CONSIDERATIONS IN SELECTING A CONCRETE FACE ROCKFILL DAM IN A WATER RESOURCES

PROJECT IN HIMALAYAS : A CASE STUDY

J. CHANDRASHEKHAR IYER, DIRECTOR

S. K. G. PANDIT, CHIEF ENGINEER

CENTRAL WATER COMMISSION, NEW DELHI

THE PROJECT

• Envisages construction of a dam of about 110m high, 430 m long at top, for irrigation and generating hydropower.

• Other project components include Intake, Head Race Tunnel, Surge Shaft, Pressure shaft and a Surface Power House with an installed capacity of 186 MW through 3 units of 62 MW each.

• The Dam site is located in alternate bands of sandstone, claystone and siltstone of Lower Siwalik Group.

SELECTION PROCESS FOR DAM TYPE

• At the pre-feasibility stage, dam was conceived as an earth and rockfill type.

• At the DPR stage, detailed investigations and studies have been carried out for assessing the feasibility of other dam types.

• Geological Mapping, Geophysical surveys and Sub-surface investigations through drilling and drifting have been carried out.

A total of 644m, consisting of 5 spreads of seismic refraction survey done in the dam area.

10 Drill holes drilled in the dam area, i.e. along dam axis and in the vicinity (50m u/s and d/s of dam axis) to the desired depth. Six drifts excavated, three on each abutment.

Laboratory Testing of samples carried out.

Construction Material Studies: Location, Quality and Quantity

Contd….

• Overburden in the river bed is upto about 20 to 25m depth. Layers of sandstone and silt /clay stones in the river bed with thin layers of fractured zone. Rock quality varies from poor to good in river bed portion. Average water loss in bore hole is 50%.

• On the right abutment, alternate bands of sand stone and clay/siltstone. Core recovery is high. While the RQD of sandstone is high, for silt/claystone it is of the order of 30%.

• On the left abutment, overall core recovery is 95-100% with RQD 84% in sandstone and 48% in claystone and siltstone. Porosity is high.

SELECT GSI FIELD INVESTIGATION INPUTS

Some of the rock samples from DH-1(river bed) and DH-6 (left abutment) disintegrated on saturation after carrying out wave velocity tests (in saturated state) and hence on those samples, UCS and deformability characteristics could not be determined.

Based on the UCS test performed on the foundation rock samples, UCS values in the range of 12 to 18 MPa has been recommended, which falls under low strength category as per ISRM classification.

Slake durability tests on samples from right and left abutment drill holes exhibit low to medium durability.

SELECT RESULTS FROM CSMRS LAB TESTS

• Borrow areas for the clay core material is about 100 km or more from the dam site.

• Coarse aggregate samples are exceeding the expansion criteria limit for AAR Test. Suitable measures need to be taken while using them.

• Fine aggregate are not suitable for use as fine aggregate in structural concrete. Crushed sand is recommended for use as fine sand after further investigations.

CONSTRUCTION MATERIALS – KEY INPUTS

COST COMPARISON

SL. NO.

TYPE OF DAM COST WITHOUT LEAD

(IN CRORES)

COST WITH LEAD

(IN CRORES)

1 Concrete Face Rockfill Dam 27.65 27.65

2 Conventional Clay Core Rockfill Dam 112.35 316.98

DIFFERENCE 84.70 289.72

CONCLUSION Based on the reports of GSI and CSMRS summarised above,

possibility of a concrete dam was ruled out. The choice narrowed down to Conventional Central Clay Core Rockfill Dam and Concrete Face Rockfill Dam.

Creating road infrastructure in the Himalayas and transporting clay for the conventional rockfill dam from a distance of around 100km worked out to be an uneconomical proposition.

A Concrete Face Rockfill Dam has been finally adopted for the

project.