Chuong 6 (bao cao)

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TRT T V CC HINTNG LIN QUANEVIRONMENTAL GEOLOGY

nguyenhuynh

1/1/2008

2008

TRNG I HCKHOA HC T NHIN

KHOA MI TRNG


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Trt t v cc hiu ng lin quan 2008

CHAPTER 6: LANDSLIDEANDRELATED

PHENOMENA

Trt t l mt hin tng t nhin rt ng s. N xy ra nhiu vng trnth gii nht l cc khu th, vi rt nhiu cng trnh trn b mt t gy nhhng tiu cc n cu trc cc tng t.

Cc vn cn nghin cu:

Nng cao nhng hiu bit c bn v cc qu trnh hot ng ca con dcv cch thc hot ng lm cho dc tr nn nguy him.

Hiu c vai tr ca cc lc di chuyn v cc lc ngn cn trn dc. Vmi quan h ca chng nh th no i vi bn vng ca dc.

Bit c thc vt, thi tit, nc, thi gian, a hnh nh hng nh thno ln d dc v phm vi nh hng ca s trt t.

Tm hiu xem con ngi s dng t nh th no m gy ra s st ln,trt l ca t.

Ph bin cc phng php nhn bit, ngn nga, phng trnh , cnh bov khc phc s trt l ca t.

Nng cao kh nng nh gi cc hot ng c th gy ra trt t

1. Gii thiu v trt t

(Introduction to landslides)

Trt t v cc hiu ng ca n gy ra nhng thit hi to ln vmt mt v nhn mng. Hng nm c khong 25 ngi M cht do trt t,v con s c th ln ti 100-150 ngi do cc vn lin quan ti trt t.

Tng thit hi hng nm hn 1 t USD

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Trt t v 1 s dng khc tng t n l nhng hin tng t nhin vchng vn xy ra khi c hoc khng c s tc ng ca con ngi. Tuy nhin,vic s dng t ca con ngi c th lm tng s trt t hoc gim nxung. V d: trt t c th xy ra nhng sn i xy dng nh ca;

ngoi ra, trt t s d xy ra nhng con dc t nhin, v chng li iuny ngi ta s dng nhng cu trc bn vng hay k thut xy dng.

Thng thng, trt t c th hiu nh sau: l s di chuyn xung dcrt nhanh ca hay t, c th l khi t ln hoc nhng mng nh.Thutng trt t c s dng cho bt c s di chuyn xung dc ca cc vtliu tri t. Cc thut ng khc ch s trt t l slope failure v masswasting. Trong chng ny ta s xem xt trt t trong 1 gii hn nh, cng

nh cc hiu ng lien quan nh: dng chy, nc bn, ri, tuyt l. v thun li, thnh thong chng ta xem tt c nhng s vic trn nh l trt t.Chng ta s tho lun v s st ln, n l 1 dng tai bin trn b mt c ctrung bi bin dng ca b mt theo thiu thng ng ( chm xung) ca vt liutri t, v chng thng to ra nhng h trn trn mt, hay c th to ra vt nthay hnh dng khng theo 1 quy tc no.

2. Cc qu trnh ca dc v s bn vng ca dc

(Slope processes and slope stability)

Dc l hnh dng thng thng nht ca cnh quan hay a hnh, mc dxut hin n nh nhng chng thay i theo mt h thng ng lc m. Nhngvt liu trn cc dc lun lun di chuyn xung pha di dc vi tc lunthay i t khng c trong lc cho ti nhng cn l tuyt v l khng khip.S di chuyn vi mt tc d rt khng khip trn cc con dc l l do gii thchcho vic dng chy nhng thung lng rng hn nhng dng chy cc khu

vc ng bng. Ging nh 1 cn l, cng ln th xy ra cng t, cng nh thxy ra cng nhiu.

xem xt s din bin ca cc con dc, th vic nhn nhn cc yu tca con dc l 1 vic cn thit, trong c 4 yu t ca dc: phn mt li (convexslope), mt phng trng (free_face), phn dc vt liu( debris slope) nghing30o _35o v phn mt cong thp (concave slope) or wash slope. Ch rng theonhng nh a cht th dc thp (wash) l mt khu vc vt liu xp mm bvn chuyn, lng xung bi dng nuc. Tt c nhng con dc l s kt hp ca1 hoc nhiu yu t v din bin ca cc con dc rt khc nhau ph thuc vo

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yu t mi trng c th. Convex slope l ni nhng vin hay t di chuynchm xung di nh l trt. Free-face th c bit vi vic l , v debrisslope l ni nguyn vt liu t free-face tch t li. Gc ca debris slope l theangle of repose, l gc ln nht m cc vt liu b ri c th gi li c. The

concave slope c to ra do cc qu trnh di chuyn ca dng nc. Dcnghing vi mt phng dc (free-face) nh trong hnh Figure6.1 xut hin ph

bin nhng khu vc kh hn v nhng ni m cc khi vng chc chnh thnh.

nhng khu vc subhumid( ph m t) v nhng khu vc c kh

nhn mn, chng ta c th tm thy nhng hnh dng dc kh n gin, minhho figure 6.1b. Nhng yu t ca con dc ny nm phn trn ca convexslope( dc li) v pha di concave slope( dc cong). Mt nghing ca nhngcon dc thoi thng c lp t ph dy v nm pha di ca nhng tng cs chng chu km. Theo ta c th thy hnh dng ca nhng con dc thlun ph thuc kh hu thi tit v sc bn ca . Tuy nhin nhng qu trnhkhc nh xi mn do dng chy hay xi mn do sng bin c th lm free-facehin ra , cng c th l qu trnh kin to a cht nh s nng ln.

Hai dng dc ch ra 6.1 th hin s phn b ca lp t ph tu theohnh dng ca n. Trn nhng dc ng, t c tm thy nh dc v trnthe wash slope, nhng khng c trn free-face, ni m s phong ha xy racng vi s xi mn nhng do xi mn qu nhanh lm di chuyn cc vt liu

phong ho. dc thoi, t dy nh v phn y dc, v mng phn giani dc nht. Lp vt liu phong ho gia con dc b di chuyn gn nh ton

b lng tch t ca n.

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Vt liu tri t trn dc c th b thay i v bin dng theo vi cchin hnh trong hnh 6.2. Dng chy (flowage) l s di chuyn xung dc canhng vt liu vng chc to thnh nhng khi ng. S di chuyn chm dip lc (flowage) th c gi l creek, cn vi vn tc khng khip th n nh

l s l tuyt. S trt (sliding) l s di chuyn xung dc ca nhng khi linkt vng chc vt liu tri t. C flowage v sliding u c quan h vi dc.Falling, ni 1 cch ngn gn l s ri ca vt liu tri t, t phn free-face cacon dc. S ln xung c th xy ra dc hay b mt phng, ln xung tikhi nguyn vt liu bao quanh chng

Trt t thng l s kt hp phc tp ca ca s trt (sliding) v sdi chuyn c p lc (flowage). V d nh hnh 6.3 ch cho ta s thay i nh

sau khi t ln pha trn s chuyn thnh dng chy khi di chuyn xungni thp ca ng trt (Slumping l 1 dng trt). Nhiu v trt t c thxy ra khi cc nguyn liu Tri t thm m nc hay bo ho v di chuynn nhng ni thp hn ca dc, v gy ra slumping (trt) ca cc khi vtliu Tri t .

Kiu dch chuyn. gc Vt liu gn kt yu.

Ht th. Ht mn.

nho,l , l. vn ,vn l.

t ,t l.

Trt.Trt xoay.Trt tnh tin.Trt ngang.

sp./khi

trt. chui.

vn sp. vn/khi

vn sp. vn chui.

t sp.Khi t/t

trt.t chui.

Chy. Chy . Chy vn. Chy t.

Phc hp. Phi hp t hai kiu c bn.

2.1. Tnh n nh ca dc

(Slope stability)

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xc nh r nguyn nhn ca trt t, chng ta nn kim tra nnh ca dc, c din t nh thut ng ch cc lc tc dng ln dc. Nhnglc c xc nh bng quan h qua li ca cc gi tr sau: dng vt liutri t, gc ca dc (a hnh), thi tit, thc vt , nc v thi gian.

2.2. Lc tc ng ln con dc

(Force on slope)

Hnh: mt ct ranh gii trt

n nh ca dc c biu hin l mi quan h gia cc lc dichuyn, lc di chuyn lm cho vt liu tri t di chuyn xung pha di dc,v cc lc chng li (tr lc) ngn cn s di chuyn. Lc di chuyn thngthng nht l khi lng ca cc vt liu thnh phn ca dc, bao gm mi thxp chng ln nhau trn con dc nh: thc vt, vt liu tch t, cc cng trnh

xy dng. Lc khng c thng thng nht l sc bn ca vt ct (sheartrength) ca vt liu trn con dc, hot ng dc theo cc ng trt tim tng.

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bn vt ct (shear strength) l 1 hm s ca s kt dnh cc vt liu tri tv s ma st trong. Nhng mt phng trt tim tng l mt phng a l yunht trn vt liu ca dc.

S n nh ca dc th c nh gi bng cch tnh ton h s an ton(SF), c nh ngh l t l gia cc lc khng c v cc lc di chuyn. Nuh s an ton ln hn 1 th cc lc khng c ln hn cc lc di chuyn v condc c xem nh n nh. Nu h s an ton nh hn 1 th cc lc di chuynln hn cc lc khng c v con dc ny thiu s n nh. Cc lc di chuyn vkhng c th khng bao gi n nh: thay i theo nhng iu kin a phng,do h s an ton c th tng ln hoc gim xung.

