用覆蓋層土的浮重與滲透力計(jì)算基坑下承壓水降深

1、第24卷第4期水利水電科技進(jìn)展2004年8月 作者簡介:葛孝椿(1936,男,江蘇南京人,教授級高級工程師,從事堤壩變形、滲流和穩(wěn)定分析研究.用覆蓋層土的浮重與滲透力計(jì)算基坑下承壓水降深葛孝椿(安徽省水利部淮河水利委員會水利科學(xué)研究院,安徽蚌埠233000摘要:通過對滲透力基本概念的分析,指出用覆蓋層土的浮重與覆蓋層內(nèi)滲透力計(jì)算基坑下承壓水降深比用覆蓋層濕土重與層底水壓力計(jì)算合理.通過兩種方法比較說明,在基坑滲透穩(wěn)定安全系數(shù)等于1,后一種方法中覆蓋層用飽和土重計(jì)算時兩者是一致的;但是,在基坑滲透穩(wěn)定安全系數(shù)大于1時,通常前者計(jì)算的基坑下承壓水降深小于后者;當(dāng)基坑下承壓水降深一定時,前者求得的基

2、坑滲透穩(wěn)定安全系數(shù)大于后者.用覆蓋層土的浮重與覆蓋層內(nèi)滲透力計(jì)算基坑下承壓水降深比較合理,宜于采用,但是相應(yīng)的基坑滲透穩(wěn)定安全系數(shù)可以適當(dāng)提高.關(guān)鍵詞:基坑降水;承壓水降深;滲透力;蚌埠閘中圖分類號:T U46+3文獻(xiàn)標(biāo)識碼:A 文章編號:10067647(200404001202當(dāng)建筑物基坑以下存在承壓水,上覆弱透水層不足以保證基坑滲透穩(wěn)定時,必須通過承壓水降深的計(jì)算,求得確保施工時期基坑滲透穩(wěn)定安全的承壓水位.計(jì)算基坑下承壓水降深(以下簡稱降深的常用方法是將承壓水層頂面以上到基坑底面之間的濕土重與承壓水層頂面的水壓力之比作為基坑滲透穩(wěn)定安全系數(shù)(以下簡稱安全系數(shù).但是,當(dāng)安全系數(shù)不等于1時

3、,上述計(jì)算方法與滲透力作用的概念并不相符,而用覆蓋層土的浮重與覆蓋層內(nèi)滲透力計(jì)算降深比較合理.下面介紹兩種降深(或安全系數(shù)的計(jì)算方法,并對兩種方法進(jìn)行比較,最后通過蚌埠閘擴(kuò)建工程降深的計(jì)算,進(jìn)一步說明兩者計(jì)算結(jié)果的差異.為了說明方便,將承壓水層頂面以上到基坑底面之間的弱透水土層簡稱為覆蓋層(見圖1.圖1基坑降水示意圖1兩種計(jì)算方法及其比較1.1按覆蓋層濕土重與層底水壓力計(jì)算安全系數(shù)方法1如圖1所示,承壓水層頂面以上到基坑底面之間的單位水平面積濕土重為(h p -h s c g ,其中h p 為基坑底面高程,即基坑開挖后覆蓋層頂面高程;h s 為砂層頂面高程,即覆蓋層底面高程;c 為覆蓋層濕土密

4、度;g 為重力加速度.覆蓋層底面水壓力等于承壓水層頂面水壓力,單位水平面積上水壓力為(h -h s w g ,其中h 為基坑承壓水降后水位;w 為水的密度.安全系數(shù)k 為承壓水層頂面以上到基坑底面之間的單位水平面積上濕土重與承壓水層頂面水壓力之比,即k =(h p -h s c(h -h s w (1基坑降水前的承壓水位與式(1計(jì)算的安全系數(shù)滿足要求的承壓水降后水位h 之差即為降深.式(1適用于覆蓋層為絕對不透水層或底部存在絕對不透水隔層的情況,但在實(shí)際工程中這種情況難以見到.1.2按覆蓋層土的浮重與覆蓋層內(nèi)滲透力計(jì)算安全系數(shù)方法2由于覆蓋層土體內(nèi)存在孔隙,下面又有承壓水,因此覆蓋層土體內(nèi)總會

