Preamble

High-pressure Water-abrasive Coupled Jet Rock Breaking Technology and In-situ Sample Preparation Device

DU Weichang¹, CHENG Peiming¹, ZHAO Shunli¹, YU Yang¹, GUO Chong¹

¹ Jianghe Anlan Engineering Consulting Co., Ltd., Zhengzhou 450001, China

Abstract

To address the challenges of harsh working conditions, low efficiency, and poor adaptability to complex geology in traditional in-situ rock sample preparation techniques, this study proposes a novel in-situ sampling device based on high-pressure water-abrasive coupled jet rock-breaking technology. By integrating an adjustable cutting head, a dual-inlet jet system, and an intelligent control module, the device achieves adaptive switching between high-efficiency hard rock cutting and non-destructive soft rock sampling. Field tests demonstrate its significant advantages in rock-breaking efficiency (35% improvement), sample integrity (fracture rate < 2%), and energy consumption (28% reduction). The results provide innovative technical support for geological exploration and engineering rock mass quality assessment.

Keywords: High-pressure water-abrasive coupled jet; In-situ sampling; Rock mass integrity; Adaptive control

Introduction

The preparation of in-situ rock samples is a critical step for obtaining rock mass mechanical parameters, yet this process faces numerous technical challenges. Due to the inherent heterogeneity and anisotropy of rock masses, sample preparation must not only ensure representativeness but also precisely control the distribution and orientation of structural planes (such as faults, fractures, and bedding) and pre-existing fissures. These factors directly influence the accuracy and reliability of subsequent mechanical parameter tests, which serve as fundamental data for engineering geology, mining operations, engineering design, resource assessment, and environmental management.

Traditional mechanical cutting techniques suffer from significant limitations, including low efficiency in hard rock cutting, thermal damage to samples, and poor adaptability to complex strata. High-pressure water jet technology has emerged as a novel geotechnical cutting method with distinct advantages, prompting extensive research both domestically and internationally. Pozzetti et al. \cite{1} proposed a numerical method for simulating water jet action, particularly in capturing cumulative rock mass damage, which enhances understanding of erosion processes induced by water jets. Raj P and Jifu Yin et al. \cite{2,3} investigated various factors affecting rock-breaking performance, concluding that both pulsed jet frequency and incident angle substantially influence breaking characteristics. Dehkhoda et al. \cite{4} studied the role of pulsed jets in rock fragmentation, discovering that high-velocity pulsed jets generate internal stress waves causing fatigue failure, and that pulse length and frequency play crucial roles in the process. Wu Wei et al. \cite{5} employed a CFD Eulerian multiphase flow model using Fluent software to compare internal and external flow fields of abrasive side-entry, center-entry, and improved center-entry nozzles under identical boundary conditions. Domestic theoretical research on water jet rock breaking has primarily focused on frameworks such as "tension-water wedge" and "compact core-splitting" theories, which explore mechanisms combining water jets with mechanical impact \cite{6,9}. Zhai Shengyu \cite{10} investigated the fracture-enhancing mechanism and slotting performance of abrasive water jets, establishing a neural network prediction model. The combined action of water jets and mechanical forces produces superimposed stress waves in rock, significantly increasing shear and tensile stresses in overlapping regions, which promotes crack generation and reduces rock compressive strength, thereby facilitating fragmentation. Zhang Jinliang et al. \cite{11} studied cutting performance of water jets under high linear velocities, developing a semi-theoretical, semi-empirical prediction model based on Crow's rock cutting theory to predict cutting depth of ultra-high pressure water jets. Although previous studies have conducted relevant experimental and numerical investigations on high-pressure water jet rock breaking \cite{12}, research on the rock-breaking mechanisms of high-pressure water-abrasive jets remains relatively scarce, particularly regarding single jet mode limitations and the inability to accommodate both soft and hard rock conditions.

