Preamble

Research on the In-Situ Mechanical Properties of Tuff Rock Mass
TANG Aisong¹, WANG Yongsheng², YAO Shaoqiang¹
(1. Key Laboratory of Geotechnical Mechanics and Engineering of the Ministry of Water Resources, Changjiang River Scientific Research Institute, Wuhan 430010, China;
2. The Yangtze River Three Gorges Survey and Research Institute Co., Ltd. (Wuhan), Wuhan 430010, China)

Abstract

Tuff, as a type of volcanic clastic rock, is widely utilized in construction and engineering fields due to its dense structure, high compressive strength, and excellent durability. This paper investigates the in-situ mechanical properties of tuff at engineering sites in Shunxi (Zhejiang), Fushun (Liaoning), and Emin (Xinjiang) through rigid bearing plate deformation tests and push-method direct shear tests, aiming to reveal the variation规律 of deformation and strength characteristics with different weathering degrees. The research results demonstrate that the mechanical properties of tuff are significantly influenced by weathering. The deformation modulus of intact, fresh crystal-fragment welded tuff reaches as high as 20.36–44.40 GPa, with shear strength parameters of friction angle 1.43°–1.95° and cohesion 1.53–3.05 MPa. In contrast, the deformation modulus of strongly weathered tuff plummets to 0.10–0.31 GPa, with strength parameters also decreasing substantially (friction angle 0.66°, cohesion 0.14 MPa). Analysis indicates that the predominant role of weak minerals such as illite and muscovite during weathering leads to deterioration of the internal rock structure, consequently causing a cliff-like decline in mechanical performance. This study systematically examines the in-situ mechanical properties of tuff, providing an important reference for engineering applications of tuff, particularly for stability evaluations of dam foundations and tunnel engineering.

Keywords: Rigid bearing plate method; Push method; Deformation modulus; Shear strength parameters

Introduction

Tuff is a fine-grained volcanic clastic sediment formed by the cementation of volcanic ash and sand ejected during eruptions. The glassy fragments within the rock are transparent and slightly yellowish or brownish, appearing as tiny flakes or foam-like chips, and additionally contain broken phenocrysts and solidified lava fragments. As a volcanic clastic rock, tuff is composed of more than 50% volcanic clastic material with particle diameters smaller than 2 mm, predominantly volcanic ash. Its appearance may be loose or dense; layered varieties are termed stratified tuff, and it exhibits diverse colors including purplish-red, gray-white, and gray-green. Based on its volcanic clastic composition, tuff can be classified into crystal-fragment tuff, glass-fragment tuff, and rock-fragment tuff. Tuff forms from magma that cools slowly under substantial overburden pressure in the deep crust, characterized by dense structure, high unit weight, high compressive strength, low water absorption, and good frost resistance, wear resistance, and durability. It is commonly used as a construction material, as raw material for cement production, and for potassium fertilizer extraction.

Numerous scholars have investigated the mechanical properties of tuff. Wang Rubin et al. \cite{1} studied the anisotropic characteristics of tuff as a dam foundation material; Zheng Mingxiong et al. \cite{2} examined its internal structure and mineral composition; Li Jian et al. \cite{3} investigated the deformation and mechanical characteristics of tuff in high-altitude cold regions through monitoring methods; Jia Lichun et al. \cite{4} studied its mechanical properties through laboratory tests; Li Xiping et al. \cite{5} investigated its rheological properties through laboratory creep tests; and Li Yuanjun et al. \cite{6} studied its dynamic properties. Other researchers have also investigated tuff as a concrete additive material. However, no studies have yet examined the in-situ mechanical properties of tuff. This paper addresses this research gap through in-situ testing of tuff at multiple engineering sites, synthesizing the in-situ rock mass mechanical properties of tuff to provide a comprehensive understanding and guidance for engineering construction.

2.1 Rigid Bearing Plate Method Deformation Test

The rigid bearing plate method for rock mass deformation testing employs a bearing area of 2000 cm², with increased plate thickness for rock masses with higher deformation moduli. At selected test sections, deformation test points are arranged. Skilled masons manually remove the loose surface layer and carve a 2 m × 2 m horizontal plane, with the central Φ60 cm area meticulously prepared to meet specification requirements. Geological sketches and photographs are taken for each test point. The test installation is shown in Figure 1 [FIGURE:1]. Loading is applied in five stages using a stepwise cyclic loading method, with deformation readings taken every 10 minutes before and after each pressure increment.

