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**Industry**

**Oilfield Places**

**Technology**

All mechanical factors of the floor heave in roadways, such as rock dilatancy, creep and buckling failure, are analysed here. The mechanism and rational parameters of two new kinds of stress relief methods are discussed by means of FEM. The stress relief method for roof was applied to a belt chamber in Yanzhou coal mining area, and serious floor heave was eliminated.

Floor heave has been a difficult problem in many underground coal mines for a long time. It shows more and more serious as the depth of extraction increases. However, the control of the floor heave has been considered less important than roof support over a long period of time in China, so the mechanism of the floor heave has not been clarified, there has been great blindness in selecting the control methods. The intense floor heave has made roadway support costs obviously rise, what is more, seriously hindered coal mining rate. The floor heave is a very complicated process. It not only relates to the physical and mechanical properties of surrounding strata, but also depends on the stress distribution around the roadway. The mechanical factors of the floor heave are studied in this paper, and the mechanism and effectiveness of the stress relief methods are analysed.

ARMA-95-0889

The 35th U.S. Symposium on Rock Mechanics (USRMS)

SPE Disciplines:

The volume-frequency distribution of rock falls and rock slides in the Yosemite Valley are well described by a simple power-law relationship, where log N(Vol) 3.48 0.57(log Vol). This relationship, based on 214 documented rock-fall and rock-slide events that occurred from 1900 to 1992, allows determination of estimated return periods and probabilities for rock-fall events of different sizes. Based on this relationship, the largest prehistoric rock fall in the Yosemite Valley at Mirror Lake has an estimated return period of 325 years. On 10 March 1987 two massive rock falls from Middle Brother, with a combined volume of 600,000 m 3, spread across the talus cone and covered Northside Drive, blocking the primary exit from the Yosemite Valley. Brittle fracture indicated by rock popping noises and suggesting release of horizontal residual stress accompanied small (<50 m 3) rock falls that preceded these two massive rock falls from Middle Brother. Removal of "key blocks" by the smaller rock falls may have released the interlocked geometry of closely jointed and fissured rock of the face of Middle Brother and permitted the failure of the much larger rock mass. During the subsequent months the number of continuing small rock falls at Middle Brother exhibited an inverse power law decay with time.

Approximately 400 rock falls and other forms of slope movement (as defined by Varnes, 1978) have been documented in the Yosemite Valley and vicinity since the 1850's (Wieczorek et al., 1992). The volume of individual rock falls has been noted, which provides volumefrequency data for analyzing the occurrence of infrequent, large rock falls. Many rock falls have been associated with triggering events, such as earthquakes, storms, and freeze-thaw cycles; however, the majority of the rock falls in Yosemite have occurred in the absence of a recognized trigger.

Beginning in March 1987, we documented an unusual sequence of rock falls from Middle Brother in Yosemite Valley (fig. 1). On March 10, two large rock falls with a cumulative volume of approximately 600,000 m 3 occurred, spread rapidly across the talus cone, and covered Northside Drive. This was the largest historical rock-fall in Yosemite Valley. The Middle Brother rock falls occurred without an apparent triggering event, such as an earthquake or storm, but they had been preceded for several days by smaller rock falls. Following the rock-fall events of March 10, numerous smaller rock falls continued at the site for several months. This sequence of rock falls at Middle Brother was unusual because of its several month long duration with smaller rock falls both preceding and following the larger events; unlike any other documented rock falls in Yosemite.

In this paper we examine the events and conditions associated with rock falls in Yosemite to better understand the behavior of large rock masses. The geologic setting of Yosemite is reviewed before describing the volume-frequency distribution of rock falls and rock slides in the Yosemite Valley and the sequence of rock falls at Middle Brother. We conclude by exploring several possible explanations for the spatial and temporal behavior of rock falls.

