Application of non-destructive methods for testing of index properties of gneissic rocks in large-scale geotechnical investigations

There is an abundance of gneissic rock formations found in Sri Lanka. Gneiss is a banded rock with fairly continuos segregation of different minerals. Foundations of most of the large-scale civil engineering structures are extended up to or into the fresh gneissic rocks. Especially, the foundation of dams, bridges and high rise buildings are extended into fresh rock. In such large-scale geotechnical investigations, boreholes are advanced up to the interested depth of exploration and rock coring is compulsorily done using rotary core drilling machines. Borehole logging is followed by arranging the laboratory testing programme for testing of soil and rock materials. Hence the identification of engineering behaviour of gneissic rocks at the detailed investigation stage is a prime necessity in such projects. Laboratory testing of a large number of rock samples is time consuming and expensive. The general practice of selection of representative rock samples on visual inspection followed by laboratory destructive testing may not lead to a precise interpretation of engineering properties of the entire subsurface rock strata. Non-destructive testing of gneissic rock is identified as a fast and effective method of selection of representative rock samples for a laboratory-testing programme. A 100m deep exploratory rock core of diameter 54mm was selected for this research. More than two hundred samples were prepared and subjected to three different nondestructive tests, followed by destructive tests. Depending on the results of the study, samples having distinct characteristics could easily be identified. In particular, the strata having low compressive strength were clearly identified and hence those samples could be specified for destructive tests. The findings of this research will be immensely helpful to organise laboratory testing programmes on rock samples effectively and economically especially in large scale geotechnical investigations

Durability of rocks under tropical conditions

Sri Lanka has a basement rock formation of metamorphic origin that the majority is gneissic type, which is intensively used in many of the civil engineering constructions. In addition, foundations of most of the important civil engineering structures are extended up to or into the fresh gneissic rock strata. Sri Lanka has a tropical climate. Therefore the study on durability of gneissic rocks under tropical conditions would be immensely beneficial to the construction industry of the country. The simulation of exact natural weathering conditions is quite complicated. This study concentrates mainly on how heat stress affects deterioration of index properties of gneissic rocks. When the rock mass gets heated, and cooled rapidly due to sudden rainfall, there is a possibility of propagating micro-fractures in the rock mass which will initiate weakening the strength properties of the rock mass. Most of the bridge abutments and dam constructions are associated with basement rock. In such locations rocks mass is alternately subjected to wetting and drying processes. Therefore the deterioration of rock resulting in its breakup, which is termed as "slaking" is also studied in this research. One hundred and sixty rock samples of gneissic rock of diameter 54mm (NX size) are tested in this research. Out of these 132 samples are fresh competent rocks and 28 are weathered to different degrees. These samples are selected from biotite gneiss and quartzo-feldspathic gneiss; the major two gneissic rock formations found in Sri Lanka. Fresh gneissic rock samples were subjected to heating and the weakening of their strength properties were studied upon cooling. Naturally weathered gneissic rock samples belonging to different weathering grades were also tested for their index properties. These were compared with the index properties of artificially weathered fresh gneissic rock samples and some correlations are established. The rate of deterioration of index properties of quartzo-feldspathic gneiss is seen to be more than that of biotite gneiss. Therefore more attention should be paid in the designing of foundations of important civil engineering structures in quartzo-feldspathic gneiss as far as the 'durability, is concerned, based on the project location

Sri Lankans built first road tunnel, underpassing a live railway track

The road and railway network to access the Central Province of Sri Lanka from the city of Colombo was built by the British, mainly to develop coffee and tea plantations, and transport the production back to Colombo, for exportation. Hence, the dimensions of the roads and tunnels in this area are as per the transport needs of that time. The existing road tunnel at Kotagala town in Hatton, which underpasses the railway track has been a bottle neck on Colombo – Nuwara Eliya main road via Hatton, over the past couple of decades, in which a twelve-foot container just manages to pass through, and the tunnel dimensions are just sufficient for one-way traffic only. Hence, the container drivers had used the road via Kandy to reach Nuwara Eliya as an alternative route, which is approximately 22km longer than the route via Hatton, up until the new Kotagala tunnel was commissioned in May last year. In order to cater the current and future traffic demands on this major road, the only solution is to build a new tunnel at Kotagala. However, the existing railway track could not be interrupted for this construction, as the trains do not have any other alternative route to access Badulla. Hence, it was decided to build a new tunnel by diverting the existing road at Kotagala. Both hard rock and soft ground tunnelling techniques had to be practiced during this construction. The steep earthen slopes at the tunnel portals were also protected by soil nailing techniques, prior to the construction of the new tunnel. This paper describes the construction methodology adopted, safety precautions taken, and the management techniques used for the project. The effective utilization of scientific knowledge and experience in Mining, Geotechnical and Civil Engineering practices have immensely helped to make this project a success

