NhQ'ng ILFU y dac biet ve sy giam do ben (kha nang chju tai cua dat nen) theo thai gian cua tram tich loai set ngam nu'ac do
"tLF bien" gay ra du'ai mong cong trinh va hieu LPng ma sat am
Mai Di Tam
ABSTRACT
One of the biggest difficulties that experts in irrigation, transportation, construction, airports, docks, dams, urban infrastructure, underground work, etc. takes more time and effort to research and find appropriate solutions for construction in soft soil of water-saturated clay deposit in the pasty state, elastic dough, half hard, water-saturated or peat soil - the soft soils distribute in areas along the urban districts of Ho Chi Minh city and Mekong delta, in the South of Vietnam. The above consistent solutions focus through a concatenation including geo-technical surveys, experiments at the construction sites and in the laboratory, foundation design, selecting technology, construction equipment, set settlement cycle for projects and design exploiting rules, management of construction works in these soft soil having settlement prolonged over time or deformed over the allowed limits, causing difficult to the use of the construction works after completion and exploitation.
In this scientific article, the author focuses on monograph on the introduction of research results on the increasing - reducing the strength of the ground of water-saturated clay deposit over time due to the effect
"creep" or is subject to the influence of geo-mechanical effects "negative skin friction" more adverse effects when reinforced concrete piling, tend to pull down the stake because the consolidation process of the foundation soil is not over. These are issues that need to be noted especially in the construction in soft soil conditions at present and in the future.
TS Mai Di Tam
Cong ty CO phan Dia Oc 10 Tong Cong ty Dia 6c Sai Gon
1 . f nghia cua van de
Khi xay dung nhUng cong trinh tren cac tram tich set d trang thai nhao, deo nhao, nUa eUng ngam nudc hay nen dat than bun - thuoc loai dat yeu phan bo d khu vUe ven eac quan npi dd thanh phd Hd Chi IVIinh va vung chau tho sdng IVle Kdng, cue Nam nude ta, mdt trong nhUng khd khan Idn nhat ma cae chuyen gia xay dung, giao thdng, thuy Ipi, san bay, cau e^ng...
phai mat nhieu cdng sUc de nghien eUu tim kiem giai phap phu hop cho qua trinh xay dung ndi tren qua tat ca cac xau chuoi:
khao sat dja ky thuat, thi nghiem hien trudng va trong phdng, thiet ke phuang an mdng, chon lua cdng nghe thiet bj thi cong, thiet lap ehu trinh quan trae lun cho cac cdng trinh va thiet ke bd quy tac khai thae, quan ly cdng trinh xay dung bdi vi cac cdng trinh xay dung tren nhUng nen dat yeu nay thudng cd dp lun keo dai theo thdi gian hole la bien dang qua gidi han cho phep gay khd khan cho viec sCrdung cdng trinh sau khi da hoan thanh. Qua trinh giam yeu dp ben - (giam C^ va cp^) cua dat nen thude loai tram tich set bao hda nudc theo thdi gian do tU bien gay ra hoac la ehju anh hudng eua hieu Ung ma sat am tae dung phu tren be cpc cd xu hudng keo cpc chuyen vj di xudng do dat nen ma cpc dang ngam vao edn dang trpng qua trinh cd ket ehua ket thiic ciJng la nhUng van de can phai dupc luu y dac biet khi xay dung cac cdng trinh trong mdi trudng dat yeu cho hdm qua va ngay mai.
TU eac ket qua nghien cUu da dan d cac bai bao trUdc, chung ta cd the dua ra nhUng nhan xet quan trpng ve dat yeu d vung chau thd sdng Me Kdng nhu sau:
1/ La tram tieh tre (DeTU) HALOXEN cd tudi tU 5.000 nam 12.000 nam.
2/ Oat yeu nay ed khoang chat thU sinh chiem ham luong Idn la Montmorilonite. Khoang chat nay ed tinh tU bien.
3/ Oat yeu nay cd do pH thap: pH < 2,5 -;- 3,5. Op pH nay ehdng ddng tu dung dich bentonite tdt nhung rat xau cho be tdng do trong nude cd dp pH nay vi dp pH nay gay ra truong nd manh be tdng tao ra nUt be tdng.
4/ Oat yeu nay cd it cac cation AL *** va H* ddng thdi cd nhieu Na^COj va CaCI^. Viec cd it va cd nhieu nay la khdng thuan Ipi eho dieu kien lam viec cua dat yeu.
5/ Oat yeu nay hau het d cac trang thai:
5.1. Trang thai nhao, deo nhao.
5.2. C < 0,06 KG/cm^ cp < 6".
5.3. E < 6 K G / c m ^
6 8 ^('if'f^n^* 08.2012
6/ 0at yeu nay c6 chieu day tCr 10m den 40m, CO noi day tdi hang tram met.
