軟土水泥攪拌樁復(fù)合地基沉降特性試驗(yàn)研究-

1、胡賀松1, 2,彭振斌1,楊坪3,楊慶光4,彭文祥1(1. 中南大學(xué)地學(xué)與環(huán)境工程學(xué)院,湖南長(zhǎng)沙,410083;2. 廣州市建筑科學(xué)研究院,廣東廣州,510440;3. 同濟(jì)大學(xué)地下建筑與工程系,上海,200092;4. 湖南工業(yè)大學(xué)巖土工程研究所,湖南株洲,412008摘要:為了分析軟土水泥攪拌樁復(fù)合地基沉降特性,通過現(xiàn)場(chǎng)載荷試驗(yàn),對(duì)軟土單樁和四樁水泥攪拌樁復(fù)合地基沉降特性進(jìn)行試驗(yàn)研究。試驗(yàn)結(jié)果表明:對(duì)于不同的荷載等級(jí),單樁和四樁水泥攪拌樁復(fù)合地基沉降可分為快速沉降和緩慢沉降2個(gè)階段;在快速沉降階段,沉降量可達(dá)到本級(jí)荷載下復(fù)合地基沉降量的90%左右,而在緩慢沉降階段,沉降量只占本級(jí)荷載下的1

2、0%;在不同荷載水平下,單樁沉降量比與時(shí)間的關(guān)系和四樁復(fù)合地基的沉降量比與時(shí)間的關(guān)系相似;其復(fù)合地基沉降量與時(shí)間的關(guān)系可以用Hill模型進(jìn)行描述。關(guān)鍵詞:軟土;水泥攪拌樁;復(fù)合地基;沉降中圖分類號(hào):TU411 文獻(xiàn)標(biāo)識(shí)碼:A 文章編號(hào):16727207(200903080305Settlement characteristic of composite foundation forcement mixing piles in soft soilHU He-song1, 2, PENG Zhen-bin1, Y ANG Ping3, Y ANG Qing-guang4, PENG Wen-xia

3、ng1(1. School of Geology and Enviromental Engineering, Central South University, Changsha 410083, China;2. Guangzhou Institute of Building Science, Guangzhou 510440, China;3. Department of Geotechnical Engineering, Tongji University, Shanghai 200092, China;4. Geotechnical Engineering Institute, Huna

4、n University of Technology, Zhuzhou 412008, ChinaAbstract: In order to analyze the settlement characteristic of composite foundation for cement mixing piles in soft soil, the settlement characteristics of composite foundation for single and four cement mixing piles in soft soil were analyzed by load

5、ing. The test results show that the settlement of composite foundation for single and four cement mixing piles, under different loads, includes two stages, i.e., quick settlement and slow settlement. At the stage of the quick settlement, composite foundation settles fast. 90% of the total settlement

6、 for the loading takes place during the quick settlement, and the remaining 10% of the settlement occurs during the slow settlement. In different load stages but in the same period, the relation of settlement ratio and time of composite foundation for single and four cement mixing piles is similar.

7、The relationship between settlement and time can be described by Hill model.Key words: soft soil; cement mixing piles; composite foundation; settlement目前,人們對(duì)復(fù)合地基沉降的研究遠(yuǎn)少于對(duì)復(fù)合地基承載力的研究,也遠(yuǎn)遠(yuǎn)落后于工程實(shí)踐的需要,復(fù)合地基沉降計(jì)算理論還不成熟,因此,對(duì)復(fù)合地基沉降量進(jìn)行計(jì)算和預(yù)測(cè)是現(xiàn)階段復(fù)合地基研究領(lǐng)域的收稿日期:20080620;修回日期:20080918基金項(xiàng)目:國家自然科學(xué)基金資助項(xiàng)目(50878212;上海市博士

8、后科研基金資助項(xiàng)目(05R214145中南大學(xué)學(xué)報(bào)(自然科學(xué)版 第40卷804一個(gè)重要課題。Pells等13認(rèn)為復(fù)合地基沉降是一個(gè)多因素作用的綜合的復(fù)雜過程;喬京生等4通過模型試驗(yàn)和數(shù)值模擬對(duì)復(fù)合地基進(jìn)行研究,提出樁體沉降法計(jì)算沉降量,并通過與工程實(shí)測(cè)對(duì)比分析,結(jié)果表明,采用樁體沉降法計(jì)算的沉降量與實(shí)測(cè)沉降量相吻合;王鳳池等56通過對(duì)復(fù)合地基沉降規(guī)律進(jìn)行分析,得出復(fù)合地基沉降時(shí)間曲線具有單調(diào)遞增性、有界性和呈“S”形的特點(diǎn),并利用復(fù)合地基的變形協(xié)調(diào)條件,推導(dǎo)了層狀土中樁體復(fù)合地基的荷載沉降關(guān)系及樁土應(yīng)力比公式,并結(jié)合工程實(shí)際對(duì)其精度進(jìn)行了驗(yàn)證;劉杰等7推導(dǎo)出路堤荷載作用下復(fù)合地基加固區(qū)樁及樁周