ARMA-95-0085

The 35th U.S. Symposium on Rock Mechanics (USRMS)

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.35)

Billions of tons of rock are shipped as overburden in surface mining operations annually worldwide. Some 40 million tons in China alone are dumped on waste piles which occupy large tracts of sometimes valuable land. Failures of these waste piles result in large scale environmental and other problems. This paper discusses the underlying mechanisms and gives some specific histories.

WASTE PILE FAILURE

The 220 m high spoil pile at the Janshah Iron mine, containing some 2.2 million cubic meters of waste rock, failed catastrophically in 1979. The resulting damage to the mine railroad disrupted production for more than six months.

Another serious waste pile failure occurred at the Shanbou Coal mine in West China in 1991. The soft foundation allowed the 20 million tons of waste rock to move downslope. Mine production was adversely affected for several months. Waste pile failures in southern China are nearly always related to heavy falls of rain. These result in what are termed mine debris flows or mudflows, arising from liquefaction and creep of waste

A wall of mud carrying with it boulders and rocks of all sizes, suddenly starts to flow. The flow which quickly widens out into a fan engulfs structures in its path, covering roads and fields until it slows and stops. The final shape of the deposit is a lobate form, with a steep terminal snout and margins. Debris flow containing 80-90% granular solids by weight, can move in sheets about 1 m or more thick over slopes with slopes as low as 5-10 degrees.

Other Chinese mines that have been impacted by debris flow from waste piles include the Hainan' Iron Ore mine, the Yonfou Nonferrous metal mine, and the Yonpin Copper mine. Millions of tons of waste rock together with large volumes of water and soil have been carried distances up to 7 km. The acid nature of the water resulted in fairly severe environmental pollution problems (table 1).

A summary description of the processes involved in pile slope failure (Johnson and Rodinc, 1984) is given below.

ARMA-95-0831

The 35th U.S. Symposium on Rock Mechanics (USRMS)

debris flow, failure, Flow, foundation, height, material, mine, pile, pore water pressure, pressure, result, rock, seepage, slope, SOIL, surface, waste pile, water

ABSTRACT :

An underground powerhouse chamber at chibro was constructed about two decades ago in dolomitic-lime stone of lower Himalayan region. Long-term instrumentation was planned during construction to check the safety and adequacy of the support system and to monitor the post construction behaviour of the excavation. The present paper essentially ?eals with the analysis of ten year data. The observed roof support pressure has been compared with the support pressures estimated from different empirical theories. The time-dependent effect has been noticed significantly where the rock mass is saturated due to seepage problem and near thick plastic shear zone.**INTRODUCTION**

Construction of underground excavations for hydroelectric projects in the lower Himalayan region is a challenging task due to support problems under complex hydrogeological conditions and tectonic influences. Further, these problems could be attributed to the non-homogeneous and anisotropic nature of rockmass and their time dependent behaviour. In this context, the Chibro underground powerhouse complex has set a major precedent by being the first venture of its type in the lesser Himalaya. The 240 MW Chibro underground power station exploits the drop of about 124m along the first loop of Tons river, a tributary of the Yamuna between Ichari and Chibro, which is part-I work of Yamuna Hydroelectric Scheme Stage-II. This was the first venture of its type in the lesser Himalaya and was necessitated because the location of a surface power station would have involved large scale excavation of steep slopes. Finally, the powerhouse complex was sited in a band of limestone which has horizontal width of 193 to 217m. The complex comprises a network of excavation for the machines, transformer, turbine inlet valves and control room and also provides operating galleries and hydraulic connections to the Part-II. This latter stage involves a 120 MW Khodri power station utilising the remaining drop of 64m along the second loop. Fig.1 shows a general layout of the powerhouse complex.

ARMA-95-0437

The 35th U.S. Symposium on Rock Mechanics (USRMS)

Industry:

- Energy > Power Industry (1.00)
- Energy > Oil & Gas > Upstream (0.32)

SPE Disciplines:

Discontinuous deformation analysis (DDA) was created by Shi & Goodman. It solves a finite element type of mesh where all the elements are isolated blocks and bounded by pre--exisfing discontinuities under kinematic conditions of dynamic and quasi-static motion. The authors introduced an elasto-plasfic yield criterion in the analysis, and added several new elements to handle with practical rock mechanics problems. The results show validity of the method for practical use.