5、受到滲透力的作用.基坑開挖前,覆蓋層在地基承壓水的長期作用下,一般已處于穩(wěn)定滲流狀態(tài),基坑開挖僅使?jié)B透力進(jìn)行調(diào)整;又由于水利工程基坑開挖面大,歷時較長,因此可按穩(wěn)定滲流考慮.覆蓋層單位水平面積土體的浮重為(h p -h s c g ,其中c 為覆蓋層土體的浮密度.覆蓋層內(nèi)平均豎向滲透比降j為j=h-h p h p-h s作用于覆蓋層單位水平面積土體的滲透力(方向向上為j(h p-h sw g=(h-h pw g.安全系數(shù)k為覆蓋層單位水平面積上土體浮重與滲透力之比,即k=(h p-h sc(h-h pw(21.3兩種計(jì)算方法比較當(dāng)安全系數(shù)k=1時,從式(1可得:h p-h s=(h-h swc

6、(3由于基坑頂面高程低于承壓水水位,所以c可用飽和密度代替,故有c=c+w,代入式(3并整理得(h p-h sc(h-h pw=1(4可見,當(dāng)k=1時,式(4即式(2,說明在k=1時,以上兩種方法是一致的.但是,在k1時,上述公式推導(dǎo)不能成立,兩者計(jì)算結(jié)果不相同.一般而言,若(h-h s/c(h-h p/c,則用式(2求得的安全系數(shù)k大于用式(1求得的;反之,則用式(2求得的小于用式(1求得的.在覆蓋層土的飽和密度約等于2倍浮密度,以及cw的情況下,可由上述不等式推求得:當(dāng)(h p-h s(h-h p,即用式(2求得的安全系數(shù)k大于1時,用式(2求得的安全系數(shù)k 大于用式(1求得的;反之,則用

7、式(2求得的小于用式(1求得的.下面用蚌埠閘擴(kuò)建工程降深的計(jì)算來說明兩種計(jì)算結(jié)果的差異.2兩種計(jì)算方法計(jì)算實(shí)例比較2.1蚌埠閘擴(kuò)建工程簡介蚌埠閘樞紐位于淮河干流,蚌埠市以西,渦河河口以下.該樞紐由節(jié)制閘(簡稱老閘、電站、船閘和分洪道等建筑物組成,其作用是蓄水灌溉,兼有航運(yùn)、發(fā)電和供水等功能.根據(jù)淮河正陽關(guān)蚌埠近期河道整治規(guī)劃,需擴(kuò)大蚌埠閘(老閘的規(guī)模,減少設(shè)計(jì)過閘落差,因此對該閘進(jìn)行擴(kuò)建.蚌埠閘擴(kuò)建工程位于老閘北端與淮北大堤之間,主要建筑物為新閘.新閘同老閘軸線一致,共12孔,每孔凈寬10m.閘底板高程為9.0m,消力池底部高程為614m(均為黃海高程,下同.閘址原地面高程為15.01715m

8、,地基砂層承壓水水位為1615 1810m,砂層厚約27m;砂層以上為17m厚的粉質(zhì)黏土.基坑開挖到閘底板和消力池底部高程時,砂層承壓水對覆蓋層產(chǎn)生的向上滲透力遠(yuǎn)大于覆蓋層浮重,將會產(chǎn)生滲流破壞.為了保證新閘基坑施工安全,并且確保老閘不致因承壓水位降低而產(chǎn)生有害沉降,必須降低砂層承壓水水頭,進(jìn)行砂層承壓水降深計(jì)算.2.2按方法1計(jì)算安全系數(shù)及降深根據(jù)文獻(xiàn)1得蚌埠閘擴(kuò)建工程基坑降水設(shè)計(jì)中的h s=-110m,c=1192g/cm3,w=110g/cm3,h=hp-h s.由于安排非汛期施工最低部工程,所以承壓水位(降前設(shè)計(jì)值用1615m,由建筑物各部位底部高程確定的h p見表1,根據(jù)各種基坑承壓

