DO BEN CHONG CAT CUA DAT
1.4. Do ben cua dat no niroc
Ddi vai dat cat no nirac bien dang cdt tang len, v] khi cat no nirac bi bien dang cat, tai doi cat do uojng thay doi do anh hirang cua ap lire thuy dong (Pld). Cat a trang thai ket cau chat, khi bi cat do rong tang len va nirac tham hirang vao dai cat (hinh 7-9), ngirgc lai trong cat xop khi bi cat do rong giam di va nirac tir dai cat thoat ra. NghTa la, trong cat chat ap lire thuy dong hirang vao dai cat lam tang ap lire vao khoi cat, trong cat xop ap lire thuy dong lai hirang ra ngoai dai cat cho nen, lire nen tac dung vuong goc vao hat cat giam di (hinh 7-10). Cho nen, do ben chong cat trong cat no nirac co ket cau chat
• • * * • -
tang, ket cau xop thi giam di.
Y ? S ) - th
W m =
Hinh 7-10. D ai eat trong c at .xop no nifdc:
a) Sit sap xep hat trudc khi cat ; h) Sau khi cdt Hinh 7-9. Ddi cdt trong c at chat no nude:
a) S u sap xep hat trudc khi c at; h) Sau khi cdt,
h- do tang chi€u cao mSu; —> hirang tham cua nirac. h- do giam chieu cao mau. —>hirang tham nirdfc.
+ Trong cat chat: x = (a + Pld))tg(p, (7-7)
+ Trong cat xop: x = (a - P(d)tg(p. (7-7')
Truong hap cat co do rong tai han, ap lire thuy dong khong gay ra anh hirang nay nghla la Ptd = 0. Ap lire thuy dong tang theo do lan cua ap lire thuy tinh (Plt). Vi vay, neu
1 ii s
<
* 0 w. Wch w
3) b)
tang nirac ngam tren dai cat co be day cang lan thi Pfd cang cao. Tmh tham nirac cua cat cang nho thi P(d cang lan. Doi vai cat co tinh tham lan, khi cat cham thi Pld khong co anh lurang den ap luc phap. Toe do cat cang nhanh ap lire thuy dong cang co anh huong nhieu han den do ben cat cua cat.
Doi vai dat loai set cac hat va cite dam hat (the ket hat) lien ket vai nhau bai cac lien ket deo (lien ket keo-nudc) va mot phan bai lien ket ciing (ket tinh-ket dinh), cho nen sue chong cat cua ehung phu thuoc nhieu vao luc dmh cua dat.
Tnrong hop thi nghiem dat loai set trong he kin (khong thoat nuoc), trong thdi gian thf nghiem do chat do am cua dat khong doi, tren thirc te chi co the thirc hien diroc khi cat nhanh. Cac ket qua thf nghiem cho thay (hinh 7-11 a), sir lien he giua sue chon^ cat cua dat loai set no niroc voi do chat hay do am cua no (vi do am va he so rong ciia dat no niroc co moi tuong quan chat che: e = YsWbh). Do ben chong cat cua dat set no niroc, trong trudng hop thf
nghiem khong thoat nude va giir do am khong doi, khong phu thuoc vao gia tri ciia ung suat nen, no chi thay dcSi khi do chat do am cua dat thay doi (hinh 7-1 lb).
Khi thf nghiem dat loai set trong di6u kien hd (thoat nirdc-co ket) cac dudng cong cat co dac trung khac. Ban dau mau dat dugc nen dudi ap suat (a ) nao do de cac mau dat set co cung do chat (e giong nhau). Ta dung nhanh do tai (1) ciia dudng cong nen den mot ap suat nao do (hinh 7-12a). Vi dac diem ciia dat set la bien thien cua he sd rong khi do tai rat nho, cho nen cd the chuan bi mot sd mau dat bang each cho nen du bi trudc dudi ap lire ldn nhat. Vf du, nen vdi ap lire a max = 4kG/cnr cho tdi khi dat on dinh lun hoan toan roi da tai tirng cap nhd: 3; 2; 1; .. cho den a () = 0,5 ~ 0,7kG/cm2. Sau khi on dinh na, tien hanh thf nghiem cat vdi cac cap ap luc do.
aj
Hinh 7-11. C ac dudng cong ryh cua dat dinh trong dieu kien he kin: a) Q uan he r - f(W );
h) K hi cat nhanh W = const.
