Gravel formations are mainly distributed in the areas surrounding the Central Range of Taiwan. Owing to the high altitude and steep slope, the rock strata of the Central Range often crumble to blocky debris after slight weathering and slide down slope with mass movements. The rock fragments are transported downstream by debris flows﹑ floods and turbidity currents and deposited in the river valleys, alluvial fans, floodplains, beaches, and eventually deep seas. After deposition, gravels may be buried to depth, compacted and consolidated to become conglomerates. If subjected to orogenic influence, the gravel beds may be uplifted as terrace and tableland deposits, or folded as tilted conglomerates.
The temporal-spatial distribution of gravel formations in Taiwan is mainly controlled by orogeny. In early Tertiary time, Taiwan was part of the China continental margin which was covered by sediments derived from Fujian. Since the late Mesozoic time, Fujian has been a hilly terrain which produced mainly sands and muds, and few pebbles. Hence the early Cenozoic strata of Taiwan are dominated by sandstones and shales with only a few thin-bedded conglomerates. After the Pliocene, the Penglai orogeny gradually reached its climax and the Central Range uplifted rapidly. A huge amount of rock fragments were derived from the Central Range and deposited in the surrounding basins to form the thick piles of Pliocene-Quaternary gravel formations.
Sedimentary structures of gravel deposits are modified by hydraulics, water depth, bedloads etc. of the environments, even in one single area the facies shift very frequently. The sedimentary facies and structures are easily neglected during the discussion of gravel slopes in engineering design, or in the investigating processes. Therefore, different opinions arise, and the actual reason is that gravel slopes are not uantitatively described . After a brief introduction of sedimentary facies and structures of gravel deposits, this paper attempts to suggest one set of quantitative description methods for gravel slopes.
With the increase of infrastructure development in Taiwan, more and more constructions will encounter the gravel formations. For a better understanding of the material properties of the gravel formations and the construction techniques, relevant data and cases are collected and reviewed. General discussions are provided in this paper. All data obtained are from the western areas of Taiwan and they are categorized into six groups based on the geological districts namely Linkou, Miaoli, Taichung, Tatushan, Pakuashan and Chushan. The material and engineering properties of the gravel formations are discussed. As for construction in gravel formations including excavation, tunnelling and grouting are all reviewed.
The geotechnical characteristics and behavior of gravel formations are significantly different from those of alluvial and sedimentary deposits or other rock formations. Due to the facts of complex material properties and orientations of particle arrangement, the results obtained from the laboratory tests and field measurements are quite different and diverse. Up to the present time, the basic data of the gravel formation in Taiwan are still limited. Hence, the reliable relationship between the mechanical properties and index properties for the gravel formation is still under developing. In the authors opinion ,the main reason of the difference may be due to the method of exploration, classification system, testing and analyses used by the engineers are quite different from each other. In other words, there is still lack of standard criteria that can be followed. Therefore, the geotechnical engineers often encounter some problems when deal with gravelly formation. This paper aims to present a rational approach of exploration and testing method for the gravel formation. Furthermore, through the existing data conducted by the authors and others, it is hoped that this paper will stimulate the understanding of the field geotechnical engineering properties of the gravel formation.
A significant amount of experience has been accumulated through years of research and development in characterizing the mechanical properties of gravel materials. However, due to deficiencies of experimental procedures and the scattering nature of test results, there are many controversial issues related to the fundamental mechanical behavior as well as practical applications of gravelly soils. This paper summarizes current domestic and overseas development efforts in laboratory and in-situ experiments on gravelly soils. Possibilities of applying these experiments in engineering practice and their future development are recommended to our peers for their reference.
The northern segment of the Sanyi Tunnel for a length of 413m with a diametr of 10.6m is being driven through gravel layer. It is a new double-tracked railway tunnel of the mountain line near central Taiwan. The gravel portion could be divided into two sections：a 183m section going underneath the existing freeway with an overburden of 8～20m, and a second section of 230m is driven southward through the Miaoli hilly district. Due to the high ground water level above the tunnel, a pretreatment of grouting and forepiling is recommended before excavation.
This paper describes the tunnelling methods adopted for the 230m section, where the progress was only 5.7m per month at the beginning, and had collapsed four times. Gradually, the progress rate was improved to 26m a month after the grouting and tunnelling techniques were applied. The results were compared with that of the other section underneath the freeway, where the grout of microfine cement and the minipipe roofing were used.
The shear strength and permeability of gravel layer are in general higher than those of sandy or clayey materials, and the behavior of deep excavation in gravel layer is different as a result. This paper describes three deep excavation sites located in Taipei county, which were mainly excavated within gravel layer, and the characteristics of monitoring data are discussed. These cases indicated that pressure acting on diaphragm wall and internal struts were relatively small for deep excavations in gravel layer, and the wall displacement was small as well, while the control of groundwater flow was a main issue during diaphragm wall construction and subsequent basement excavation.
The effects of using rock bolts and prestressed anchors are highly affected by the geological conditions and the construction processes. In recently years, many types of rock bolts and prestressed anchors have been developed in Taiwan. The young Liushuang Formation of the western foothills geological province in southwestern Taiwan is composed of loosely cemented sandstone and mudstone alternations. Both of their PH value and silt content are high. These alternations also contain expansive clay minerals, and thus show slaking through absorption of water. In consequence, these rocks show high strength when dry, but the strength drops drastically when wet. There is very little case history of engineering application in this rock formation. In order to gain some engineering insights to such rock formations during tunnel excavation and slope protection, in-situ tests were conducted on nine types of rock bolts and six types of prestressed anchors. During these tests, the following engineering characteristics were studied :(1). The effects of drilling operation, materials for the fixing section, type of bolt or anchor, and grouting method on the pulling behavior of rock bolts and prestressed anchors;(2). Long term creep effects when rock bolts or prestressed anchors are installed in sandstone and mudstone alternations; and (3). Considerations for design safety factor.
The test results indicated that applying an expansion end rock bolts and prestressed anchors through dry drilling of boreholes would achieve a more desirable pulling strength in the sandstone and mudstone alternations. Taking long term creep into consideration, appropriate drilling of borehole, adequate installation method and safety factor will enable correct mastering of the pulling behavior of the rock bolts and prestressed anchors in sandstone and mudstone alternations.