Simbología De Rocas

LUGEON TEST INTERPRETATION, REVISITED Camilo Quiñones-Rozo, P.E.1 ABSTRACT The Lugeon test is widely used to estimate average hydraulic conductivity of rock masses. Interpretation methods currently available in the literature were developed at a time when measurements were made in an analogous fashion and data was subsequently recorded by hand at rather large intervals of time. Current technologyallows measuring and digital recording of data in real time, thus granting us an opportunity to update the interpretation procedures for Lugeon tests. This paper provides an interpretation method that expands the current procedures to benefit from the recent advances in data acquisition equipment. INTRODUCTION The extents of grouting and cut-off depths required in a dam foundation are directlyrelated to the hydraulic conductivity (permeability) of the rock masses involved. In contrast to other geotechnical parameters for which variations can usually be measured in percentage terms (e.g., shear strength, density, compressibility, etc.), variations in hydraulic conductivity are usually measured in terms of magnitudes (e.g., 10-2 to 10-3). Selecting a representative value of hydraulicconductivity becomes of the outmost importance during design; especially, since under such a wide variation range, averaging the measured values will not suffice. Unlike soils, where seepage takes place through a series of small, closely spaced, interconnected pore spaces, seepage through rock masses occurs mostly along discrete planar discontinuities (e.g., joints, foliations, shears, etc.). Thereby,whereas in soils hydraulic conductivity is mostly controlled by the size, shape and arrangement of its voids (Terzaghi et al., 1996), in rock masses the conductivity depends on the aperture, spacing and infilling characteristics of its discontinuities (Goodman, 1980). Discontinuity aperture plays a particularly important role in the hydraulic conductivity of a rock mass. Consequently changes inthe stress condition of the rock mass can produce significant changes on its hydraulic conductivity. The existence of an interrelation between stress and hydraulic conductivity ultimately means that accurate estimates of the hydraulic conductivity of a rock mass can only be obtained using in-situ tests. The Lugeon Test The most commonly in-situ test used to estimate hydraulic conductivity of rockmasses is the Lugeon test – also called the packer test. The test, which derives its name from Maurice Lugeon (1933), is a constant head type test that takes place in an isolated portion
Senior Civil/Geotechnical Engineer, URS Corporation, 1333 Broadway Suite 800, Oakland, CA 94612, camilo_quinones-rozo@urscorp.com
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Lugeon Test Interpretation

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of a borehole. Water at constantpressure is injected into the rock mass through a slotted pipe bounded by pneumatic packers (Figure 1). A pneumatic packer is an inflatable rubber sleeve that expands radially to seal the annulus space between the drill rods and the boring walls.

Figure 1. Lugeon test configuration Prior to the beginning of the test a maximum test pressure (PMAX) is defined. PMAX is chosen such that it does not exceedthe confinement stress (σ3) expected at the depth where the test is being conducted, thus avoiding the development of hydraulic fracturing or hydraulic jacking. As a rule of thumb, PMAX is usually established using Equation 1, where D is equal to the minimum ground coverage – depth in the case of a vertical boring in a flat site or minimum lateral coverage in the case of a test conducted in ahillside. PMAX = D × 1 psi ft (1)

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Collaborative Management of Integrated Watersheds

The test is conducted in five stages, with a particular water pressure magnitude associated with each stage. A single stage consists of keeping a constant water pressure at the test interval for 10 minutes by pumping as much water as required. The first stage is held at a low water pressure, increasing...