鑽掘樁在複層地層中不同軸向壓載承載力評估模型之探討

Performance of Selected Analysis Models for Axial Compression Capacity of Drilled Shafts in Multiple Strata

陳彥霖(Yen-Lin Chen)

指導教授 ： 陳皆儒

國立暨南國際大學/科技學院/土木工程學系/碩士(2017年)

摘要

由於鑽掘樁本身具低施工噪音、震動小，對環境危害程度較輕等優良特性，已是工程中常選用之深基礎型式。過去十幾年來，學者們對鑽掘樁的研究有相當大的進展，儘管如此，現有鑽掘樁評估大多採用單一地層主導為前提所做的探討。事實上，鑽掘樁隨著土層深度的貫入而遇上兩種或者更多種類的地層狀況是相當常見的，因此評估鑽掘樁承載力時，能綜合考量樁身貫入土層所遇到之所有地層類型及其互制行為之模式才是較合理的作法。本研究選用三組能評估基樁於複層地層中之承載力評估模式，透過建置之試樁資料庫來探討不同評估模型之適用性或合理性。此三組評估模式分別為:(1)以Kulhawy等學者所發展出之一系列可用於評估各式地層條件下的基樁承載力評估方法(簡稱為：Kulhawy方法)；(2)依據台灣建築物基礎構造設計規範建議之設計模式(簡稱為：TGS (2001) 方法)；(3)依據加拿大基礎工程手冊建議之設計模式(簡稱為：CFEM (2006)方法)。本研究透過蒐集世界各地試樁資料建立一資料庫進行相關探討，本資料庫共取得165個可用之鑽掘樁試樁案例，其中試驗基樁之樁徑介於0.3公尺至2.5公尺之間、樁長介於4.6公尺至76公尺之間而其細長比則介於0.018至0.25之間。基樁所貫入之地層概可分為：(1)純黏土地層、(2)純砂土地層、(3)砂黏土互層地層及、(4)複合地層。各試樁載重試驗結果經詮釋可得破壞載重之數據，再利用模型評估值與試驗詮釋值進行比較。根據比較之結果，基樁於純黏土地層之承載力評估，以Kulhawy方法及TGS (2001)方法有較佳預測成果。對於砂性土層之評估，就樁身阻抗而言，三種模式皆有不錯的相關性但都有低估之趨勢，而對於樁底阻抗來說，三種評估模式估算結果皆有高估之趨勢。而在複層土層中，其整體預測之離散程度相對較大。另外，本研究亦使用評估模式的偏差係數來進行修正，使承載力的評估更加合理。結果顯示，對於基樁軸向壓載乘載力之預測，採用修正的模式對Kulhawy方法有較佳的預測改善。本研究根據分析成果提出相關之建議，可供未來工程分析設計使用之參考。

關鍵字

鑽掘樁 ； 軸向承載力 ； 分析模式 ； 偏差係數

並列摘要

The drilled shaft has been the preferred foundation system employed in projects involved with use of deep foundation owing to its beneficial features of low noise and vibration during installation. Significant advances in drilled shaft researches have been made over the past few decades. Most previous researches have focused on drilled shafts in a single predominat geomaterial. However, it is not uncommon for a drilled shaft to encounter multiple geomaterials over the depth. For rational evaluation of axial compressive capacity, approaches that can taking account contributions from various types of geomaterial are needed. In this study, three analysis models that can be applied for computing axial capacity of drilled shafts in multiple strata were selected for critical evaluation of their performance on capacity evaluation. The selected models include : (1) The Kulhawy Approach, which is a generalized capacity evaluation model that can be applied for various types of drilled foundation in several differernt gomaterials developed by Prof. Kulhawy and his associated researches, (2) The TGS (2001) Approach, which is the computation model prescribed in the Taiwanese Foundation Design Spectification for Building, and (3) the CFEM (2006) Approach, which is the computation model given in the Canadian Foundation Engineering Manual. The groud profiles can be categorized into four different types, including (1) pure clay profile, (2) pure sand profile, (3) interbededly clay and sand profile, and (4) profile with multiple starta. Evaluation ofperformance of the selected models were carried out through analyses using a compiled database consisting of load test case histories from all over the world. A total of 165 load test case histories were obtained. The diameters of shafts range from 0.3 m to 2.5 m, the depthes range from 4.6 m to 76 m, while the diameter over depth ratio for these shafts range from 0.018 to 0.25. The measured failure loads were interpreted from the load test results. Compariosn were made between the interpreted loads and those predicted from the analysis models. For drilled shafts in clay profile, the axial capacity can be better predicted by the Kulhawy and the TGS (2001) approaches. For shafts in sand profile, all three approachs can result in consistent predictions. However, all three approaches tend to underestimate slightly the side resistance and overestimate the tip reistance. The predicted results are somewhat scattered for shafts in multiple strata. In this study, model factors for shafts in different ground profiles were computed for all approaches. The model factors characterized the mean bias for the prediction model and can be used as the corrections for capacity evaluation. Satisfactory results can be obtained when corrections were applied for evaluation of the compression capacity for drilled shafts in various profiles. Based on results of this study, recommendations are made for consideration on analyses of axial compression capacity for drilled shafts.

並列關鍵字

drilled shafts ； axial compression capacity ； analysis model ； model factor

參考文獻

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