2.2 Push Method Direct Shear Test

The push-method direct shear test for rock mass involves preparing rock specimens measuring 50 cm × 50 cm × 35 cm (length × width × height). A concrete protective jacket is cast around the specimen or a steel mold is used to encase the sample, with a 2 cm shear gap reserved between the bottom of the protective jacket or steel mold and the rock mass (structural plane). Testing is conducted after the protective jacket has achieved sufficient stiffness and strength. The test installation is shown in Figure 2 [FIGURE:2].

3.1 Deformation Data Processing

The average effective surface deformation of the plate is calculated, and the pressure-deformation (P-W) relationship curve is plotted. The rock mass deformation modulus $E_0$ and elastic modulus $E_e$ are calculated using the following formula:

$$
E = \frac{\pi}{4} \frac{P D (1-\mu^2)}{W}
$$

Where:
$E$—Rock mass deformation modulus or elastic modulus, MPa; when total deformation is substituted, it is the deformation modulus $E_0$; when elastic deformation is substituted, it is the elastic modulus $E_e$;
$\mu$—Poisson's ratio of rock mass;
$P$—Pressure calculated per unit area of bearing plate, MPa;
$D$—Diameter of bearing plate, cm;
$W$—Surface deformation of rock mass, cm.

3.2 Rock Mass Direct Shear Data Processing

For the push-method direct shear test, normal stress and shear stress under each normal load are calculated using the following formulas:

$$
\sigma = \frac{P}{A}
$$

$$
\tau = \frac{Q}{A}
$$

Where:
$\sigma$—Normal stress acting on the shear plane (MPa);
$\tau$—Shear stress acting on the shear plane (MPa);
$P$—Total normal load acting on the shear plane (N);
$Q$—Total shear load acting on the shear plane (N);
$A$—Shear plane area (mm²).

For each specimen, shear stress versus shear displacement and normal displacement curves are plotted based on test results. The peak shear strength and residual shear strength values under each normal stress are determined from these curves. For a set of test data, the shear stress versus normal stress relationship curve is plotted based on normal stress and its corresponding peak shear strength values. The shear strength parameters are determined using the Coulomb expression as the rock mass shear strength parameters; similarly, the rock mass residual shear strength parameters can be determined.

4.1 Deformation Test Results and Analysis

Comprehensive test results from various engineering sites are listed in Table 1 [TABLE:1]. The results indicate that the deformation modulus of tuff is higher when the rock is fresher, and lower when more weathered, exhibiting a cliff-like decrease. This phenomenon may be related to internal composition and structure. According to research by Zheng Mingxiong et al. \cite{2}, tuff contains minerals such as quartz, illite, and muscovite. As weathering progresses, the weak minerals like illite and muscovite play a dominant role, increasing rock deformation and causing a rapid decrease in deformation modulus.

4.2 Rock Mass Direct Shear Test Results Summary

The direct shear test results exhibit the same characteristics as the deformation modulus: the more weathered the rock mass, the more significantly its strength parameters decrease, particularly the cohesion $C'$ value.

Table 1 [TABLE:1] Comprehensive Deformation Test Results
Crystal-fragment welded tuff
Deformation modulus/GPa: 20.36–44.40, 10.28–19.50, 0.10–0.31, 5.80–14.62, 1.07–4.00

Table 2 [TABLE:2] Comprehensive Direct Shear Test Results
Rock mass direct shear strength parameters: Friction angle 1.43–1.95, $C'$/MPa 1.53–3.05

5 Conclusions

[5] Li Yuanjun. Testing and analysis of rock dynamic characteristic parameters for Zhejiang Sanmen Nuclear Power Station [J]. Engineering Science, 2010, 12(2): 27-30.

When tuff is intact and fresh, it exhibits dense structure, high unit weight, high compressive strength, high deformation modulus, and high strength parameters. As weathering progresses, its internal structure rapidly changes due to the presence of weak minerals, causing a sharp decline in its deformation and strength parameters.

[1] Wang Rubin, Zhang Yu, Zhang Zhiliang. Experimental study on anisotropic rock mechanical properties of dam foundation [J]. Journal of China Three Gorges University (Natural Sciences), 2010, 32(2): 5-9.
[2] Zheng Mingxiong, Li Baozhu. Study on characteristics and collapse prevention of tuff in Dapingzhang Copper Mine, western Yunnan [J]. Geotechnical Investigation & Surveying, 2020, 7: 6-10.
[3] Li Jian, Zou Xiaoxin, Fan Fangfang, et al. Study on deformation and mechanical characteristics of tuff in high-altitude cold region tunnels [J]. Modern Tunnelling Technology (Supplement 2), 2018, 55(S2): 1166-1174.
[4] Li Xiping, Yang Junsheng, Wang Lichuan, et al. Study on rheological test and constitutive model of tuff in Sanlian Tunnel [J]. Journal of Railway Science and Engineering, 2015, 12(1): 1-7.