Surface Geology of uma oya project-srilanka

:Purpose of the basic studies and its reports

The basic studies for the feasibility design of the project consist of detailed geological sur-face mapping, cored exploratory boreholes, including water permeability tests of the bedrock and rock mechanics in-site and laboratory tests
Since these site investigations can not be performed parallel and will have unlike durations, it is not possible to document all information and results in one report. Therefore a documentation concept is chosen as following

Geology of Sri Lanka

Tectonic map of Sri Lanka with project area

The centrally located Highland Complex (HC) is bound on ei-ther side by the Vijayan complex (VC) to the east and by the Wai complex (WC) to the northwest. … The Highland Com-plex is composed of supracrustal and metaigneous rocks. Most of the rocks in the complex have older model ages (2.0-3.4 billion years) than in the Wai and Vijayan complexes. The supracrustal rocks of the highland complex comprise shallow-water metasediments consisting of metaquartzites derived from sandstones; quartzo-feldspathic gneisses derived from arkose (type of sandstone); pelitic gneisses derived from greywacky and shale; marbles de-rived from limestones and dolomitic limestone; as well as a subordinate bimodal volcanic suite. … These supracrustals had been intruded by magmatic rocks between ~1950 Ma and ~670 Ma, and are now tectonically intercalated with magmatic rocks and orthogneisses. … The metaigneous rocks of the HC are now charnockites and charnockitic rocks

Structure and tectonics of Sri Lanka

These crustal unites have undergone repeated deformations at various stages, forming a variety of geological structures. Despite the fact that the ductile deformation of the crustal blocks in Sri Lanka before the Gondwana assembly is unclear, evidence for pre-Gondwana deformations have been found

General hydrogeological aspects

In Sri Lanka, rainfall is the major source for supply of the freshwater that seeps through the land surface for circulation and storage as ground water. The upper surface of the saturated zone forms the water table, with its own undulations like the surface topography, but to a much-reduced extend. It rises and falls according to the amount of water that percolates through to the saturated zone. Some water stored underground, confined between impervi-ous formations, possesses a certain amount of pressure. Shallow aquifers play an important role in providing domestic supplies from traditional wells between 6 m to 9 m depths, and also in discharging water to rivers and other water bodies during low flow periods. They also support wetlands and native vegetation. The water discharged in many springs is generally of very desirable quality. Many springs in the hill-country regions of Sri Lanka convey water emerging mostly through rock fissures or joints

Throughout several parts of the metamorphic hard rock (mainly within metaquartzites) area in each of the climatic zones, deep aquifers can be found. These are present in joints, frac-tures and fissures and are therefore discontinuous and sporadic, and can be exploited only through deep tube wells

Floods

Sri Lanka has 103 rivers. Most major rivers drain radially outwards from the central massif over the coastal plains, except those in the north. The southwest quadrant of the country re-ceives the most precipitation; consequently the highest flood risk is from the western and southwestern rivers during the months of May to October, with the Southwest Monsoon

Earthquakes

earth quake hazards are not given consideration in the design and construction of buildings. Recent research has shown that some damage can be expected if a mild earth-quake of magnitude 6 on the Richter Scale occurs in an urban area such as Colombo. Such an earthquake has an approximate return period of 200 to 400 years. It is necessary to flag this possible hazard … The possibility of the aggravitation of landslide hazards by even mild earth tremors should also be considered

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General geology of the project area

Based on the site inspection held in March 2010, a geological-geotechnical investigation programme was established and issued in a separate report

Geomorphology of the project area

The project area lies on the south eastern slope of the central Sri Lankan highlands. The re-gion is characterised by a rugged, agricultural landscape (incl. tea plantations and paddy fields) with an altitude of approximately 1100 to 1200 m asl. Some hills and ridges of the highland in the project area may reach elevations between 1300 and 1600 m asl. These highlands are interrupted by river valleys with a general trend from SW to NE and an altitude of approximately 900 to 1000 m asl. In the SE project corner, where the tail race tuel de-livers the diverted water into the Kirindi Basin, the landscape reaches elevations between 200 and 400 m asl

Geological units of the project area

Outcrops of the different bedrock types are well observable in the riverbeds at the two dam locations, in numerous outcrops in the steep valley slopes and in fresh cuts of the main roads. However, in the area of the peneplains and of the tailrace tuel, downstream of the pressure haft, fresh bedrock outcrops are very seldom and hidden in the jungle or covered by the soil of agricultural areas