2. Qua trinh gi^m yeu dp ben (khd nang chju tSi) theo thdi gian cua tram tich set bao hoa nUcTc dudi nen mong cong trinh
Khci nang chiu t^i cua dat nen loai tram tich set 6 trang thai nhao, ngam nUdc bj giam yeu theo thdi gian khi xay ra hieu urng tCr bien bdi U'ng suat clit (T) gay pha hoai nen dat du6i mong cong trinh vdi mpt muTc dp nhat djnh doi vdi dp ben ket cau giufa cac hat ran cua dat (su b^ gay cac lien ket cu'ng (C^). Hieu iTng tCr bien cua nen dat dudi cong trinh cd the tat dan ma cung cd the tiep tue phat then theo thdi gian, dieu dd phu thupc vao mijrc dp pha hoai nhieu hay it cac mdi lien ket cau true trong tram tich set deo nhao ngam nUdc, ca hai qua trinh nay deu lam giam yeu dp ben cua dat nen theo thdi gian khai thae sir dung cac cong trinh xay diing tren nen cac tram tich loai set, khi tien hanh xay dang cac cdng trinh trong mdi tradng dat set yeu bao hda nadc, phSi luu y dac biet den dieu Xciyrasagi^myeu kha nangchiu taicuadat nen theo thdi gian la:
<T<6tg(p + C + I
C : lac dinh toan phan,
C^: Lac dinh ket cau cufng I : Lac dinh nhdt Trong d d :
T: la Lfng suat cat tai mat bat loi nhat trong dat nen.
T|^^:gidi han t a bien.
6: t^i trpng gay ra do lac ngoai.
(p ; gde npi ma sat cua dat nen.
C : la lac dinh toan phan cua dat nen, bao gom 2 thanh phan.
C^: lac dinh ket cau cUng.
I : lac dinh nhdt.
TCr thae nghiem xay dang cdng trinh tren
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ai r^ng c u a mong cOng
Hinh 1. So do xac dinti troong hop nguy hiem nhat cCia cong trinh khi dong thoi CO ca iJng suat phap (6) va ijfng suat tiep (T) ciing dong thoi tac dung len nen mong daiJi cong trinh
dat yeu, dac biet la cae tram tieh loai set bao hoa nudc d trang thai mem nhao dudi mdng cac cdng trinh vdi su giam yeu dp ben eua dat nen theo thdi gian do hien tuong tU bien gay ra, tae dpng anh hudng rat nghiem trpng vdi dieu kien lam viec cua eac cdng trinh la do lam tang bien dang cua cdng trinh, dan den lam mat on dinh cae cdng trinh xay dung tren nen dat tram tich set va lam giam dp ben chiu tai cua dat nen. Dac biet nhat la doi vdi cac cdng trinh nhu: tudng chan dat, de, dap tran, cau cang... nhUng cdng trinh loai nay chju luc ngang (gid, ddng dat, sdng than, nd min khai thae mo...) la ehu yeu, lam tang dp chuyen vi ngang va lam lat tudng ehan dat... lam tang ap lUc dat tac dpng, ddng thdi lam giam ap luc dat bi ddng tae dung len cdng trinh gay mat on djnh va bien dang qua gidi han eho phep ddi vdi eac cdng trinh xay dung ben tren.
Oac biet la khi cdng trinh eung mdt lue chiu dong thdi tac ddng tU bien cua Ung suat tiep (T) va Ung suat phap (6), dd la trudng hpp nguy hiem nhat eho cdng trinh.
3. Van de hieu ufng ma sat am gay ^nh hudng len cac cong trinh xay dung tren nhufng nen dat yeu thuoc tram tich set ngam nUdfc - cac giai phap thUc tien han chetacdongcuaMA SAT AM
3.7. Nhgn dien hieu ijtng MA SATAM tac dgng len cgc va mong cgc ditdi nen dot cong trinh
Hien tupng ma sat am xuat hien khi coc ddng qua khdi dat mdi dap ma qua trinh cd ket trong cac Idp chiu nen nam dudi mdi bat dau thi trpng lupng eua khdi dap xung quanh cum cpc cung ed the truyen vao cpc. Tai trpng tae ddng len mdt cpc khi dd them mpt luong xap xi (A y H/n), trong dd: A la dien tieh cua dat ben trong mat cat ngang gidi han bdi ehu vi ciia cum cpc y va H la trpng lupng don vi va be day cua khdi dap va (n) la sd cpc trong cum cpc.