V d xem xt cu trc mt ct ca ng (roadcut) chn dc vi cc

ng trt tim tng (figure 6.4). ng lm gim cc lc di chuyn trncon dc bi v 1 vi vt liu dc c di chuyn. Tuy nhin, vic ng ctqua dc cng lm gim cc lc khng c bi v chiu di ca nhng mt phngtrt c gim bt, v cc lc khng c (shear trength) hot ng dc theomt phng ny. Nu bn kim tra hnh 6.4, bn c th thy rng khng ch cnhng phn nh ca dc b di chuyn, m cn c phn ln hn ca mt phngdc cng di chuyn. V th, tc ng ca con ng ct qua dc (the roadcut) lm gim h s an ton, bi v gim ca cc lc di chuyn nh hn

gim cc lc chng . H s an ton thng thng c tnh ton cho nhngcon dc t nhin hoc nhn to nh nhng khu vc pht trin hoc xy dngng cao tc.

2.3. Vai tr ca cc dng vt liu tri t

(The role of earth material type)

Hnh: dng trt trng( slump)

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Vt liu to nn dc c nh hng n dng di chuyn xung dc v tns ca n. S trt c hai m hnh chuyn ng c bn: quay trn v tnh tin.i vi cc loi trt quay trn hay trt (slump), s trt xy ra dc theocc b mt cong (hnh 6.5a). Bi v s chyn ng theo mt ng cong, nhng

khi (slump block) c khuynh hng to nhng a hnh ging nh gh ( ikhi b quay v lt hng ln trn) ging nh hnh 6.5a. Slumps l hnh thcthng thy nht trn t dc (soil slope), nhng n cng ch xut hin 1 vidc (rock slope), c bit nhng ni yu nh l phin st. Trt tnhtin c 2 chiu, chng xut hin dc theo mt phng trt nghing trn con dc.(S trt trong hnh 6.2 l trt tnh tin). Cc mt phng trt tnh tin thngthng trn cc dc bao gm c khe nt trong tt c cc loi , mt phngy (hnh6.5b), cc ng nt t st trong cc trm tch, cc mt lt mng

trong bin cht. mt vi khu vc, cc ng trt rt cn ch 1 phn lpt ph trn lp , c bit nh l trt t (soil slip), mt dng ca trt tnhtin, cng xy ra (hnh 6.5C). i vi trt t (soil slip), mt phng trtlun trn gc nhng bn di t, nm trong lp vt liu ca dc c

bit nh l lp t ri rc- mt hn hp ca b phong ho v cc vt liukhc.

Loi t l nhn t trong s ri v trt. Nu mt rt bn nm trn

mt loi km bn, khi lp bn b ct ri ra nhanh chng v c th lmhng cc phin mng ( ri hnh 6.6)

bn ca vt liu trn dc c th tc ng mnh m n cng vtn s ca s trt t gy ra cc tai nn lin quan ti n. V d : creep (schuyn ng xung dc rt chm ca t hoc ), cc earthflows (dng chyxung dc ca vt liu t bo ho nc, c th chm hoc nhanh) trt t vtrt (slumps and soil slip) th ph bin trn dc phin st hoc trn ccdc c cc trm tch ni la(volcanic), trn cc dc trn bn chng hn ct

kt(cemented sandstone), vi, hay granit th t gp. Tht ra, phin st khhin nhin i vi hot ng l t, iu ny gi l a th phin st(shaleterrain), c trng cho a hnh bt thng nhiu g ng c to ra bi ccqu trnh chyn ng xung dc a dng. i vi tt c hot ng s dng t,t nng thn n thnh th, trn phhin st hoc cc dc yu khc, trckhi pht trin phi xem xt k cng kh nng tai bin trt t.

2.4. Vai tr ca dc v a hnh

(The role of slope and topography)

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Dc nh hng mnh m n cng cc lc di chuyn quan h vinhau trn cc con dc. Khi gc ca mt phng trt tim tng tng ln, cc lcdi chuyn cng tng ln; v th kt qu ca tt c iu ny l trt t s xy rathng xuyn hn trn cc dc ng. V mt nghin cu v s l t xy ra

trong hai ma ma vnh San Francisco ca California xc nh c l 75-85% hot ng trt t c lin quan mt thit vi cc khu vc th c dcln hn 15% hay 8,5o. V mt quc gia, cc vng ni ven bin ca California vOrezon, cc ngn ni Rocky, v Appalachian c tn s trt t ln nht. Tt ccc lai chuyn ng xung lit k trong bng 6.1 u xy ra nhng v trny.

Hnh: cc dng vn ng ca trt t

1 vi phm vi no , cc hot ng trt t l mt hm s ca dc v a hnh. V d: l v l tuyt (debris avalanches) (s chuyn ngxung dc rt nhanh ca t v mnh vn hu c nh cy) th c lin quan

ti cc con dc rt ng, v min Nam California trt t nng th ph bintrn cc dc ng bo ha nc. Nhng trt t (soil slip) ny thng b bin

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i thnh cc lung t hay lung bn khi xung dc v iu ny c th ht scnguy him (hnh 6.7). kha cnh khc, cc lung t (earthflow) c th xy ratrn cc dc va v tc ng ca vic l t c th quan st c trn cc dcch c vi . Thut ng khc l dng chy rc (debrish flow), l dng chy

xung dc ca cc vt liu tng i th. Hn 50% cc ht trong dng chy rcth hn ct. Dng chy rc c th chuyn ng rt chm hoc nhanh cn tythuc iu kin. Dng chy bn (mudflow)v cc dng chy rc c quan h vicc qu trnh ca ni la c tho lun chng 8

2.5. Vai tr ca kh hu v thc vt

(The role of climate and vegetation)

Kh hu v thc vt c th nh hng n dng trt t hay chuynng xung dc khc. Thi tit iu khin t nhin v phm vi ma do m (moisture) s cha trong cc vt liu ca dc.V d: c hai dng chy t(earthflow)(thng xuyn c trn dc) v dng chy bn hay rc (mudflow haydebris flow)( ban u c th c gii hn trong cc con knh) lin quan ti sdi chuyn xung dc ca cc vt liu tri t bo ho nc. Tuy nhin, dngchy t thng thng v dng chy bn kh him cc khu vc m t, nim c s phn chia tt lng nc thm vo dc, to iu kin thun li chocc dng chy t. Hn na, vng m t c rt nhiu dng chy lu nm, vn lin tc vn chuyn cc trm tch t nhng con dc ra khi lu vc.V th,nhng nguy him t dng chy rc chc chn s xy ra vng m t, v tcng c n ging nh l ca 1 cn bo ln

Vai tr ca thc vt i vi trt t v cc hiu ng c lin quan rtphc tp, bi v thc vt 1 khu vc ph thuc vo nhiu yu t, bao gm thitit, loi t, a hnh, nhng v ho hon trong lch s. Mi yu t trong c

nh hng rt ln n nhng g xy ra trn cc con dc. Thc vt c ngha rtquan trng i vi tnh bn vng ca con dc v 3 l do sau:

+ Thc vt cung cp che ph lm gim bt va chm ca nhng trnma ri trn dc, gip cho nc thp vo t d hn, v lm chm qu trnhxi mn trn b mt.

+ Thc vt c h thng r s to ra s lin kt chc chn i vi cc vtliu trn dc.

+ Thc vt lm tng sc nng cho dc

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Hu ht cc vn trt t u lin quan n tnh n nh ca dc vthm thc vt rt phc tp nhng u c lin h vi s xo trn, thay i hocloi b thm thc vt trn dc.

Hnh 6.8 ch v hnh nh mt trt t phc tp xy ra vo nm 1995.ng dc ng m s trt xy ra l mt vch bin 40000 nmtui. trn nh dc, a hnh tr nn khc bng phng, v 1 vn l c phttrin ti y. V mi nguy him ang dn gia tng do vic ti nc cho vncy n qu lm tng m trong t, lm gim tnh vng ca dc do lmgim bt lc chng chu. Vic xy dng 1 con ng trn dc gp phnthm cho s tht bi. Nhng nhn t phc tp khc bao gm ng trt cto ra do trn ma d di v n kch hot ng trt c v nh hn trn dc

ng vng, v trc y n c nhiu ng trt t nhin. Do cnguyn nhn v s trt u phc tp bao gm s lin kt gia vt liu dc yu,a hnh, nc v cch s dng ca con ngi.

Vic xo trn hoc loi b thm thc vt bi vic n h cng gp phnlm tng nguy c trt t. R rng vic loi b hay cht tt c cy ci gyra nhiu vn :

+ Tc thot hi nc ca cy gim mt cch r rt, m t tng

ln v s n nh ca dc gim i.+ Trong 1 vi trng hp c th, lng nc thm vo dc c th tng

ln. y l iu c bit rt ging nh t trn dc tng i thp c baoph bi lp tuyt dy vo ma ng v tan chy t t vo ma xun.

+ Cng vi vic loi b nhng cy c g mu (ti sinh sau khi n),r ca nhng cy b cht s thi ra theo thi gian, kt qu hin nhin l lmgim sc chu ng v tnh dnh kt ca t. V iu cng lm gim cc lc

chng ca con dc, v iu ny gip gii thch vic trt t tng ln tngln sau khi thu hoch nng sn nh cht cy.

