TINH BIEN DANG CUA DAT
2.1. Dac diem tinh nen lun cua dat
Day la mot dac trung quan trong ciia dat, nd th6 hien kha nang thay doi the tich ciia dat dudi tac dung ciia tai trong nen (hoac do lun co, do sir dong tu keo,..) va no khac co ban so vdi vat lieu ran lien tuc. V6 ban chat tinh nen lun ciia dat la sir di chuyen va sap xep lai vi tri tuong doi giua cac hat, lam cho dat dac chat them, tinh rdng xop giam va nude trong dat thoat ra. Ddi vdi dat loai set thi mang keo-nudc va nude lien ket vat ly co chigu day giam. Khi ap suat ngoai tang len dii ldn con lam cho cot dat (hat khoang va nirdc lien ket chic) bi bien hinh ciia mang tinh the khoang vat va do sir chay-nhdt cham chap ciia mang nude lien ket chic. Ngoai ra, ddi vdi dat bao hoa nirdc, tinh nen lun con do kh bi bao (bot khi) trong 16 r6ng giam the tich. Tinh nen lun ciia dat phu thuoc nhi<Su yeu to: loai dat, thanh phan khoang vat, cap phoi hat dat, trang thai aim, do chat ket can, do trdn nhin cua hat (dat loai cat), do ldn, loai tai trong va tdc do gia tai...
Dei vdi dat loai cat, tinh nen lun ciia ehung co dac diem la dudi tac dung cua tai trong lap tire xay ra bien dang liin do sir di chuyen vi tri sip xep giua cac hat lam cho khoi cat dac chat hon. Vi khi co tii trong tac dung, nude trong cat d6 dang thoat ra khoi 16 rOng. irng suat ngoai lap tire truyen cho hat cat. Nhu vay, dudi tac dung cua tii trong tfnh bien dang ciia dat loai cat hiu nhu khong he phu thuoc vao trang thai im , nhunj dudi tac dung ciia lire chan dong, cat bao hoa nude bj nen lun rat manh. Bdi vi, dudi Lie dung ciia luc chan dong cac hat cat, d di6u kien bao hoa nude, ehung d trang thai “ a lung” (bi triet tieu ma sat) rat de di chuyen vi tri tuong ddi giua cac hat. Mot khi ap luc nen tinh rat ldn cac hat cd the bi pha hoai. Tuy nhien, tii trong do cong trinh
truyen xuong, trong thirc te khong lan tai muc nhir vay nen khong can de cap Jen.
Khac vai dat loai cat, dat set khi chiu tai trong lan cac hat set cung khong bi pha hoai ma dat set chi bi lun nhi6u. Theo tai lieu thi nghiem, cho dat set vao trong ong co thanh day cirng va sau khi nen vai ap lire tai 2000kG/cnr, thanh phan hat ciia set van khong thay doi. Tri so nen lun cua dat loai cat thuong nho, dirang cong nen doc thoai han dat loai set va chung nhanh dat trang thai dn dinh lun. Trong qua trinh bien dang lun do am cua cat hau nhir khong thay doi vi chung hoi phuc do am rat nhanh. Tinh nen lun ciia dat loai cat phu thuoc vao: kfch thuoc do dong diu ciia hat, do tron nhan, thanh phan khoang vat, trang thai ket cau va trang thai am,.. Cat thach anh gom cac hat goc canh, su si, nhi6u ca hat va ket cau chat chung co tinh nen lun rat nho va do on dinh cao, do cac hat rat kho di chuyen vi tri. Cac hat cat cang tron nhSn di chuyen cang d i dang va ket cau xop rai tinh nen kin cang tang (hinh 6-5). Chi thi nghiem nen cac mAu cat khi chung co lan nhieu hat set hoac co ket cau xop rai.
Dat loai set co thanh phan khoang set thu sinh khac nhau, tinh giu* nuac cua chung cung khac nhau, do do tinh nen lun cua chung cung khong giong nhau. Dat cang chua nhieu hat phan tan min dien tiep xuc giua cac hat tang, ty dien tfch b i mat lan kha nang tirong tac vai nirac cang manh, giu nuac cang nhieu. Do do, tfnh nen lun (bien dang the tfch) lan nhung cung lam tang sue chong nen cua dat. Thanh phan cation trao ddi cung gay anh hudng den tfnh nen lun cua dat loai set. Vf du, set-Na co tfnh ua nirac (giu niroc) lan han set-Ca va no co do phan tan lan nhat. Do vay, tfnh nen kin cua set-Na rat manh.