A new numerical methoddiscontinuous deformation analysis (DDA) was invented by Shi & Goodman (1984, 1985) and further developed by Shi & Goodman (1988, 1989)since then. This method uses the displacements and strains as unknown variables in an element block, and solves the equilibrium equations in the same manner as the matrix analysis of structures in the Finite Element Method.

The original DDA only uses an elastic material property for a block and the friction is activated along a block-to-block interface. In this paper improvements of the original DDA are described and the new code is applied to solve rock stability problems. The block element in the new code can deform as an elasto-plastic material following the DruckerPrager associated constitutive law. The main purpose to take into account the block yielding is to analyze soft rock mass behavior subjected to various loading conditions such as excavation and embankment. The interface between blocks behaves according to the Mohr-Coulomb's criterion including cohesive force. Damping coefficient was implemented to take into account the block collision. The rockbolt element was introduced to represent the effect of confinement for rock masses. The bonding element which fuses two blocks was also invented to represent shotcrete in a tunnel structure.

The new DDA code was calibrated in comparison with laboratory model tests. Stability of rock slope and tunnel in a discontinuous rock mass is analyzed, and the effect of lining and rock bolts is discussed. Rockfall on a very steep slope was also calculated. Some of the results were compared with those of the finite element method. The method of DDA with improvement has been proven to be very effective to analyze rock stability problems in discontinuous deformable rock masses.

ARMA-95-0045

The 35th U.S. Symposium on Rock Mechanics (USRMS)

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.69)

The premature failure of rock used as construction stone is a fundamental concern in many engineering projects, such as dams, roads, and embankments. As part of an effort to develop better procedures for determining rock durability, we conducted seismic and strength tests on rock from two different dolomite rip-rap sites that have begun to seriously and prematurely deteriorate. For comparison, we also tested a clastic siliceous rock, Berea Sandstone, and a crystalline calcitic rock, Carthage Limestone. Seismic tests were consistently sensitive to structural anisotropy, but strength anisotropy was only apparent in the Berea Sandstone. The dolomites, which came from rip-rap samples categorized as 'failed' were very strong in compression but weak in indirect tension. Conversely, the Berea was stronger in tension than the dolomites, yet weaker in compression. These puzzling results warrant further research into the connection between strength and durability.

The use of stone as a construction material is on the increase worldwide in response to the need to rehabilitate and maintain existing infrastructure and meet the requirements for new facilities. The economics of using natural materials depends critically upon their availability and accessibility, as well as their durability and degradation in service. As infrastructure projects such as roads, dams, and embankments are usually expected to last on the order of decades, the construction materials must also endure as long, the determination of which must be made prior to the start of the project.

Two Army Corps of Engineers rip-rap sites have weathered more severely and earlier than anticipated, possibly requiring unforeseen costly maintenance. Consequently, the Army has enrolled our assistance in reevaluating its testing procedures and standards for selection of rip-rap stone to better identify natural and induced weaknesses that accelerate the degradation of rock when subjected to weathering and freeze-thaw cycling.

We tested samples from the two weathered rip-rap sites and tested two reference rock types, Berea sandstone and Carthage limestone. Our aim was to identify laboratory tests which might indicate rip-rap durability in a broad range of service environments. For this study, we tested small specimens extracted from the rip-rap samples of poor quality sento us by the Corps on Engineers. We also hope to eventually examine whether larger-scale features are important to the weathering process but are currently limited in variety of sample size and origin.

The four rock types tested the two dolomites, the sandstone, and the limestone all exhibited transverse anisotropy in bedding planes or fractures. In the case of the two types of rip-rap which are tabular in shape the transversely anisotropic fractures appear to have controlled the shape of the blocks and were perhaps also responsible for the accelerated weathering.