Trong vi trng hp, s hin din ca thm thc vt li lm tng khnng xy ra trt t, c bit l trt t nng trn nhng ng dc ng.Mt dng trt t pha nam khu vc b bin California xy ra trn nhngng dc ng, c bao ph bi thc vt c l ph nhng tinh th bng (hnh6.9). c bit trong nhng thng ma ng m t, nhng r ngn ca thc vtny ht nc, thm 1 trng lng ng k vo dc ng (mi l cy d trnc v trng nh 1 cn tin nh) v tng lc di chuyn. Thm thc vt dng

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chn nhng trn ma ro cng l nguyn nhn lm tng s thm nc vo trongng dc, lm gim i lc chng chu. Khi n 1 mc no th thm thcvt v mt vi cm r cy v t s trt .

Trt t xy ra trn nhng ng dc t nhin gy ra nhiu vn nguy him ti pha nam California (hnh6.10). Trong trng hp ny, nhng bicy rm rp lm tng s thm nc vo trong dc, lm gim i h s an ton. 1nghin cu kt lun rng, trong mt vi trng hp nhy ca trt t c thrt ln trn dc c thm thc vt so vi dc m thm thc vt gn nh

b loi b do t. iu ny khng c ngha l l vic t thm thc vt khnglm tng tai bin trt t. Thm ch, thnh thong trt t li gim xung dovic loi b thm thc vt, v vn ny khng th b qua. Thm vo , s

xi mn cc ht vi nhau tng ln rt ln do nhng cn ma lan ra v chy trnvng dng ln. Nhng trm tch xi mn s tch t hm ni cng vi rc(cnh cy, si), v iu s to ra nhng dng chy rc v dng chy bnvo ma ng.

2.6. Vai tr ca nc

(The role of water)

Nc hu nh u c lin quan trc tip hoc gin tip n trt t, vth vai tr ca n l c bit quan trng. Nhiu cht ho hc c to ra t qutrnh phong ho , v n s t t lm gim sc bn ca nhng vt ct(shearstrength), do nhng phn ng ha hc ca nc c lin quan n t v gn

b mt tri t. Nc t nhin thng c tnh acid bi v n phn ng vi CO2trong khng kh v t to ra acid yu H2CO3.

H2O + CO2 H2CO3

Qu trnh phong ha ha hc ny c bit c ngha nhng vng vi, y l loi rt d b nh hng phong ha ha hc v phong ha vt l

bi acid cacbonic.

Kh nng gy xi mn ca nc cng nh hng n tnh n nh cadc. Xi mn do dng chy hay do sng c th lm di chuyn vt liu v lmtng dc, v vy lm gim i h s an ton. iu ny tht nguy cp, nu nhchn dc c k, v tnh n nh gim th nhng cn trt t ngng hotng s hot ng tr li. Do , iu quan trng trc khi thi cng l phinhn ra nhng v trt t c dc theo nhng con ng ct qua dc cng nh

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nhng cng trnh khu vc khc, phn lp v sa cha nhng vn timtng .

V tranh ci xy ra, nc c lm tc ng lm trn nhng ht t

ring l v ng trt tim tng (potential slide planes), nhng iu ny khngng. Nc khng phi l cht bi trn. Ngc li, b mt cng ca nc baoquanh nhng ht t s cung cp lc dnh kt.

V d, ct kh s to thnh cn ct hnh nn khi cht ng, trong khi ct t gn nh to thnh thng ng bi v b mt cng ca nc gi nhnght t li vi nhau. Nu tt c l hng c lp y, ct t sng c thchy xung nh bn. Nhng tc ng ca nc i vi ng dc v trt tl c thay i c. u tin, s bo ho nc ca nhng vt liu tri t l

nguyn nhn lm tng p lc ca nc nhng l hng. Ni chung , p lcnc nhng l hng (pore-water pressure)(p lc nc tng ln khongtrng gia cc ht, khi cc khong trng lp y nc) trn nhng con dc tngln, sc bn cc vt ct (shear strenth)(lc ngn cn) trn dc gim xung vtrng lng tng ln (lc di chuyn). Do tc ng mng li (net effect) slm gim h s an ton. y l h s rt c ngha trong vic nh gi trt tv l tuyt cng nh nhng dng trt t khc. p lc nc nhng khongtrng s tng trc khi xy ra trt t, v nht nhiu v trt t l do s tng

p lc bt thng.

Ni chung, trt t xy ra trong sut nhng trn ma d di khi c shin din ca mc nc ngm gn mt t (figure 6.13).Trong nhng trnma dng, tc thm nc b mt nhng vng cha bo ho nc hay t ri rc vt qua tc thm nc qua lp di lp t ri rc, thmch mt t nc di chuyn trn dc cng thm vo t, lm tng mc ncngm.. V tai ho s xut hin khi lc chng gim n mc va - khi h

s an ton nh hn 1. H s an ton s thp nht khi mc nc ngm dng caoln ti b mt, v du hiu bit l khi khi t trn mt phng trt tim tngbo ho nc.

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Cch th hai m nc c th lm gim s n nh ca dc l nc rt i

nhanh chng, s h thp nhanh chng ca mc nc h cha hay sng (vigi tr thp nht l 1m/ngy). Khi mc nc v tr tng i cao, th mt khilng ln ln c th trn ln b, v hin tng ny c gi l bank storage (stch lu hai bn b). Sau , nu mc nc bt thnh lnh gim xung, th nc hai b rt i m khng c g ngn li. iu ny to ra s phn b p lc nc

bt thng nhng khong trng, v lm gim lc chng ; cng lc khilng nc trong t s lm tng lc di chuyn. Chnh v l do , nhng tainn b sng hay xut hin dc dng sau khi nc l rt xung.

Cch th ba m nc c th l gy ra trt t l do s ho lng t nhinca cc trm tch giu st hoc lm mm st (quick clay). Khi b xo trn, tst s mt i sc bn bin dng( shear trength) v n s c tnh cht nh chtlng v chy. S xo trn t st bn di Anchorage, Alaska, trong trn ngt nm 1964 to ra hiu ng trn v ph hu nng n.

Nhng v d khc v trt t c lin quan n tnh nhy ca t st cth tm thy Quebec, Canada, ni nhng v trt t ln tn ph mt s

lng ln nh ca v git cht khong 70 ngi. Nhng v trt t xut hin nhng thung lng sng dc, nhng vt liu rn ban u chuyn thnh bnlng v s trt li bt u. iu c bit y l s ho lng t st xut hinm khng cn s rung ng ca ng t. Ban u, s trt xy ra thng dos xi mn sng chn dc mc d ch bt u nhng khu vc nh, nhng cth pht trin thnh thnh nhng thm ho ln. Bi v chng thng lin quann s kch hot cc ng trt c, nhng vn ny trong tng lai c thtrnh c bng cch pht trin gii hn. May mn thay, nhng ng trt c

mc d b che ph bi thc vt nhng thng c th nhn bit c t nhngbc nh trn khng.

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2.7. Vai tr ca thi gian

(The role of time)

Cc lc tc ng trn nhng dc thng thay i theo thi gian. V d,lc di chuyn v lc ngn cn cng thay i theo ma nh m hoc do mcnc ngm thay i. c bit, nhng thay i ny th nhiu hn trong nhngnm m t, do tn s trt t tng trong nhng nm m t. Trong nhngcon dc khc, vic gim bt lin tc lc cn c th xy ra theo thi gian, c ldo qu trnh phong ha lm gim i s dnh kt cc vt liu trn dc, hoc victng p lc u n do cc iu kin t nhin hay nhn to. Theo thi gian, dcs tr nn t vng chc v t l l t ngy cng tng cho n khi tai ho xut

hin (figure 6.14a)H s an ton ca dc ngy cng gim theo thi gian do s m t ngy

cng tng, gy ra s xo trn nhng ht t trn dc, lm gim ma st bn trongdo lm gim sc bn ca nhng vt liu dc.

Hnh 6.14b m t kt qu ca vic xy dng v lp t h thng nclm tng lng nc xung dc nh k. Trng hp ny nhn mnh 1 imquang trn nh sau: chng ta c th thit k sao cho con dc bn vng khi xy

dng, nhng s bn vng s thay i theo thi gian . Do , vic thit k dcnn bao gm vic d phng lm gim i qu trnh lm yu cc vt liu dc.

2.8. Nhng nguyn nhn gy ra trt t

(Causes of slandslide)Nguyn nhn thc s ca trt t-lm tng lc di chuyn hoc lm gim

lc cn- thng c che giu bi nhng nguyn nhn trc tip trc mtchng hn nh va chm ng t, chn ng hay nh t ngt tng mt khilng nc trn dc. S khc bit gia nguyn nhn thc s v nguyn nhntrc tip l rt quan trng. Khi mt trng hp trt t c nu ln trongmt hi ngh, cc nh khoa hc tri t mong ch cung cp mt tuyn b

cui cng lin quan n nguyn nhn trt t. V d, s trt dch chuyn(translation slide) c th xem nguyn nhn trc tip ca n l do ma lnlm

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vt liu tri t bo ha nc, nhng ngc li nguyn nhn thc s l khnng lm trt do tng t yu trn, nh lp t st. V d khc: s kmcht lng ca dc nhn to trong s pht trin nh , m nguyn nhntrc tip l ng t nhng nguyn nhn thc s l con dc c xy dng

mt cch t hi.