Hinh 6-5. Dudng cong nen cua cat Hinh 6-6. Anh huong ciia toe do gia tai nen co do chat han dau khac nhau a dieu kien tir nhien (nen lun circ chain); II- nen dat 1- ket cau xop; 2- can dirai chat vira; dtfdi cong trinh; III- tai tang cham len mau tu nhien
3- ket cau chat khi thf nghiem; IV- gia tai nhanh len mau xao dong
Dat co he so rdng tu nhien cang lan do lun tuyet ddi cang cao, dieu nay dac biet the hien ro khi thf nghiem mau xao dong ngay ca a cap tai nho. Va ca khi so sanh do kin ciia mau co cung trang thai vat ly nhung khac nhau v i thanh phan hat va thanh phan
IV
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a, kG/cm2
khoang vat. Tinh nen lun cua dat loai set con phu thuoc vao do Ion ciia tai trong va toe do gia tai. Toe do tang tai cang nhanh bien dang lun nhieu hon cach gia tai tang chain va theo giai doan (hinh 6-6), dieu nay dac biet to doi voi dat co tfnh deo cao; dat co tinh deo binh thudng va thap nhu set pha va cat pha the hien yeu hon. Toe do gia tai nho nhat doi vdi dat d dieu kien tu nhien va ldn nhat d didu kien thi nghiem. Ndn dat dudi mong cong trinh thudng o dieu kien co toe do gia tai binh thudng. Trong dieu kien tu nhien, qua trinh tram lang ciia cac vat lieu tram tich bi nen lun rat chain, nen lun it nhat. Va mail thi nghiem trong phong bi nen lun nhieu nhat. Bdi vay, do lun tinh duoc theo tai lieu thi nghiem trong phong thudng nho hon do lun thuc cua cong trinh. Trong vat lieu tram tich set, do rdng hau nhu giong nhau d toan bo ldp vi d dieu kien tu nhien toe do gia tai cuc ky be.
Trudng hop dat loai set co do aim nho, phan ldn 16 rong chua khi co lien he vdi ddi thong khi. nude d dang bi giam ham. Du lam mat nirdc hoac khong khi khong bi nen thi mau dat van bi lun. Luc chong lun d trudng hop nay tuy thuoc vao luc hut giua cac hat, luc khang nen cua mang thuy hoa noi tiep xuc giua cac hat va ma sat giua cac hat dat...
Trudng hop dat clida nhieu nude, dac biet trong lo rong co bot khi hoac khi d the bi bao, qua trinh nen lun trd ndn phuc tap. Hien tuong lun dien bien phu thuoc the tich khi bi nen. luong khi hoa tan vao trong nude va luong nude thoat ra tu man dat.
Tinh nen lun cua dat loai set phu thuoc vao nhiet do. Bdi le, qua trinh bien dang lun cd lien quan den tinh tham nude cua dat. Nhiet do cao do nhdt cua nude giam, nude tham nhanh hon va nguoc lai. Do do, khi nhiet do tang len, toe do bien dang lun cung tang (vi du ndn dat dudi lo cao, ong khoi,..).
Qua trinh bien dang lun cua dat loai set bao hoa nude g6m 2 giai doan nhu sau:
a) G iai doan bien dang so cap.
Khi gia tai tinh lien tuc khong doi len dat, trudc tien ung suat tac dung vao nude lo rong va lien ket kien true. Neu ung suat ngoai chua du ldn de thang duoc ap lire nude 16 rong (u) va do ben kien true (Pkl) thi su nen chat dat chua xay ra do bien dang phat sinh dudi tac dung cua tai trong ngoai. Bien dang cua dat luc nay gan giong nhu vat the ran lien tuc. Khi tang tai trong ngoai den gia tri ldn hon ap luc nude lo rong va do ben kien true (cung) cua dat, nhung lien ket kien true bi khac phuc va nude lo rong tham thoat ra, dat bi nen chat dang ke, kern theo do do aim cua dat giam theo. Su ep thoat nude trong dat loai set chi co the xay ra khi cot nude phat sinh dudi tac dung cua tai trong ngoai phai ldn hon gia tri gradien thuy lire ban dau (I0). Tai trong nen tang do lun tang theo, va tiep \iic giua cac hat cung tang ldn lam tang luc dinh cua mang thuy hoa, noi tiep xiic giua cac hat. Do vay, lam tang sue chong nen theo sir tang cua tai trong ngoai. Dieu kien thoa nude Id rdng cd anh hudng ldn den tinh nen lun cua dat. Khi tang set nam xen kep vdi Ling cat co tinh tham tot, set xay ra bien dang lun ldn hon so vdi tang set dong nhat cd cing chieu day.