This study employs an integrated experimental and theoretical analysis approach to propose a high-pressure water-abrasive dual-mode adaptive jet system. By combining independently developed high-pressure water-abrasive coupled jet rock-breaking technology with an in-situ sample preparation device, we investigate the system's application and advantages in rock fragmentation. The system leverages the synergistic effect of high-pressure water jets and abrasive injection technology to effectively enhance rock-breaking efficiency while minimizing rock damage, maintaining high precision and stability, and improving working conditions for sampling personnel. Research demonstrates that this technology holds significant potential for applications in rock breaking, geotechnical engineering, and in-situ testing, with promising prospects for widespread implementation in future engineering practice.

2 High-pressure Water Jet Rock Breaking Mechanism and Jet Dynamics Model

High-pressure water-abrasive coupled jet technology represents an efficient method for in-situ rock mass sampling and processing, with its core mechanism stemming from the synergistic effect between high-pressure water jets and abrasive particles. This technology elevates water pressure to ultra-high levels, forming high-velocity jets through specially designed nozzles. These high-velocity jets possess tremendous impact kinetic energy and can produce significant damage to rock surfaces through multiple mechanisms including water wedge effects, dynamic impact, and surface erosion.

To further enhance cutting effectiveness, high-hardness micron-scale abrasive particles can be introduced into the high-pressure water flow. These particles acquire substantial kinetic energy through acceleration by the high-velocity water stream, producing composite effects of micro-cutting, plowing, and fatigue damage upon impact, thereby significantly improving material removal efficiency. According to stress wave fragmentation theory, the rock damage process induced by high-pressure water-abrasive coupled jets occurs in two key stages: first, compressive stress waves generated by jet propagation travel through the rock interior; subsequently, when these waves reflect to form tensile stress fields, material fracture occurs if the tensile stress exceeds the rock's tensile strength \cite{13}.

From a fluid dynamics perspective, the initial stage represents the first region upon jet exit, extending from the end of the conical constant-velocity core to the nozzle outlet. This region is critical for target distance selection in high-pressure jet cutting and consists of the constant-velocity core zone and the mixing zone. The constant-velocity core zone forms a conical region where no energy exchange occurs with the surrounding medium. The mixing zone is where energy exchange with ambient air takes place, causing jet boundary expansion and gradual axial velocity decay through turbulent entrainment. This turbulent mixing region extending outward from the nozzle is termed the shear layer (Figure 1 [FIGURE:1] (a)). The initial stage comprises the region from nozzle exit to the end of the conical constant-velocity core, which is essential for target distance selection. The constant-velocity core zone maintains relatively uniform flow velocity with minimal energy exchange with surrounding air or media, representing a stable central flow region. The mixing zone immediately follows, where energy exchange with ambient air occurs. Within this zone, the jet boundary gradually expands while axial velocity decays, with jet-air interaction generating turbulence and mixing effects that alter flow velocity and morphology. Additionally, the turbulent exchange region between jet and surrounding medium is called the shear layer, a region extending outward from the nozzle where velocity differences between jet and ambient air create turbulence that drives boundary expansion \cite{14}-\cite{17}.

The coupling effect between high-pressure water and abrasives arises because the high-velocity water flow provides sufficient kinetic energy for abrasive particles to impact target surfaces at high speed, effectively destroying material through impact and grinding actions. These high-hardness particles rapidly erode or cut rock surfaces, promoting crack propagation. Based on a multiphase flow model derived from Navier-Stokes equations describing particle-laden fluid dynamics and particle-fluid interactions, the coupled fluid-abrasive equations are:

$$\partial(\rho_m v) + \nabla \cdot (\rho_m v \otimes v) = -\nabla p + \mu_m \nabla^2 v + F_{particle}$$

where $\rho_m$ is the mixture density representing the overall density of the fluid-particle mixture; $v$ is fluid velocity; $\nabla \cdot (\rho_m v \otimes v)$ denotes the convective term of fluid momentum, describing spatial variations in fluid motion; $-\nabla p$ represents the pressure gradient term; $\mu_m \nabla^2 v$ is the viscous diffusion term describing internal friction and viscous effects; and $F_{particle}$ is the abrasive particle momentum exchange term.