Nguyn nhn ca trt t c th chia ra thnh nguyn nhn bn ngoi vnguyn nhn bn trong. Nhng nguyn nhn bn ngoi lm tng p lc ln vtct (lc di chuyn trn 1 n v din tch) khi bn mt ct l tng i nnh (lc ngn cn trn 1 n v din tch). Nhng v d v nguyn nhn bnngoi nh: s vn chuyn trn con dc, lm dc su hn do xi mn hay ong, hay do rung lc ca ng t. Nhng nguyn nhn bn trong to ra trt

t bao gm cc qu trnh lm gim bn mt ct . Cc v d ny bao gmnhng thay i nh s gia tng p lc nc nhng khong trng hoc lmgim tnh dnh kt ca cc vt liu trn dc. Thm vo , 1 vi nguyn nhngy ra trt t l trung gian gia 2 nguyn nhn trn, n c nhng c imca nguyn nhn bn ngoi v bn trong. V d : khi mc nc h nhanhchng, n s lm tng p lc ln mt ct (gy ra bi khi lng ca nc trncon dc) cng vi vic gim shear strength (gy ra do p lc khong trngcao). Mt vi nguyn nhn trung gian khc bao gm ho lng t nhin, v

phong ho lp t di b mt v xi mn.

3. Cc hot ng ca con ngi v trt t

(Human use and lanslide)

Tc ng do vic s dng t v hot ng ca con ngi lm cho cng v tn s xy ra trt t thay i t gn nh khng c g n mt thm hirt nghim trng. Trong mt s trng hp, hot ng ca chng ta s phi lmthay i cng v tn s ca trt t, th chng ta cn bit trt t s xyra u, khi no v ti sao xy ra trt t, trnh nhng vng c nguy ctrt t v lm gim thit hi. Trong nhng trng hp khc m cch sdng t ca con ngi lm tng s lng cng nh tnh khc lit ca trt

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t, chng ta cn phi bit cch nhn ra chng nh th no, kim sot v lmgim kh nng xut hin ca chng n mc c th.

S kt hp nhng iu kin a l bt li chng hn nh t hoc yu

km, nhng mt phng trt tim tng trn nhng ng dc thng ng vinhng cn ma xi x (chy xit), tuyt ri, mt t ng bng theo ma s tiptc to ra hin tng trt t, dng chy bn, l tuyt v n khng lin quann cc hot ng ca con ngi. Nhng qu trnh t nhin phn ng vi cciu kin t nhin. Tuy nhin, nhng hot ng s dng t ca con ngi vnhng dng st ln lm tng mc nghim trng ca cc thm ho .Chng ta hy so snh. V d, nh hng ca nhng v l tuyt nhng vngdn c tha tht vi nhng vng ng dn.

3.1. Timber Harvesting and landslide

CASE HISTORY

Vaiont Dam

Thm ha p ngn nc nng n nht trn th gii din ra vo 9-10-

1963, khi , khong 2600 ngi b mt tch ti Vaiont Dam Italy. Theo boco ca George Kiersc , thm ha c lin quan n con p hnh cung cao nhtth gii (267m) , tht k l, v ko c 1 s bo v c th duy tr mi bi mt lpv ca p nc hay tr chng . Thm kch gy ra bi mt v trt t rt ln,vi hn 238,000,000m3 t v cc mnh vn khc di chuyn vi tc 95km/hr xung sn pha Bc ca ni pha trn h nc, v lp y n 1,8kmdc theo trc thung lng n chiu cao 152m trn mc nc h (hnh 6.16), sdi chuyn nhanh chng to nn 1 lung khng kh d di t di ln, yt v nc ln pha Bc thung lng, cao hn 250m so trn mc nc h. Strt km theo ting n ca hi, nc v t to nn trn ng t mnh mc th bit c mc d xa hng cy s. N thi nhng mi nh ca mtngi hn 250m trn h v nm ngi n ng vi cc t v mnh vn. Slp h to nhng cn sng cao hn 90m cun theo tr chng ca p nc.

Xui dng chy hn 1.5 km, sng vn cn cao hn 70m v mi th trn ngi u b hy dit hon ton. Ton b nhng s kin din ra (trt v l lt )trong vng cha n 7 pht.

Theo d liu v nhng v trt t trong khong thi gian 3 nm, gitr ca chng l rt khc nhau t t hn 1cm cho n 30cm mi tun, n thng9 1963, tng ln n 25cm hng ngy. Cui cng, mt ngy trc khi xy

ra v trt t, n hn 100cm mi ngy. Cc k s d on l trt t,nhng mt trong chng c cng nh hn, v h khng khng nhn ra

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cho n 10-8, mt ngy trc khi xy ra trt t, l mt vng rng ln dichuyn nh mt kh ng nht nhng khng bn vng. Cc ng vt trn sni cm nhn c nguy him v di chuyn ra xay vng vo 1/10.

Trt t c to ra bi s kt hp ca nhiu nhn t. u tin l cc

yu t a cht i nghch, bao gm t yu v vi vi nhng vt nt gy,nhng ch t st v lp t st, cc yu t c khuynh hng tc ng xu tih nc, to ra nhng khi khng bn vng (hnh 6.17 trn trang 145), v ahnh rt dc to nn lc di chuyn mnh m (sc ht). Th 2, p lc nc nhng khong trng c tng ln trong t thung lng, do s ngn nctrong h. S di chuyn ca nc mt bank storage lm tng p lc nc nhng khong trng v gim lc ngn cn trn dc. Tc ca s l trctrc khi trt t tng ln, khi mc nc ngm tng ln cng vi mcnc ca h cha. Th 3, ma to t cui thng 9 n ngy thm ha lm

tng trng lc ca cc vt liu trn dc, lm tng p lc nc trong , v lmtng lng nc chy trn vo h cha thm ch sau cc k s c gnglm gim mc nc trong h.

Mt kt lun l, nguyn nhn ca thm ha gia tng l do s gia tngcc lc di chuyn cng vi s gim xung rt ln ca cc lc ngn cn, lmtng lng nc mt trn dc t lm tng p lc nc nhng mt phngtrt yu trn .

Cho n by gi chng ta vn cha thng nht kin khi xem xt vn

trt t c lin quan vi thu hoch g.

3.2. TH HA V TRT T

( Urbanization and landslide)

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Hnh : trt t th

Cc hot ng ca con ngi i vi cnh quan th rt ging nhng nguynnhn gy nn trt t trong nhng vng th, ni c mt dn s cao v cc

cng trnh xy dng nh ng ph, nh ca, v cc ngnh cng nghip. V dminh ha t Rio de Janeiro, Brazil, LosAngeles, California.Rio de Janeiro, vi dn s hn 6 triu, c th c rt nhiu vn v tnh n

nh ca dc hn bt c thnh ph no c cng din tch. Thnh ph ni tingv nhng nh ni granite p, to nn cnh quan p mt trong thnh ph(hnh 6.18). S kt hp ca nhng con dc ng v cc vt nt gy ca cche ph bi b mt trm tch gp phn gy ra cc vn . Lc trc , nhngcy g trn dc b cht lm nh, cht t v to khong trng cho nngnghip. Hot ng ny ko theo sau l nhng trn trt t c lin quan ti

nhng cn ma ln xung. Gn y hn, do thiu t trn nhng vng bngphng nn s pht trin th gia tng trn nhng con dc. Thc vt b loib, thay vo l lm nhng con ng ti cc a im pht trin nhng ningy cng cao. S o bi ct chn cc dc, v ct t lp t che ph tiim ti hn. Thm vo vic lp y cc vt liu nhng ni o bi trncon dc lm tng cc l di chuyn trn con dc. Bi v vng ny nh k triqua nhng cn bo d di, v tht d dng thy rng Rio de Janeiro cnhng vn rt nghim trng (13).

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Trong thng 2-1988, mt cn dng rt mnh xung hn 12cm ncma xung Rio de Janeiro trong 4 gi. Cn bo l nguyn nhn dn n l ltv l bn v lm khong 90 ngi cht, khong 3000 ngi khng nh ca.vic khc phc tiu tn hn 100 triu USD. Mt vi trn trt t c bt u

t nhng con dc ng ni m nh ca khng chc chn v vic kim sotlng nc chy trn do bo khng cn thc t na.. nhng khu chut trni l ni c nhiu ngi cht nht do l bn. Tuy nhin, 1 phn ca vin dnglo trn cnh dc mt ngn ni cng b cun tri bi mt trn trt t, gitcht 25 bnh nhn v mt s nhn vin. Rio de Janeiro nhn ly nhng huqu nng n, do trnh nhng thm ho lp li trong tng lai, h cnnhng phng php kim sot lng nc chy trn ca cn bo v lmtng bn ca dc.

Los Angeles, v ni chung vng nam California, tri qua mt tn sutln ca trt t c lin quan ti s pht trin trn sn i. Trt t NamCalifornia l kt qu ca nhiu iu kin vt l phc tp, 1 phn trong l dos thay i phc tp v a hnh a phng, cc loi t, kh hu v thc vt.S tng tc gia mi trng t nhin v hot ng ca con ngi l phc tpv khng th on trc c. V l do ny, khu vc ny thnh thong c ara nh 1 v d in hnh v s tng ln cha tng thy v cc gi tr ca a cht

th. Los Angeles lnh o quc gia trong vic pht trin cc b lut linquan (s o nhn to v lp y) cn thit cho s pht trin.