— — / 3
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w /M W /m m
h) G iai doan bien dang thi? cap
Khi tai trong tang hon niia, ung suat se tru\Sn cho cot dat (hat khoang + mang nirdc lien ket ehac) lam bien hinh mang tinh the; khoang vat. Doi vdi dat co lien ket keo-nudc, mang keo-nudc ciia cac hat khoang bi giam chieu day, mot luong nude nao do se thoat ra lam cho dat bi nen chat.
Nhu vay, qua trinh thoat nude 16 r6ng va nude lien ket vat ly trong dat co anh huong ldn den tmh nen lun cua dat loai set va day la can nguyen lam cho dat loai set co tinh tir bien. Tir bien ciia dat set la qua trinh bien dang ciia dat theo thdi gian dudi cap tai trong khong doi.
2.2. Dudng cong nen lun
De xac dinh cac chi tieu co ban ve tfnh nen lun ciia dat, thuong lam tin nghiem nen chat dat dudi tac dung ciia tai trong d dieu kien bai toan mot hudng, thf nghiem nen don true khong nd ngang (hinh 6-7). Tin' nghiem nay dugc diing rat pho bien va don gian, nhung co nhugc diem nhat dinh (tfnh nguyen dang ciia mSu dat, kfch thuoc m5u nho, mo hinh bien dang khong phii hgp vdi di6u kien lam viec thuc te ciia n6n dat dudi cong trinh,..). M lu dat dugc dat trong dao vong bang kim loai co chiSu cao h = 2~4cm, tiet dien ngang F = 25~30cm2. Neu ty le dudng kfnh va chi6u cao
d/h = 3, khi gia tai ma sat giua mSu va thanh dao vong co the len tdi 8-10% ap luc nen. De giam anh hudng ciia ma sat nay can tang ty le nay d/h > 3. Gia tai len mau dat theo tirng cap tang dan, tri sd ciia m6i cap tai trong hon kern nhau: 0,25; 0,5 hoac l,0kG/cnr, tuy theo loai dat va trang thai cua no. Sd cap tai thf nghiem n > 4 cap. Dat co do dac chat thap (dat yeu) nen chon cap gia tai nho; dat co kha nang chiu tai trung binh va tot gia tri ciia m6i cap lay hon kern nhau l,0kG/cnr. Tai m6i cap phai duy tri tai trong khong d6i va theo doi bien dang lun ciia mSu theo quy dinh cho tdi khi mau dat on dinh lun (tat lun). Khi thf nghiem nen nhanh co hieu chinh, co the; coi mfui da tat liin neu sau 2h sd gia do liin AS < 0,01mm. Qua trinh bien dang ciia mSu dugc doc tren d6ng ho do lun, co do chfnh xac den 0,005mm. Tri sd bien dang ciia mfiu dat set no nude, co tfnh tham nho, thi m6i lugng tang ciia ap suat ngoai hoan toan tuong irng vdi lugng bien thien nhat dinh ciia do am. Quan he giua do am (W ) va ap suat the hien tren hinh 6-8a va goi la dudng cong nen. Bdi vi doi vdi dat set no nude ton tai quan he e = W h hys, cho nen de dang lap dudng cong nen tren he toa do e — P.
Tuy nhien doi vdi dat loai cat, co tfnh tham nude tot, thi khong the’ lap dudng cong nen theo bien thien ciia do am bdi vi, khi gia tai len dat loai cat do aim dirge hoi phuc gdn nhu tire thi.