Optimization of the distance between twin tunnels by stress analysis

In this research, the stress characterization around twin-tunnels passing through a hard rock mass mostly consisting of Charnockitic Gneiss, Biotite Gneiss and Garnet Granulitic Gneiss in Kaluganga Development Project was analyzed by Boundary Element Method numerical solutions. The distribution and magnitude of major and minor principal stress contours, mean stress, differential stress, total displacement, maximum shear strain, and strength factor contours around the tunnels were simulated using the “Examine 2D” software. Examine 2D is a plane strain boundary element programme for calculation of stresses and displacements around underground and surface excavations in rock. Modeling results show that the countour values of the strength factor around the tunnel is greater than 1 when the distance between tunnels is 4.2 m which is the actual designed distance between the twin tunnels. When the distance is less than 4.2 m, model results shows that the strength factor reaches 1 in 1.88 m, and the excavation becomes unstable

Integration of Direct and Indirect Techniques to Optimize Subsurface Exploration

Subsurface exploration is one of the major activities conducted to extract information for Geotechnical applications. Borehole construction is the most common direct exploration technique which provides exact information on a particular location whereas, Electrical Resistivity Method is one of the commonly practiced indirect exploration techniques. Since the exploration costs are relatively high, minimizing the cost while obtaining adequate information is of everyone’s interest. For larger constructions with deep foundation requirements, determining the overburden thickness (bedrock level) and water table is a mandatory requirement. Hence, the use of a proper combination of direct and indirect subsurface exploration methods could result a considerable cost reduction and time saving. This study was conducted to find an optimum integration of resistivity method and borehole construction for selected subsurface exploration activities currently in progress. Accordingly, resistivity surveys were conducted at sites proposed for a twelve storied residential tower at Malabe and Matara – Kataragama Railway Extension Project, near piers for new railway track. Apparent resistivity data were gathered using ABEM – Terrameter SAS 1000 instrument, were interpretated using “IP 2 Win” software. The results were validated using the borehole information. The resistivity survey information revealed a close relationship with the borehole data and resides within the statistically acceptable range. Hence, an optimum combination of resistivity surveying and borehole construction can be proposed, for cost controlling at large-scale subsurface explorations

Fluctuations in groundwater level and corresponding earth resistivity changes

The earth resistivity at a known location was frequently measured, through resistivity sounding. Resistivity of subsurface layers could be influenced by the permeability, porosity, transmissivity, composition etc., where moisture content was the concern of this study. ABEM-Terrameter SAS 1000 instrument was used for surveying and data were interpreted using “IPI2-win” software. A borehole was established to reveal accurate subsurface stratification information and monitor the groundwater level, at the surveying location. Groundwater level was recorded through a datalogger in the borehole, at each 12 hour interval. The precipitation data of the area were obtained from the meteorological department to understand the wet & dry seasonal characteristics. Attempts were made to identify the changes in apperant resistivity data in response to the changing groundwater levels/moisture content and the accuracy of corresponding layer thicknesses, computed through resistivity data

Structures of water column and sediment sub-bottom in the Tangalle Bay, Sri Lanka

Water quality parameters and local morphology are important for monitoring aquatic life, and for planning any coastal development project. So far, few studies have been carried out in Sri Lanka for identifying water column structure and subbottom lithology in coastal zones. Tangalle Bay is situated in lowlands in the intermediate zone of southern Sri Lanka. Tangalle Bay replenish freshwater by minor tributaries (e.g., Kirama Oya) and Rekawa Lagoon. Field excursion in Tangalle Bay was carried out during the northeastern monsoon which is relatively less influenced by high wave-energy compared to the southwest monsoon. In this study, water quality parameters were measured using Valeport Midas CTD instrument. In addition, sub-bottom morphology was identified using Bathy 2010PCTM CHIRP sub-bottom profiler. Pre-decided systematic sampling locations were identified for covering three cross-shore and six long-shore survey lines. Water quality parameters of cross-shore and long-shore survey lines are represented separately. In cross-shore survey lines, temperature variations show warm pool in shallow water (depth 4 km) water can probably indicate well-mixed nature, perhaps due to upwelling. Less dense water can be observed in shallow water (depth < 3 m). Density variations also indicate upwelling of dense water at a distance of 4 km. The lowest salinity was detected close to the nearshore area due to freshwater supply by tributaries. Salinity variations also indicate possible vertical zonation of upwelling beyond 4 km distance from the shore. Dissolved oxygen values are high up to a 3 km distance in surface and bottom water, due to well mixing in the nearshore area. However, dissolved oxygen values are decreased in bottom water at distance of 3-4 km from the shore. In long-shore survey lines, temperature and density variations show strong stratifications same as that of cross-shore survey lines. High salinity on the bottom and low salinity on the surface water in the nearshore area can probably indicate freshwater supply from terrestrial margins. Sub-bottom profile data indicate the attached coral reef on the bed rock. This attached coral reef creates two depositional features such as layered sedimentary features on the landward side and homogeneous sediments on the seaward side of the bay

