IPRM-2014 Template

National Seminar on Innovative Practices in Rock Mechanics, Bengaluru, 2014

Evaluation of Deformability Characteristics of Rock Mass at Pare Hydroelectric Project for the Design of Dam Foundation G.Shyam*, D.S.Subrahmanyam, R.K.Sinha and T.Y. Moses Immanuel National Institute of Rock Mechanics, Bangalore, *[email protected]

Abstract Modulus of deformability (Em) is one of the essential inputs for the design of dam structure. It provides an insight to the behaviour of the rock mass when loaded and unloaded incrementally. The deformability parameters cannot be predicted on the basis of case histories or by empirical approach. This is due to the variation in rock mass properties influenced by the discontinuities. Therefore, the best way to determine the deformability parameters is to carry out in situ testing. This paper deals with deformability parameters of rock mass determined for the design of concrete gravity dam by conducting in situ deformability test. The tests were carried out on the right bank drift along the proposed dam axis across river Pare in Arunachal Pradesh. The in situ deformability tests were carried out for five samples as per the ISRM suggested method. The modulus of deformability of rock mass encountered ranged from 0.08 GPa to 0.66 GPa. Keywords: Modulus of deformability, concrete gravity dam, plate loading experiment

1. Introduction Pare Hydroelectric Project is a run off the river scheme proposed on river Pare in Papum Pare District of Arunachal Pradesh (Fig.1).The project envisages construction of a 78 m high concrete gravity dam and a surface powerhouse comprising two turbines each having a capacity of 55 MW. The total power generation being 110 MW.

understanding the loading and unloading characteristics of any rock foundation can through in situ tests of the dam foundation level. If a dam is seated on varying rock types having dissimilar deformability properties (Fig.2), this will result in development of shear stress and diagonal tension due to unequal deflections in the foundation (Sinha 2010). The dam can be designed to handle these deflection tendencies if the properties of the rock are known and the variations of properties are determined. Plate loading/jacking test is one such method to evaluate the deformability modulus of the rock mass.

Fig.1. Location map of Pare Hydroelectric Project

Fig.2. Development of shear stress due to variation of deformability in the foundation of the dam

Concrete gravity dam being relatively narrow in section requires a firm foundation to transfer the load. This is possible only when the rock mass encountered at the foundation level is competent enough to sustain the applied load. In order to design the dam foundation, deformability modulus of the rock mass plays a vital role. The best approach to be followed in

The aim of such tests is to find out the response of rock mass in terms of deformation on loading and unloading. The rock mass is loaded with a circular plate using hydraulic jacks. Deformation of rock mass beneath the plate is picked up by sensors installed inside a borehole drilled at the centre of the ground surface 121


loaded by circular plate.

instrumentation.

2. Local Geology of the Project The rocks exposed at the dam site belongs to Upper Siwalik Group (Kimin Formation) of Plieocene Pleistocene Age and is represented by medium to coarse grained grey to buff coloured, micaceous soft and friable sandstone. The sandstone displays a typical salt-pepper texture and is gritty at places with pebble layers along the bedding plane. The strike of bedding shows a variation in attitude from N35°W to N45°-55°E with their dip ranging from 15-20° in S35°W to S35°-45°E direction.

5. Equipment Setup The equipment setup consisted of two modules one being the measuring module and other the loading module. These two modules are discussed in following two sections. 5.1 Measuring module

The measuring unit was modular in design and comprised of the following components: i. Potentiometer ii. Mechanical anchor (BoF-ex) iii. Extension tubes of varying lengths iv. Centralizers The mechanical anchor (BoF-ex) comprised of a disk supporting three pads spaced at 1200 having a central cylinder that housed a jacking screw is shown in Fig.3. The anchor was designed to fit inside boreholes having finished diameter of 76 mm. One end of the anchor was terminated with a bayonet connector and a screw with hexagonal head. The anchor was expanded inside the borehole using a string of concentric setting rods. The inner rod with the hexagonal head was turned clockwise to actuate the screw jack. The other end of the anchor has a short threaded cylinder to attach the extension tubes or transducer module. The transducer module was attached to one or more extension tubes in order to span the required distance between anchors. The centraliser was used to prevent swaying and to support the transducer modules along with extension rods.

