Comparison of Permeability and Groutability of Ostur Dam Site Rock Mass for Grout Curtain Design

Abstract

The focus of this study is the empirical hydromechanical behaviour of the Ostur dam site rock mass. The area surrounding the dam mostly consists of diorite and andesite, with primary fractures and hydrothermal veins. The hydromechanical behaviour of the rocks was determined using 500 water pressure tests at 5-m intervals. The hydrothermal veins and 2,739 discontinuities were studied and mapped along the dam axis. As a result, it was possible to design an optimum grout curtain for the dam axis. The empirical hydromechanical behaviour of the rock was studied to determine water flow and grout pressurised flow during the field tests that were conducted on two representative A-series grouting operation boreholes (one borehole for each abutment). The secondary permeability index (SPI), Lugeon value (LU), rock quality designation (RQD) and cement take (CT) values are presented and compared in this article. It is concluded that permeability and groutability are mostly controlled by the specifications and characteristics of the veins, especially in shallow areas and lower depths. A procedure is proposed based on a comparison of the trends in the RQD–SPI and LU–CT, and it is suggested that the areas with diverging trends require no treatment and that those with converging trends require heavy treatment. Additional complementary studies that were conducted during the construction stage have validated these results.

Appendix A

The water pressure test (WPT) (in which pressurised water is injected into the borehole) is an effective field test of rock mass permeability and is mostly conducted using the Lugeon method. The water take (discharge) is measured under increasing (loading) and decreasing (unloading) pressure steps (amount of pressure changed) at proper time intervals along the borehole.

1.1 Permeability (LU) and Hydromechanical Behaviour

The Lugeon (LU) value should be calculated for a selected amount of pressure applied during the test and based on the tangent slope of the P–Q diagram (Ewert 1997a, b, c, d). Nonveiller (1989) believes that the tangent slope of the P–Q diagram is a proper expression of the LU value at any point on the diagram and may explain many complex cases.

According to Nonveiller (1989), the LU value may be calculated as:

$$ {\text{LU}} = \frac{10Q}{{P_{\text{e}} L}} $$

(1)

where:

LU:

Lugeon value

Q :

Water take (discharge), l/min

L :

Length of tested (injected) interval, m

P _e :

Effective pressure at the middle of the tested interval, bar

The LU value is the volume of water (l) in a unit of time (min) over a unit of length (m) for a tested interval at a pressure of 10 bar (Nonveiller 1989; Houlsby 1990).

Following Houlsby (1977a, 1990), one can calculate the LU values by dividing the discharge by the pressure (column C, Table 7) for each step in the loading/unloading process (the slope of the P–Q diagram tangent is given by Eq. 1). The rounded values are graphically presented in a bar chart (as in column D of Table 7). The equivalent group is determined based on the similarity between the prepared chart and the standard chart (column D of Table 7).

Table 7 Typical pattern for the determination of the hydromechanical behaviour and permeability (Shroff and Shah 1999; Kutzner 1985)

Finally, one value is chosen from the LU values that are calculated for each of the five steps. This value represents the permeability of an interval that is 5 m in length based on the pattern in column E. The discharge value is substituted into the LU and SPI equations (Eqs. 1, 2) and is chosen based on the behaviour type as indicated in a particular step or steps for the selected type.

The LU value may be expressed as a range, particularly when fewer than five steps are used.

1.2 Secondary Permeability Index (SPI)

Foyo et al. (2005) combined a modified form of the Lugeon relation (Eq. 1) with the radial permeability of a rock mass. They also included the borehole geometry (the radius) to propose a new equation that yields a value that is closer to the permeability coefficient than that yielded by the Lugeon relation:

$$ {\text{SPI}} = C \times \frac{{{\text{Ln}}\left( {\frac{{2L_{\text{e}} }}{r} + 1} \right)}}{{2\pi L_{\text{e}} }} \times \frac{Q}{Ht} $$

(2)

where

SPI:

Secondary permeability index, l/s per m² of the borehole test surface

C :

Constant that depends on the fluid viscosity at 10 °C (equal to 1.49 × 10⁻¹⁰ for water)

L _e :

Length of the tested borehole interval, m

r :

Borehole radius, m

Q :

Water flow absorbed by a fissured rock mass, l

t :

Duration of the pressure applied in each step, s

H :

Total pressure expressed as a water column, m

The SPI establishes a new permeability-based rock mass classification (Table 8). Based on this classification, different considerations regarding ground treatment are proposed (Foyo et al. 2005).

Table 8 Rock mass classification based on the SPI and ground treatment considerations (Foyo et al. 2005)

The proposed classification differs from classical geomechanical classifications. Most critically, it does not reflect the strength of the intact rock. Instead, the classification defines the quality of the rock mass based on the permeability of the discontinuities (Foyo et al. 2005).

1.3 Rock Quality Designation (RQD)

The RQD was proposed by Deere (1968) as follows (Bieniawski 1989):

$$ {\text{RQD}} = \frac{{\sum {{\text{Length\; of\; core\; pieces}} \ge 10\;{\text{cm}}} }}{\text{Total core run length}} \times 100. $$

(3)

The International Society for Rock Mechanics (ISRM) recommends that a core size of at least Nx in diameter (54.7 mm) be drilled using double-tube core barrels.

Although the RQD is a simple and inexpensive index, it cannot provide an adequate description of rock masses because it disregards joint orientation, tightness and gouge (infilling) material. Essentially, the RQD is a practical parameter based on a “measurement of the percentage of ‘good’ rock (core) interval of a borehole” (Deere and Deere 1988).