The main petrographic units are represented by quartz-feldspathic gneisses partly, with frequent gar-net and biotite minerals; by undifferentiated charnockitic (greenish, medium to coarse grained) biotite gneisses; by pure coarse grained quartzites and by calc-gneisses and mar-bles

Overburden material

The overburden material is mainly formed by residual soils on the peneplains, a mixture of residual soils and colluvial material in the slopes of the valleys

Residual soils

Residual soil on the peneplain, which is superficially often reworked for tea and other planta-tions, paddy fields or covered by forest and scrub . In portal zones residual soil, mixed with colluvial material, may affect construction activities

Generation of residual soil above bedrock: below the geological hammer (centre of photo) slightly weathered quartz-rich garnet-feldspar-quartz gneiss crops out. Above it, moderatly to completely weathered feldspar-rich garnet-quartz-feldspar gneiss is observable, which is (to the right behind the roots) completely altered to residual soil

Colluvium

Colluvium on the steeper slopes leading to hill tops above or river valley below the pene-plain. This material is, due to agricultural activities, superficially often reworked, too, or cov-ered by pine forest or grassland, as on the steep slope towards the Vertical Vertical Pres-sure shaft area. In portal colluvial material, mixed with zones residual soil, may affect con-struction activities

A misture of colluvial material and residual soil (with vegetation) covers the less steep bedrock areas

Alluvium

Alluvium is mainly present in the erosion scarps of the small rivers northeast of Banda-rawela, near Ambadandegama village and in the tailrace area. By man reworked alluvial de-posits will be crossed by the tailrace chael, leading the water from the tuel portal to the Alikola Oya

Bedrock units

Leucocratic garnet-quartz-feldspar gneisses

The dominant rock types are bright, banded gneisses with reddish garnet minerals, weather-ing to red, iron-rich residual soils (garnetiferous quartzofeldspathic gneiss). Often the portion of quartz minerals is considerable higher than the feldspar, which gives the rock a much higher strength and reduces its weatherability drastically

massive, fresh to faintly weatherd quartz-rich garnet-feldspar-quartz gneiss

dark gray charnockitic gneisses

Dark gray, undifferentiated charnockitic gneisses were observed as interlayers in between thegarnetiferous quartzofeldspathic gneisses

The different bedrock types described above and documented in the geological map as un-differentiated charnockitic gneisses show a very low variation in mineral content. The grain size is usually fine to medium, with quartz and garnet as the largest individuals. According to the thin section analyses this bedrock unit comprises: quartz: 40 - 50%, feldspar: 40 - 50%; orthopyroxene hypersthen

Garnet-biotite charnockitic gneiss

The mineralogy of this isolated bedrock unit is according to the thin section analysis (approx. amounts): othopyroxene (hyper-sthen?): 30-20%; garnet: 20-30%, biotite: 20%; quartz: 15%, feldspar: 15% and ore/graphite: All mineral grains, but especially quartz and feldspar, show signs of high strain, implicating a high potential for alkali aggregate reaction

Quartz-rich gneisses and pure quartzites

Quartz rich bedrocks (ca. 70% quartz and ca. 30% feldspar) or pure quartzites (>95% quartz) are frequent occurring bedrock units. Due to the very high quartz content and the banded texture of theses rocks, the density of discontinuities (joints and foliation) is re-markably higher than in the other rock types

Marbles and calc-silicate gneisses/granulites

Thick units of very massive, medium to very coarse grained marbles are often observed. They show solution features on their surface only, but due to their massive texture without joints no relevant internal karst features were observed

Very coarse re-crystallized marbel with calcite crystalls up to 2-3 cm

Marble and impure crystalline limestone may ran into a problem when encountered in tuels, shafts and dam foundations or across reservoir periphery (especially when associated with faults and when found in the solution cavitated zones or in the karstic zones)

Transitions between different units

The transitions between the different geological units are mainly parallel to the main foliation S, but are not always sharp boundaries. Often the transition is a slow change of the portions of the different minerals as e.g. between quartz and feldspar (and garnet) in the different va-rieties of the leucocratic garnet-quartz-feldspar gneisses or between quartz-rich gneisses and pure quartzites

Suitability of bedrock units as concrete aggregate

The thin section analysis of the different geological bedrock units show that all minerals, but especially the quartz have suffered high to very high strain, resulting in many micro cracks. Therefore it has to be stated that all bedrock material from the project area bears a high to very high potential for alkali aggregate reaction. Therefore it will be required to perform ex-tensive laboratory tests with potential bedrock material for concrete aggregates