Deep Underground Structures Research Articles

微動を用いた空間自己相関法によるS波速度構造の実務利用への評価

The SPatial Auto-Correlation (SPAC) method applied for array records of microtremors provides a S-wave velocity structure by surface wave inversion with sufficient accuracy. The efficiency of the SPAC method has mostly been verified for relatively deep underground structures as a tool of geophysical prospecting. However, efforts to increase its accuracy and develop the tools for measurements and analysis are still necessary for extending and applying the method in practical engineering use, especially for determining shallow velocity structures. We carried out array measurements of microtremors at 7 sites around Kanto area in Japan, where strong-motion stations have already been installed and the underground structures have been determined by other methods, such as PS logging and/or reflection survey. Through the comparison of S-wave velocity structures by various methods, we can conclude that the SPAC method has sufficient precision in practical engineering use.

Construction of HATS Stage 1 deep shafts in Hong Kong Special Administrative Region (SAR), China

Shafts play an essential part in the construction, operation and maintenance of tunnels and deep underground structures but are rarely given exclusive prominence in technical publications. The aim of this paper is to summarise the experiences gained in Hong Kong SAR during the construction of the Harbour Area Treatment Scheme (HATS) Stage 1 works where seventeen shafts were constructed. The function of the shafts varied from providing construction access to the tunnel works to conveying sewage to and from the tunnel system. The excavated diameters of these shafts range between 2.5 and 50 m. At a maximum depth over 150 m these are the deepest shafts below sea level in Hong Kong SAR at present. All shafts were located in reclaimed land and in close proximity to the sea. The upper shafts in soils and weak rock were constructed by diaphragm walling method and the lower shafts in rock by mainly drilling and blasting. Raise boring and blind shaft drilling methods were also employed. The upper shafts and permanent shaft linings were designed using conventional methods and the primary support selection for the lower shaft was based on Barton's (1974) 'Q' system. Settlement, water level monitoring and inclinometer measurements were undertaken during excavation to confirm the design assumptions. During construction several difficulties were met that had to be overcome. All shafts with the exception of one were successfully excavated and completed. This paper addresses the key design and construction issues and the difficulties that were encountered which may be common for deep shafts constructed in an urban setting near coastlines. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

Development and applications of diaphragm walling with special section steel—NS-Box

Diaphragm walls are used throughout the world to construct deep underground structures, conventionally in reinforced concrete (RC). The NS-Box diaphragm wall is a composite steel section and concrete technique, first developed in Japan in 1992 to meet the demand for strong, stiff walls of reduced thickness, to save space on congested sites and to achieve manpower savings by maximizing prefabrication. This paper gives guidance on how to design and construct the NS-Box walling, and comparisons of different aspects of the performance between the NS-Box and conventional RC diaphragm walls. Details of how the method was successfully employed during the construction of some projects are given.

3次元深部地下構造がやや長周期地震動の特性に及ぼす影響

The effects of 3-D deep underground structures in and around Yokohama City on the characteristics of rather long-period ground motion are examined. The detailed 3-D deep underground structure was modeled by interpreting the time delay between PS converted waves and the direct P waves (PS-P time) calculated from earthquake records. This PS arrival time was determined on the stacked receiver function waveforms. The results show that the basement depth varies between 2.5 to 4.0km. The propagation characteristics of rather long-period ground motions in the southern part of the Kanto basin were examined by analyzing records pertaining to the east off Izu Peninsula earthquake of May 3rd, 1998. The results show that Love wave packets traveling across the basin turn into the main contributors to the observed rather long-period ground motions. The amplitudes of the Love waves appear well correlated with the basement depth. The observed phase velocities for periods between 5 and 10s show good agreement with those of fundamental mode Love waves calculated for the estimated model. However, in cases where steep dipping of the basement is detected, a simple underground model beneath the sites cannot explain the amplitudes and phase velocities. This is caused by the effect of these irregularities of the deep underground structure on the waves propagation.

