Preamble

Analysis of Shear Characteristics of Strongly Weathered Granite in a Nuclear Power Plant in Guangdong Province

Liujun¹
¹Nuclear Industry of China Geotechnical Engineering Co., Ltd., Shijiazhuang 050021, Hebei, China

Abstract

To provide suitable and accurate shear strength parameters for highly weathered granite at a nuclear power plant site, this study conducted both outdoor in-situ shear tests and indoor simulated shear tests based on analysis of the rock's fundamental characteristics. Research indicates that the cohesion and friction angle obtained from outdoor direct shear tests are significantly greater than those from indoor tests. The cohesion from indoor direct shear tests shows direct proportionality to both saturation and natural density, while the relative variation in friction angle remains comparatively small. Water exposure demonstrates a more pronounced weakening effect on cohesion than on friction angle. In practical engineering applications, the highly weathered granite masses in this area are influenced by multiple factors including structural conditions, discontinuities, moisture content, climate, and construction disturbance, resulting in actual shear strength values substantially lower than experimental results. Therefore, determination of shear strength for such strongly weathered granite masses should not rely solely on specific test values, numerical simulations, or engineering analogies, but rather requires comprehensive analysis and judgment. This study provides a valuable reference for investigating the shear strength of similar rock masses.

Keywords: Strongly weathered granite, Shear strength, Cohesion, Friction angle, Direct shear test

2 Basic Characteristics of Strongly Weathered Granite

The strongly weathered granite at this nuclear power plant site exhibits highly developed fractures, with mineral composition primarily consisting of quartz, feldspar (including orthoclase and plagioclase), and biotite. The degree of weathering is severe, with rock mass integrity classified as Grade IV [FIGURE:1]. The granite shows a cataclastic structure with fragmented blocks, where the original rock structure is partially preserved but with significantly reduced strength.

Field investigations reveal that the granite mass is extensively fractured, with rock blocks primarily appearing as sub-rounded to sub-angular shapes. The particle size distribution is non-uniform, with block sizes ranging from 2 mm to 20 cm. The rock mass contains numerous weathered fractures and joints, with fracture surfaces showing iron staining and clay mineral coatings. The structural plane characteristics significantly influence the mechanical properties and shear behavior of the rock mass.

The geological setting of this granite formation belongs to the Yanshanian period, with the rock mass affected by multiple tectonic movements. The primary structural planes develop along NW-trending faults, with secondary sets oriented NE and near EW directions. These structural planes control the overall stability of the rock mass and serve as primary pathways for groundwater infiltration. Weathering along these discontinuities is particularly intense, forming zones of weakness that substantially reduce the overall shear strength.

Mineralogical analysis indicates that feldspar minerals have undergone significant alteration to clay minerals, with biotite showing pronounced chloritization. The weathering process has created a heterogeneous material composed of residual rock fragments within a matrix of weathered products. This transformation results in increased porosity and water absorption capacity, which directly affects the shear strength parameters under saturated conditions.

3 Shear Test Research

Based on the fundamental characteristics of the strongly weathered granite at this site, systematic investigations were conducted using field direct shear tests and various laboratory simulation tests to determine appropriate shear strength parameters.

3.1 Field Direct Shear Tests

Field direct shear tests were performed on undisturbed samples of strongly weathered granite collected from the site. Test locations were selected in representative areas with relatively intact rock mass conditions. The test apparatus consisted of a portable direct shear device capable of applying normal stresses up to 30 cm in thickness [FIGURE:2]. The test procedure involved preparing the sample surface, applying normal load, and shearing at a controlled displacement rate [FIGURE:3]. Shear strength parameters were calculated based on the failure envelope obtained from multiple tests under different normal stresses [FIGURE:4] [FIGURE:5].

The field test results yielded shear strength parameters under natural moisture conditions [TABLE:1]. Sample preparation involved careful extraction of undisturbed blocks, with immediate sealing to preserve in-situ moisture content. The testing process accounted for the heterogeneous nature of the weathered granite by conducting multiple tests across different locations. The obtained cohesion and friction angle values represent the integrated strength of the rock mass, including the influence of structural planes and weathering features.

