DO BEN CHONG CAT CUA DAT
2.3. Dieu kien ben ciia dat loai set
Bieu thuc the hien do ben chong cat cua dat loai set:
I = C7tg(p + C.
Day la ducmg thdng tao vai true a mot goc cp, cat true x tai H cach goc toa do mot khoang C. Ducmg tiep tuyen nay co tiep diem tai T tren vong tron Mohr (hinh 7-18). Diem T tuong ung voi mat truot OpT, tao vdi mat irng suat chinh Ion nhat mot goc a = 45° +
cp/2. NghTa la phuong cua mat cat khong lien quan den luc dinh C. Va goc tru'ot tai diem T ldn han goc ma sat trong va nho hon goc lech ldn nhat: q> < 0 < 0max.
Trongo han, giira quan he s
Trong
0 ,F
Hinh 7-18. D i eu kien ben trong dat loai set
Do do: FT = R = —— — = a, + a3 + C
tg<P
sin cp,
Hay la sincp + Ccoscp. Vay difiu kien can bang gioi han trong dat dfnh:
C = coscp tg<p> (7-13)
Neii Pc = C/tg(p thi Pc (tri sd tirong dirong vdi do ben kien true hay luc dmh) co the coi vicc khao sat vdi ung suat phap ma do ben cat chi do lire dmh tao ra. Luc dau do b6n eat khong chi do ngoai luc ma con co ca lire Pc dam nhan. Trong trudng hop nay cac ung suat chmh:
a, = a, + Pe va o', = + Pe.
Vong tron Mohr, giong trudng hop thu nhat, diem goc cua toa do C dich chuyen v i F\ Ta co:
x = atgep + C = otgcp + Pctgcp = (a + Pc)tg(p.
1 a'ic nay, goc lech ldn nhat bang goc ma sat trong (Qmax = cp ). Va dieu kien can bang gioi han lai giong nhu trong trudng hop dat loai cat:
a.t
a
O’ | 4" P „ *), a rt\
1 M g - (4 5 °- J),
a , + P. 9 (7-14)
NghTa la di6u kien bfin can bang gidi han khi 9max = (p.
§3. C \ C P H U O N G P H A P X A C D IN H DC) B E N C H O N G C A T
Xac dinh do bfin chong cat cua dat la xac dinh trang thai can bang gidi han cua khoi dat. Khi ung suat ngoai tang len vuot qua do ben chong cat, pha vd trang thai can bang gidi lun, mau dat bi cat theo mat piling nhat dinh ma tai do, do ben chong cat la nho nhat.
Cd ill i6u phuong phap xac dinh do ben chong cat cua dat d trong phong tin' nghiem:
a) Phuong phap cat true tiep hay la cut theo matphdng dinh sun;
b) Phuong phap cat bang may nen dan true hoac may nen ba true,
c) Phuong phap xac dinh gan dung goc ma sat trong cua eat bang dung cu do goc doc tu nhien.
3.J. Phuong phap cat true tiep
Cc the tien hanh thi nghiem cit true tiep theo mat phang dinh sin (may cat phing), theo mot mat hoac 2 mat. Khi thf nghiem, miu dat co hinh tru tron (dudng kfnh d = 50 ~
70mm, chidu cao h0 = 15 - 20mm) dirge dat vao hop cat (mau dirge nen truac hioac khong nen truac). May cat mot mat gom 2 phan; may cat 2 mat gom 3 phan (hinh 7-119).
Dung cu cat mot mat co nua mSu phia tren hoac phia dudi di chuyen ngang duoi tac dung cua lire cat, dong thdi dam bao kha nang tang hoac giam the tich cua mAu khi icat.
Dung cu cat 2 mat cd phan tren va phan dudi dugc giu cd dinh chi cd phan giirai di chuyen ngang dudi tac dung ciia lire cAt va cung dam bao cho mAu dat cd the tang hôoac giam the tfch khi cAt.