9、水降后水位h,對不同部位的h p代入式(1求得相應(yīng)的安全系數(shù)k,見表1.表1按方法1計(jì)算與基坑承壓水降后水位h相應(yīng)的安全系數(shù)項(xiàng)目名稱h p/mh/m不同h時的安全系數(shù)k18.0m1615m11.0m10.0m9.0m 消力池6147140181111811291142閘底板6197190187112611381151鋪蓋7108100188112811401154拋石槽7108100188112811401154堤基清淤7158150193113611481163下游引河910101011011110116011751192上游引河915101511061115116811832102堤基回填

10、1010111011111121117611922117施工組織設(shè)計(jì)采用k=1110,從表1中可見,消力池是控制情況,所需要的承壓水降后水位h= 1110m,所以降深為1615m-1110m=515m.2.3按方法2計(jì)算安全系數(shù)及降深覆蓋層土的c=0192g/cm3,其余數(shù)據(jù)同前,但是代入式(2求安全系數(shù)k,計(jì)算結(jié)果見表2.從表2可見,仍為消力池控制,所需要的承壓水降后水位為1215m,所以降深為1615m-1215m=4.0m.表2按方法2計(jì)算與基坑承壓水降后水位h相應(yīng)的安全系數(shù)項(xiàng)目名稱h p/mh/m不同h時的安全系數(shù)k1810m1615m1310m1215m1210m1110m1010m

11、910m 消力池61471401590167110311121122114811892162閘底板61971901650176111911301143117721343146鋪蓋71081001670176112311341147118421453168拋石槽71081001670176112311341147118421453168堤基清淤71581501740187114211561174212331135121下游引河91010101102112221302163310741609120上游引河915101511141138217631223186614419132堤基回填1010111

12、01127115631374105510610112從兩種方法計(jì)算結(jié)果可見,方法2計(jì)算的降深比方法1小115m;如果降深仍取方法1計(jì)算值515m,則從表2中可得用方法2計(jì)算的安全系數(shù)為1148,比方法1求得的大.(下轉(zhuǎn)第32頁性能,但索引的創(chuàng)建是以浪費(fèi)存儲空間為代價的,若在一個經(jīng)常修改的數(shù)據(jù)域上建立索引,則效果適得其反,最終將造成系統(tǒng)性能的下降和存儲空間的浪費(fèi).通過分析,可得使用索引的幾個原則:為經(jīng)常出現(xiàn)在檢索條件中的數(shù)據(jù)域建立索引,如果數(shù)據(jù)域是經(jīng)常一起出現(xiàn)在檢索條件中,那么建立復(fù)合索引;為了提高多表連接效率,對經(jīng)常用于表連接操作中的數(shù)據(jù)列應(yīng)建立索引,如果是多域連接則建立復(fù)合索引;在利用外鍵連

13、接的表中,外鍵上也應(yīng)該建立索引.c.調(diào)整內(nèi)存分配及相關(guān)的初始參數(shù).正確分配內(nèi)存資源可以改善高速緩存性能,減少S Q L語句的分析,減少內(nèi)存的分頁和交換.主要調(diào)整庫緩沖區(qū)、數(shù)據(jù)字典緩沖區(qū)和緩沖區(qū)高速緩存的內(nèi)存分配.d.優(yōu)化磁盤I/O.磁盤的I/O速度對整個系統(tǒng)的性能有較大的影響,將文件分散存儲在不同的可用磁盤上,這樣事物處理所執(zhí)行的磁盤訪問不妨礙對相應(yīng)的事務(wù)日志登記的磁盤訪問,從而減少對數(shù)據(jù)文件和事務(wù)日志文件的競爭,有效改善服務(wù)器性能.e.優(yōu)化回滾段.數(shù)據(jù)庫數(shù)據(jù)表的事務(wù)(delete, insert,update在回滾段產(chǎn)生登記項(xiàng),它保持?jǐn)?shù)據(jù)塊在事務(wù)開始以前的狀態(tài),回滾段控制著數(shù)據(jù)庫處理事務(wù)的能

