土工格柵加筋方法在軟基處理工程中的應用

1、城市道橋與防洪第3期200年9月土工格柵加筋方法在軟基處理工程中的應用戴慶民蓋希光(黃石市水利局)摘要:本文以黃石合興閘重建工程軟基處理為例,對土工格柵加筋軟土地基的設計、施工及處理效果進行了較詳細的介紹。關鍵詞:土工格柵加筋軟基處理應用1前言土工合成材料在軟土地基加固工程中已得到廣泛應用,但在堤防工程穿堤建筑物的地基處理工程應用較少。黃石合興閘重建工程落座于軟土地基之上,閘底以下1018m深為淤泥質(zhì)粉質(zhì)粘土,塑性指數(shù)Ip=17.1,含水量 =45.3%,地基承載力fk=100kPa, =18.4kN/m, =16°,壓縮模量Es=10MPa。原設計地基處理方案采用樁基,后考慮到要在

2、汛前竣工,施工工期太緊,且投資高的原因,經(jīng)論證采用了土工格柵加筋方案。3式中:!fR加筋土增加的地基承載力設計值!f筋材提高的承載力設計值Z加筋墊層厚設計取!fR=p-fk=280-100=180kPa,查規(guī)范得,d=1.1,#=25°,則反算!f:180=1.1×18.4×(6.27+1.6-0.5)+280×2×1.6×tan25°/(5+2×1.6tan25°)+!f!f=25.17kPa(3)用反算法求筋材破壞時的極限拉力,假設破壞時筋材拉力T方向沿朗肯主動破壞面,則T與水平面的夾角=45

3、6;+%/2,!f可用下式表示:!f=NT(2sin/(b+2zntan#)+cos/Du×tan2)/K式中:N加筋層數(shù)Du滑動面最地深度,Du=bcos×etan%2方案設計設計方法:極限分析法。合興閘基底寬度b=5m,基底壓力設計值P=280kPa,基礎埋深d=6.27m。(1)擬布置方案如圖1所示。(2)考慮墊層對基底壓力的擴散作用和基礎埋深的影響。!fR=!f+d (d+z-0.5)+p×2ztan#/(b+2ztan#)/(2cos)K安全系數(shù),取K=1.85計算得,Du=5.2m,T=46.61kN/m<50kN/m(選用的格柵極限拉力)。(4

4、)設計筋材錨固長度。計算筋材上摩擦力時,只計算上覆土重引起的正應力,不計算基底壓圖1土工格柵布置圖及其斷面圖62城市道橋與防洪第3期200年9月力產(chǎn)生的附加應力,則各層筋材料的的長度可按下式計算:2L=B+2zitan#+2Le=B+2zitan#+T/fp (d+zI)取fp=0.8tan%c,%c為砂墊層的內(nèi)摩擦角,計算得,從上而下三層土工格柵的長度分別為6.79m,7.12m,7.25m。(5)沉降量驗算:推薦公式S=4P×Z×/Es計算沉降量,沉降計算深度Z=b(2.50.4lnb)=9.3m,即加筋砂墊層底下厚Z=9.3m的淤泥質(zhì)粉質(zhì)粘土層驗算沉降量S。綜合地基土

5、的地質(zhì)條件,取Es=10MPa,得:最終沉降量S=4×99.4×9.3×0.141/10000=0.052m,符合規(guī)范要求。綜上計算的結(jié)果,該工程采用三層極限抗拉強度為50kN/m的土工格柵加筋處理,按7.25m的長度等長布置。(6)其它:由于該工程為穿堤建筑物,加筋砂墊層將給堤基滲流帶來一定的影響,為此,在砂墊層的外江側(cè)鋪設土工膜作為防滲措施。圖2速度時間沉降曲線圖表1黃石市合興閘閘室基礎沉降觀測匯總表觀測觀測日期本次沉降(mm)235675432111121累計沉降(mm)2510162328323537383940414344沉降速度(mm/d)0.070.

6、00.070.030.030.030.020.020.01序號(年.月.日)11999.2.6234567891011121518241999.3.81999.4.81999.5.61999.6.51999.7.81999.8.91999.9.81999.10.101999.11.21999.12.52000.1.62000.4.82000.7.102001.1.63施工(1)清基。(2)鋪設土工格柵,土工格柵強度大的方向垂直于箱涵軸線,搭接尺寸5cm,綁扎間距20cm。(3)填筑砂料,砂料進倉前,土工格柵一次性鋪好。填砂20cm一層并夯實。取

7、樣試驗,密實度達90%以上,進入下一循環(huán)。5結(jié)論(1)運用觀測資料表明,用土工格柵加筋方法處理合興閘的軟土地基是成功的。工程已經(jīng)受了1999年洪水的考驗,且經(jīng)兩年運用,地基沉降已趨于穩(wěn)定,到目前為止沉降量僅44mm,滿足堤防工程設計規(guī)范(GB50286-98)的要求。(2)利用土工格柵加筋方法處理軟基,除了工程效果良好的優(yōu)點,還有就是施工簡易,造價低廉。合興閘重建工程竣工后,對土工格柵加筋方案和樁基方案比較,前者的造價僅為后者的25%。(收稿日期:2001-12-20)4竣工后的觀測合興閘于1999年5月15日建成,6月份開始試運行,經(jīng)1999年、2000年兩個汛期的考驗,到2001年沉降量為

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