CHAPTER – II
2. Literature Review
Engineering geology encompasses the geology of the project and its technical implications for the surface and subsurface work. After a brief description of the regional geology, a detailed description is offered for the geology of the hydropower project stretching from the intake in the Neelum river at Nauseri and about 32 Ian SW to the outlet at Zaminabad. On the way the tunnel crosses under the Jhelum River and the smaller Agar Nullah River and develops a head of about 420 m. The basis for planning and executing the study for the feasibility report has been the geological work documented in the Terms of Reference (TOR). This includes geological maps for alternative projects and subsurface investigations performed for same. In addition to this a satellite photo of the project area in scale 1 : 850 000, aerial photos in scale 1 : 30000 covering parts of the project area and topographical maps in scale 1 : 50 000 from Survey of Pakistan have been used. The underground design is based on a rock mass classification founded on interpretation of lithology and tectonics and on relevant results from physical site investigations. This has resulted in a rock support design for underground openings ranging from 20 to 90 m2 in size in all types of rock mass qualities. The special design considerations applying to tunnel boring machine (TBM) tunnels are treated separately.
2.2 Sounding And Core Drilling
The purpose of the investigation was to collect information on:
depth of weathering
rock mass quality
soil and rock mass permeability and groundwater levels
geotechnical properties of overburden soil
allow for rock stress measurements at depth
The scope of work has been defined as follows:
At Agar Nullah and the outlet four drill holes with a total length of 600 m. One drill hole of 300 m is located in the transition zone between the unlined and the steel lined part of the headrace tunnel close to the underground power station. The objective is to performed in-situ stress measurements to control the data input to the numerical model used. The rest of the holes are dedicated to the shallow river crossing in Agar Nullah and the tailrace channel at Zaminabad. The last hole is primarily dedicated to permeability measurements in order to plan the excavation of the deep tail water channel joining the Jhelum River.
2.3 Rock Mechanics Parameters
For early evaluation of rock mass characteristics a few tests have been conducted on rock samples taken at surface. These are:
Drilling parameters DRI (Drilling Rate Index), BWI (Bit Wear Index) and CLI (Cutter Life Index) on S8-I, 88-II and shale material.
S (brittleness) and degree of compaction on all three materials
SA (abrasion) on all three materials
F (flakiness and elongation indices) on all three materials
The majority of tests, however, have been planned for material representative for the tunnels which are obtained as cores from deep drill holes. The laboratory analysis of rock samples, representative for each rock type 1? as been defined as follows:
Mineralogical and petro graphical analysis including thin sections (types of minerals and quantities)
Uniaxial compressive strength
Point load index
Hoek shear box test on existing joints and on cut surfaces with indication of cohesion and friction angle, both peak and residual values.
2.4.1 Geological History
The project is located in the Himalayas. The Himalayas are geologically young mountains with spectacular heights, developed as a result of collision between various continental and micro continental plate fragments during the Mesozoic to late Cenozoic periods. The tectonic activity still continues today. The North Western part of the Himalayas which enters into Pakistan is subdivided into Distinct units, such as the Sub Himalayas, Lesser Himalayas, Higher Himalayas and the Kohistan sequence. All of these units are separated by parallel to sub-parallel series of thrust faults, i.e. the Main Boundary Thrust (MBT), the Main Central Thrust (MCT) and the Main Mantle Thrust (MMT). These structures are developed as a consequence of the south verging compression which was caused by the collision between the Indian and Eurasian plates. Another peculiar feature of the North Western Himalayas is the development of the Hazara .Kashmir Syntaxial bend during the Miocene age. The syntax is is dominated by a 1600 bending of the geological lineaments. The molassic sediments of Miocene age were deposited in the core of the syntax is and are highly deformed by major faults. These rocks constitute the Murree formation in which the project is located.
2.4.2 Stratigraphy and Structures
The Himalayas have been divided into three longitudinal stratigraphic zones; the Outer or Sub Himalayan zone which is composed of Tertiary rocks; the Central Himalayan zone which is composed of crystalline and unfossiliferious slaty sediments, and the Tibetan zone which is composed of fossiliferious marine sediments. Thus the orogeny spans from the Cambrian to Eocene age. The project is located in rocks belonging to the Murree formation except at the intake, which is partly in ignometamorphic rocks belonging to the Panjal formation. This latter group of rocks is located to the right side of the Neelum River.
Figure.2.1: Regional geological map of Neelum-Jhelum Hydroelectric project
The Murree formation consists of sandstone and shales -a monotonous series of alternate beds with shales dominating the sandstone beds. The rocks are widely exposed with hundreds ofmetres thickness in the project area. Structurally the formation shows a high degree of compression in the form of tight folding with repeated faulting and fracturing. At places it shows open broad folds which have been weathered into steep ridges and valleys with a succession of escarpments and steep slopes. The Panjal formation is a group of volcanoc1astic rocks. They are generally greenish grey to blackish grey and consists of clastic, volcanic and carbonitic rocks. The rocks are metamorphosed into metabasites (greenstone), agglomerates, marble and quartzite.
Table.2.1: Stratigraphic subdivision of The North Western Himalayas
The geologic sequence in the region indicates a polyphase deformation resulting in complex folding and thrust faulting with crustal thickening. This can be seen as a series of thrust faults which later deformed into sharp bends and closed arches called ''syntaxes''.