The jet energy transfer efficiency is calculated as:

$$P_{pump} 2P_{pump}$$

where $\rho$ is fluid density; $Q$ is flow rate representing fluid volume per unit time through a cross-section; $v$ is jet velocity; $P_{jet}$ is jet kinetic power; and $P_{pump}$ is pump output power. This formula measures the ratio between pump-supplied energy and actual energy transferred to the jet. Higher efficiency $\eta$ indicates better pump energy utilization and more effective energy transfer to the external system.

3.1 Test Platform Construction

The high-pressure water-abrasive coupled jet cutting experimental system comprises several key components, including a CNC water cutting platform, cutting head, CNC system, and ultra-high pressure pump unit.

The CNC platform precisely controls the cutting head's motion trajectory based on preset parameters, including horizontal and vertical adjustments, enabling automated execution of complex cutting tasks as shown in Figure 2(a). The cutting head serves as the core component of the high-pressure water-abrasive coupled jet, directly responsible for water jet and abrasive injection. Equipped with high-precision nozzles, it concentrates and accelerates water flow to extremely high velocities, ensuring powerful impact force (Figure 2(b)). Abrasives mix with high-pressure water through dedicated channels before co-injection through the nozzle, with abrasive flow rate and injection velocity being critical control parameters. The CNC system coordinates platform, cutting head, and other components (Figure 2(c)), enabling precise control of water jet pressure, flow rate, abrasive concentration, nozzle trajectory, and standoff distance to achieve optimal rock-breaking performance. Through preset programs, the system operates automatically, reducing manual operation errors, enabling high-efficiency and high-precision operations, and monitoring experimental parameters in real-time with feedback-based adjustments to ensure stability and safety. The ultra-high pressure pump unit elevates water pressure to hundreds or thousands of MPa to ensure jet impact force (Figure 2(d)), employing an efficient pumping system for high-pressure output.

3.2 Experimental Design and Results Analysis

This study compared three different in-situ test specimen preparation schemes: traditional mechanical cutting, high-pressure water-abrasive coupled jet cutting, and pneumatic cutting, using granite—a common igneous rock—as the test material to evaluate practical cutting performance and analyze each scheme's effectiveness during specimen preparation.

Standard granite samples (50 cm × 50 cm × 50 cm cubes) were selected to simulate cutting processes under actual mining conditions. Three different cutting technologies were tested and compared across multiple dimensions: average cutting efficiency, fracture rate, compressive strength retention, construction quality, labor intensity, and operational safety. The comparison is presented in Table 1 [TABLE:1]. Average efficiency was calculated as the mean unit-time cutting volume (m³/h) from multiple tests. Fracture rate represents the mean percentage of fracture area relative to total surface area (%) across multiple specimens, reflecting sample integrity through post-cutting surface scanning or CT imaging. Strength retention indicates the mean ratio (%) of post-cutting compressive strength to original strength, characterizing damage extent.

Based on comparative experiments and engineering practice data, systematic evaluation of the three schemes yielded the following results:

The high-pressure water-abrasive coupled jet cutting scheme achieved average efficiency of 2–4 m³/h, significantly exceeding traditional mechanical cutting (0.5–2 m³/h) and pneumatic cutting (1–3 m³/h). Under hard granite conditions, optimizing abrasive mixing ratio (25% emery, 0.5 mm particle size) and jet pressure (380 MPa) enabled cutting efficiency up to 3.4 m³/h—4.2 times higher than traditional mechanical cutting. This efficiency advantage stems from: (1) non-contact cutting where jet energy directly acts on rock surfaces without friction losses; (2) energy concentration effects where gradient contraction nozzle design increases jet velocity to 800 m/s with 72% kinetic energy transfer efficiency (compared to <30% for mechanical cutting); and (3) adaptive switching where pneumatic valves enable rapid (<50 ms) activation of abrasive mixing mode under hard rock conditions, reducing ineffective energy consumption.