3. Theory For a uniformly distributed pressure on a circular area, the displacement at any point beneath the centre of the loaded area is given by the following relationship (ISRM Suggested method 1979): 𝑊𝑧 =

2𝑞(1 − 𝜇 2 ) 2 2 [ √𝑎 + 𝑧 2 − 𝑧] 𝐸𝑚 2𝑞(1 + 𝜇) 1 − [𝑧( − 1] 2 𝐸𝑚 𝑧 × √𝑎2 + 𝑧 2

Where, 𝑊 𝑧 = Displacement inside the borehole in the direction of the applied pressure 𝑧 = Distance from the loaded surface to the point where displacement is measured 𝑞 = Applied pressure on the circular area 𝑎 = Outer radius of the loaded area 𝜇 = Poisson’s ratio of rock mass 𝐸𝑚 = Modulus of rock mass

After determining the depth of the deepest anchor, the bayonet head was connected on the installing tool to the anchor. The outer installing tube was then connected to the anchor installation tool and the whole assembly was inserted into the borehole. The anchor was then expanded and installed with the help of inner rods. Once anchor was installed to its desired depth the inner and the outer assemblies were withdrawn. The transducer module was then installed.

4. Site Preparation The size of the test pad for the in situ investigations was 55 cm. An area with diameter of 1.50 m, slightly larger than twice the diameter of the test pad was demarcated at each site both on invert and crown of the investigation drift. All the loose rocks were chipped out to expose fresh surface of the rock mass. A concrete pad of diameter of 55 cm having thickness of 3-5 cm were casted on the prepared area. These pads which were co-axial and parallel to each other were allowed to cure for 28 days. Later, an NX-size borehole of 3 m length was drilled. The borehole was drilled only on invert of the drift, over the already prepared test surface, for the purpose of 122


6. Testing Programme The test was conducted by applying and varying the vertical pressure on the aluminium plate at the bottom, and recording vertical displacement of the anchor placed at different depths from the collar of the borehole. The pressure was raised in four steps to 3.18 MPa in five cycles of almost equal intervals. All the pressure and displacement reading was acquired with a digital data acquisition system. The readings were recorded every second on a Laptop used for the purpose.

Fig.3. Mechanical anchor (Bofex)

5.2 Loading module

The loading module comprised of load distribution plate, stiffeners, hydraulic jacks and extension columns. The complete setup of the system is shown in Fig.4. The jack base plate rested on the load distribution plate of 50.5 cm diameter placed on the concrete pad.

Firstly the pressure was raised up to 20% of 3.18 MPa at the rate of 0.3 MPa every minute. Displacements were recorded every second by the digital data acquisition system. The peak pressure of the first cycle was maintained for 10 minutes, recording the pressure and corresponding potentiometer reading every second. Then the pressure was lowered gradually to zero at the rate of 0.3 MPa every minute. The zero pressure was maintained for 10 minutes, recording the pressure and corresponding potentiometer reading every second. ii. After 10 minutes of zero pressure had elapsed, the pressure was raised up to 40% of 3.89 MPa. The rate of pressure increment of 0.3 MPa per minute was maintained till the peak was reached. The peak pressure was maintained for 10 minutes, and was lowered to zero at the rate of 0.3 MPa every minute. The pressure and corresponding reading of the potentiometer was recorded every second during the entire cycle. iii. Next, the pressure was raised to 60% of 3.89 MPa to attain the peak pressure of the third cycle and lowered back to zero in the same manner. iv. In the fourth cycle of pressure increment and decrement, the peak was raised to 80% of 3.89 MPa. v. In the last cycle the peak pressure was increased to 3.89 MPa. i.

Three numbers of jacks each of 100 tons capacity were placed above the jack base plate with the spherical seats. The extension columns were placed one above another with sufficient stiffeners up to the top flattened rock leaving enough space to accommodate a bearing plate, a load distribution plate of the same diameter as the bottom one.

Fig.4. Complete set up of the Plate loading equipment used at Pare hydroelectric project 123