Applicability of ground penetration radar (GPR) technique to optimize soil Nail Wall designs

Soil nailing is used to stabilize existing natural slopes or excavations using soil reinforcement technique. Due to lack of information on ground profiles and thickness of soil layers, the required soil nail lengths cannot be determined in advance, for most of the steep slopes. Furthermore, the cost of drilling in fresh rock is much higher than the cost of drilling in soft soil and weathered rock. Without proper information on the depth at which the fresh rock is found, a cost-effective soil nail wall design cannot be finalized. This research was focused on studying the methods that can be used for soil nail wall design optimization by applying Ground Penetration Radar (GPR). The study was carried out on an unstable slope near Nursing Training School located in Kandy in the central part of Sri Lanka.The physical properties of soil were determined by direct shear tests, and stability analysis was done by means of “Slope-W” software. GPR techniques were also used in this study. The investigation results showed that the existing slope is unstable, and necessary to be protected. Further, it was identified that the basement rock cannot be encountered at already designed depths of the soil nails, which was subcequently proven as correctbased on data from ongoing drilling for soil nail installations. With the precise knowledge about the underground geological structure using GPR technique, the drilling cost, nail transporting cost, nail off cutting time to complete the total work can be reduced

Applicability of pre-heating techniques for recovery of garnet from garnet biotite gneiss

In mineral processing industry, the highest energy is consumed by crushing and grinding operations. However, if we can initiate micro cracks in the hard rock pieces in advance, we should be able to reduce the crushing energy and liberate useful minerals economically. Various pre-heating techniques have been tried by the scientists around the world to liberate minerals from rocks. In this investigation, pre-heating techniques were used to liberate Garnet from Garnet Biotite Gneiss. The market price of Garnet varies with the particle size. Hence, the samples were crushed and sized into industry needed three size fractions: (+250-850) μm, (+180-250) μm and (-180) μm. Afterwards, by means of Wilfley Table, Garnet was separated from rocks. Final Garnet recovery percentage was recorded for different temperatures. Furthermore, energy calculation was done for each sample to get an idea about the power consumption. It was observed that the crushing strength decreases with the temperature increment and it has been proven by identifying propagation of micro cracks in thin sections, prepared from rock samples pre-heat treated in Muffle Furnace and Microwave Oven. The analysis reveals that the preheat treated and quenched samples give higher Garnet recovery than Garnet recovered from as-received samples of Garnet rich Biotite Gneiss

Soil Nail Wall Design Optimization by Geotechnical Applications and Geophysical Techniques

Soil nailing is a soil reinforcement technique which is used to stabilize slopes by insertion of slender elements, called nails. When steep slopes are to be stabilized by soil nailing, practical problems are encountered related to subsurface investigations, as drilling machines cannot be placed on such slopes. Hence, bedrock levels cannot be determined in advance, which is disadvantageous to produce cost effective designs. This research was focused on studying the methods that can be used for soil nail wall design optimization by applying geotechnical and geophysical techniques. The study was based on an unstable slope situated near Victoria dam. The physical properties of soil were determined by direct shear tests, and stability analysis was done by means of “Slope-W” software. Determination of the profile of weathered quartzite layer, inter-beded with Charnokite bands was the major emphasis of this research. Three techniques were used: Ground Penetration Radar (GPR), Earth Resistivity Measurements and Geological Mapping. This investigation scientifically showed that the existing slope is unstable, and to be protected. Further, it was identified in advance that the basement rock cannot be encountered at designed depths of the soil nails, which was subcequently proven as correct by the ongoing drilling for soil nail installations