gifra caolinit va monmorilonit.
K = 10
(cm/ph)
4
10
10
Ham luang khoang set, %
0 25 50 75 100
Ham lirang khoang set, %
10
10
10
1
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! \ \ 1 ^ 1 \ 1 \ \
X 2 1 '
1 \ 1
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'b
\* \
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..
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" <
\ N
100 75 50 25 0
Ham lirang thach anh, %
Hinh 2-4. Sif thay doi K theo thanh phan vd ham lifting khoang set, khi nen 1,5kG/cnr
l- thach anh + mica; 2- thach anh + caolinit;
3- thach anh + Ca-bentonit;
4- thach anh + Na-bentonit.
Ham li/gng thach anh, %
Hinh 2-5. Si/ thay doi cua f vd cp theo ham lifting vd thanh phan khoang vat set
1- thach anh + Na-bentonit;
2- thach anh + Ca-bentonit;
3- thach anh + caolinit; 4- chi co thach anh.
Sau day se khao sat anh hirdng cua thanh phan va ham lirang khoang set ddi vdi mot sd tmh chat cua dat. Ty dien tich be mat cua cac khoang set rat khac nhau (Caolinit la 10nr/g, Hidromica la 8()m2/g va Monmorilonit la 800nr/g) va cd anh hirdng rat ldn den cac tinh chat hoa ly cua dat loai set. Thanh phan va ham lirang khoang set cua dat anh hirdng den tmh chat ky thuat cua dat da trong xay dung.
Tren hinh 2-4 cho thay, khoang vat Na-bentonit cd anh hirdng ldn nhat den tinh tham nude cua dat da, tiep den la Ca-bentonit va cuoi cung la caolinit. Neu tron dat vdi mica, thanh phan mica khong gay anh hudng den tinh tham nude cua hon hgp nay. Tren hinh 2-5 cho thay, he sd ma sat f va goc ma sat trong (p cua dat se thay doi theo ty le ham lirang cua khoang vat set va thach anh. Vf du, khi ham lirgng thach anh tang do bin chong ct\l cua dat tang len; neu cho them lugng khoang vat set vao thach anh thi he sd ma sat f se giam manh, dac biet la khoang vat Na-bentonit.
Khi thf nghiem nen ddi vdi mSu dat tron iSn thach anh va khoang set vdi ty le 1:3, ket qua thf nghiem the’ hien tren hinh 2-6. Qua day cho thay hdn hgp thach anh + Na- bentonit cd tfnh dam chat thap nhat, tiep theo la hdn hgp thach anh + Ca-bentonit, con hon hgp thach anh + caolinit va cat thach anh thuan tuy thi gan giong nhau.
0 1 2 3 4 5 6 7 Ung suat nen, kG/cm'
H inh 2-6. D if dug cong e - f ( P ) theo thanh pha n khoang vat
I - thach anh + Na-bentonit;
2- thach anh + Ca-bcntonit;
3- thach anh + cao lin it; 4- thach anh.
H i n h 2-1. K ha nang hap thu nudc theo ham I icon g
vd loai khoang set
1- N-bentonit; 2- Ca-bcntonit;
3- h idrom ica; 4- caolinit.
Hinh 2-8. S u hien doi W ,„ W (l theo ham lugng,
loai khoang set
1 - Na-bentonit;
2- Ca-bcntonit.