7. Test Results The results corresponding to reduced distance (RD) of 28 m, 22.6 m, 17.2 m, 11.8 m and 6.4 m with respect to portal of the drift are given in the Table-1 through Table-5. A sample of the pressure versus deformation graph is shown in Fig.5. Table-1. Moduli at RD 28 m Pressure on Modulus (GPa) concrete pad Em Ee (MPa) 0.63 0.31 0.51 1.32 0.42 0.57 1.86 0.49 0.61 2.53 0.57 0.64 3.16 0.66 0.70 Table-2. Moduli at RD – 22.6 m Pressure on Modulus (GPa) concrete pad Em Ee (MPa) 0.62 0.29 0.57 1.33 0.42 0.54 1.85 0.46 0.54 2.49 0.51 0.58 3.19 0.59 0.65 Table-3. Moduli at RD – 17.2 m Pressure on Modulus (GPa) concrete pad Em Ee (MPa) 0.63 0.02 0.07 1.43 0.04 0.09 1.94 0.07 0.08 2.60 0.11 0.11 2.66 0.11 0.13 Table-4. Moduli at RD – 11.8 m Pressure on Modulus (GPa) concrete pad Em Ee (MPa) 0.49 0.06 0.10 1.38 0.10 0.14 1.36 0.09 0.11 2.55 0.14 0.16 3.18 0.16 0.18 Table-5. Moduli at RD – 6.4 m Pressure on Modulus (GPa) concrete pad Em Ee (MPa) 0.63 0.2 0.05 1.30 0.05 0.07 1.95 0.07 0.09 2.59 0.08 0.10

Ee/Em 1.66 1.36 1.24 1.12 1.07 Fig.5. Representative Plot of Pressure versus Displacement at RD -28 m, Pare hydroelectric project Ee/Em

8. Conclusion

1.98 1.27 1.15 1.15 1.10

The test results indicate that the deformability modulus of the rock increases as the RD of the test point increases. There exists an abrupt increase of almost fivefold in the deformability modulus of the rock indicating possibility of rock weathering (Fig.6). This is corroborated by the fact that only four cycles of loading and unloading could be carried out at RD 6.4 m. The fifth cycle could not be conducted due to differential settlement of the test pad after the fourth cycle. The test pad failed due to uneven rock strength which is an inherent characteristic of weathered rock mass.

Ee/Em 4.44 2.32 1.16 0.98 1.13

Em 0.08 GPa

0.16 GPa

0.11 GPa

RD 6.4 m

11.8 m

17.2 m

0.59 GPa

0.66 GPa

Ee/Em 1.57 1.40 1.21 1.17 1.14

22.6 m

28 m

Fig.6. Deformability modulus of rock mass vis-à-vis RD

Rocha (1974) based on his extensive study of dam foundations came out with a set of guidelines with respect to the deformability modulus of the rock mass and the deformability modulus of the concrete. The guidelines are as follows:

Ee/Em 0.24 1.42 1.33 1.30

i. The ratio between moduli of deformation of rock mass (Em) and concrete (Ec) is 𝐸 1 important only when 𝐸𝑚 < 4. For 𝐸𝑚 / 𝑐

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investigations conducted for the Design of Dam at Pare H.E Project, GE-1103C. NIRM

𝐸𝑐 ≥ 1/4 the modulus of deformation plays a very slight role. ii. For 𝐸𝑚 /𝐸𝑐 < 1/16 the behaviour of dam is entirely governed by the modulus of deformation of the foundation. This is particular important when the foundation of rock is inhomogeneous. iii. For homogeneous rock the structure is not much influenced by values as low as 𝐸𝑚 /𝐸𝑐 ≤ 1/16.

Sinha R.K, 2010. Course material, Comprehensive training course on “Rock Mechanics” for THDC Engineers, April 05-30, 2010, Rishikesh, Uttarakhand

The average value of the deformability modulus of the rock mass in the investigation drift is 0.32 GPa. Considering the deformability modulus of concrete as 30-50 GPa the ratio 𝐸𝑚/𝐸𝑐 is much lower than 1/16. Even if the stripping zone for the dam abutment is considered up to RD 17.2 m, the average deformability modulus of the rock mass for RD 22.6 m and 28 m being 0.62 GPa results into the ratio of Em and Ec still lower than 1/16. This indicates that according to the guideline (iii) of Rocha (1974) the dam structure will not face any problem. 9. Acknowledgement We are thankful to Director, NIRM for the permission granted to present this paper. The management of Pare hydroelectric project is also thankfully acknowledged for extending necessary facilities during the course of fieldwork. 10. References Chappel B.A. 1984. Determination of rock mass modulus. Proc. 4th Australia New Zealand Conference on Geomechanics. Perth. Pp.514-518 ISRM (1979) International Society for Rock Mechanics, Commission on Standardization of Laboratory and Field Tests: Suggested method for deformability determination using a plate test. Rocha M, 1974. Present possibilities of studying foundations of concrete dams. Lab. Nac. De Eng. Civil (LNEC), Lisbon, Mem No. 457, 1974 Sengupta S., Subrahmanyam D.S., Sinha R.K., Shyam G. and Moses Immanuel T.Y., 2011. Report on Different Rock Mechanics 125