【摘要】：從歷次發(fā)生地震的震害調(diào)查表明,可液化土層中樁基礎(chǔ)容易產(chǎn)生破壞而導(dǎo)致上部結(jié)構(gòu)損壞,引起嚴(yán)重的經(jīng)濟(jì)和生命財(cái)產(chǎn)的損失。研究液化土層中樁基礎(chǔ)承載特性具有非常重要的意義,直接關(guān)系到國(guó)內(nèi)外經(jīng)濟(jì)建設(shè)和工程的抗震減災(zāi)問題。雖然現(xiàn)行的相關(guān)規(guī)范和國(guó)內(nèi)外相關(guān)學(xué)者在樁基礎(chǔ)抗震設(shè)計(jì)方面已經(jīng)提出了一些方法,但是這些方法對(duì)于存在液化土層時(shí)樁基礎(chǔ)的設(shè)計(jì)還存有明顯不足,其原因主要是現(xiàn)有的樁基抗震設(shè)計(jì)方法是基于靜荷載條件下的理論與經(jīng)驗(yàn),對(duì)動(dòng)荷載作用下低承臺(tái)樁——液化土之間相互作用機(jī)理理解不夠,使計(jì)算方法本身缺乏可靠的理論依據(jù),存在許多不妥之處。 在國(guó)內(nèi)外相關(guān)研究的基礎(chǔ)上,通過一系列單樁和群樁的小比例模型振動(dòng)臺(tái)試驗(yàn)和MIDAS GTS數(shù)值模擬,對(duì)復(fù)合樁基體系的豎向承載性能進(jìn)行了深入的分析和系統(tǒng)的總結(jié),取得以下研究成果： (1)基于模型試驗(yàn)動(dòng)力相似性無量綱理論,通過對(duì)不同配比材料的密度和彈性模量的測(cè)定,首次研制了滿足試驗(yàn)要求的混凝土樁的模型材料,保證了室內(nèi)模型試驗(yàn)的可行性,為今后類似試驗(yàn)提供了經(jīng)驗(yàn)。 (2)室內(nèi)模型試驗(yàn)表明：飽和砂土在水平周期荷載作用下,超靜孔隙水壓力隨著振動(dòng)時(shí)間逐漸產(chǎn)生和發(fā)展,土層中的孔壓比隨振動(dòng)時(shí)間增長(zhǎng)而逐漸增大,土層自上而下逐漸發(fā)生液化。振動(dòng)停止以后,由于下部土層中的孔隙水壓力大于上部土層的孔隙水壓力,孔隙水產(chǎn)生向上滲流,因而導(dǎo)致下部土層孔隙水壓力消散速度比上部快。 (3)單樁試驗(yàn)中樁身應(yīng)變測(cè)試分析表明：樁側(cè)摩阻力隨土層中的孔壓比的提高而降低,當(dāng)土層產(chǎn)生液化后,樁側(cè)摩阻力降低顯著,但是明顯滯后于液化產(chǎn)生時(shí)間。隨孔隙水壓力的消散樁側(cè)摩阻力又有所提高。 (4)低承臺(tái)3×3群樁體系的振動(dòng)臺(tái)試驗(yàn)表明：復(fù)合樁基體系對(duì)于土層液化產(chǎn)生有抑制作用,且樁間距越小,抑制作用越明顯。這是因?yàn)殚g距越小,擠密效應(yīng)越顯著,樁—承臺(tái)對(duì)土的夾持作用越大,因此樁間土產(chǎn)生液化需要的振動(dòng)時(shí)間就越長(zhǎng)。這一點(diǎn)通過對(duì)模型箱場(chǎng)地剪切波速的測(cè)定(本文發(fā)明的用于室內(nèi)模型箱場(chǎng)地剪切波速測(cè)定裝置)進(jìn)一步予以了證明。 (5)通過對(duì)群樁試驗(yàn)不同工況沉降時(shí)程的分析,引入沉降動(dòng)力放大系數(shù)分析了不同樁間距條件下,SDAF隨振動(dòng)時(shí)間的變化規(guī)律,建立了線性統(tǒng)計(jì)表達(dá)式,為復(fù)合樁基動(dòng)力設(shè)計(jì)的靜力計(jì)算轉(zhuǎn)化提供了基礎(chǔ)。 (6)有限元數(shù)值模擬分析結(jié)果進(jìn)一步證明：復(fù)合樁基對(duì)樁間土的抑制作用隨樁間距增大而減小,當(dāng)樁距為6D(D為樁徑)時(shí),抑制作用幾乎完全喪失。有限元分析還表明,承臺(tái)剛度提高對(duì)樁間土液化產(chǎn)生有一定的延緩作用,且承臺(tái)剛度越大,這種延緩作用越大。 (7)對(duì)不同樁間距、承臺(tái)剛度條件下,復(fù)合樁基體系豎向承載力隨振動(dòng)時(shí)間減小的變化規(guī)律進(jìn)行了計(jì)算分析,通過對(duì)樁側(cè)摩阻力和樁端阻力隨振動(dòng)時(shí)間變化的進(jìn)一步深入研究,提出了動(dòng)力荷載作用下的樁基豎向承載力計(jì)算公式：Q=β1η5Qsk/γs+β3ηpQpk/γp+β2ηcQck/γ,其中β1、β2、β3分別為考慮動(dòng)力荷載作用的樁側(cè)摩阻力、樁端阻力和承臺(tái)下土抗力折減系數(shù),與動(dòng)荷載的作用和土層條件有關(guān)。在本次研究條件下,當(dāng)樁周土體部分液化時(shí),β1和β2可取0.7,當(dāng)樁周土完全液化時(shí),β1和β2可取(0～0.55),荷載作用時(shí)間越長(zhǎng),取小值。當(dāng)土體未發(fā)生液化時(shí),可取β3=1,當(dāng)土體一旦發(fā)生液化取β3為0,即不計(jì)入承臺(tái)下土的抗力。
[Abstract]:The investigation of earthquake damage shows that the pile foundation in liquefiable soil layer is liable to damage the superstructure and cause serious economic and property losses. Although the current relevant codes and domestic and foreign scholars have put forward some methods in pile foundation seismic design, but these methods for the existence of liquefied soil layer pile foundation design still has obvious shortcomings, the main reason is that the existing pile foundation seismic design method is based on the static load theory and experience, dynamic. The interaction mechanism between pile cap and liquefied soil under load is not well understood, which makes the calculation method itself lack reliable theoretical basis and has many defects.
Based on a series of shaking table tests and MIDAS GTS numerical simulations of single pile and group piles, the vertical bearing capacity of composite pile system is analyzed and systematically summarized.
(1) Based on the non-dimensional theory of dynamic similarity in model test, by measuring the density and elastic modulus of materials with different proportions, the model material of concrete pile was developed for the first time, which ensured the feasibility of indoor model test and provided experience for similar test in the future.
