Tünellerde, ön araştırma, zemin iyileştirmesi ve stabilite analizleri

Abstract

Tüneller, yeraranın bilinmeyen ortamlarında çalışmanın bir sonucu olarak yapım güçlükleri ile çok sık karşılaşılan, inşaat mühendisliğinin zor bir dalıdır. Tünel projelendirmesinin ve inşasının en önemli adımlan sırası ile ön araştırma (tüm jeolojik, geoteknik, hidrojeolojik vb. durum araştırmaları dahil olarak), toplanan verileri doğru olarak yorumlama ve bunlardan yararlanarak stabilite ve ön boyutlandırma analizleri yapmak olarak sıralanabilir. On araştırma sonuçlan Tünel kazı yöntemi seçimi de dahil olmak üzere tüm inşaat süresince birçok konuda kararlara katkıda bulunduğu için, bu konuya özel bir önem verilmesi -ülkemizde tam olarak uygulanmamakla birlikte- bir zorunluluktur. Bu çalışmada, tünellerde, yapılması bir zorunluluk olan ön araştırmanın neleri içermesi gerektiğine detaylı olarak değinilmiştir. Ayrıca hemen her tünelde uygulanmak zorunda kalman zemin iyileştirme yöntemlerine, ençok uygulanan enjeksiyonlama konusunda ağırlıklı olarak geniş bir çerçevede değinilmiştir. Çalışmanın son bölümünde günümüzde tüm analiz gerektiren konularda biz mühendislere büyük kolaylıklar sağlayan Sonlu Elemanlar Yöntemine dayanan bir paket program ile İstanbul Hafif Raylı Sistemi, Mevhibe İnönü Tünel'ine ait üç kesit yeniden, sonuçlan karşılaştırarak paket programın güvenilirliğini kontrol etmek amacıyla, analiz edilmişlerdir.

