土釘拉出行為的實驗與理論探討

Experimental and theoretical studies of soil nail pullout behavior

朱志峯(Chih-Feng Ju)

指導教授 ： 洪勇善

淡江大學/工學院/土木工程學系碩士班/碩士(2006年)

https://doi.org/10.6846/TKU.2006.00340

摘要

土釘加勁技術主要靠密集的土釘打設，使土釘與周圍土壤形成一連貫性實體，並利用土釘抗張的特性對邊坡產生約束作用，然密間距情況下可能導致土釘周圍應力影響範圍重疊，而造成拉出阻抗下降。有鑑於此，本研究以單釘拉出試驗瞭解土釘拉出行為，並建立釘-土界面之重要參數“粗糙因子”R，以描述界面粗糙程度與拉出阻抗的關係。此外，亦藉由孔穴擴張理論，模擬土釘拉出時土釘周圍土壤因受剪而產生膨脹及其轉嫁於土釘表面的應力發展；以單釘試驗配合孔穴擴張模式分析，將對於土釘拉出行為有更清楚的認知。最後，透過雙釘試驗結果，剖析尺寸效應對雙釘拉出阻抗之影響。
本研究透過四種砂土材料之三軸試驗結果，建立足以描述砂土力學行為的參數，並將所得的相關參數應用於試驗分析及孔穴擴張理論模式中。經分析結果可得以下結論：(1)在直徑、牙距及顆粒平均粒徑影響下，粗糙土釘的尖峰及殘餘視摩擦係數隨直徑增加而遞減，且隨牙距及顆粒平均粒徑增加而遞增；而光滑土釘視摩擦係數則不受直徑影響，僅與兩材料間之界面性質相關。(2)粗糙因子主要係與砂土粒徑、土釘直徑及牙距等相關變因有關，亦即直接影響尖峰及殘餘視摩擦係數；試驗所得的經驗模式顯示尖峰及殘餘拉出力誤差皆分別小於9.28%及16.08%。(3)以建立的粗糙因子，引入理論模式中的剪力帶厚度關係，可模擬土釘拉出時，周圍土壤的膨脹及土釘表面徑向應力變化情形；結果顯示粗糙因子愈大則土壤膨脹量及土釘表面正向應力增量比皆遞增，最大正向應力增量比發生在No.313砂土，可達初始覆土應力的3.09倍；而土壤膨脹情形亦為大顆粒的No.313砂土最為明顯，最大膨脹量可達0.39 mm。(4)雙釘間互制效應及影響範圍受粗糙因子及土釘直徑影響極為明顯，尖峰狀態下，達到100%群釘效率所需正規化間距S/D與粗糙因子成正比關係，與土釘直徑則成反比關係；且分析結果亦驗證由本研究所詮釋的粗糙因子R，對於模擬群釘效率達100%時所需之埋設間距較為合理。

關鍵字

土釘 ； 拉出 ； 粗糙因子 ； 群釘效率 ； 孔穴擴張理論 ； 覆土應力

並列摘要

The soil-nailed structure is a retaining structure formed by inserting inclusion into predrilled hole, or by hammering reinforcing inclusion into soil matrix. The nails provide resistance against slope slip through nail tensile strength activation or by a frictional force at the soil-nail interface on the nail portion protruding from the active zone into the resistant zone. However, nails are usually installed as a group to form a reinforced structure. Closely spaced nails may cause stresses overlap, which to cause reducing the effectiveness of individual nail. The purpose of this study is to investigate the pullout behavior of the nail through the tests of single and double nails. A non-dimensional surface roughness factor R was described the interface between nail and soil. Additionally, an attempt is made to construct a theoretical model for description of the relationship among the nail pullout and the dilation of soil around the nail using the cavity expansion theory. The influence of size effect for the pullout resistance of double nails was concerned.
The conclusions via the tests and theoretical analysis are summarized: (1) The peak and residual apparent friction coefficients at the soil-nail interface are all increased with increase of the screw pitch and particle size but decreased with increase of the nail diameter and reaches a constant value. Besides, such of smooth nails are not influenced by the nail diameter. (2) The surface roughness factor of soil-nail was defined as the function of a single asperity characteristic, the ratio of the soil particle size to nail diameter, and asperity number per unit length. The test results showed that the apparent friction coefficient at the soil-nail interface were dependent upon the surface roughness of the nail. An experiential regression analysis results reveal the error to be smaller than 9.28% and 16.08% for the peak and residual pullout resistance respectively. (3) Analytical results using the cavity expansion theory show that soil dilation and confining pressure around the nail are increased with increasing the surface roughness factor. The maximum soil dilation can be reach 0.39 mm and confining pressure on the nail can be 3.09 times initial normal stress. (4) Because the stress influence around the nail has the phenomenon of interference and superposition in the experiment of two nails under dense spacing, the pullout force of soil nailing will reduce. The diameter-spacing ratio S/D as peak group efficiency reaches 100% is linear proportion to the surface roughness factor.

並列關鍵字

soil nail ； pullout ； surface roughness factor ； group efficiency ； cavity expansion theory ； surcharge stress

參考文獻

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