(2) Laboratory model tests show that the excess pore water pressure of saturated sands under horizontal cyclic loading is gradually generated and developed with the vibration time, the pore water pressure ratio in the soil layer increases with the vibration time, and the soil layer liquefies gradually from top to bottom. The pore water pressure in the soil layer causes the pore water to flow upward, which results in the pore water pressure dissipating faster in the lower layer than in the upper layer.
(3) The strain analysis of pile body in single pile test shows that the side friction of pile decreases with the increase of pore pressure ratio in soil layer. After liquefaction, the side friction of pile decreases significantly, but lags behind the time of liquefaction. The side friction of pile increases with the dissipation of pore water pressure.
(4) The shaking table test of 3 *3 pile group with low cap shows that the composite pile system has an inhibitory effect on soil liquefaction, and the smaller the pile spacing, the more obvious the inhibitory effect. This is further proved by the measurement of the shear wave velocity at the model box site (the apparatus for measuring the shear wave velocity at the model box site invented in this paper).
(5) Based on the analysis of settlement time history of pile group under different working conditions, the variation of SDAF with vibration time under different pile spacing is analyzed by introducing settlement dynamic amplification coefficient, and the linear statistical expression is established, which provides the foundation for the static calculation and transformation of composite pile foundation dynamic design.
(6) The results of finite element numerical simulation further prove that the restraining effect of composite pile foundation on soil between piles decreases with the increase of pile spacing, and the restraining effect is almost completely lost when the pile spacing is 6D (pile diameter). The finite element analysis also shows that the increase of pile cap stiffness has a certain retarding effect on soil liquefaction between piles, and the greater the pile cap stiffness, this is the case. The greater the delay is.
(7) Under the condition of different pile spacing and pile cap stiffness, the variation law of vertical bearing capacity of composite pile system with the decrease of vibration time is calculated and analyzed. K / gamma S + beta 3_pQpk / gamma P + beta 2_cQck / gamma, in which beta 1, beta 2 and beta 3 are related to lateral friction, pile tip resistance and soil resistance reduction factor under cap, dynamic load and soil condition respectively. under this study condition, beta 1 and beta 2 can be taken up to 0.7 when the soil around the pile is partially liquefied, and beta 1 and beta 2 can be taken into account when the soil around the pile is completely liquefied And beta-2 is preferable (0-0.55), the longer the loading time is, the smaller the value is. When the soil is not liquefied, beta-3=1 is preferable. When the soil is liquefied, beta-3 is 0, that is, the soil resistance under the cap is not included.
【學(xué)位授予單位】：太原理工大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2014
【分類號(hào)】：TU473.1

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