岩盤 つくばでの最近の研究 （２） 稲田花こう岩の高圧下の強度とクリープ寿命の分布に関する検討

In designing the long-term stability of deep underground structures in a rock mass, it is necessary to estimate the creep failure time of rocks under confining pressure. In the case of nuclear waste disposal tunnel, the creep failure time should exceed 10,000 years. In this paper, experimental results of tri-axial and multi-stage creep experiments are presented. Both kinds of experiments were conducted under confining pressures up to 40 MPa, including uni-axial stress state. In order to eliminate the loading rate dependency of strength in analysis, the concept of standardized strength is introduced. The Janach's failure criterion was successfully applied to explain the relation between standardized strengths and confining pressures. We introduce a new concept of normalized failure time that is thought to be most substantial value in determining the strength and / or creep failure time of a specific rock specimen. It is shown that the dispersion of normalized failure times, which was estimated from the experimental results, exhibited approximately a Weibull distribution. A conversion formula from normalized failure time to strength and / or creep failure time is presented. Using this conversion formula, we showed that creep stress level should be less than 79 % for survival of 99 % of test pieces after 10,000 years under the confining pressure of 40 MPa. It is also shown that not only the distribution of creep failure times, but also the strength distribution under any loading rate and confining pressure can be calculated from the normalized failure time.

Creep behaviour of natural rock salt and its description with the composite model

Natural rock salt in deep underground structures is planned to act as the host material for the permanent storage of radioactive and toxic wastes. Dimensioning and safety analysis of such repositories require a model which allows to predict the creep behaviour of rock salt on the basis of the microstructure and the physical mechanisms of deformation. Already before testing natural rock salt contains a subgrain structure. During deformation under constant stress the creep rate changes until a steady state is reached. The same holds for the microstructural parameters subgrain size and spacing of dislocations inside subgrains. The composite model of plastic deformation takes the observed heterogeneity of the dislocation structure explicitly into account. Using this model in combination with appropriate kinetic laws for dislocation movement, transient as well as steady-state creep of rock salt can be calculated.

微動観測とその工学的利用

Microtremors have been utilized in the earthquake engineering field as an effective tool to investigate underground structures from the ground surface measurements indirectly. In this paper, studies on measurements of microtremors are reviewed focusing on the engineering utilizations. Reviewing is carried out classifying the microtremors into two kinds of short and long period.Initially, short period microtremors are discussed. At the beginning of this century, the first measurement of short period microtremors was carried out, finding that microtremors reflect well ground vibrational characteristics during an earthquake. After that many studies continued to investigate the relation with the dynmamic characteristics of shallow underground structures. Among these studies Kanai and his colleagues' works are distinguished and along their results a standardized method for measurements and analyzing of microtremors is described here. Typical examples for practical applications are referred to including the Kanai's discrimination method for types of ground.Secondly, studies of long period microtremors are discussed which have been developed during the past 20 years in order to investigate deep underground structures up to several kilometers. These studies have been initiated and accelerated by the circumstances of the recent expansion of building height and width. Examples of the field measurements and their interpretation in earthquake engineering use are also introduced.In conclusion an applicability of microtremors to earthquake engineering field is summarized and further problems to be solved are pointed out.

Concerning the technology of constructing pumped-storate stations of a new design

1. The technological methods of sinking underground structures under the effect of their own weight recommended by the building code and those traditionally used (and also the recommendations on eliminating tilting and sticking) do not provide the normal occurrence of the sinking process and do not preclude the possibility of failures. 2. Reliable control of the process of sinking drop shafts can be accomplished by using the external active (adjusted in magnitude and place of application) forces, by using a jack-cable system with simultaneous control of the stress-strain state of the drop shaft (stresses in the concrete, reinforcement, pressure under the cutting curb, skin friction of the curb part). 3. Construction of deep underground structures should be carried out in two and more stages with the use of reliably controlled jack-cable sinking systems.