Sample integrity is crucial for accurate rock mass parameter determination. The high-pressure water-abrasive jet scheme controls fracture rate below 2% through non-contact operation without thermal damage, significantly outperforming traditional mechanical cutting (>5% fracture rate) and pneumatic cutting (>3% fracture rate). Uniaxial compressive strength tests show 97.3% strength retention. Considering average efficiency, fracture rate, strength retention, construction quality, labor intensity, and operational safety, high-pressure water-abrasive coupled jet cutting proves most suitable for in-situ sampling. It offers superior efficiency, lower fracture rates, and higher strength retention while ensuring construction quality with reduced labor intensity and enhanced safety, avoiding mechanical wear and potential safety risks associated with pneumatic cutting.

3.3 Microscopic Damage After Impact

CT scanning analysis further examined microscopic damage within granite. Figure 3 [FIGURE:3] reveals that under impact loading, granite impact crater cross-sections exhibit irregular morphologies. Impact action causes not only localized surface damage but also internal microcrack propagation, with CT scans revealing crack depth and distribution characteristics.

Traditional mechanical cutting employs rotating cutters to fracture rock through applied pressure. Contact between cutter and rock surface generates localized stress, producing relatively regular fracture patterns through mechanical crushing and compressive stress, with cracks tending to follow rock bedding planes and penetrating to greater depths.

High-pressure water-abrasive coupled jets utilize synergistic water jet and abrasive action, projecting high-pressure streams onto rock surfaces to generate impact forces and abrasive wear. This process doesn't merely crush rock but creates fractures through intense water jet cutting and abrasive physical destruction, producing typically complex, irregular, and finer fracture distributions with shallower depths. Impact point peripheries show relatively neat fracture surfaces with observable microcracks and stepped fault crystals. During initial impact, combined shock and tensile wave action forms preliminary micro-pores, with weakly bonded planes preferentially failing through stripping. These irregular micro-pores subsequently undergo shear fracture under sustained jet scouring, allowing jet penetration into weak planes and promoting microcrack propagation \cite{14}.

Pneumatic cutting uses air pressure to drive cutting heads, with airflow impact causing compressive damage and fracture generation. Pneumatic cutting produces relatively localized and concentrated fractures, typically deep and fine, with pressure and frequency influencing fracture distribution. High air compression is usually required to overcome rock compressive strength, producing detailed fracture features with strong directional orientation.

Considering fracture development patterns, high-pressure water-abrasive coupled jet cutting produces relatively shallow fracture depths, making it most suitable for in-situ test sampling. Under optimal cutting conditions, this method better preserves overall rock mass integrity compared to traditional mechanical cutting and pneumatic cutting, which typically generate deeper fractures and more significant damage that may substantially affect rock structure.

4 Conclusions

This study investigated high-pressure water-abrasive coupled jet technology for rock cutting, exploring its potential for in-situ sample preparation and comparing performance with traditional cutting methods. The main conclusions are:

1. High-pressure water-abrasive coupled jet cutting demonstrates superior efficiency. Compared with traditional mechanical cutting, this technology can effectively cut hard rock in shorter time, particularly excelling in high-strength rock such as granite.

2. The technology causes minimal damage to rock mass during cutting, better preserving sample integrity. Compared with traditional methods, water jet technology produces lower fracture rates and shallower cracks, enabling precise cutting under high pressure and appropriate abrasive concentration.

3. Rock samples processed by high-pressure water-abrasive coupled jets maintain favorable compressive strength and other mechanical properties. Experimental data show that strength retention rates are significantly higher than those achieved with traditional mechanical cutting, indicating effective cutting improvement without damaging rock mass integrity.

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