Hinh 7-12. Cac dudng cong Tyh cua d a t dinh, dieu kien he h d a) D udng cong nen (1) va n d (2); h) T h i nghiem cat.
Luc nay, co the xem sire chong cat cua cac mlu dat (da ehuan bi nhu tren) dirge thf nghiem dirai ap suat nen khac nhau, tuang irng vai do am-do chat cua no khi nen dudi ap suat lan nhat.
Doi vai trang thai khong co ket cua dat dmh no nirac, chua dat tai do chat hoan toan dirai tai trong da cho, thi phan sue chong cat cua dat phu thuoc gia tri ung suat phap se nho hon do ap suat chua truydn toan bo len cot dat ma mdi chi truydn ap suat hieu qua (a ) . Luc nay, sire chong cat cua dat loai set chua cd ket hoan toan se co gia tri trung gian cua sue chong cat ung vdi do am ban dau va sue chong cat ung vdi trang thai can bang gidi han cua dat, xac dinh theo bieu thirc:
xgh = (a - u)f + C hay x = a f + C, (7-8) trong dd:
u - ap suat trung hoa, ung vdi do cd ket da cho;
C - gia tri luc dinh hieu dung, ung vdi do chat - do am da cho.
§ 2 . K H A O S A T D I E U K I E N B E N
2.1. Khai quat chung
Mat cat (mat truot) trong dat phat sinh khi ung suat tiep, tac dung vao diem tiep xuc giua cac hat, thang dirge lire chong cat cua dat. Mat khac, do ldn va phuong cua noi ung luc tai diem da cho trong khoi dat cd lien quan den do ldn va phuong cua ngoai lire, den phuong cua mat phan to trong khoi dat dang xet.
Khao sat di6u kien bin tai mat phan to trong khoi dat cho thay, mat trirgt di qua mot diem cd ling suat phap va ung suat tiep tac dung, diem dat khac nhau. Ung suat phap (a) la phan lire tac dung thang dung vao dan vi dien tich ciia mat trirgt; ung suat tiep (x) la ung suat cat tac dung song song vdi mat trirgt. Goi mat phfmg khong cd ung suat cat (mat cd goc nghieng bring 0 ) la mat chinh. Nhu da biet trong ca ly thuyet, tai mot diem cd 3 mat phdng nhu vay vuong goc vdi nhau. chia thanh cac mat phang chinh to, vira va nho theo do ldn cua ung suat phap (irng suat chmh) cd d cac mat nay. Dudng OA bieu thi mat chinh ldn cd ap suat chmh a, ldn nhat tac dung, dudng O B bieu thi mat chinh nho cd irng suat chmh a 3 nho nhat tac dung, con mat chinh trung binh la mat trirgt.
Hay xac dinh cac ung suat tai mat phang bat ky (mat A B) vuong goc vdi mat chmh vira va tao thanh goc a vdi mat chinh ldn OA. Neu irng suat tai tat ca cac diem xung quanh diem O ma bang nhau thi do ldn cua O AB khong cd y nghia gi. Nhung vi ung suat khong bang nhau cho nen, do ldn cua O AB dugc coi la vo cung nho do bo qua ung suat tai 3 bien. Goi do dai ciia A B la 1, thi tren dien tich dan vi cd ca ung suat a va ung suat cat tac dung.
a)
Hinh 7-13. Xac dinh do
Tren hinh 7-13b cho thay, dieu kien can bang doi vai phuong phap tuyen tai A B va phuong song song duoc xac lap bai phirong trinh sau:
Ga = a , cos2 a + a 3 sin2 a, xa = (a, - a 3)sinacosa.
a (x =a | +ct3 a, - a 3
cos 2a hay
a, - a
-sin 2a.