Hinh 6-7. Sa do hop nen don true
1- dao vong; 2- tam nen; 3- da tham;
4- ong thoat nircfc; 5- niroc trong hop non
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b)
Hinh 6-8. a) Quan he W = f(P ); b) So do man dat trudc va sau nen:
V (), V , - the tich ban dau va sau khi nen; V s, V r, - the tich hat dat, the tich lo r6ng;
A V - do giam the tich dat; e0, e, - he so rong ban diiu va sau khi nen.
Tren hinh 6-8b cho thay, dirai tac dung cua tai trong Pj chidu cao cua mSu bi giam mot luang la Ahr luc nay chidu cao cua mSu la hr V I thi nghiem khong na ngang cho nen tiet dien cua mSu vfm la F. The tich ban dau cua mSu V 0 = Fh0; the tich mau sau khi bien dang on dinh V s = Fhj va lirang giam the tich tirong ung la AV, = FAhr Qua day ta thay, ty le bien dang the tich bang ty le bien dang thang dung:
A V Ah;
nhung
s iu y ra:
V„ ho
V 0 = V s( 1 + e0) va AVj = V s( 1 + e0) - V s( 1 + c,), Ah, e0 -ej
VAy ta co: Ah
e , = e o (1 + e0), hay e, = e0 -
d day: eF - he so ep co cua dat, bien dang tuong doi tnong pham vi ap suat da cho, mm/m.
O ng thurc (6-10) duoc dung de tinh he so' r6ng e, ung voi cap tai trong (P,) da cho va nhd cac :ap gia tri nay ta lap duoc toan bo dudng cong nen tren hinh 6-9.
Be ve dudng cong nen, chon true tung bieu thi £ia tri he so rdng e; true hoanh bieu thi gia tri ung suat nen P(kG/cnr). Trudng hop tri so umg suat nen dung he toa do loga thi quan he
1 P =2,72
P,kG/cm'
Hinh 6-9. Tim a() tren duong cong nen he true thong thuong
p( 1 + e0), (6-10)
e = f(lnP), thi bien thien he so rong se phu thuoc tuyen tinh voi loga bien thien cua ap suat nen - la dudng thang. Luc nay, phuong trinh dudng cong nen trong khoang ldn cua ap suat duoc bieu thi dudi dang:
hay
e, = e0 - tga0(lnPj - lnP0), P
e i = e 0 — a oln — >
U)
(6-11) trong do: a0 = tga0 - he so goc cua dudng thang hay he so nen.
Khi P; = e = 2,718 kG/cnr thi InP; = 1,0 va khi P() = l,0kG/cm2 thi lnP0 = 0, vay a0 = e()- e^ He so nen a0 dung danh gia tfnh bien dang ciia dat udt lun va khi xet tdi tmh phi tuyen nen lun vdi pham vi ap suat ldn.
Hinh 6-10. Xac dinh he sd nen lun Hinh 6-11. D if dug cong nen e - f(P ) cua man xao dong va a = f(e.P)
Tren dudng cong nen, neu gidi han doan co ap suat khong ldn (1 ~ 3kG/cm2), thudng xay ra trong nen cong trinh, thi vdi do chfnh xac du cho thuc te, cd the xem doan dudng cong A B la doan thang (hinh 6-10). Phuong trinh dudng thang nay la:
e2 = e, - tgcx(P2 - P|), hay e2 = e, - a(P2 - P,)
d day: P2 — P, - sd gia tai trong;
a = tga - lie sd goc cua dudng thang hay he sd nen lun.
Tii day suy ra:
, cnr/kG, (6- 12)
He so nen lun (a) la ty sd giua tri sd bien thien ciia he sd r6ng va sd gia ciia tai trong nen.
Khac vdi he sd a(), he sd nen lun a la bien sd. Dat cd he sd nen lun ldn tfnh bien dang nhieu, hay he sd nen lun giam theo do dac chat ciia dat. He sd a cd the xac dinh
dug; tai bat ky khoang ap lire nao, va do do ta lap dirge do thi quan he a - f(e,P) nhir Iren hinh 6-11.
Trong thirc te, de danh gia kha nang bien dang ciia dat theo modun bien dang E|_2, tlurrng dung lie sd nen lun a( (khoang ap lire P, = 1 ~ 2kG/cm~) danh gia tfnh nen lun ciia dat theo a,^ va theo he sd ep co ep neu d bang 6-1.