Trt t tc ng n 60% chiu di ca bi trn bin min NamCalifornia (thy hnh 6.12), v s lui vo ca cc vc chc chn ciu khin bi trt t (14). T cc c trng a cht phc tp ca vng,cng nh bng chng t nhng nhng vt trt t li hay trm tch catrt t, m ta kt lun rng trong lch s nhng con dc hot ng. Tuynhin, hot ng ca con ngi lm tng cng v c bit l tn sut

ca trt t.

Qu trnh phn loi (nhng vt ct hnh gh trn con dc khu vc nh, gi l pads t cu ni vo nhng nm 60 come on over to my pad )

Nam California chu trch nhim cho nhng trn trt t. Trong qu khqu trnh t nhin ny mt hng ngn nm to ra cc vc su, dy ni, vi. Trong th k ny, chng ta phi pht trin cc my mc xp loi chng.L.B.Leighton vit: Vi nhng k thut hin i v s phn loi r rng, v ti

chnh thch hp, s khng c ngn i no tr nn th do cc hot ng phttrin trong tng lai.(14). Khng 1 vt liu tri t no c th chng chi s

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tn cng nghim trng ca khoa hc k thut hin i. V vy, hot ng cacon ngi l mt tc nhn a cht cht c kh nng to hnh cho cnh quan nhl sng bng hay dng sng. Chng ta c th bin i dc ca nhng qu ich trong 1 m thnh nhng dy t bng phng v nhng con ng, v s

thay i dn ti 1 s lng ln v trt t nhn to. Nh thy trnhnh 6.19, nhng con ng c lm qu dc kt hp vi vic tng lngnc chy trn sau nhng cn ma, v s gia tng khi lng cc vt liu tcht hay nh ca, lm cho nhng con dc bn vng tr nn km bn. Mt sd n lm dc, ng hay lp y vt liu s lm tng chiu cao ca n, hoclm tng tng ti trn n t gy ra trt t.

Trt dt trn vng t t v chung Halmiton County, Ohio, li

nhiu vn nghim trng. S trt xy ra trn cc trm tch bng ( a s l tst, lakebed, till) v t ri rc trn phin st; thit hi trung bnh minm hn 5 triu USD.

S thay i ca nhng con dc c lin quan ti vn th ho Allegheny Country, Pennsylvania, c tnh vic th ho chu trch nhim cho90% trt t gy ra y, v lm thit hi trung bnh khong 2 triu USDhng nm. Hu ht trt t di chuyn chm, nhng thnh ph k bn, l bp mt chic xe but v git cht 22 hnh khch. Hu ht cc trt t

Allegheny County l kt qu t hot ng xy dng, n lm nng thm dc trn cao v ct vo nhng v tr nhy nhng di chn dc, hay lm thay iiu kin nc trn hoc trong dc (15).

4. Gim thiu ri ro trt t

(Minimizing and the Landslide)

gim thiu ti a ri ro ca trt t, iu cn thit l nhn ra nhngvng c nhiu kh nng xy ra trt t, thay i con dc hay to ra cc cutrc k thut ngn nga trt t, v cnh bo cho nhng ngi trong vngnguy him c nguy c trt t, v kim sot trt t khi h bt u chuyn i.

4.1.Nhn ra trt t tim tng

(Identification of Potential Landslides)

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Hinh: cc v tr xy ra trt t

Nhn ra nhng vng c nguy c trt t tim tng cao l bc u tintrong vic pht trin cc k hoch trnh tai bin trt t. Nhng ni ckhuynh hng trt c th c nhn ra bng cch kim tra cc iu kin acht trong cc m khai thc hoc l kim tra qua cc hnh nh v tinh xcnh cc v trt t trc . Thng tin ny c th c dng nh gi ccri ro v to ra cc bn dc n nh .

SPECIAL FEATUREXc nh tai bin v ri ro trt t

(Determing landslide hazard and risk)

Phng php dng nh gi tai bin trt t l bc u tin lmbng danh sch trt t, n c th l mt bn vin thm cho thy nhngvng dng nh c nhng thm ho v dc. iu ny c th c lm nhcc hnh nh v tinh cng vi s hiu bit v vng kim tra. Ti mt mc chi

tit hn, nh gi trt t c th dng bn c th xc nh nhng trm tchdo trt t trong gii hn ca cc hot ng lin quan, nh vn ng v

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khng vn ng ( thuc a cht tr ) hay ko vn ng (thuc a cht c). Mtv d l bn cho mt b phn ca Santa Clara County, California, c thhin trn figure 6.20(a). Thng tin lin quan ti hot ng trt t trc kiac th sau c kt hp vi cc vn s dng t pht trin nhng dc

n nh hay lm bn ri ro trt t vi li khuyn s dng t , nh thy trn hnh 6.20(b). Bn cng mi cng c s dng nhiu, trong nhng ngi lm bn cung cp cc thng tin hu ch cho cc nh a cht.

Nhng bn ny khng din t chi tit ca tng vng nh gi tng aim c th nhng a cho chng ta 1 hng dn chung cho vic ln k hoch

s dng t v nh gi a cht chi tit hn.Xc nh r ri ro trt t v lm cc bn ri ro trt t th rt

phc tp, bao hm nhng vic nh tnh xc sut xut hin hay nh gi khnng tn tht, thit hi. Nguy c c th (Rs) vi 1 cng xc nh ca trt

t l : Rs= E x H x V, a im

E: l yu t ca ri ro trong vng nghin cu k (i.e.,gi tr v tisn, x hi v hot ng kinh t).

H: l kh nng m trt t vi cng c th s xy ra. ( trong mtthi gian cho trc ).

V: l h s thit hi , nh ngha nh mt phn ca yu t ri ro E, nh

hng bi trt t c th. Gi tr ca V t 0 (khng thit hi) n 1 (ph hyhon ton).

V d: nu mt vng th tr gi 100 t USD v kh nng xy ra trtt rng ln trong khong 10 nm l 1 phn 1000 hay 0,001 v h s thit hil 1%(0,01). Tnh

Rs = 100 x 109 x10-3 x 10-2 = 100 x 104 = 1 triu USD. Tng t, nguy ctrt t ca cc vng khc cng c th xc nh, v cc thng tin c th kt

hp vi nhau lm bn nguy c trt t. phng php v pht tho ny lm nhng bn ri ro ny c nu ra trong chng 4 v c p dngcho cc tai bin khc nh ng t, chy rng, v bo.

Ch nh, ngi mua hay ngi xy dng c th nh gi nguy c trtt trn nhng c ngi trn sn i, khi nhn thy nhng bng chng vt l cth, v nhng du hiu c th cho bit vn trt t tim tng hay thc ss xy ra. Du hiu bao gm : cc vt nt n cng trnh xy dng hay nhng

bc tng gn xung quanh , cc ca chnh v ca s b nn hay p li; nhngbc tng hay hng ro b mo i, nt nhng ng ng ngm di mt t hay

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nhng cng trnh khc, xut hin l thng trong cc h bi, lm nghing cy vct in, lm nt mt t hay c nhiu g ng , v n ging nh 1 c trngtrn mt t , v s r r nc di chn dc.

S c mt ca mt hay nhiu c im khng l bng chng hon ton ni rng trt t s xy ra. V d: nhng vt nt trn tng c th to ra dot trng ra. Cc c im khc nh nhiu g ng hay steplike ground trnnhng con dc va ( ln hn 15%). Nhng chc chn, vic hnh dung kh nngtim tng ca trt t cn thit phi c nh gi bi cc nh a cht hc.Xa hn na, iu l thch hp khi s quan tm ca chng ta ch l ti sn;trt t thng rng hn nhng l t c nhn; nhng vng gn k nhau, c

bit l nhng vng trn dc v di dc. (15)Vic phn chia b lut nhm hn ch ti a ri ro trt t nh hng

n vng trong Los Angeles t 1963. s thc y xy dng nhng b lut ny dn ti hu qu l lm thit hi rt ln v nhn mng v ti sn vo nhngnm 50 v 60.

T khi nhng nghin cu chi tit v k thut a cht c p dng, phntrm s nh trn i b tn ph bi trt t v l lt gim i 1 cch ng k.Mc d, chi ph xy dng nhng cng trnh th nhiu hn khi p dng nhngquy nh, nhng chng s cn bng hn khi gim nhng tn tht t nhng nmma. V thm ch thm ho trt t trong nhng nm m t tip tc e do

chng ta, th vic p dng nhng hiu bit v a cht v k thut trc khi xydng vng i c th gip lm gim tai bin.

4.2. Phng nga trt t

(Prevention of landslide)

Vic phng trnh nhng v trt t t nhin l rt kh khn, nhng vi thc v nhng vic thc hnh tt k thut c th gip ch nhiu hn trong vic

gim thiu tai bin. V d, ti lng trn nh dc, vic ct vo nhng con dcnhy, placing fills trn nhng con dc hoc thay i tnh trng nc trn condc nn c trnh hoc lm vi 1 s thn trng. Nhng k thut xy dngthng thng ngn nga trt t bao gm cung cp vng thot nc trn vdi mt t, loi b nhng vt liu ri rc trn con dc, xy dng nhng bctng hay nhng cu trc h tr cho con dc, hay kt hp c 2.

Kim sot vng thot nc: cc cch thc kim sot thot nc b mt

v di t c nh hng rt ln n vic to s n nh cho con dc. Nhngbin php c bn bao gm gi lng nc chy qua hay thm vo trong dc.