Nhu vay, khoang thach anh co tmh ira niroc (gifr nirac) kem nhat, tiep theo la caolinit va hidromica. Tuy nhien, hai khoang vat nay lai khac thach anh a cho, khi cho them nirac vao thi chiing the hien tfnh deo. Khoane vat co tfnh ira nirac lan nhat la nhom monmorilonit, dac biet la khoang Na-bentonit co tfnh ira nirac va tfnh deo cao nhat. Cac thuoc tinh da khao sat tren day la cac ket qua thu dirac trong dieu kien phong thf nghiem sir dung nirac cat va khoang vat set de thf nghiem. VI vay, khong the dung nguyen xi cac quan he phu thuoc (ket qua) nay cho cac dat loai set ma phai xem xet den di£u kien dia ky thuat thirc te. Boi vi, tfnh chat ciia dat khong chi phu thuoc vao thanh phan khoang vat ma cluing con phu thuoc vao nhieu nhan to khac, dac biet la trang thai vat ly, hoa ly ciia cac khoang vat nay va thanh phan thuy dung dich trong lo rong, khe nirt ciia dat.
2.3. Cac khoang vat co tinh hoa tan
Cac khoang vat nay ton tai trong dat a trang thai ran phan tan, via mong hoac dang than kfnh, via nho hay dang muoi hoa tan trong nirac lo rong va nirac khe nirt ciia dat da.
Cac khoang vat nay co the la khoang nguyen sinh hay khoang vat thu sinh do cac hgp chat bi nirac hoa tan roi tai ket tinh tao thanh, ton tai a nhieu hinh thai khac nhau: phan tan nho, dang xi mang gan ket, cac the bi bao xunh quanh cac tinh the hoac hat dat.
Co the chia cac khoang vat nay ra thanh ba nhom; cac khoang vat de hoa tan gom:
cac muoi clorua, sunfat, cacbonat natri, kali, magie va canxi (haiit- NaCl, xinvin- KC1, Tren h'inh 2-7 va hinh 2-8 the’ h ien tmh hut nirac, cac dac diem ve sir tac dung tirang ho gifra khoang set va nuac.
W max, % Han luang khoang set, % Ham luang khoang set. %
e 0 2) 40 60 80 100 25 50 75 100
100 80 60 40 20 0 100 75 50 25
Ham iuang thach anh, % Ham luang thach anh, %
inirabilit- Na2S 0 4, xoda- NaoC03,..); cac khoang vat co tinh hoa tan trung binh g6m:
sunfat canxi (anhidrit- CaS04, thach cao- CaS042H00,..) va cac khoang vat kho hoa tan lion gom: cacbonat canxi, cacbonat magie (canxit- CaC03, manhezit- MgCO?, dolomit- CaM g[C O ,]: ,..). Khoang vat dd hoa tan thi tan nhanh trong nirdc, tham chi' vdi luong nirdc khong nhidu. Khoang co tinh hoa tan vua thi hoa tan cham, con khoang kho hoa tan can thdi gian lau dai va co tac dung cua dong tham nude dudi dat. Khoang de hoa tan (nlnr trong hoang tho) lam cho dat rat khong on dinh khi do am tang, dac biet khi lirong khoang nay chiem hon 0,3%; cac khoang kho hoa tan thuc te' co tac dung rat yeu den tinh chat cua dat da khi tac dung vdi nude.
Nhin chung, mot khi cac khoang vat nay d trang thai cung trong dat hoac trong dieu kien hoa ly thuan loi, dung dich (muoi hoa tan trong nude) se ket tua tao thanh xi mang gan ket cac hat dat, lam tang do bdn, giam tinh nen lun. Chung thuong ton tai trong dat set, set pha hoac da maclo (marl). Khi dat bi kiem hoa lam thay doi do chat ket cau, do dinh bam, do On dinh va do ngam nude. Khi bi hoa tan va qua trinh lira lua cac muoi, thanh phan cation hap thu trong dat co the bi thay dOi lam anh hudng ldn tmh chat cua dat. Ngoai ra, su hoa tan va lira lua nay con anh hudng den thanh phan, tfnh chat cua nude dudi dat tham qua dat va co the gay ra hien tirong an mon cac cau kien xay dung.
