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Development of stress measurement system by overcoring method

suitable for soft rock

Hem Nath Ghimire,
Rock Mechanics Laboratory,
Hokkaido University.


The measurement of rock stresses has become more common in recent years. But most of the data are from hard or moderately hard rock, with very few from soft rock. It has been felt necessary to develop a convenient method for stress measurement in soft rock. Overcoring methods, although the most reliable methods of rock stress measurement, require much time as well as heavy equipment and a skilled technician. This research aims to develop a system that measures in-situ rock stresses in hard, moderately hard and soft rock, while affording greater efficiency than existing methods, keeping in mind that the expense and measuring period be small as possible.

For the development of stress measurement system for the soft rock, certain restrictions had to be considered. The time required for the measurement should be as short as possible to overcome the problems due to time-dependent deformation of soft rock. Inserting type stress meter composed of cantilever beams where strain gauges are attached and which measures the deformation of the pilot borehole during overcoring is the best option for soft rock.

Considering all the restrictions for soft rock, a stress meter has been developed that measures seven components of diametrical and axial deformations in a pilot borehole during overcoring and records the measurements on a small data logger installed within it. From these deformation data, three-dimensional stress states in rock are determined using the observation equation derived from analytical elastic solution. The stress meter is inserted into a 40-mm pilot borehole that can range in water content from dry to completely wet. The advantages of this stress meter are its smallness, ease of use, reusability, and ability to measure in borehole of any orientation at any desired depth. At the same time, as the pilot hole does not require any finishing and overcoring can be started immediately after the stress meter is inserted in to the pilot borehole, this system is more efficient than any existing overcoring methods. Moreover, elastic properties of the rock can also be measured using the core in which the stress meter sits. Although the system is particularly suited to soft rock, it is applicable to any type of rock since the stress meter is highly sensitive.

Laboratory experiments were conducted to confirm the functioning of the stress measuring system by using three welded tuff blocks (40 x 40 x 40 cm) with boreholes of 40 mm in diameter at different orientations. The stress meter was inserted into the borehole and two cycles of uniaxial loading and unloading were carried out to simulate the stress relief. At the same time, elastic properties of the rock were measured using the radial and axial deformation data recorded by the data logger during the loading and unloading cycles. The calculated stresses were found to correlate closely with the applied stresses in all the three cases, indicating that the stress meter functions well.

In-situ stress measurement system was developed with the introduction of certain special equipment for drilling to overcome some difficulties in the conventional drilling system. A tapered bit is used to finish the bottom of the large diameter hole. This tapering helps for the centering of the pilot hole and also to insert the stress meter smoothly into the pilot hole. While drilling the pilot hole, a special centralizer is used. This helps in collecting the drill cuttings inside a hollow cylinder capped at the bottom so as to minimize the chance of clogging the pilot hole by the cuttings, and at the same time minimizes the fluctuation of the pilot hole. Double core tubing is used for the overcoring, so as to retrieve the core with minimum disturbance.

A vertical borehole of 50m depth was drilled in the diatomaceous mudstone, which is classified as weak rock, at Horonobe, Hokkaido and line-wise stress measurement by the borehole deformation method was carried out at different depths. The rock is massive with density as low as 1.48 g/cm3, reflecting high porosity of 45%, and mean value of the compressive strength is 3.21MPa. Fresh water and bentonite was inevitably used as the drilling fluid to maintain the fragile borehole wall.

In spite of the frequent occurrence of fracturing of the core due to overcoring, seven results were obtained. Maximum principle stress directs EW and the stress is biaxial with one of the principal stress being nearly equal to zero in the subsurface region of Horonobe district. This characteristic of stress condition is in harmony with the direction of the active folds distributed in the vicinity of this district. Two types of fractures, longitudinal and cross, frequently developed in the thick walled cylindrical core could have been induced by overcoring under relatively high stress condition with low rock strength. Effect of these cracks to the accuracy of the measured stress has been discussed.

The efficiency of measurement was highly dependent on the condition of borehole. Up to three sets of measurements were taken within eight hours working period, at section with comparatively good rock condition with fewer cracks. It has been proved that the stressmeter is applicable in weak rock and that the measuring system adopted is practical, although the stressmeter suffered some damages at sections where the core was highly fractured.


Characteristics of flow and sediment diversion at open channel junction


Lama Sunil Kumar

Department of water Resources
and Environment engineering

Hokkaido University

     Many applications involving diversion of flow through the junction are found in various field of engineering for examples in irrigation, flood relief, sanitary, water supply and power engineering etc. Flow characteristics at the channel junction are very complex, so the general analysis of junction flow with taking all the existing parameters is almost impossible. Throughout the different flow and dynamic condition occurring at the junction, main governing parameters are selected among the existing numerous parameters and junction structure is hydraulically designed with the particular engineering interest.

    The interest found to be providing on the junction flow study are to find out the discharge division, flow characteristics around the junction and if the problem is related with the mobile bed then sediment diversion in to the branch channel with the shoaling taking place at main and branch channels. In most of the engineering interest, entering of sediment and shoaling near the intake are the serious problems, so these problems are need to be solved with the comprehensive hydraulic study of the junction flow for the various flow conditions.

     Besides the withdrawal of the clear water through the intake, diversion of sediments to the downstream of the dam or barrier, constructed across the river is also being the emerging interest for the researchers as a tool on counter measurement of the reservoir sediment management. In order to increase the useful life of the reservoir and to have the sediment deficit replenishment downstream of the dam, sediment diversion could have the major role in this regard. In this research, hydraulic study of the junction flow for the understanding of the sediment bypass technique is considered to be the prime objective, however this study could also provide some lights on the others objectives of the junction flows study too.