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Nc b mt c th chuyn hng xung quanh con dc do cc dng thot nc.Thng thng chng ta p dng cch ny bng cch lm con ng ctqua( hnh 6.22a). Khi lng nc thm vo trong dc c th c kim sot

bng cch che ph dc bng cc lp khng thm nc, nh l t-xi mng,

nha ng, thm ch c cht do (hnh 6.22b). Nc ngm c th b hn chdo vic o rnh mng (a cutoff trench). Nhng rnh mng c lp y siv th v c t ch c th ngn chn hoc chuyn hng dng chy raxa con dc tim tng khng bn vng.

Phn loi: mc d s phn loi dc cho s pht trin lm tng tai bintrt t trong nhiu khu vc, nhng s phn loi c k hoch 1 cch cn thnc th c s dng lm tng s n nh ca dc. 2 k thut thng thng

lm gim dc l single cut-and-fill operation, and benching. Trong trnghp u, vt liu pha trn ca dc c di chuyn xung gn v tr cn bngca dc. C nh th khi ton dc gim xung, v vt liu nhng chgp phn to ra lc di chuyn b di chuyn i v t di chn dc, y l nilm tng lc chng . Phng php ny khng c p dng nhng dcng. La chn khc l con dc phi c ct thnh nhng bc hay 1 s

benches. The benches, c to ra cng vi vng thot nc b mt ihng nc chy trn, lm gim dc, thm vo , l s la chn tt cho

nhng khu vc l v trt t nhCng trnh h tr dc: retaining walls, c xy t concrete

cribbing(hnh 6.23), st ng t, hoc ct chng (ct b tng, st hay gin g),n s gp phn h tr mt nm ngang ca dc. Chng chng rt tt khi chndi chn dc, lp t li vi si thm nc hoc th, v cung cp nhng lthot nc lm gim s thay i p lc nc khi xy dng trn cc con dc.Mt phng php khc cng lm tng bn ca dc nhng t thng dng hn,

bao gm s chen nhng nng vo cc l m c o xuyn qua lp ri

rc tim tng ti lp dnh kt. Phng php ny c s dng bo v chocon dc p Glen Canyon trn sng Colorado v p Hanson trn sng Green Washington.

Vic ngn nga trt t c th rt t, nhng li ch ca n ca th rtln. Ngi ta c tnh rng t l li ch-chi ph cho vic ngn nga trt tkhong t 10 ti 2000. C mi 1 USD dng cho vic ngn nga trt t, thgi tr tch lu s thay i t 10 ti 2000 USD. Tn tht do khng ngn nga

trt t c minh ho bi 1 v trt t khng khip Utah. ng trt di chuyn ngang quang hm ni vo thng 4/1983, to ra 1 con p t nhin

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cao khong 60m v gy ra l khu dn c ca Thistle (Utah), ng ray nilin the Denver- Rio Grande v phn ln ng cao tc U.S (hnh 6.24). Thithi do trt t v cn l gy c tnh khong 200 triu.

V trt t Thistle c lin quan n s hot ng li ca cc ngtrt c, m n c bit trong nhiu nm v khi xy ra nhng cn ma lnth n hot ng tr li. V vy, n c th c bit khi c 1 cn ma lnkhng khip vo nm 1983 v gy ra thm ho trt t. Thc vy, khi xemxt li lch s ca v trt t, cc nh a cht ng rng v trt t Thistle l c th nhn ra c, c th d on c v c th ngn nga c!Vic phn tch nhng d liu thch hp ng rng v tr ca vng thotnc ngm v vic kim sot nc chy b mt s lm h thp mc nc ngm

trong cc khi trt ngn nga thm ho. Chi ph c vic ngn nga trtt c tnh khong t 300.000 ti 500.000 USD, mt con s rt nh so vithit hi. Bi v t l li ch-chi ph trong vic ngn nga trt t l qu thunli d dng, nn chng ta phi thn trng trong vic nh gi trt t hotng v trt t tim tng hot ng trong nhng khu vc m chc chn thithi c th ngn nga.

4.3. H thng cnh bo trt t(Landslide warning systems)

H thng cnh bo trt t khng th ngn chn trt t nhng n cth cho chng ta thi gian s tn ngi dn v ti sn, dng cc tuyn tuha hoc i cc tuyn giao thng. Gim st l hnh thc cnh bo n ginnht. Nhng khu vc nguy him c th nhn bit bng mt bi nhng thay i

bn ngoi v nhng v l nh trn ng v nhng khu vc khc c th

c ch do s di chuyn nhanh chng. Vic cho ngi gim st nhng taibin c u im l tin cy v linh hot nhng n s tr nn bt li trong iukin thi tit nguy him v nhng vng nguy him. Nhng phng php cnh

bo khc bao gm h thng in, dng c o dc, my d m thanh di tc th nhn c nhng chn ng t nhng tng ri. Nhng ci ging cnc th gim st bng cc du hiu khi con dc cha 1 lng ln nc nguyhim. 1 s vng, quan st l cng hu ch cho vic pht hin khi nolng ma vt qu ngng cho php v nhng trt t ch nng tr nnd dng hn.

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Thay i trt t(Landsildes Correction)

Sau khi qu trnh trt t bt u, cch tt nht ngn chn n chnh l

tn cng n tin trnh bt u trt t. Trong hu ht cc trng hp, nguynnhn gy ra trt t l s gia tng p lc nc v trong nhng trng hp, 1 h thng thot nc ph hp nn c tin hnh. N c th bao gm thotnc b mt phn u ca ng trt gi cho lng nc b mt thmvo bng qu trnh thm hay dng ng thot nc ngm hay ging tho ncv lm gim p lc nc. S thot nc c khuynh hng tng cc lc ngn cntrn cc vt liu ca dc v do lm n nh dc.

Thnh cng ln nht ca h thng thot nc c chng minh thng

qua m t ca Kart Terzaghi. Sau 1 cn bo c cng ln, con dc 30

0

xy ra s di chuyn lp bin cht b phong ho su sc bi thi tit. Mtphng trt cch mt t khong 40 m v khu vc trt t khong 150m brng v 300m b di. Trt t xy ra gn trm nng lng thu in, v vyvic hnh ng ngay lp tc l rt cn thit. Nhng nghin cu thc t chngminh rng nu mc nc c th h xung khong 5m, th sau cc lc chng s tng ln lm n nh ng trt. H thng thot nc khi honthnh s gm nhng con knh mng v nhng ci h khoan su n vng cth cha nc ca . Sau khi c h thng thot nc, s di chuyn s dng liv ngay c khi n ma ma tip theo mang theo 1 lng ma k lc th cngkhng c s di chuyn no quan st c.

5. L tuyt(Snow avalanche)

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Hin tng l tuyt l hin tng tuyt di chuyn theo dc 1 cch nhanhchng. Nu c 1 lng ln , t, cy kt hp cht ch vi nhau, n ging nh1 trn l tuyt cng vi cc mnh v. Cng nh trt t, l tuyt cng c cc

lc di chuyn v cc lc ngn cn trn con dc.

C khong 10000 v l tuyt xy ra hng nm vng ni pha ty ncM, v c khong 1% trong s gy tn tht v ngi v ti sn, git chttrung bnh 7 ngi v thit hi 300.000 USD. S ngi mt tch ngy cng tngcng nh s ngi thm him nhng vng ni trong sut ma ng.

L tuyt c th xy ra vo ma tuyt kh hay tuyt t, v c 2 loi ltuyt chnh.

+ Loi th 1: l tuyt do tuyt khng kt dnh (loose-snow avanlaches),xut hin khi tuyt ri rc v c khuynh hng gy thit hi nh v shallowfailure.

+ Loi th 2: slab avalanches, tuyt l theo tng khi c din tch khongt 100 n 10000m3 v dy 0.1 n 10m. Large slab avalanches c sc tn

ph v nguy him nht, n gii phng 1 nng lng rt ln bng cch huy nghng triu tn bng tuyt v ri xung theo dc vi tc 5-30m/s (18-100km/h) v c th hn na. Sc p nm ngang t nhng v l tuyt c khuynhhng tng t 5-50 tn/m2, nhng cng c trng hp tng n 100 tn/m2. hiu r v ln ca sc p, chng ta c th tng tng nh sau vi sc pch khong 3 tn/m2 l nh sp c khung ngi nh v 100 tn/m2 c thlm sp c nhng cu trc b tng c gia c chc chn.

L tuyt c th bt u khi 1 khi lng tuyt v bng trt xung dcdo c 1 lng ln tuyt mi ri xung, hay khi c nhng thay i bn trongkhi tuyt to ra ra nhng khi tuyt yu ( bn vt ct yu) dc theo nhngni m tai ho s xut hin. Khi tnh trng tr nn bt n, ch cn khi lngca 1 ngi trt tuyt thi cng c th gy ra l tuyt. Mt khi bt u, ltuyt c khuynh hng theo nhng ng c sn (hnh 6.25). Tuy nhin l

tuyt cng c th xy ra trn nhng con ng thng thong trn con dc.Nhng con ng tuyt i thng c nhiu nhnh trn nh v tp trung li khicng xung dc. Do , trong 1 vi trng hp l tuyt s i qua nhng conng chnh trong 1 khong thi gian ngn, nh s tch t tuyt cc nhnhtrn cao ri t ngt v ra. Tai ho c th xy ra khi nhng cng nhn ang dnmnh v hay rc t trn l tuyt u tin v khng phng trn l tuyt th 2s n sau .