9、土壓縮量計(jì)算的解析式;章定文等8通過二維有限元方法分析了路堤荷載下復(fù)合地基變形特性?，F(xiàn)有各種復(fù)合地基沉降計(jì)算方法中,由于對(duì)壓縮土層剖面分布、計(jì)算深度、計(jì)算模型、荷載條件等都進(jìn)行了簡(jiǎn)化,使沉降理論計(jì)算結(jié)果與工程實(shí)測(cè)結(jié)果有不同程度的差異,特別是沉降過程及沉降量的預(yù)估誤差更大912。常見的指數(shù)曲線法、雙曲線法等沉降曲線擬合方法存在一些不足,如采用指數(shù)法時(shí),在初期沉降預(yù)測(cè)誤差較大,采用雙曲線法所得預(yù)測(cè)值偏于保守。為此,本文作者在前人研究的基礎(chǔ)上1317,通過現(xiàn)場(chǎng)載荷試驗(yàn),分別對(duì)軟土單樁和四樁水泥拌攪樁復(fù)合地基沉降特性進(jìn)行分析研究。1土的基本性質(zhì)采用數(shù)理統(tǒng)計(jì)方法,對(duì)試驗(yàn)場(chǎng)地軟土物理力學(xué)性質(zhì)進(jìn)行統(tǒng)計(jì)分析,

10、得到軟土的物理力學(xué)指標(biāo):含水量為65.96%,孔隙比為1.72,飽和度為94.95%,液限為48.96%,塑限為34.13%,液性指數(shù)為1.85,塑性指數(shù)為14.83,固結(jié)系數(shù)為1.29×103 cm2/s,壓縮模量為1.98 MPa,凝聚力為5.82 kPa,內(nèi)摩擦角為4.69。2實(shí)驗(yàn)方案分單樁復(fù)合地基和四樁復(fù)合地基進(jìn)行實(shí)驗(yàn)。選用水泥攪拌樁,采用濕噴工藝,樁長(zhǎng)為15.5 m,樁徑為500 mm,四樁復(fù)合地基樁間距為1.5 m,呈正方形布置,設(shè)計(jì)承載力為100 kPa。單樁復(fù)合地基載荷試驗(yàn)和四樁復(fù)合地基載荷試驗(yàn)所采用的承壓板長(zhǎng)×寬分別分別為150 cm×150 c

11、m 和240 cm×240 cm,上部最大載荷均為200 kPa。試驗(yàn)采用鋼梁和混凝土塊組成的壓重平臺(tái)作為施荷反力系統(tǒng),重物平臺(tái)總質(zhì)量為預(yù)定最大試驗(yàn)荷載的20%以上,載荷在試驗(yàn)前一次性加上平臺(tái)。試驗(yàn)時(shí),由置于試驗(yàn)承壓板上的油壓千斤頂逐級(jí)加、卸荷載,每級(jí)荷載所需的油壓讀數(shù)由千斤頂?shù)暮奢d油壓關(guān)系標(biāo)定曲線經(jīng)換算得出,千斤頂作用中心線與承壓板中心線重合。在承壓板對(duì)稱位置垂直安裝4個(gè)WBD30型機(jī)電百分表,測(cè)讀承壓板在各級(jí)荷載作用下的沉降量,百分表精度為0.01 mm。載荷試驗(yàn)參照建筑地基處理技術(shù)規(guī)范(JGJ 792002要求,分8級(jí)加載,每級(jí)荷載為25 kPa,卸載分4級(jí)進(jìn)行,每級(jí)卸載減量為

12、預(yù)定最大試驗(yàn)荷載的1/4,即50 kPa。3試驗(yàn)結(jié)果分析3.1 單樁和四樁復(fù)合地基沉降量與時(shí)間的關(guān)系進(jìn)行荷載試驗(yàn)時(shí),單樁復(fù)合地基在荷載為150 kPa 和175 kPa及四樁復(fù)合地基在荷載為100 kPa和125 kPa時(shí),單樁和四樁復(fù)合地基沉降量與時(shí)間的關(guān)系分別如圖1和圖2所示。從圖1和圖2可以看出:單樁和四樁復(fù)合地基在小于地基允許承載力的荷載作用下,其沉降量與加載時(shí)間的關(guān)系相似,對(duì)于不同的荷載,只是沉降量不同,而沉降發(fā)展的規(guī)律相同。沉降曲線存在2個(gè)階段:快速沉降階段和緩慢沉降階段,分別見圖1和圖2中的AB段和BC段。在快速沉降階段,沉降速率快,沉降量可達(dá)到本級(jí)荷載下復(fù)合地基沉降量的90%左