Va khi chuyen vj thang dUng (lun) cua dat Idn hon chuyen vi eua mdng thi xuat hien ma sat am. Tri sd eua lue ma sat am cd lien quan vdi sued ket cua dat, phy thude true tiep vao Ung suat ed hieu eua dat xunh quanh cpc. NhU vay luc ma sat am phat trien theo thdi gian va cd trj sd Idn nhat trpng qua trinh khai thae IVIA SAT AM la mdt hien tuong phUc tap vi nd phu thude vao nhieu thdng sd nhu dp liin eiia dat sau khi ddng coc, sU co ngan dan hdi ciia cpc, quy
luat phan bo ciia ma sat am len cpc... vi vay de dpn gian khi tinh toan lay tri sd ma sat am Idn nhat luc ket thuc cd ket.
Ma sat am la luc keo cpc xudng bdi dat nen xunh quanh cpc dang trong qua trinh cdn dien ra cd ket.
3.2. Oieu kien xay ra MA SATAM - Cpc ngam trong dat rdi va nam tren la dat lap loai set dang trong qua trinh ed ket. Qua trinh cd ket do tao ra lue keo cpc xudng gpi la MA SAT A M .
- Dpe than cpc tdn tai lop dat set yeu chju nen ma tren nd la dat lap. Oat lap lam eho set yeu lun xudng va qua trinh co ket nay da tao ra lue MA SAT A M de keo cpc xudng.
- Qua trinh bam hut ngam lam ha thap mue nude, lam tang ap luc hUu hieu cot dat va khi cpc ngam qua Idp dat loai set yeu thi hien tupng cd ket xay ra tao thanh ma sat am keo cpc xudng.
3.3. Binh lugng MA SAT AM trong tinh todn thiet kemong cgc va tinh todn dubdo do liin cdc cong trinh
Odi khi ma sat am dat den gan gia tri Idn den mUe lam cho cpc cd the bi pha hdng nen viec tinh toan du bao ma sat am trong thiet ke la rat can thiet.
0 6 lun keo theo don vj (MA SAT AM don vj) khdng the vUpt do ben ehdng cat ciia dat nen, thudng dupc coi bang nUa (1/2) dp ben chong nen khdng nd hdng dien tich tren dd luc keo theo tac dpng la be mat thang dUng bao quanh toan nhdm cpc hoac toan mdng cpc.
Mac dii khdng the danh gia vdi dd ehinh xac cao tai trpng phy them len cpc do bj keo theo chieu lun xudng, nhung cd the tinh phae qua de danh gia tai trpng phy them cd kha nang gay ra hau qua nghiem trpng hay khdng ma cd bien phap xU ly thich hpp, nhieu vi du ve dp lun qua Idn eua cong trinh la do khong xet tdi MA SAT AM mat ngoai gay ra dp lun phu them.
Anh hudng ciia MA SAT A M Gay:-Liin lech 1 ben - NUt gay ben dd.
NhUng kha nang ed the xay ra tai cho tiep giap giUa doan dudng vao eau vdi md cau dat tren be cpc. Dudi tac dyng ciia tai trpng nen dap, nen dat yeu se bj lun va gay ra eac lUc ma sat am tae dung ien cpc, gia trj cua cac luc ma sat am nay cd khi cdn lOn hon nhieu kha nang chju tai ciia cpc.
Mat khac bien dang eua nen dat yeu cdn tae dung len cpc eac lUc day ngang kha Idn. Vi vay, khi thiet ke ket cau mdng cpc can phai xet den day dii cac luc nay, neu khdng cd the gay ra bien dang khdng cho phep cho cpc.
3.4. Tai trgng khi co tinh den hien tugng MA SAT AM
Tai trpng an toan len nhdm cpc ddng qua cac Idp de nen tdi tang dat chju lue cUng chae bang s6 cpc trong nhdm nhan vdi tai trpng an toan len moi cpc. Khdng can chiet giam vi
08.2012 ^EJniEilSl 6 9
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Hinh 2. Cac bien dang ciJa nha tren cac cpc treo khi xay di/ng gan no 1 nha tren mong be
1. Nha tren cac coc; 2. Nha tren mong be; 3. fiat yeu; 4. Bat dtfoc nen chat lai
R6i lai m^l duang bu hi
Ldp ddt cilng (dd)
Hinh 3. Cac van de phat sinh tai cho tiep giap giiia doan di/dng vao cau va mo cau dat tren be coc.
oat ASP Y , HJ
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Hinh 4. Dat yeu bi chuyen vi tao nen life day ngang tac dong vao than cpc.(Tchabotarioff - fond -72)
khoang each giaa cac cpc gan nhau.
Trong thae te nen gia khoang each bang 2,5 lan dadng kinh eua cpc neu thay MA SAT AM mat ngoai cd the phat trien do eo ket cua Idp tram tich yeu, tai trong phu them len cpc do MA SAT A M mat ngoai can duoc xet den trong tinh toan thiet ke cae loai mdng cpc.