As a diagnostic tool for identifying defects in rock, we have concentrated on using seismic testing to identify mechanical weaknesses in rock, such as bedding plane weaknesses and fractures. Theoretical microcrack models (Nur, 1971; Crampin, 1982) predicthat the presence of oriented flaws will cause shear wave splitting. We experimentally observed shear wave splitting and learned that amplitudes are a more sensitive indicator of structural anisotropy than velocities, static moduli, or strength. Seismic testing also offers a distinct advantage over conventional strength testing in that it can be conducted at scales varying from the laboratory specimen to the rock quarry. We also studied the relationship between rock strength from oriented unconfined compression and Brazilian tests a

ARMA-95-0367

The 35th U.S. Symposium on Rock Mechanics (USRMS)

Industry:

- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > US Government (0.54)

Oilfield Places:

- North America > United States > Wyoming > Table Rock Gas Field (0.89)
- North America > United States > Kansas > Beaver Oil Field (0.89)

Stability of keyblocks subject to self-weight and surface forces is examined using linear programming methods. The results are applied to determination of the maximum size of unstable keyblocks in tunnels. We find that there is usually an upper limit to the size of unstable keyblocks, based on in-situ stresses, excavation geometry, fracture orientations and shear strength. In many cases this upper limit can be much less than the maximum keyblock region predicted by block theory.

A widely used model for predicting tunnel support requirements in hard, jointed rock is the block theory method developed by Goodman & Shi (1985). Block theory assumes that block size is limited only by the size of the excavation. Unstable keyblocks, however, are limited in size by two additional factors. One factor concerns the stabilizing (or destabilizing) effect of in situ stresses and the other the sizes of the bounding discontinuities. These controls on the sizes of unstable keyblocks have great practical importance for the optimization of excavation geometry and the prediction of support (e.g., rock bolt) requirements. In this paper we discuss the effect of in situ stresses in limiting the sizes of unstable keyblocks. We restrict our attention to the case of keyblocks forming in the roof of a square 2-D tunnel.

One measure of stability for a roof block is the factor of safety, defined as the net stabilizing force divided by the net destabilizing force (including, in particular, the block weight). In a homogeneous stress field, for two blocks of the same shape, the larger should have the lower factor of safety, everything else being equal. This is because block weight is proportional to z ?, while the frictional resistance is proportional to z a, where z is block altitude. The factor of safety is then inversely proportional to z. This would suggest that the maximum keyblock, i.e., the largest block consistent with the excavation dimensions, should always be the most critical. However, this is not the case; in many cases larger blocks may be stable in cases where their smaller counterparts fail (e.g., Goodman 1989). The primary reason for this discrepancy is that stresses are not homogeneous; they tend to arch around underground excavations (Fig. l) so as to make larger blocks relatively more stable. Keyblock stability has previously been discussed by Brady and Brown (1993) and by Karzulovic (1988), among others. In general the problem is statically indeterminate. In this paper we propose a new approach to the problem based on linear programming (Mauldon 1995).

ARMA-95-0113

The 35th U.S. Symposium on Rock Mechanics (USRMS)

SPE Disciplines:

**ABSTRACT:**

We have measured ultrasonic Rayleigh and interface waves along laboratory induced planar fractures on Anstmde limestone under dry conditions. Comparing measurements of the Rayleigh wave along rough and smooth surfaces we observe that the effect of the roughness on the surface wave propagation is to induce attenuation and velocity dispersion. The attenuation increases linearly with frequency; the phase velocity goes through a maximum at 240 kHz. We put forward a physical model describing the effect of the roughness in terms of an increase in the effective length of propagation. This model predicts a frequency dependence of an order that scales with the magnitude of the roughness. We have also compared measurements of the interface wave with measurements of the Rayleigh wave, along a single rough surface, to observe the effect of stress on the interface wave propagation. Within the range from 0 to 10 MPa, the interface wave velocity and its amplitude increased with the applied stress.