Ri ro l tuyt c th gim c bng cch trnh xa nhng khu vc nguy

him, tng n nh ca dc bng cch t thuc n dn sch tuyt, xydng nhng cu trc lm chuyn hng hay lm chm s l tuyt, v trng

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li rng trn nhng con ng tuyt l v nhng v l tuyt ln him khi xy ratrn nhng dc c khu t rng rm rp.

Tuyt l ch yu e da n nhng ngi trt tuyt trn cao, ni dc

ni ng, nhng n cng e da n cc khu resort trn ni, lng, ng xe la,ng cao tc, v thm ch s khu vc ca thnh ph. V d nh th Alaska,Juneau, xy 1 trn l tuyt rt ln. Trong 100 nm qua, nhng v l tuyt lntrn Juneau 6 ln gii phng 1 s lng bng v tuyt khng l, v 1 s vl tuyt i ra ti tn bin. Khng c 1 thit hi do l tuyt xy ra sut thp kqua, tuy nhin, c 1 khu vc pht trin hon chnh c xy dng doc theodc tuyt. V nu v l tuyt ln na xy ra, n s tn ph khong 30 ngi nh,trng hc, khch sn v thm ch qut n cng ni m hng trm tu angneo li. Ngi ta c lng rng 1 ngi nh khu vc dc tuyt c tui th

khong 40 nm, th chc chn 96% kh nng s b ph hu bi tuyt l, cngvi nhng ngi sng , nhng a s h u h hng vi l tuyt.

6. St ln(Subsidence)

S tng tc gia iu kin a cht v hot ng ca con ngi to ranhiu nhn t c lin quan mt s lng ln tai nn do st ln. St ln l schm xung 1 cch nhanh chng ca nhng vt liu tri t. Hu ht st ln gy

ra l do s rt nhng cht lu bn di hay bi s sp b mt v lp t gnb mt v trn nhng khong trng di mt t. S rt i ca du cng vica kh v nc, ca nc ngm v ca hn hp hi nc v nc to ra nnglng a nhit v gy ra st ln. Trong tt c trng hp, ngun gc chungto ra st ln u ging nhau. Nhng lu cht trong vt liu tri t bn di

b mt tri t c 1 p lc lu cht ln v c khuynh hng gi li lp vt liupha trn. l l do ti sao phn di y h bi c v nh hn: sc nito ra bi cht lng c khuynh hng nng ln. Nu s chng hay sc nic to ra t vt liu tri b loi b t bng cch bm lu cht ra, lc y s

i v st ln b mt s xy ra.

Cch thc hot ng thc t ca st ln c bao gm s lin kt tng htring l vt liu tri t hay lm tng lc hp dn gia cc ht vi nhau bi plc lu cht thp. S st lt nhng vng c du ni chung bao gm s suygim p lc lu cht, t 2.8 x 107 Pa su nht nh (hng ngn m) trn 1din tch tng i nh, t hn 150 km2. Hay ni cch khc, st ln l kt qu ts rt xung ca nc ngm c lin quan n s suy gim tng i tho p lclu cht, thng thp hn 1.4 x 106 Pa, su tng i nng (thp hn

600m), trn 1 din tch rng, thnh thong l hng trm km2

.

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Hng trm km2 trung tm thung lng California b ln xung lkt qu ca vic bm qu mc nc ngm. Hn 5000 km2 Los BanosKettleman b ln 0.3m v trong khu vc ny, 1 on 113 km b ln trung

bnh hn 3m, ln nht khong 9m. Khi b khai thc, p lc lu cht c gim

v nhng ht dnh st vo nhau (hnh 6.27), v tc ng ti b mt gy ra stln. V d tng t ca st ln gy ra bi vic bm qu mc c ghi nhn gnPhoenix, Arizona: Las Vegas, st ln c th gy ra nhng khe nt di (hngtrm mt) v su (vi mt) trn b mt hnh thnh trm tch.

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6.1.H sp

(Sinkholes)

S l t cng c gy ra bi s di chuyn ca cc vt liu bn dimt t theo cc qu trnh t nhin. Nhng l hng thng hnh thnh bn trongnhng vt liu ho tan c nh vi v khong v khi chu tc ng canhng khi nng bn trn c th lm n sp . Kt qu l hnh thnh nnsinkhole, khu vc xung quanh ca ni t l gy ra bi s sp ca cc lhng bn di mt t. Vi l hng c th rng hn 30m v su hn 15m.

Trong t nhin mt vi l hng c th b che lp bi cc vt liu v vn,do s thiu trch nhim trong vic kho st, chng ta c th xy dng cc cng

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trnh bn trn v chu tc dng ca hiu ng chng cht chng c th sp v li nhung hu qu to ln. in hnh l trng hp ca l hng Allentown.

Mt ni gn Tampa, Florida bt ng sp lm bin mt mt phn vn

cam vo nm 1973. L hng ln nht c tn Decmeber Giant rng 120m v su45m hnh thnh nm 1972 gn Motevallo, Alabama, rng hn t ngt do cctng t mt b sp bi cc lp vi bn di b khot thng. L hngcng c gy ra bi s mt mt ca tng t st, nhng ngi dn gn khu vcny k rng h nghe c mt ting n ln gy ra bi s sp ca cyci cng vi cc chn ng lm nh h lung lay.

L hng li nhiu hu qu to ln n cc cng trnh nh: ng caotc, nh ca, cc cu trc v c s vt cht cc dao ng t nhin v nhn to

trong mc nc ngm c th l mt b phn c gii gn gng. Mc nc ngmcao l iu kin cn thit cho vic m rng cc hang ln, v s ni ca ncgip cho vic chng nhng cht qu nng. Mc nc h xung lm l ra cctr ni v cc c s vt cht b sp . iu ny c minh chng vo ngy 8thng 5 nm 1981, cng vin nc ma ng, Florida, khi mt l hng ln btu pht trin, n rng ln nhanh chng trong vng 3 ngy v nut chng mt

phn h bi, nhiu t v mt ngi nh. Tn tht do l hng gy ra vt hn 2triu USD. L hng hnh thnh mi nm gn trung tm Florida khi mc ncngm xung thp nht. L hng cng vin ma ng c hnh thnh trong

sut ma hn kh, khi ma nc ngm thp k lc. v tr l khng th lngtrc v trong sut ma hn xut hin rt nhiu l hng nh xung quanhCng vin nc ma ng.

Case history

Lehigh Valley, Pennsylvania.Ngy 23 thng 5 nm 1986, mt l ln t ln c hnh thnh

thung lng Lehigh gn Allentown min dng Pennsylvania. Trong khong thigian khong vi pht, v sp li mt h dt su 14m v c ng knhti 30m, gy thit hi cho mt con ng, nhiu bi u xe, va h, nhiu hthng dn nc cng vi nhiu vt cht thit thc khc. 17 ngi dn khng ththot ra c khi l hng xut hin, cng tc n nh v sa cha ln n 1,5triu USD. Phn pha Bc thung lng c nn tng l khong dip thch, tri li vi bao gm mt phn pha Nam. Thung lng hnh thnh bi s nng ca ct kt pha bc cn pha nam l grant tin k camri v g nai.

Nhng hnh nh t nhng nm 1940 n 1969 l nhng minh chng chocc l hng sinh ra do l t trong lch s. nm 1940, l hng c v l mt

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ci h vi ng knh khong 65m. nm 1958, ci h cn nc to thnh mth rng v c trng cy ln v khu vc xung quanh c trng trt nhnhng cnh ng. bc nh nm 1960 cho thy nhng t liu v vic con ngiang s dng h rng cho vic ti nha ng v nhiu vt liu v ri khc.

n nm 1969 th khng cn bc nhng t liu din t v b mt ca h rng,n dng nh b lp kn v xut hin nhng cch ng bn trn n.

Thm ch l hng cn c lp y bi rc v cc loi vt liu v vnkhc, n vn nhn c nc thi t cc cng trnh v ngy cng tng ln biqu trnh th ho. Ngun nc bao gm ma, bo, cc nc thi t cc khuchung c v thnh th.theo mt vi phng on th vic nc r r cng gp

phn lm tng lng nc nhim khu vc c l hng, ngoi ra, do s th

ho lm nhu cu nc ngm v ti nguyn tng cao dn n s suy gim camc nc. cc cht thi th tc ng lm v vn hoc di di cc vt liu baoph b mt ca phn t c kh nng sp ln trong khi mc nc ngm gim lnguyn nhn lm gim kh nng che y chng ca cc tng bn ci dnn hnh thnh l hng trn t nh trng hp l hng cng vin ma ng.

6.2.Trm tch mui v s st ln

(Salt deposits and subsidence)

Nhiu v l t cng c lin quan n cc m mui, du v cc m khc.Mui thng c khai thc bng phng php: nc c tim vo ho lntrong m mui, sau cc mui lng xung khi nc qu bo ho, cui cngmui c bm ra ngoi. Mui c ly ra li mt l hng trong gc lmn yu i v kh lp y li c. iu ny lai hng hu qu to ln.