Vay ung suat tai mot diem trong dat da co the bieu thi bang vong tron ung suat viet diroi dang phuong trinh sau day:
a, a \-
C T .
a, - a ,
+ Ta - (7-9)
Cong thuc (7-9) la phirong trinh cua vong tron Mohr lay ctu,t w lam toa do true. Dira vao vong tron nay tim dirge ung suat tren mat nghieng cua phan to d trang thai irng suat mat (hinh 7-14). Moi diem tren vong tron Mohr co toa do <5a va xa tuong ung voi mat co do nghieng xac dinh voi mat chinh. Vong tron Mohr cho thay, trang thai ling suat tai mat khac nhau. Tir diem hieu thi do ldn cua irng suat phap ldn nhat ke dudng song song voi phuong ciia mat chinh ldn (dudng OA tren hinh 7-13), thi tim dugc 0 P tren vong tron ling suat. Diem nay chinh la diem giao cua 3 mat chinh (goi la cuc cua vong tron ling suat). Tir Op ke dudng thang vuong goc vdi (),>D. dudng nay cat true cua ung suat phap tai E. Dudng 0 PE la dudng song song vdi mat chinh be. Toa do ciia diem ma dudng thdng tir diem Op ke song song vdi mat bat ky, giao cAt vdi vong tron Mohr chinh la toa do ciia cac phan to ung suat tac dung vao mat do. Dieu nay cd the dugc chiing minh nhu sau:
ldn cua Ung suat tai diem bat ky
Ta co: xa = Rsin2a.
R - ban kinh vong tron. xH va R duofc bieu thi tren hinh 7-14b. Goc CFD = 2a va la goc o' tam cho nen goc COPD = a. Tuc toa do (a axu) cua diem C. Bien dang cat tren mat A B xuat hien theo phucmg ngugc chicu kim d6ng h6, nlumg tren hinh 7-15a ung suat cat theo chieu (+) chieu chmh cua bien dang cdt ngugc chieu kim dong ho.
Hinh 7-14. V ong trdn Ung suat M ohr (ting suat cua hinh 7-13) Co vong tron irng suat co the trirc tiep tim cac gia tri ca ban sau:
1. Ung suat cat Ion nhat (irng suat cat chinh) bang — - va chmh la R cua vong tron irng suat. Ung suat cat Ion nhat xay ra tai mat co a = 45° (tren hinh 7-15a).
2. Ung suat cat, tai mat tao vai mat chinh 90°, v£ gia tri bang nhau nhung khac nhau ve dau (hinh 7-15b);
3. T6ng irng suat tai mat bat ky la +x2 , goc tao bdi tong irng suat va irng suat phap la:
0 = arctg— , tren hinh 7-15c.
4. Goc 0 cd gia tri ldn nhat goi la goc lech ldn nhat (0max), tim goc nay bang cach dira vao tiep tuyen vdi vong trdn Mohr ke tir goc toa do (hinh 7-15d). Toa do cua tiep diem bieu thi irng suat tai mat cd goc lech ldn nhat. Ung suat cat tai mat nay nhd han irng suat cat chmh.
Hinh 7.15. Chi so Ung suat xac dinh nhdvdng trdn Mohr
Mai cat nguy hiem nhat khong phai la mat irng suat cat chinh ma la mat co goc lech Ion nhat. Dua vao vong tron Mohr ta co cac cong thirc sau:
1 + sin 0.
sinOm a x TU | (7 ^ CT | m a x
CTi + ct3 ' ct3 ! sin 6max a.0 = 45a+ ; o.O = a , (1 + sin0max)
(7-10)
(7-1 I ) a.0, a.0 - irng vdi do ldn cua a va a tai mat ldn nhat.