\ \ \ A
...^
W / M
Hinh 7-19. S o do hop cat (m ay cat phang) a) C at 1 m at; h) C at 2 m at: 1- khi m5u bi c k
Trudc tien dat tai trong thang diing (P), cd tri so: P, = 0,25; 0,5; 1,0;...5kG/cm2, len mat tren mau, qua tam nen cd gAn dong hd do kin theo phuong vuong goc vdi mat cat.
De tao lien ket tot giua tam nen vdi mfui dat, be mat tam nen cd cac dudng gd ranh nlho, chiing an sau vao mat tren va mat dudi mau dat va dam bao cho ung suat cat phan bo d6u tren mat cat. Gia tang ap luc T (toe do ddu) theo plurang ngang cho den khi mSui bi cat (bien dang ngang dugc A/ > 5mm, theo doi d dong hd do chuyen vi ngang), ta x<ac dinh dirge ung suat tiep (x,).
Thong thudng can ehuan bi 2~3mAu dat cung loai va tien hanh cAt dudi 3 cap tai men (Pj) khac nhau. Tu ket qua thf nghiem lap dd thi quan he x = f(a), tren he true toa do x - a. de xac dinh gia tri C va (p cua dat thf nghiem. Ung suat phap thAng dung a = P ’/F, irng suat cat x = T/F (F la dien tfch mat cat). Trong qua trinh thf nghiem phai theo doi bien dang thing diing va bien dang ngang vdi so doc chinh xac den 0,005mm. Tuy thieo sa do cat, mau dirge ehuan bi bang 3 cach sau:
1. Phuong phap nen (du bj) mAu trirdc khi thf nghiem bang cac cap ap lire khac nhuau.
Tai moi cap ap luc giu cho mau dn dinh lun hoan toan. Sau dd, chat tai nen va gia tai c;At.
Phuong phap nay da noi d tren;
2. Trudc khi thf nghiem cat, gia tai nen (chi mot cap) len mau va duy tri cho tdi khi mAu lun hoan toan, dd tai hoac chat tai nho han len mau rdi gia tai cAt;
3. Plurang phap khong nen du bi trirdc.
Phuong phap nay dirac dung cho dat loai set, d trang thai cung - deo cung. Mau dat hinh tru, thudng cd d = 3,75mm, cao h = 75mm, gia tai nen (khong khong che nd ngang) cho den klu nuiu bi pha hoai cat. Mflu bi pha hoai theo mat nghieng bat ky hoac khi thanh mau bi bong nut. Ap luc nen lam cho mau dat bi pha hoai la ung suat thang dung ldn nhat a, = P. ung suat ngang bang 0. Mat truot tao vdi mat ung suat chinh ldn nhat (mat phang ngang) mot goc 45c+ cp/2 (hinh 7-20). Goc a do true tiep d m3u a = 45° + cp/2 do do: <p = 2a - 90°.
Khi biet goc a co the tim duoc diem T la tiep diem cua tiep tuyen vdi vong tron Mohr, nam tren dudng thang noi vdi goc toa do va tao vdi true a mot goc a (hinh 7-21).
3.2. Phirong phap dung may non don true
Hinh 7-21. Tun C (Ufa vao vong trdn ifng suat H inh 7 20. C at m an khi nen clan true
Tit T ke dirang tiep tuyen vai vong tron Mohr lam vai true hoanh mot goc (p. Biet cr, va cp ta tim duac lire dmh C:
a, a, C x .
— = (— +---) sin (p.
2 2 tgcp
^ CTiU-sintp) in
Suyra: C = —1--- , (/-15) 2coscp
Neu dat chi cd luc dinh chi phoi mat truot se lam vdi phuong nam ngang mot goc 45°. V khi cp = 0 thi C = a,/2. Dat trong tu nhien khong the dong nhat ly tuong do do mat c i khong tao thanh goc nghieng ly thuyet 45° + (p/2. Thanh thu viec xac dinh gia tri cua C' 'a (p dua Iheo thi nghiem nen don true chi la cac gia tri gan diing.