3.2 Laboratory Shear Tests at Different Saturation Levels

To investigate the effect of water content on shear strength, laboratory tests were conducted on samples at various saturation levels. Strongly weathered granite samples were prepared at controlled saturation states ranging from dry to fully saturated, representing different moisture conditions encountered in the field. These tests aimed to quantify the relationship between saturation degree and shear strength parameters [TABLE:2] [TABLE:4].

Sample preparation involved compacting crushed granite material at a controlled density of 1.50 g/cm³ to create standardized specimens [FIGURE:6]. The specimens were then saturated to different degrees using a controlled water injection method [FIGURE:7]. Direct shear tests were performed using a ZJ-type shear apparatus, with shear strength parameters determined for each saturation condition [TABLE:2].

Test results demonstrate that cohesion increases proportionally with saturation and natural density, while friction angle exhibits relatively minor variations. The weakening effect of water on cohesion is particularly significant, whereas its impact on friction angle remains limited. As saturation increases, the cohesive strength shows a marked decreasing trend, particularly at saturation levels above 80%. The friction angle, however, remains relatively stable across different saturation states, typically varying within a narrow range of 30-32 degrees.

3.3 Laboratory Shear Tests at Different Natural Densities

Given that strongly weathered granite exhibits variable density due to differing weathering intensities, shear tests were conducted on samples prepared at various natural densities. Specimens were compacted to target densities representing the range observed in the field, followed by direct shear testing under consistent moisture conditions. This series of tests established the relationship between density and shear strength parameters [TABLE:3].

The results indicate that both cohesion and friction angle increase with higher natural density. Cohesion shows a particularly strong correlation with density, increasing substantially as density rises from 1.4 to 1.7 g/cm³. The friction angle also demonstrates an increasing trend, though with less sensitivity compared to cohesion. This relationship reflects the influence of particle packing and interlocking on the shear resistance of the weathered granite material.

3.4 Saturated Shear Tests at Different Densities

To further investigate the combined effects of density and saturation, a series of saturated shear tests was conducted at different density levels. Samples were prepared at various densities and then fully saturated before shearing. The test results provide insight into the shear behavior under worst-case saturation conditions [TABLE:4].

Analysis reveals that under saturated conditions, shear strength parameters show more pronounced density dependence. Cohesion values under saturation are significantly lower than those at natural moisture content, with the reduction magnitude varying with initial density. The friction angle under saturated conditions remains relatively stable, typically ranging between 30-32 degrees regardless of density variations. These findings highlight the critical role of water in weakening the inter-particle bonds within the weathered granite matrix.

3.5 Comparative Analysis of Field and Laboratory Direct Shear Tests

(1) Differences in Test Conditions and Sample Preparation

Field direct shear tests utilize undisturbed samples that preserve the original rock mass structure, structural planes, and weathering features, representing the integrated behavior of the rock mass. In contrast, laboratory tests employ remolded samples with controlled properties, isolating specific factors such as density and saturation. Field tests capture the influence of natural fractures, joint fillings, and localized weathering patterns, whereas laboratory tests provide fundamental strength parameters of the weathered material under idealized conditions.

Laboratory testing allows precise control over sample density, moisture content, and saturation degree, enabling systematic investigation of individual parameter effects. However, the remolding process destroys the original structural fabric, potentially underestimating the influence of discontinuities. Field tests, while more representative of actual conditions, are limited by sample heterogeneity and testing constraints.

(2) Comparative Analysis of Strength Parameters

Comparison between field and laboratory results reveals significant differences in measured shear strength parameters. Field tests consistently yield higher cohesion and friction angle values compared to laboratory tests on remolded samples. Specifically, field cohesion values are approximately 2-3 times higher than laboratory values at comparable densities, while field friction angles are typically 5-15 degrees greater.

This discrepancy arises from several factors: field samples retain structural integrity and interlocking features that are destroyed during sample remolding; natural cementation and weathering products along discontinuities contribute additional strength; and the scale effect of testing larger, more representative volumes in the field. The presence of incomplete fractures and partial cementation in field samples creates a composite strength that exceeds that of homogeneous remolded material.