14、力,因而即使數(shù)據(jù)庫其他部分設(shè)計(jì)得再好,但如果回滾段設(shè)計(jì)得不合理,仍將會嚴(yán)重影響系統(tǒng)的性能.為回滾段建立一個獨(dú)立的表空間數(shù)據(jù),可以包括在不同驅(qū)動器上的多個文件,使回滾段與數(shù)據(jù)字典、用戶數(shù)據(jù)、索引等分離開來,減少I/O的競爭.同時獨(dú)立使用回滾段,可以減少用戶數(shù)據(jù)表空間碎片的產(chǎn)生.4結(jié)語清江防洪與梯級調(diào)度仿真系統(tǒng)前期開發(fā)已經(jīng)完成,應(yīng)用程序設(shè)計(jì)采用C+,界面設(shè)計(jì)采用VC+,各計(jì)算子系統(tǒng)利用Oracle提供的Pro3C訪問數(shù)據(jù)庫,GIS與三維仿真顯示利用ODBC訪問數(shù)據(jù)庫.多次系統(tǒng)聯(lián)合調(diào)試結(jié)果表明,數(shù)據(jù)庫管理子系統(tǒng)能為多個子系統(tǒng)提供有效的數(shù)據(jù)服務(wù),下一步的工作是繼續(xù)完善功能,進(jìn)一步提高系統(tǒng)的性能.參考文

15、獻(xiàn):1薩師煊.數(shù)據(jù)庫系統(tǒng)概論M.北京:高等教育出版社,2000.204.2R owss opoulos N.M odern client/server DBMS architecturesJ.S igm od Record,1991,120(3:78.3邵佩英.分布式數(shù)據(jù)庫系統(tǒng)及應(yīng)用M.北京:科學(xué)出版社,1998.12.4郭生練,楊金星,郭井.水庫調(diào)度綜合自動化系統(tǒng)M.武漢:武漢水利電力大學(xué)出版社,2000.78.5敬錚.Oracle8i數(shù)據(jù)庫開發(fā)與專業(yè)應(yīng)用M.北京:國防工業(yè)出版社,2002147.(收稿日期:20031118編輯:熊水斌(上接第13頁3用方法2計(jì)算的最小安全系數(shù)取值范圍鑒于方

16、法2比方法1合理,一般(h p-h s(h-h p時計(jì)算的安全系數(shù)比方法1大,因此,用方法2計(jì)算時可將安全系數(shù)適當(dāng)提高.就像用圓弧滑動條分法分析邊坡穩(wěn)定一樣,簡化畢肖普法考慮了條塊間作用力,比瑞典法合理,但是計(jì)算的穩(wěn)定安全系數(shù)比瑞典法大,規(guī)范(S D J218842規(guī)定了用瑞典法計(jì)算的最小安全系數(shù),而用簡化畢肖普法計(jì)算時,安全系數(shù)提高5%10%.由于簡化畢肖普法比瑞典法合理,因而新規(guī)范(S L27420013則規(guī)定用簡化畢肖普法計(jì)算安全系數(shù),而用瑞典法計(jì)算時,安全系數(shù)減小8%,說明新規(guī)范比老規(guī)范突出了較合理的簡化畢肖普法.能否參照這種精神確定兩種降深計(jì)算方法的最小安全系數(shù)值得探討.目前G B5

17、0072002建筑地基基礎(chǔ)設(shè)計(jì)規(guī)范4附錄W規(guī)定,用方法1計(jì)算的最小安全系數(shù)為111,所以用方法2計(jì)算的最小安全系數(shù)宜大于111,但應(yīng)小于新規(guī)范(S L27420013中81214節(jié)規(guī)定的安全系數(shù)下限值115,因?yàn)橥潦瘔螢橛谰眯越ㄖ?而基坑降水是臨時性施工措施.4結(jié)論a.對承壓水以上覆蓋層為絕對不透水層,或者其底面存在絕對不透水隔層的情況,按方法1計(jì)算安全系數(shù)或降深是正確的.但是,覆蓋層通常為黏性土層,土體內(nèi)存在孔隙,相對于承壓水層而言均為弱透水層,因此,實(shí)際工程中難以出現(xiàn)上述情況.b.覆蓋層為弱透水土層時,按方法2計(jì)算降深比按方法1計(jì)算合理.c.當(dāng)(h-h s/c(h-h p/c時,按方法2