**INTRODUCTION**

Rough interfaces, in the form of hydraulically induced fractures or naturally occurring faults and fractures, play an important role in determining the productivity and final recovery of hydrocarbon reservoirs. Moreover, stress changes, as a result of the reservoir depletion, may affect considerably their hydraulic properties. To evaluate the presence and the properties (mechanical and hydraulic) of these interfaces using seismic waves, it is imperative to understand the various modes of wave propagation that can take place along them.

Non-welded interfaces have shown theoretically to support the propagation of interface waves (Pyrak-Nolte, 1987). Experiments on roughened aluminum interfaces (Pyrak-Nolte, 1992) showed that fast and slow modes of interface wave propagated along the interface. Moreover, the predominance of these modes and their velocities were strong functions of the stress applied across the interface (i.e., of the interface stiffness). It was shown that the interface wave velocity, for example, increased from the Rayleigh wave velocity, at zero stress, to the body shear wave at high stress, and that these observations were in agreement with the theoretical expectations (Pyrak-Nolte, 1992). Furthermore, numerical modeling of interface wave propagation (Myer, 1994) showed that several modes of interface wave propagate simultaneously and that these may interfere extensively with each other.

We are not aware, however, of similar experimental work conducted on sedimentary rocks. Our intention with this work is to present experimental data of interface wave propagation on rocks and to evaluate these measurements within the context of the theoretical work by Pyrak-Nolte, (1987), experimental measurements on aluminum samples (Pyrak-Nolte, 1992), and the numerical modeling of Myer (1994).

ARMA-95-0161

The 35th U.S. Symposium on Rock Mechanics (USRMS)

Brazilian tensile strength of welded Apache Leap tuff was analyzed to investigate the effect of specimen size. Five different nominal diameter specimens were used -the ratio of largest to smallest diameters was about seven. Mean tensile strength is minimum when the specimen diameter is 50 mm. Tensile strength increases somewhat and then becomes independent of specimen size. Significant scatter in tensile strength is observed at each nominal specimen diameter. Evan's power law did not reproduce the trend of tensile strength variation with specimen diameter adequately. A Weibull plot shows one distinct line belonging to each specimen diameter. By normalizing the measured tensile strength of each specimen by the mean tensile strength of all the specimens having the same nominal diameter, a Weibull plot of the combinedata can be fitted by a straight line. Weibull modulus of the rock can be estimated from this plot using the combined data. This plot also indicates the presence of another flaw population especially noticeable in samples with low strength. A scale-dependent conceptual model of flaw population distribution is proposed for explaining the size effect observed.

Rock contains discontinuities or defects at different scales. These discontinuities include microfractures, pore space, inclusions, grain boundaries, joints, bedding planes, and other inhomogeneities. Tensile strength of rock exhibits significant scatter about the mean value due to the existing discontinuities. Tensile strength is a material property. Consequently, tensile strength should not depend on specimen geometry and conditions of the tests (Hudson, 1993). But, in reality, tensile strength is observed to be dependent on the size of the specimen (Bernaix, 1969; Lundborg, 1967). A common belief is that the measured strength decreases as the sample size increases giving rise to the scale effect. The discontinuities, being weakness planes, control the behavior of rock specimens, and, thereby, introduce an intrinsic scale effect which depends on the size distribution of these discontinuities. These inherent defects or discontinuities have their own distribution of size, shape, and orientation with respect to the applied load direction. Each defect population may have originated at different times and from different sources. Each population of defects may have its own characteristic size with specific variation about the mean. Small scale tensile strength as obtained in the laboratory may not be appropriate for designing large scale structures. Some method to extrapolate the laboratory data to field scale is necessary.

