壁樁垂直承載力試驗之案例分析

Abstract

近年來，由於連續壁施工機具與技術的進步，採用連續壁直接作為承載樁(簡稱為壁樁)的工程案例越來越多。壁樁與一般傳統圓形斷面基樁，除了施作方式不同外，其斷面形狀也有所差異，然而目前工程界均直接引用一般圓形樁之設計分析方式來評估壁樁之極限承載力，其適用性應進ㄧ步驗證。因此，本研究主要目的是探討壁樁受垂直載重的行為及其承載能力。 本研究規劃進行大安變電所之壁樁試驗，包括下壓與拉拔試驗各一組，並蒐集近年來台北鄰近地區之壁樁靜力垂直承載試驗資料，共計有8組資料，壁樁的尺寸介於80cm×260cm~100cm×270cm之間，樁長則介於30m∼50m之間，分析這些基樁之垂直承載力，以及相關土層之t-z與q-z曲線，繼則比較壓力試驗與拉拔試驗所能發揮樁身摩擦力之差異，以及矩形壁樁與圓形基樁承載能力之差異；最後則利用統計回歸的方式，歸納出壁樁的垂直承載曲線特性，建立出圖表與關係式，以供工程設計分析使用。 本研究利用有限元素法模擬不同斷面形狀之基樁受垂直載重作用行為，並由分析所得之t-z與q-z曲線探討基樁斷面形狀所造成之影響；另針對本研究規劃施作之大安變電所壁樁現地試驗進行垂直承載之模擬分析。分析模型同時考慮壁樁基礎的三向度結構、土壤之彈塑性反應及樁-土壤間之互制作用。分析結果顯示，不論下壓或是拉拔模擬，在砂性土壤中，壁樁樁身摩擦應力之發揮均較圓樁為大，且二者之差異隨深度增加而降低；而在凝聚性土壤中，下壓模擬亦是壁樁樁身摩擦應力之發揮均較圓樁為大，但二者差異甚小，拉拔模擬仍以壁樁樁身摩擦應力之發揮均較圓樁為大，但二者之差異隨身度增加而降低；在大安變電所案例模擬部份，在黏土層之模擬結果與現地試驗相近，至於對砂土層與礫石層之模擬結果，則與現地試驗仍有差距。

In recent year, owing to the improvement of construction machine and technique of diaphragm walls, more and more cases use the diaphragm walls as supporting piles for structures. The diaphragm wall differs from the traditional circular pile not only by on the shape of cross section, but also on the method of construction. However, in engineering applications, the design method for wall-type pile still use the conventional method for circular piles. The suitability for this kind of analysis needs to be investigated. The purpose of this study is to investigate the behavior and bearing capacity of axially loaded wall-type piles. In this study, a set of wall-type pile load tests for the Daan Extra-high Voltage Substation(E/S), including a compressional test and a uplift test, is conducted. Besides, 8 sets of diaphragm wall load test cases in Taipei area are collected for analysis. All the test diaphragm walls are constructed by MASAGO-type machine with size ranged from 80cm×260cm to 100cm×270cm, and length ranged from 30m to 50m. From the analysis, the characteristics of the t-z curves and q-z curves for both the compressional and uplift cases are obtained. Besides, based on all test data, a normalized load-deflection curve and corresponding parameters for stiffness are deduced for engineering applications. In addition, the finite element method by using the compute code ABAQUS is adopted in this study to simulate the response of axially loaded piles with various shapes of the cross sections. The characteristic t-z and q-z curves are deduced from the simulation analyses and discussed. The results of numerical analyses reveal that the skin friction developed in the wall-type pile is generally greater than that of circular pile for the cases of sand layers. However, for the case of clay layers, the skin friction developed in both the wall-type pile and circular pile are almost same. Then, the finite element method is used to simulate the wall-type pile load test for the case of Daan E/S. A three-dimensional elasto-plastic analysis with frictional interface between the pile and soil are considered in the analysis. Results obtained are agreed well to the field data.