Tri so cua ma sat am luc ket thue eo ket daoc cpng vao tinh tai tac dung len mdng ma khdng cpng vao hoat tai.
3.5. Cdc gidi phdp thi/c tedehqn che tdc dung cua hieu vfng MA SATAM ien cpc va mong cpc dUdi
mong cac cong trinh xay dung tren ddt yeu
x a ly vi tri noi tiep giCa nen dap va cdng trinh cung la mot trong nhdng van de khd giai quyet. Ndi chung cac cdng trinh cau, cong xay dang tren nen dat yeu deu dat tren be cpc vi vay can dac biet chu y khi dap cac doan dUdng vao eau, de tranh nhdng ha hdng cd the xay ra do tac dung eua nen dap len tren epe.
Cac mdng sau trong nen dat yeu dang co ket thadng chju mpt lac cd the rat Idn do MA SAT A M
gay ra. Neu khdng xet tdi day du hoac la bd qua hien tUpng nay thi ve lau dai (theo thdi gian) ket cau cd the bj hu hdng do bj lun qua mde (phai tinh them dp lun do MA SAT A M gay ra khi xay dang cdng trinh tren dat yeu - cung nhu thiet ke chieu dai cpc va tiet dien cpc).
Giam lac ma sat am:
MA SAT AM ty le thuan vdi K tg 6, vi vay can tim bien phap giam trj so cua dai lapng nay bang cac giai phap sau day:
Quet nhaa bitum (chieu day 5 - 7mm) la tam vao vai thd quan quanh than cpc mdt phaong phap cd hieu qua, cd the giam K tg 6 xuong 15 lan.
Dung nhdm cpc cung la mdt bien phap giam ma sat am vi cac cpc d giaa chiu ma sat am it hon eac epe d chung quanh.
- M A S A T A M c h i p h a t trien khi dat bi lun, vl vay ap dung bien phap dap bSng vat lieu nhe hoac bien phap gia tai tam thdi {cho nen dat dadi cdng trinh lun trade) hoac thay the khoi dat dap bang be tdng, tro si... de nen dadng khdng lun hoac ket thue lun trade khi ddng epe eung la cac each giam ma sat am rat cd hieu qua.
Phdng ngda ma sat am:
Cae coc thang ddng dadi tadng hay mo eau cd the ngan can mpt phan chuyen vj ngang, d dang nay vi dp ben chong nen cua cpc taong ddi nhd va chi ed the phat then khi phan dadi
cua cpc ngam chae vao tang dat tot ehju lac cd sde khang cao (tang dat cdng dadi mui cpc) - eac cpc nghieng ed the ed hieu qua hon. Khi tadng bat dau chuyen vi ve phia trade, tai trpng len cac epe nghieng tang kha Idn va mui epe se xuyen them vao dat ngay vdi mot dd xuyen nhd tai mui cpc cung sinh ra chuyen vi hdng Idn d day eong trinh. Vi the nen cae cpc nghieng khdng vdi tdi tang dat chat d dadi mdng cd sde chiu tai tot thi cung it hoac khdng cd gia tri. Vi vay phai thiet ke coc xien du chieu dai va lua chpn bua ddng cpc thieh hop dam bcio ddng cac cpc xien sao cho mui coe xuyen vao tang dat cdng it nhat 0,5m.
Nhieu mo cau van xay ra chuyen vj tang dan, mac du da ed cac epe nghieng. Neu dat nen qua yeu khdng chong d d noi trpng laong eua khoi dap, de khoi sinh ra chuyen vi ngang, tang dan, thi can bdc bd vat lieu nen khdng thich hop hoac la sd dung dadng tiep can den eong trinh bSng eau can hoac eau treo va loai bd khoi dapdi edkhi sddung giai phap vat ep nudc cho dat set yeu xung quanh cpc eo ket xong mdi thi cong ha cpc hay la cd the ddng (ladi cd tram gia CO dat yeu hoac la xd ly gia co nen dat yeu bSng gieng cat, cpc ban n h a a - t h o a t nadc theo phaong ngang - lam tang nhanh qua trinh co ket cua nen dat dadi mdng eong trinh; cpc eat, cpc xi mang - dat, cpc vdi... de lam chat dat nen loai tram tich set yeu bao hda nddc) trade khi ddng cpc.
TAI LIEU THAM KHAO
1. Tran Hijij Nhan, Oat xay dimg, Nha xuat ban giao due, 1997.
2. Boan The Tadng, Le Thuan Dang, Thi nghiem dat va nen mong cong trinh, Nha xuat ban giao thong van tai, Ha Noi, 2002.