13、右。而 圖1單樁復(fù)合地基沉降量與時(shí)間的關(guān)系Fig.1 Relationship between settlement and time forsingle pile第3期 胡賀松,等:軟土水泥攪拌樁復(fù)合地基沉降特性試驗(yàn)研究 805 圖2 四樁復(fù)合地基沉降量與時(shí)間的關(guān)系Fig.2 Relationship between settlement and time for four piles在緩慢沉降階段,沉降速率很小,沉降量只占本級(jí)荷載下的10%。3.2 復(fù)合地基沉降規(guī)律單樁和四樁復(fù)合地基在不同荷載水平作用下,沉降量與時(shí)間的關(guān)系相似,只是沉降量不同,為了便于分析,將某級(jí)荷載水平作用下不同時(shí)間的沉

14、降量進(jìn)行歸一化處理,即將某級(jí)荷載水平作用下某時(shí)刻的沉降量除以本級(jí)荷載水平作用下的最大沉降量,得出沉降量比與時(shí)間的關(guān)系曲線。單樁復(fù)合地基沉降量比與時(shí)間的關(guān)系如圖3所示。四樁復(fù)合地基沉降量比與時(shí)間的關(guān)系同圖3相似。對(duì)圖3中的點(diǎn)采用Matlab 軟件編程擬合,得出沉降量比與時(shí)間的關(guān)系式為:nnn t tm t ks s +=0/。 (1 圖3 單樁復(fù)合地基沉降量比與時(shí)間的關(guān)系 Fig.3 Relationship between settlement ratio and time forsingle pile式中:s t 為t 時(shí)刻的沉降量;t 為時(shí)間;s 0為該級(jí)荷載下的最大沉降量;k 為修正系數(shù)

15、;m 和n 為試驗(yàn)參數(shù),與復(fù)合地基的性質(zhì)有關(guān),其中,m 的量綱與t 的量綱一致,n 為無量綱。式中擬合參數(shù)見表1。其中,R 2為相關(guān)系數(shù),該值越大,說明相關(guān)性越好。表1 擬合參數(shù) Table 1 Regress parameters類型 k m n R 2 單樁復(fù)合地基1.030.329 990.563 820.847 46從圖3及表1可知,擬合曲線與實(shí)測(cè)點(diǎn)具有良好的相關(guān)性。因此,可以用式(1來描述復(fù)合地基的沉降發(fā)展過程。對(duì)式(1進(jìn)行變換,便可得沉降量與時(shí)間的關(guān) 系式:nnn t tm t ks s +=0。 (2因此,可以用式(2來描述軟土水泥拌攪樁復(fù)合地基的沉降發(fā)展過程,即復(fù)合地基沉降量與

16、時(shí)間的關(guān)系可以用Hill 模型進(jìn)行描述。圖47所示是在荷載為75 kPa 和175 kPa 時(shí),用式(2計(jì)算的計(jì)算值與實(shí)測(cè)值的數(shù)值對(duì)比結(jié)果?？梢?無論是單樁復(fù)合地基,還是四樁復(fù)合地基,在不同荷載作用下,在不同時(shí)刻,計(jì)算所得的沉降量與實(shí)測(cè)沉降量基本一致,說明用式(2描述復(fù)合地基的沉降發(fā)展過程是正確的。圖4 75 kPa 時(shí)單樁復(fù)合地基沉降量與時(shí)間的關(guān)系 Fig.4 Relationship between settlement and time for singlepiles under 75 kPa中南大學(xué)學(xué)報(bào)(自然科學(xué)版 第40卷806 圖5 175 kPa時(shí)單樁復(fù)合地基沉降量與時(shí)間的關(guān)系F

17、ig.5Relationship between settlement and time for singlepiles under 175 kPa 圖6 75 kPa時(shí)四樁復(fù)合地基沉降量與時(shí)間的關(guān)系Fig.6Relationship between settlement and time for four pilesunder 75 kPa 圖7 175 kPa時(shí)四樁復(fù)合地基沉降量與時(shí)間的關(guān)系Fig.7Relationship between settlement and time for four pilesunder 175 kPa 4結(jié)論a. 對(duì)于不同的荷載等級(jí),單樁和四樁水泥拌攪樁復(fù)

18、合地基沉降可分為快速沉降階段和緩慢沉降階段。在快速沉降階段,沉降量可達(dá)到本級(jí)荷載下復(fù)合地基沉降量的90%左右;而在緩慢沉降階段,沉降量只占本級(jí)荷載下的10%。b.當(dāng)荷載小于復(fù)合地基允許承載力時(shí),在不同荷載下,單樁沉降量比與時(shí)間的關(guān)系和四樁復(fù)合地基的沉降量比與時(shí)間的關(guān)系相似。c.利用現(xiàn)場(chǎng)試驗(yàn)得到的單樁和四樁復(fù)合地基的沉降量比與時(shí)間關(guān)系曲線,采用Matlab軟件編程擬合,得到復(fù)合地基沉降量與時(shí)間的關(guān)系可以用Hill模型描述。對(duì)于單樁和四樁復(fù)合地基,在不同荷載水平作用下,利用Hill模型計(jì)算的沉降量與實(shí)測(cè)沉降量基本一致。參考文獻(xiàn):1 Pells P J N, Turner R M. Elastic

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