The laboratory results, however, provide valuable baseline parameters that represent the minimum strength of the weathered granite matrix. These values are essential for conservative design approaches, particularly when the rock mass is highly fractured or subjected to significant disturbance during construction.

4 Comparison Between Test Values and Actual In-Situ Conditions

Based on comprehensive test results, the shear strength of strongly weathered granite is significantly affected by water content, weathering degree, and structural conditions. In actual engineering scenarios, the rock mass experiences complex environmental and construction-induced factors that substantially reduce its effective shear strength compared to laboratory or even field test values.

Several key factors contribute to this strength reduction: (1) Seasonal variations in groundwater level create cyclic wetting and drying conditions that progressively degrade inter-particle bonds; (2) Construction activities induce vibrations and stress redistribution that activate latent structural planes; (3) The presence of clay-rich weathering products along discontinuities leads to strain softening behavior under shear; and (4) Long-term climatic effects promote ongoing chemical weathering, particularly along exposed surfaces.

The actual shear strength of in-situ rock mass must account for these temporal and environmental factors. Field test values represent instantaneous strength under specific conditions, while long-term strength may be considerably lower due to time-dependent degradation processes. Therefore, engineering design should incorporate appropriate reduction factors based on local hydrogeological conditions, anticipated construction impacts, and long-term environmental exposure.

For this specific nuclear power plant site, the highly weathered granite mass exhibits complex structural features including multiple fracture sets, variable weathering profiles, and localized zones of intense alteration. These characteristics create a heterogeneous strength distribution that cannot be fully captured by limited test data. Consequently, a comprehensive approach combining test results, geological mapping, and numerical modeling is necessary to establish reliable design parameters.

5 Principles for Direct Shear Strength Analysis and Evaluation

(1) Comprehensive Consideration of Multiple Factors

Shear strength parameters should not be determined from single test values alone. Field test results must be integrated with laboratory data, geological conditions, and engineering experience. The final parameter selection should account for structural plane characteristics, moisture conditions, weathering heterogeneity, and potential construction effects. A comprehensive database incorporating multiple test methods provides the most reliable basis for parameter determination.

(2) Accounting for Environmental and Temporal Effects

Strength parameters must be adjusted to reflect actual service conditions. This includes considering long-term water saturation effects, cyclic wetting-drying degradation, and construction-induced disturbance. The potential for strength reduction over time should be evaluated based on local environmental conditions and rock mass quality. Conservative parameters should be adopted for critical structures or unfavorable geological conditions.

(3) Integration with Engineering Geological Conditions

Parameter selection must be closely integrated with detailed engineering geological investigations. For highly weathered granite masses, this includes thorough characterization of fracture systems, weathering profiles, and hydrogeological conditions. The spatial variability of rock mass quality should be mapped and incorporated into zoned strength parameter assignments. Special attention should be given to areas with adverse structural orientations or concentrated water flow.

(4) Validation Through Multi-Method Approaches

Final shear strength parameters should be validated through multiple approaches, including back-analysis of existing slopes or excavations, numerical modeling calibration, and comparison with similar engineering projects. This cross-validation process helps identify potential biases in test data and ensures parameter reliability. For nuclear power plant projects, which demand the highest safety standards, this multi-method validation is particularly critical.

Based on comprehensive analysis of test data and field conditions, recommended shear strength parameters for design purposes are: cohesion in the range of 200-300 kPa and friction angle between 35-43 degrees for natural moisture conditions. For saturated conditions at a density of 1.50 g/cm³, cohesion reduces to 26-30 kPa with friction angle of 30-32 degrees. These values represent conservative estimates that account for the various reduction factors discussed above.

In practice, the shear strength of strongly weathered granite should be determined through integrated analysis rather than direct adoption of test values. The final parameters must reflect the combined influence of rock material properties, structural conditions, environmental factors, and engineering requirements. This comprehensive approach ensures both safety and economy in the design of structures founded on or within highly weathered granite masses.

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