The deformation in the Higher Himalayas is ductile with polyphase folding and penetrative schistosity. Characteristic of the Lesser Himalayas is the imbrication and folding of the sedimentary sequence which is in part or completely detached from its basement substratum. The number of brittle thrust planes increase downwards into the Lesser Himalayas. Folding and thrusting is likewise developed in the Sub Himalayas. The defonnation of the Sub Himalayas is youngest and is characterised by the "Hazara Kashmir Syntaxis" around which all the regional trends and faults are curved. The project lies in the axial zone of the syntaxis and is surrounded by regional thrust faults. The Main Boundary Thrust (MBT) locally called" Murree Thrust" and the Panjal thrust are located to the north. The Himalayan Frontal Thrust (HFT) is located to the south. These faults run roughly parallel in a north-west to a south-east direction, dipping towards the north-east. They run the full length of the Lower Himalayas and wrap around the syntaxis where they are truncated by another regional fault called the "Jhelum Fault'. The Jhelum fault is the youngest structural feature and is considered to be active. It runs along the right bank of Jhelum for some distance about 3 km west of the proposed power house location, and continues in a southerly direction through the rock formation to the west of the Jhelum river. The fault is considered responsible for a number of landslides. Some of the slides can be seen where the fault runs along the road of the Jhelum river banks. The Murree Thrust, with· a general north-west trend and a north-east dip, cuts across the Neelum river at the proposed dam axis and passes into the Kaghan valley. It separates the greenstone of the overlying Punjal formation from the underlying shales and sandstones of the Murree formation.
2.4.3 Plate Tectonics and Seismicity
The tectonics of the entire region are closely related to the Himalayan orogeny. The Himalayan ranges originated from the collision of the Indian and Eurasian plates. The Indian plate drifting to the North rammed into the southern flank of the Eurasian plate which acted as a rigid mass. This intercontinental collision resulted in intense deformation and crustal thickening as a consequence of the continental subduction process. The seisrmcity of the region originates from the Himalayan tectonics and is particularly intense in the north-western comer of the Indian plate, where northern Pakistan and Kashmir are located. A detailed seismic hazard analysis has already been carried out and is presented in the report '''Neelum-1helum Hydroelectric Project. Seismic Hazard Analysis", dated 1991. The report indicates that the project area has a somewhat moderate seismicity, classified as 3 in a rank of low (0) to high (4).
2.5 Geology of The Area Between The Agar Nullah and The Outlet
The tailrace tunnel will pass through this narrow part which is comprised of low relief hills with heights up to 1050 m. The Agar Nullah has a bed level of about 650 m and the geophysical survey indicates approximately 5 m thick alluvium on top of the bedrock. The Jhelum river at the power outlet flows at an elevation of about 590 m. The Agar Nullah has developed a "Wide and long valley with steep slopes. The Nullah transports large quantities of sediments "With big boulders during the monsoon. The rock mass is tectonised and weathered. Slopes on both sides of the nullah exhibit some rock outcrops, mostly at lower levels. The slopes are normally covered "With loose residual soils. The area between the Agar Nullah and the power outlet is characterised by steep slopes facing the Jhelum river and the surface is mostly covered with soil and cultivated land. Rock outcrops appear along road cuts and nullahs. The slopes near the road along the Jhelum become unstable with slides during rainfalls.
Near the confluence "With the Agar NulIah (about 2 km downstream of the tunnel crossing), thick alluvial deposits are found on both sides of the river. The Nullah itself is covered with large quantities of alluvium. The material is composed of fairly consolidated rounded boulders and gravel "With some fines. A relatively thin cover of soil is common in most of the area and consist mostly of weathered bedrock. This residual soil is loose and easily erodable. At higher levels the slopes are covered "With thick vegetation and erosion is less pronounced.
The rock units in the area are as described earlier.
The rock distribution is as follows:
Generally 6-8 m thick beds, exceptionally up to 10m; one 15m thick bed at outlet
Shale + SS-II sandstone
Generally 10-15m thick beds, some up to 18 m thick towards Agar NulIah.
The rocks are highly deformed with some weak zones towards the Agar Nullah and the power outlet. The central and higher parts are characterised by open folds. A synclinal structure is located in the middle part. The strike of the layers swings frequently between N30W to N30E. Some micro faults and weakness zones are common. The rocks are heavily jointed with three joint sets plus random joints. Major joints are generally repeated after one to two metres in SS-1. The bedding joints are more closely spaced; less than 1m in SSI and less than 0.1 m in shales. Shale is mostly laminated, weathered and disintegrated into splinters at the surface. Shearing is observed along the bedding and along some other major joint sets. Some local faults in different directions are observed and mapped near the confluence of the Agar Nullah and the Jhelum river. In this area different geological blocks appear to have moved and rotated with respect to each other. These anomalies may correspond to the regional tectonics of the area. The Jhelum River also takes sharp bends in this geological segment, conforming to the structural weak zones.
Although the area exhibits low relief with precipitation mostly in the form of rain, the higher parts receive heavy snow during winter. The sandstone beds lying in the synclinal area might act as an aquifer due to the fracturing and jointing. Numerous springs are observed all along the tunnel alignment right down to the outlet.