Mt v l t gn Detroit nm 1970, l hng rng 120m v su 90m. Mtni khc Vigirna,l t lm mt tch hai h gia nh. Theo li k ca ngidn a phng th mt gia nh dn i trc khi s vic xy ra khi nm mthy ni sp xung. Khai thc mui bng nhng phng php khc cng c thgy ra cc v l dt ( trng hp Lake Peigneur, Louisiana )

Nhng l hng ln lin quan n vic xy dng nn mng c th xy rabn trong m mui. V d: thng 5 nm 1980, vng ng nam Kermit, Texas,mt l hng rng khong 110m v su 34m hnh thnh sau 48 gi c ttn l Winksink. Wink Sink v cc biu hin tng t l kt qu ca cc qutrnh t nhin, khi vt nt gy trong nn mng cn bn ca mui b nc ngmho tan t n mt kch thc nht nh no s gy ra l t v d li nhng

l hng. V iu ny l mt qu trnh t nhin v nhng vt t gy lun tn ti,

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nu khng c cnh bo sm, trong tng lai s lan rng n nhiu khu vckhc.

Case historyLake Peigneur, Louisiana

Mt in hnh cho vic sp ln t c lin quan n m mui c nyra vo ngy 21 thng 11 nm 1980 pha Nam Louisana, khi mc nc h

Peigneur qu thp, trung bnh ch khong 1 mt do s rt tho nc bi s sp ca m mui bn di, nguyn nhn l do cc hot dng ca cc m khoan

c mt l bn trong ca m mui.H Prigneur tho nc nhanh n mc 10 x lan, 1 tu ko, 1 x lan my

khoan du b bin mt trong xoy nc trn m. May mn l 50 cong nhn mdu v 7 ngi tren tu thot him. V sp t xy ra phm vi rt rnghn 25ha, bao gm mt khu vn sinh thi, nhiu nh knh vi mi cn tr gin 500.000USD. Khu vn b ph v bi bi nhng khe nt ln c nhng bcdc g gh v t bn trn trt xung nhng g mi to thnh ca h. sau, h peigneur bt ng c lp y nc tr li do nhn c nc t conknh ni lin vi Gulf, Mexic. 9 x lan b chm lc trc bt cht li ni trnb mt h 2 ngy sau . Vo nm 1983, thit hi ca h Peigneur c bp bi 30 triu USD t cc cng ty khai thc du c lin quan, ng thinhng ngi s h vn sinh hc v cc h c nhn cng c bi thng13 triu USD.

Vic nc tng gim h Peigneur t ra nhiu cu hi ln c linquan cu trc v tnh trng khng b phn chia ca cc m mui. ng thi ln

k hoch v c cc chin lc v vic d tr du th v cng tc khai thc cc m khong sn.

6.3.Khai thc than v s st ln

(Coal mining and subsidence)

Trong cc m du la, cc thao tc ly du trn b mt v ang gy ranhiu vn v st l t.

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vng Pinsburgh, ni c m du hot ng hn mt th k nay, trongnhng nm trc cc cng ty ly du trc tip khng c trch nhim bo v li nhiu h tn trn b mt m du. kt qu tht nghim trng khi du cdn n t canh tc, mt khc th ho nhanh cng li nhng hu qu do

vic cn du lm nguyn liu. Nu tt c du c ly ln, nguy c l t scao. Nu ch dng 50% s du trong m th du ch thng cung cp cho cc cquan c thm quyn.

Nhiu trng hp c bo co li, hu qu do st l t trn cc m du ngng hot ng hn 50 nm. in hnh, thng 1 nm 1973, vo mt buisng vi ngi dn Wales, Britain trong khi ang li xe th mt on ng

bt ng sp xung to thnh mt l su khong 10m, xe ca h sp d ngay bvctrong khi h c gng thot ra an ton. V sp xy ra ngay trn mt phnca m du, n hu hoi nhiu dch v thit yu. Nhiu trng hp tng t

xy ra trong qu kh v c th cn tip din trong tng lai.

6.4. Nhn thc v s nguy him ca l t

(Perception of the landslide hazard)

Nhng phn ng thng thng ca nhng ngi dn nam Californiakhi ni v s l t l: n c th xy ra nhng sn i khc ch khng baogi l dy, v v th ch vi mt tm bn v s lan rng ca l t kh c

th ngn nhng ngi dn ny di chuyn ln nhng vng nguy him tim tng,v v sau nhng ngi chim gi sn i ny cng khng nhn thc c snguy him ca l t v khng lng nghe cc thng tin k thut, d rng trongqu kh c nhng minh chng v s l dt r rng. V khi iu khng mayxy ra, n thng li hu qu thm khc, nh trng hp, v st l t khuvc Laguna Hill, California, lm tn hi ln v ti sn v nh ca, gy ra vn ln v ch ca nhiu ngi. Tuy vy, trong lc y, nhiu ngi dn RockyMountain, Appalachian Mountain, v nhiu ni khc vn tip tuc xy dng nh

ca nhng ni m l t c th xy ra trong tng lai.

7. Tng kt

(Summary)

L t v nhng hin tng lin quan gy ra nhng hu qu to ln vgim cht lng cuc sng. D l cc hin tng ca t nhin nhng n c thtng hoc gim l do cc hot ng ca con ngi.

Hnh dng ph bin nht ca tri t l nghing v lun lun bin chuyn,s tin trin ca h thng dt trong cc vt liu lun chuyn ng theo chiu

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nghing to nn cc hn tng t mt cn st l rt nh cho n mt trn botuyt vang nh sm. dc c to nn t nhiu yu t, bao gm: nh, b ntthong, vt liu rn, b ri v cc vt liu ra tri. S hin din ca cc yu tring bit v dc cu dt thng c lin quan n cc nh hng ca kh hu

v dng . Ngoi ra dc cn chu nh hng bi s trt, chy trn v rica cc vt liu trn tri t. L t l mt s kt hp ht sc phc tp ca trtv chy trn.

L t sinh ra c xc nh bi s tng tc ca nhiu yu t: dng vtliu ca tri t trn sn dc, kh hu, thc vt, cy ci, nc, nh dc vthi gian. Nguyn nhn ca hu ht cc trn l t thng c xc dnh bngcch kho st mi quan h gia lc lm cc vt liu tri t trt v lc ngncn s di chuyn . Hu ht cc lc y l do khi lng ca vt liu tri theo nghing cn lc cn l do s ct xn chiu di ca cc vt liu tri. an

ton ca mt con dc c tnh bng cng thc l t s gia lc cn v lc tri.Nu t s ln hn 1, iu c c ngha l mt con dc n nh. Hnh dng vloi hoc t trn con dc cng c nh hng dn tn s sut hin v loi lt.

Nc c mt vai tr quan trng trong s xut hin ca cc trn l t.Nc trong sng h hay i dng lm xi mn chn cc khu vc dc lm tnglc tri. Khi nc c dn vo lm tang khi lng ca cc vt liu tri trndc, p sut nc cng tng ln, dng thi lm gim lc cn ca cc vt liutrn dc. iu ny c th gy ra nhiu trn l t v tht s hu ht cc trn lt l do s tng cao t ngt, bt thng ca p sut nc.

Hot ng ca con ngi cng c nh hng n ln v tn sut cal t. V th: nhng ni l t xy ra c lp vi hot ng con ngi, chngta cn tm hiu k lng v chng trnh s pht trin xy dng nhng khuvc nguy him ng thi cung cp nhng bin php o lng v bo v. Cn nhng ni s lng v tnh nghim trng ca l t tng th chng ta cn hccch lm th no gim nhng trn l t n mc ti thiu.

gim nguy c l t n mc ti thiu chng ta cn thit lp cc binphp nhn dng, phng chng v tin trnh lm vic chnh xc. Chng ta nn ck thut iu hnh v nh v c cc vng c nguy c l t tim tng. Ngn

cn nhng trn l dt ln trong t nhin l rt kh nhng nhng phng php kthut tin tin c th gim rt nhiu nhng nguy hi do l t gy ra khi chngta khng th trnh c. Nhng cng ngh k thut cho vic phng chng l t

bao gm: kim sot s tho nc ca cng rnh, sng, knh mng, lm phnghc l nhng ni dc c nguy c l t, thit k cc cng trnh phng h nhcc dy tng chn t l. Vic phng chng l t phi tin hnh ngn nga nhng ni pht sinh, ic ny ng ngha chng ta phi lm gim p sut nctrong nhng khu vc ( nh ao , sng, knh, mng ) c dc v c nguy cl t cao.

Ngy nay, nhng ni c tuyt ph, c dc thng xy ra mt hintng nguy him rt nghim trng l tuyt l. c bit nhng khu vc ni

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cao, l tuyt cp i sinh mng ca rt nhiu ngi trong nhng chuyn imo him vo ma ng.

Hin tng cc cht lng nh du, nc c ly ra khi cc qung mnh du nc v cc loi khong sn khc cng l nguyn nhn gy nn hin

tng cc h dt ln lan rng trn mt t. Trong nhng trng hp cht lngc rt ra, s ln xung sinh ra l do p sut cht lng gim chng chu vis che y cc vt liu tri t. trong trng hp, cc vt liu rn b di di, ccl hng c hnh thnh trong cc vt liu ho tan nh vi lc phn

bn di khng th chng chu c na, v s sp ca cc vt liu che ybn trn to thnh cc sinkhole.

Nhn thc ca ngi dn v nguy c l t cn hn ch, d rng h cnhng kinh nghim v l t nhng vn cn rt thp, c bit h khng tipnhn v b nh hng ca cc thng tin k thut hin i, nht l nhng ngi

dn sng vng i ni.

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Chuong 6 (bao cao)