18、計(jì)算的降深比按方法1計(jì)算的小;在降深一定時,按方法2計(jì)算的安全系數(shù)比按方法1計(jì)算的大.當(dāng)安全系數(shù)等于1時,兩者一致.d.按方法2計(jì)算降深,且(h-h s/c(h-h p/c時,所采用的最小安全系數(shù)可適當(dāng)加大;但控制部位宜小于115.參考文獻(xiàn):1周國林.深井降水在蚌埠閘擴(kuò)建工程施工中的應(yīng)用J.治淮,2002(12:3334.2S D J21884,碾壓式土石壩設(shè)計(jì)規(guī)范S.3S L2742001,碾壓式土石壩設(shè)計(jì)規(guī)范S.4G B50072002,建筑地基基礎(chǔ)設(shè)計(jì)規(guī)范S(收稿日期:20031125編輯:熊水斌ADVANCES IN SCIENCE AN D V ol.24N o.4 TECHN OL

19、OG Y OF WATER RESOURCES Aug.20th2004Measures for sustainable development and utilization of w ater resources in G uanzhong area/ZH OU Wei2bo (College o f Environmental Science and Engineering, Changan Univer sity,Xian710054,China Abstract:An analysis is made on the current situation of water res our

20、ces development and utilization in G uanzhong area,and s ome problems are pointed out,including the low efficiency of water utilization,overmining of groundwater,aggravation of water pollution,and aging, out of repairing and insufficiency of water supply facilities.From the angle of sustainable deve

21、lopment of national economy and s ociety and sustainable utilization of water res ources,s ome measures are proposed,such as actively developing water saving technology to im prove the utilization ratio of water res ources,practicing reserv oir management by unified netw ork,limiting the v olume of

22、groundwater mining,paying attention to the cure and protection of groundwater s ources,strengthening water pollution remediation,converting wastewater into res ources,and im plementing the project of water diversion from s outh area to north area of the province.K ey w ords:water res ources;sustaina

23、ble utilization;G uanzhong areaW ater resources optim al allocation model for northw estern irrigation area/ZH AO Dan,et al(State K ey Laboratory o f Water Resources and Hydropower Engineering Science,Wuhan Univ.,Wuhan430072, ChinaAbstract:Aiming at the serious water shortage and eco2 environmental

24、problems in northwestern arid and semi2arid regions,an ecology and water saving oriented water res ources optimal allocation m odel is developed for the irrigation area based on the idea of system analysis,and a multi2objective simulation calculation method is proposed with consideration of the fact

25、ors of water2saving,water right,and ecological environment.With the Nanyang canal irrigation area as an exam ple,a reas onable scheme for water res ources optimal allocation is educed.The application shows that,with the development of s ocial economy,the shortage of water res ources is getting m ore

26、 and m ore serious,and that the v olumes of water res ources the irrigation area can provide for industrial and agricultural development and residentslife are56.415 million m3,57.967million m3,and56.572million m3 respectively at current,2010,and2030,but the v olume of water shortage will reach15.446

27、million m3,21.003 million m3,and36.279million m3.K ey w ords:optimal allocation of water res ources; planning for irrigation area;water saving;eco2 environmental water demand;Nanyang canal irrigation areaStability analysis of N o.2compound landslide on the right tail w ater bank of Ankang H ydropow

28、er Station/ ZH U Y ue2ming,et al(College o f Water Conservancy and Hydropower Engineering,Hohai Univ.,Nanjing 210098,ChinaAbstract:In consideration of the factors influencing the stability of the com pound landslide,including the structural surface of main rock faults,concrete anti2slide piles,seepa

29、ge,nonlinearity of material structure,and earthquake2induced inertial force,the stability and safety of three typical surfaces of the landslide with or without rein forcement by anti2slide piles,on which slip2induced cracks might occur,are calculated under three kinds of load combination by use of t

