Results of obtaining the dimensions and the structure of the zone of weakness, time history of their change in the course of excavation of the machine hall are described. Comparison of parameters of loosed zone with characteristics of rock falls during mining are Presented. On the basis of in-situ geological-geophysical observations a geomechanical model of unloading of the rock mass is described.


Dans le present rapport sont envisages les resultats d'etudes par ultra-son des dimensionset de la structure de la zone faible, du developpement de leurse variations Pendant l'excavatlon des salles des machines et des transformateurs de la centrale de Hoabinh. Sont compares les parametres de la zone faible avec les caracteristiques des eboulements des roches ayant lieu Pendant les travaux de mine. sur la base des observations geologiques et geophysiques in situ est decrit un modele geomechanique de ia decompression du massif.


Die resultaten der ultraschalluntersuchung der ausmamasse und der strukture der geschwachte zone, der dinamik ihrer veranderung wahrend der Projektierung des maschinen und der transformatorensalen des wasserkraftwerks Hoabinh betrachtet warden. Die parameter der geschwachte zone mit der charakteristiken der hinauswerfungenjinfolge des bergbaues vergleichtwerden. Auf grund der geologischen und geophysischen naturbeobachtungen die fur die korrektierung der wanoverstarkungprojekt aus gebescnrebt wird.


The machine hall (MH) of the Hoabinh Project is of an underground construction in effusive rocks, by the design the MH is unique construction of 260m in length. 20m in width and 57m in heigth from the bottom of cavern to the arc portion Structures of he power waterway are located in the minimum faulted tectonic block composed mainly of clastolavas of basalt porphyrites and lava breccias formed by clastolavas and lavaclastites the distinguishing features of construction of the machine hall complicating the process of construction are the following: distinctive characteristics of the design (large chamber excavation located nearby - machine and transformer halls), complicated engineering-geological conditions (Shear-risk dikes steeply dipping towards excavations and tectonic fractures of order-V), employment of explosions of a mass UP to 2000kg and intensive mining operations resulting in a rapid exposure of large areas of the MH walls favouring an embrittle-blocky de-stressing of the rock mass. The most unfavourable combinations of the abovementioned factors is observed in the interchamber pillar (ICP) between the machine and transformer halls (TH). At the initial stage the construction of the MH was accompanied by an intensive large-block inrush of rocks occuring on dipping surfaces of V-order fractures. This situation generation a need for monitoring of effectiveness of supporting of the rock mass and stability of MH walls especially at the sector of the ICP. Two kinds of ultrasonic observations are carried out. In the first stage of construction mass ultrasonic investigations are produced in the anchor bore holes. Latter on in the course of excavation of the MH a network of observations were carried out which include: engineering-geological, geophysical (ultrasonic, acousticemission, seismometric), dynamo-deformometric observations. It gave a chance to observe the development of unloading zones in the time under influence of the building works. The location scheme of observation bore holes and instrumentation in the ICP is shown in Fig. 1 in which the main elements of the engineering-geological structure of rock mass - tectonic fractures of order-V and likes of diabase-dacite and gabbro-diabase are indicated on the lower wall of the machine hall. The observation points are arranged regularly along the whole length of the MH at elevation 11.0, 15.0 and 26.5m. Geophysical bore holes are arranged parallel with anchor bore holes at a distance from 0.5 to 1.0m from measuring press stressed anchors.

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