Cac khoang vat co tmh hoa tan thudng gap trong dat thuoc ky tlu'r Tu d nhung viing kho han, noi co do am thay doi. Vung nhiet ddi am nhieu mua khien cho luong khoang nay giam, do hien tuong hoa tan va rua troi cua nude mua va nude ngam. Dat boi veil bien, chau tho song Hong va dong bang song Ciru Long thudng chua cac muoi co ti'nh hoa tan. Tinh chat co ly cua cac loai dat chua muoi don gian (cac khoang vat) nay phu thuoc vao thanh phan, ham luong va dac diem lien ket kien true ciia chung. Dat chira khoang cacbonat lam cho ki'ch thudc hat tang ldn, do long ciia dat lai giam di va sire chiu tai tang ldn. Trang thai phan bo cua khoang cacbonat cung gay anh hudng den tfnh chat cua dat. Neu trong moi trudng co Ca+: chiem uu the thi ti'nh chat dac trung cua dat set giam manh hoac mat han. Nguoc lai, neu cation Na+ chiem uu the thi cac ti'nh chat dac trung (ti'nh deo, ti'nh truong nd, tmh di'nh bam,..) cua set lai the hien rat ro ret. Nhu vay. co the cai tao ti'nh chat cua dat bang cach thay dcii thanh phan va ham luong cac khoang vat co ti'nh hoa tan co trong dat loai set (phuong phap voi hoa, phuong phap ximang hoa,..).
2.4. Thanh phan chat huu co trong dat
Trong thanh phan cua dat con co chat huu co da, dang hoac chua bi phan liuy thanh chat mini, co nguon goc thuc vat hay dong vat. Luong huu co - di tich sinh vat, nhidu hay ft tuy thuoc vao sir phat trie’n ciia sinh vat trudc day va didu kien moi tnrong thanh tao cua dat da tram tich. Muc do phan giai cua chung rat khac nhau, nhung tan tich nidi
nga mau nau cua cac than cay, rd cay hay cac khoi inun da phan huy ky. Neu luong hiru co cao va muc do phan huy cua chung Ion th 1 tmh chat keo cua dat, do ua niroc, do chua aim va tinh bien dang cua dat tang len ro ret, nhung do ben va tinh tham nude lai giam nhieu. Cac loai dat bun, theo thanh phan hat co the la cat hat nho chua bui, cat pha, set pha va set. Chung deu co nguon goc tram tich hien dai, tai cac vung co khi hau am udt, dia hinh trung, long chao thap, muc mrdc ngam gan mat dat la nhung dieu kien thuan lo'i cho viec hinh thanh cac loai bun thuc vat.
Cac dac didm chinh cua bun la: do am (70 ~ 80%)va do rong xop rat Ion (vai don vi), luong chua tan tfch huu co tu hoi cao (2 ~ 3% ) den cao (10 ~ 12%) va giam theo chieu sau, khoi luong the tich kho yK = 0,8 ~ l,lg/cm\ Bun co lire lien ket kien true rat nhd, ft khi dat 0,1 kG/cnr. Do bdn cua bun rat thap cho nen, khong nen phan chia sue chong cat cua chung ra thanh luc ma sat va luc dfnh ket. Sue chong cat cua bun khong chi phu thuoc vao ap luc thang dung ma chu yeu phu thuoc vao toe do bien dang. Goc doe tu nhien cua bun ~ 0. Khi co tai trong du nho thoi thi khoi dat bun cung de chuydn sang trang thai chay. Sue chong ban dau nao do clnnh la do bdn cua cac lien ket kien true cua bun. Dudi tac dung cua tai trong nen, bien dang cua bun thd hien bang su nen chat tu do kem theo sir ep thoat nude tu do. Khi gia tai tuong ddi nhanh, trong bun se hinh thanh ap luc thuy dong lam cho bun bi ep phe ra. He sd nen lun a,^ ô 2 ~ 3cnr/kG, modun bien dang bun set: E 0 = I ~ 5kG/cnr, bun set pha va bun cat pha E () = 10 ~ 25kG/cm2.