Tunnels are underground structures which are recognized as means of attaining convenient transportation through conditions posing natural difficulty or special hazards. Surmounting such natural obstacles as mountainous terrain, rivers and seas by tunnels allow safe and convenient transport at all times irrespective of weather conditions. The expectations of modern transportation networks in allowing continuity of flow for both passengers and goods in transit have become an important social need for developing civilizations. Consequently the scope of tunneling is to embrace convenience of direct transportation coupled with safety, efficiency and continuity of operation. Tunnels have played an important role in the development of civilization. The value of tunnels was appreciated by the earliest civilizations, but primitive man in his attempts to the extend natural caves or enlarged subterranean passages formed by water recognized a need to develop skills in the formation of underground excavations and, notably, tunnels. Tunneling was given considerable impetus with the emergence of the Industrial Revolution, especially for application to transport. The development of canals was assisted by tunneling and this contributed significantly to industrial development during the 18th and 19th Centuries in the UK. Tunnels were seen to form an integral part in the development of railways, and many notable engineering achievements were made in constructing-tunnels through major ranges and mountains. An element of engineering appears to have been introduced into tunneling operations even in early times. The importance of designing tunneling works and specifying their method of construction was appreciated by the Romans in several projects. Tunnel engineering, however, made its greatest contribution during the last 200 years in allowing the excavation, support operations and control of environmental factors to be assessed and an appropriate design produced. vt With the current advanced state of tunnel engineering, the status of geological knowledge pertaining to the site conditions has assumed a more important role. Modern tunneling machines can be sensitive to changing geological conditions which introduces a need to plan for any special provisions. Legget drew attention to the need for a thorough geological investigation, before the construction stage, as being of paramount importance in all work related to the tunnel. He states that accurate geological sections are the first requirement together with knowledge of the rocks' characteristics. Legget remarks that geological data will provide the engineer with an appreciation of the tunneling conditions and so allow improved estimates to be made of anticipated progress. Conversely the contractor involved with the tunnel work, is furnished with geological data which are vital to taking decisions on his construction program and selected methods; pre-knowledge of the geological conditions is the sole means of judging the likelihood of the main tunneling hazards, for example, underground water. It is appreciated, however, that economic considerations enter into the nature of program for determining geological conditions relating to specific tunneling sites. Legget has argued, however, that the rock conditions of the tunneling location need thorough exploration in order to satisfy the chosen design and construction method; the first task of the geological investigation for such a project that of determining whether the geological conditions are favorable for the design and construction of the tunnel. This discussion includes the response of the immediate rock surrounding the tunnel to the disturbance caused by excavation; rock strength, collapse potential and rock pressure need consideration. All tunneling projects have a common aim - namely that of successfully mastering the range of geological difficulties encountered. The geological problems, however, differ from site to site and at each site; the type of difficulties experienced may not necessarily be anticipated. The geological environment presents the major challenge to both the tunnel designers and the builders. Additionally there is often a measure of uncertainty concerning the response of the rock mass to the chosen tunneling method. Although there is, however, general recognition of the importance of knowledge on the geological aspects of tunneling projects, the depth of such knowledge and its appreciation in respect of particular sites frequently gives rise to differing opinions. It follows from this reasoning therefore that each tunneling site calls for higher orders of investigation, evaluation and appreciation of the geological aspects of relevance to the tunneling project. The nature of the geological setting of a tunneling site has a major bearing on the choice of construction method and many aspects relating to safety, design and VH subsequent service, operation and maintenance. Detailed geological factors essentially influence the choice of a particular tunneling method and in some cases govern the feasibility of an entire project. It has long been recognized that detailed and prior knowledge of the geological character of a tunneling site is a pre-requisite to successful design, construction and operation for the majority of such projects. Legget has pointed to the study of geology in tunneling as being of significant benefit, as geological, problems effect both design and construction. Virtually all tunnels are driven in the Earth's crust, with exceptions being, for example, immersed tube tunnels and those formed by excavating trenches in superficial deposits. It is the nature of the immediate rocks surrounding the tunnel and the condition of such natural factors as rock pressure, water presence and general susceptibility to weathering that greatly govern the ease or otherwise of construction and the type of tunnel support desirable for short- and long-term stability The conclusion drawn by Legget is that it is of paramount importance to tunnel work that a thorough geological investigation is conducted before the commencement of construction work, and this would involve producing geological sections along proposed tunneling routes. Additionally, the engineering behavioral characteristics of the rocks are also seen as being of equal importance by Legget. This information Legget argues provides a basis for making judgments on the practical and economical feasibility of the project. He draws attention to Precambrian and other geologically old rock types as presenting excavation difficulties in view of rock strength being generally high and thus increasing the cost of construction. Conversely.geologically younger rocks such as the Paleozoic present greater eases of excavation by virtue of decreased strength and thus reduces construction costs. With the decreasing age of rocks, however, approaching towards the younger formations including recent sand and gravel deposits, tunneling encounters operational difficulties and may require specialized techniques involving grouting, or even freezing, and consequently can result in costs increasing. Ground treatment in tunneling is aimed at achieving ground improvement sufficient to allow excavation to progress with safety, without undue delay and with an acceptable degree of control over water and debris inflow and associated collapse of rock into the tunnel. Ground improvement for tunneling operations is generally recognized as being associated with one or more of the following: lowering the ground water level by controlled drainage; dewatering subterranean water course or major aquifer within vm proximity of the tunnel; controlling water inflow to the tunnel by compressed air; and ground stabilization by grouting or freezing. Many tunneling projects generally need to employ some form of ground treatment process during the life of their drivage. This may be of a very localized nature involving ground treatment of some geological disturbance or significant change in hydrogeological conditions. In some situations extensive ground treatment may be required as, say, when tunneling through water bearing limestone, chalk or highly porous sandstone formations which call for specialized improvement measures: There are various ground treatment methods available to facilitate ground water control and/or ground improvement, namely: 1) Dewatering using well bores 2) Electro- osmosis 3) Grouting 4) Ground freezing Ground dewatering prior to the commencement of tunneling operations is probably the most used and economic water control method. The technique basically involves the artificial lowering of the ground water table by drilling a series of boreholes on either side of the projected tunnel line and pumping out the ground water using either a surface or submersible pump. Electro-osmosis is a dewatering technique which can be used specifically for stabilizing soft clays and silts which present problems to dewatering by conventional well-pointing. The.method is based on the principles of electrolysis. (Further information is given in the following pages) Grouting is the injection of a liquid under pressure into void spaces either in naturally occurring substances such as soils or fissured rocks, or in artificial cavities such as those found in faulty masonry, behind tunnel linings, etc. The basic aim of grouting in tunnels to either block the voids and flow paths in the soil/rock in order to prevent the passage of ground water through them into the excavation and/or to increase the overall strength of the ground to allow tunnel construction to proceed unhindered by running ground, and to promote increased safety in the tunneling operations. IX Freezing is a temporary ground improvement option, which is suitable to a wide range of water-bearing soil types including mixed ground where grouting or compressed air working may not be applicable. Tunneling stability analysis is consist of very complicated and hard mathematical operations and these analyses are done by computer software programs, that commonly depend on Finite Element Analysis. In this thesis SIGMA/W geotechnical software program is used to analyze three sections of Mevhibe İnönü Tunnel which is a part of Istanbul Light Rapid Transit System.