30、he finite element calculation m odel and the limit equilibrium method for rigid body analysis.The results show that the landslide is in the steady state,but the daily safety m onitoring of the landslide should still be em phasized.K ey w ords:finite element method;structural surface of rock fissure;

31、slope stability;anti2slide pile;seepageC alculation of dew atering depth of confined w ater under foundation pits by use of submerged w eight of overburden soil and seepage force/GE X iao2chun (Water Conservancy Research Institute,Anhui&Huaihe Water Conservancy Commission,Bengbu233000,China Abstract

32、:An analysis of the basic concept of seepage force shows that,for calculation of the dewatering depth of confined water,it is m ore reas onable to use the submerged weight of overburden s oil and the seepage force acting on it than to use the wet weight of overburden s oil and the water pressure at

33、the bottom of the layer.The com paris on of the tw o methods shows that,when the safety factor of seepage stability equals one,the calculated results of the tw o methods con form to each other if the weight of the saturated s oil of overburden layers is used for calculation in the latter method; how

34、ever,if the safety factor is larger than one,normally, the dewatering depth calculated by the former method is less than that by the latter method.Furtherm ore,it isconcluded that,for a certain dewatering depth,the safety factor of seepage stability obtained by the former method is larger than that

35、by the latter one.Therefore,the former method is m ore suitable for calculation of the dewatering depth of con fined water.H owever,the safety factor of seepage stability should be properly increased.K ey w ords:foundation pit dewatering;dewatering depth of con fined water;seepage force;Bengbu sluic

36、eE nhanced assumed strain(EASelement2based analysis of thin plates on elastic foundations/LI Wen2 hu,et al(College o f Water Conservancy and Hydropower Engineering,Hohai Univ.,Nanjing210098,China Abstract:The E AS element method is used to discretize the thin plate on the elastic foundation.Under th

37、e assum ption that the contact forces on the interface of the plate and foundation are in the distributive form,the relationship between the distributed force on the surface of foundations and nodal displacement is established according to the theoretical s olution of the displacement at nodes in th

38、e semi2in finite space subjected to vertical and horizontal forces.Then,based on the conditions of com patibility of displacement and static force equilibrium between the plate and the foundation,FE M equations are formulated with displacements at nodes on the middle surface of the plate taken as un

39、known variables.An exam ple is given to verify the rationality of the E AS element for analysis of thin plates and to show the necessity of taking the distributed force on the interface between plates and foundations into account.K ey w ords:E AS element;distributed reaction force; semi2infinite fou

40、ndation;G auss integral;finite elementExperimental project of environmental protection vegetation on gangue dike slope of H uaihe River/SH U Y i2ming,et al(College o f Water Conservancy and Hydropower Engineering,Hohai Univ.,Nanjing 210098,ChinaAbstract:An introduction is given to the key points for

41、 design of experimental project of environmental protection vegetation covered with32D geonet on a gangue dike slope on the Huaihe River in Huainan diggings,and the techniques for construction,as well as the categ ory selection and management of vegetation.The pavement of 32D geonet on the dike slop

42、e can prevent the erosion of thin layer of s oil and plant seeds;the mixing s owing of plant seeds of the pioneer type and artery type is fav orable for the formation of lawns and en forcement of the slope; and the effective management measures can keep the plants in green for a long period.Practice

43、 shows that the experimental project has g ood effect on environmental and ecological protection.K ey w ords:gangue dike slope;32D geonet;environ2 mental protection vegetation;Huaihe RiverN ew technique for bank protection on N anjing reach of Yangtze River/X U X i2rong,et al(College o f Water Conse

44、rvancy and Hydropower Engineering,Hohai Univ., Nanjing210098,ChinaAbstract:Based on the current situation of regulation w orks for bank protection on Nanjing reach,the mechanism of different structural types for bank protection is analyzed.Simulation experiments are performed on several structural types,including the riprap,tetrahedron frame structure,hinged fascine mattress,and s o on.A com paris on of experimental results shows that the tetrahedron frame