地盤凍結工法在地下開挖工程之應用

Abstract

本論文說明以凍結工法進行輔助隧道與豎井開挖的設計及施工方法，並對台北捷運系統施工現地所測得之凍土成長資料進行分析討論。首先介紹地盤凍結工法之基本原理、此工法之優缺點、常用的施工方法、凍結管型式、凍土形成之監測、防熱工程及凍土壁的設計原則。在國外案例方面，探討芬蘭赫爾辛基捷運隧道、紐約豎井連續壁復舊工程、美國雪城鐵路下方隧道、密爾瓦基下水道系統跌井、密西根 Kentwood 礦區工作井、日內瓦地下大型實驗室聯絡井、挪威水力發電廠隧道維修及南迴鐵路隧道通風豎井之凍結工法施工。研究結果顯示，由於凍土具有足夠之強度及良好的止水性，適合做為支撐擋土及阻隔地下水之用途。凍結工法在我國捷運隧道工程之首樁應用案例為新店線 CH221 標意外事故的復舊工程。本研究依據 CH221 標凍結工法施工所測得之凍土成長資料，建議以三段式線性模式來模擬凍土溫度下降與時間的關係。溫度監測資料顯示，凍土溫度下降速率可分為以下三階段：(1)土層之自然溫度至純水的凍結點（0℃）；(2) 0℃至凍土設計溫度（-12℃）；(3) -12℃至不凍液溫度（-25℃）：其中第一階段之溫度下降速率最快。在凍土成長速率方面，由於松三層、松二層土壤中皆含有不少粉土，故土壤種類對凍土降溫速率之影響不明顯。松二層凝聚性土壤因含水量略高一點，其溫度下降的速率較松三層砂土層稍快一些。本研究並發現，不論是第一、第二或第三階段，隨測溫點距凍結管距離之逐漸變遠，凍土溫度下降速率逐漸減緩。

The application of ground freezing method to underground tunnel and shaft construction is demonstrated in this article. The principles of ground freezing, advantages and limitations of the method, types of freezing pipe commonly used, monitoring of the frozen soil, and methods for design are discussed. Case histories of underground excavation associated with freezing method reported at Helsinki, New York, Syracuse, Milwaukee, Michigan, Geneva, Norway, and Taiwan have been summarized. It is concluded that frozen soil demonstrates both high strength and extremely low permeability. The method is an effective measure to resist earth and water pressure, and to cut off ground water during underground excavation. The application of ground freezing method to the construction of Taipei Rapid Transit Systems (TRTS), Hsintien Line lot CH221 is investigated. Based on the field data, it is suggested that the temperature versus time relationship could be simulated with a three-stage linear model. Since both the silty sand and silty clay layers contain significant amount of silt, therefore, the rate of temperature drop for both soils are similar. However, since the clayey soil has a slightly higher water content, as the result, the rate of frozen soil development is little faster than that silty sand layer. It is also found that the rate of temperature drop of the frozen soil decreases with increasing distance from the measurement point to the freezing pipe.