In this study, the scale effect in Brazilian tensile strength of welded Apache Leap tuff has been investigated. Fracturing in the Brazilian tensile strength test takes place along the weakest plane of the specimen (most critical flaw) after originating from an existing fracture. As a result, the measured tensile strength is the minimum (smallest extreme) from a set of possible values and can be considered as a random variable with Weibull distribution Cl'ype III smallest extreme value distribution) (Ang and Tang, 1984). In this study, both Evan's power law (Jaeger and Cook, 1979) and Weibull statistics (Weibull, 1951) were used to investigate the scale effect of welded Apache Leap tuff.

ARMA-95-0459

The 35th U.S. Symposium on Rock Mechanics (USRMS)

Guyer, R.A. (Los Alamos National Laboratory) | McCall, K.R. (Los Alamos National Laboratory) | Johnson, P.A. (Los Alamos National Laboratory) | Rasolofosaon, P.N.J. (lnstitut Francais du Petrole) | Zinszner, B. (lnstitut Francais du Petrole)

**ABSTRACT:**

The frequencies of the fundamental resonances of a suite of rock samples have been measured as a function of drive amplitude. Representative results from a measurement on Fountainbleu sandstone are reported. The resonant frequency shifts downward with increased drive amplitude exhibiting a softening nonlinearity. The traditional theory of the nonlinear elastic response of rock is reviewed. When applied to resonant bar measurements this theory predicts qualitative and quantitative features that are markedly unlike experiment. The new paradigm introduced by McCall and Guyer (1994) to describe the nonlinear behavior of consolidated materials is reviewed. This paradigm is applied, using extant stress-strain data on Berea sandstone, to describe resonant bar measurements. Good qualitative and quantitative agreement with the experimental observations is found.

**INTRODUCTION**

The traditional theory of elastic wave propagation in a nonlinear material is based on expressing the energy density as a function of the scalar invariants of the strain tensor. Landau and Lifshitz (1959) find the equation of motion for the displacement field u from

[Equation available in full paper] (1)

[Equation available in full paper] (2)

[Equation available in full paper] (3)

where å is the energy density, p0 is the constant mass density, ó is the stress tensor and ª is the strain tensor,

[Equation available in full paper] (4)

The constants ì, K, A, B, and C are found from experiment; for example, K is the bulk modulus of the material. This formulation, in which the stress is assumed to be an analytic function of the strain, has been very successful in describing the dynamics of a wide variety of materials and has been extended to describe the dynamic elasticity of inhomogeneous, consolidated materials such as rock. It is well known, however, that rocks have a stress-strain equation of state with hysteresis and discrete memory (Holcomb 1981). Thus the stress is not an analytic function of the strain. The traditional formulation does not provide a consistent theoretical framework for the description of the elastic properties of rock.

Recently McCall and Guyer (1994) introduced a new paradigm for the description of the elastic properties of rock and other consolidated materials. The central construct of this paradigm is Preisach-Mayergoyz space (P-M space) in which the response of the microscopic mechanical units in the rock, collectively responsible for the rock's macroscopic elasticity, is tracked. Given the density of mechanical units P(Pc, Po) in P-M space, where Pc and Po are pressures characterizing the hysteretic response of an individual unit, one can forward model (calculate) static and dynamic rock properties. Hysteresis with discrete memory, harmonic generation, nonlinear attenuation, and Mdynamic > Mstatic, where M is a modulus, are direct consequences of the model. Equally importantly the P-M space picture provides a recipe for experimental determination of P(Pc, Po). Thus the paradigm gives a complete description of rock elasticity; suitable laboratory measurements lead to p(Pc, Po), which, in turn, allows the prediction of all static and dynamic elastic properties.

The purpose of this paper is to demonstrate quantitative application of this paradigm. In this paper we describe a resonant bar experiment, the traditional theory of this experiment (giving an un-suitable answer), and the theory of this experiment using the new paradigm and an empirically determined p(Pc, Po) (giving results in qualitative and quantitative accord with observation).

ARMA-95-0177

The 35th U.S. Symposium on Rock Mechanics (USRMS)