The proposed Karura hydroelectric power plant (HPP) tunnel route, herein referred to as “the study”, is located about 200 km northeast of Nairobi city in Kenya, and stretches over a distance of about 9km from a proposed intake weir in upper Tana River. Run-of-river water, estimated to be about 190 m/s of discharge from an existing 40 MW Kindaruma HPP,, expected to serve Karura HPP which will be located about 15 km downstream in Tana river. The proposed HPP will further exploit the residual head between Kindaruma HPP to the southwest and Kiambere HPP to the northeast in the upper Tana River, estimated to be about 40 m.

The multichannel analysis of surface waves method (MASW) was performed for reverse end shots of 131 spreads. The analysis involved determination of dispersion curves followed by inversion that generated model output (such as P-wave and S-wave velocity, density and N values) used to compute the site response parameters i.e. poisson ratio, shear modulus and young modulus. To determine the distribution of these parameters along the entire tunnel route, the data was plotted using surfer software.

Distribution Poisson ratio depicts a w-shaped structure, where the low values, largely disposed at deeper levels, stretch towards the earth’s surface at the southwestern, central and northeastern segements of the proposed tunnel profile. The central elevated segement corresponds to a prominent topographic feature, the Ngia Hill, and indicate that the fresh basment rocks are shallow within this hilly area. Low Poisson ratio values are characteristic of relatively shallow depths of about 20m.

Shear modulus values range from 0 - 11Gpa. The highest values are recorded at deeper levels where competent basement rocks are encountered from shallow drilling. The low values (0-1 Gpa) at the near surface are representative of the soil layer, while intermediate values represent fractured and weathered rock material. As such, the zone of low values at distances between 2-4km can be interpreted as a fault as well as that from 7km.

Young’s modulus values range from 0 to 25000Mpa, with the highest recorded values ranging between 10000 – 20000 Mpa from a depth of 15m. A general observation is that Young’s modulus values increase with depth. From the results of this study, zones of low young’s modulus at depth, herein interpreted as zones of weakness (not rigid) and that would require attention, occur bet ween 2 and 4 km and between 7 and 9 km along the proposed tunnel route from the intake weir. Such zones are herein interpreted as either fractures (faults) in filled by soil material or weathered rock.

Presentation Date: Tuesday, October 16, 2018

Start Time: 9:20:00 AM

Location: Poster Station 5

Presentation Type: Poster

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