This paper summarizes the current empirical methods for estimating ground surface settlements caused by excavation. The different points among the current empirical methods are discussed. Four excavation cases with well documented measurements are used to compare the suitability of these methods. The present study indicates that Hsieh and Ou's method can give good concordance with field measurements for concave type as well as spandrel type of settlement troughs. Besides, ground settlements at various depths behind the wall are studied using finite element method. Based on the analysis results with the field observations,a tentative method for predicting settlement at various depths is then proposed.
This paper evaluates earth pressures recorded during the excavation of C.K.S Memorial Station of Hsintien Line of the Taipei Rapid Transit Systems. The main theme is on wall friction and vertical earth pressures. It can be demonstrated that the assumption normally adopted in the design of retaining structures for braced excavations that the vertical earth pressures equal to the overburden pressures could be erroneous if wall friction is considered. As a result, the vertical pressures on the active side are often over-estimated and those on the passive side under-estimated. Based on field measurements, it is appropriate for soft to medium stiff sites to assume that the angles of wall friction equal to the angles of internal friction of soils in computing the limiting active and passive earth pressures in designing retaining structures of braced excavations.
Design and analysis of diaphragm wall requires knowledge on soil mechanics as well as behaviors of strut and wall. The design is generally based upon assumptions that deviate a lot from practice, which may lead to erroneous results. This paper discusses the nature of a few design assumptions, including analysis model, soil strength, modulus of subgrade reaction, water pressure, surcharge loading, preload of struts,etc. The effects of construction control on the behavior of diaphragm wall are also addressed. Basically, the design must rely heavily on experience, or else it may be completely incorrect.
Most of the current high-rise buildings use raft foundation as their main foundation system, and the modulus of subgrade reaction (Ks) is considered as one of the essential design parameters. Basically, modulus of subgrade reaction is not just a soil parameter, it is affected by the structural stiffness as well. This paper lists various approaches for the selection of Ks；an example is provided for comparison purpose. The effects of diaphragm wall and structural stiffness is addressed, and the authors also recommend a set of Ks values for reference. For critical structural design, it is suggested that a sensitivity study on Ks be always carried out.
Pier A15 of Chunghsiao Bridge is founded on 16 steel pipe piles with a diameter of 500mm. Six of them blocked the way of two stacked tunnels of the Taipei Rapid Transit Systems and had to be removed before the shield machine passed under the pier. The bridge structure is a continuous prestressed beam. The settlement of the pier was limited to 10mm. A pit was made first to expose the pile cap and to provide sufficient headroom for subsequent operations. Five cast-in-place concrete bored piles were installed and cast into a load-transfer slab. The load from the bridge was transferred to these new piles by jacking the pile cap against this transfer slab before the cutting of the old piles. A total jacking force of 483 tones was applied. After all these six piles were cut, the transfer slab was structurally connected to the pile cap to form a composite cap and the operation was then completed. Instrument readings indicate that the pier heaved up by an insignificant amount of 1.5mm during underpinning. The settlements of new piles were only 0.2mm. The lower tunnel has been completed and the settlement of the pier during the passing of the shield machine was only 1mm.
The authors have incorporated hyperbolic model into finite differential computer program FLAC to make it possible to simulate non-linear behavior of soils, and subsequently, to develop a deep excavation analysis model. A chosen case of Taipei tunnel affected by adjacent excavation is evaluated by the above-mentioned to find the possible changes in stress and deformation of the adjacent shield tunnel during deep excavation. Effectiveness of some protect measures are also assessed in the paper for future engineering practices.
Drilling is one of the most popular techniques of site investigation. Its accuracy is fundamental to a successful site investigation program. The drilling supervisor should have a sound knowledge of construction methods and design process and be familiar with drilling techniques as well (e.g. their limitations and ways to interpretation). Unfortunately, in most cases, a junior engineer instead of a senior one is sent to the site as the supervisor. Therefore, it is highly possible that accurate and useful results of underground conditions may not be obtained. Based on the author's experiences on site investigation, checking lists concerning the scope and process of drilling are presented in this paper.