Pebble Layer Research Articles

Study on the Diffusion Law of Grouting Slurry at the Pile Tip of Bored Piles in Gravel Pebble Layers

The post-construction improvement technology of bored pile tips has been widely used in the construction of gravel pebble layers, but the diffusion law of the grouting slurry remains to be revealed. To study the diffusion law of the grouting slurry at the tip of bored piles in gravel pebble layers, an equivalent relationship between the seepage continuity equation and the mass diffusion continuity equation was established based on an in-depth comparative analysis; further relying on the Diluted Species Transport Module in Porous Media of Comsol Multiphysics, a three-dimensional numerical model of slurry diffusion was established to systematically study the impact of key factors such as different porosities, grouting times, grouting pressures, slurry diffusion coefficients, and the ratio of vertical to horizontal slurry diffusion coefficients on the diffusion radius of the slurry. The calculation results show that with the continuation of grouting, the increase in porosity, grouting pressure, and slurry diffusion coefficient, or the decrease in the ratio of vertical to horizontal slurry diffusion coefficients, the diffusion range of the slurry increases accordingly. Furthermore, comparatively speaking, the impact of porosity is the smallest, while the impact of the slurry diffusion coefficient is the greatest. The research findings reflect the diffusion trend of the slurry in the gravel pebble layer, which has guiding significance for the quality control of the actual design and construction of grouting.

Rising damp in heritage sites under urban expansion: A comprehensive case study of the Jinsha Earthen Site

Rising damp poses a significant threat to cultural heritage, yet its dynamics within actual heritage sites remain inadequately understood, particularly regarding its evolution and the impact of urbanization. The Jinsha Site, an earthen site from the Shang periods, is primarily composed of silt, with a layer of sand beneath and pebbles at the bottom, making it susceptible to rising damp. This study investigates the Jinsha Site as a case, monitoring soil water content and electrical conductivity, tracking changes in groundwater, precipitation, and river water levels, and investigating the relationship between urbanization and rising damp. The study aims to understand the process, causes, and influencing factors of rising damp and evaluate previous preservation strategies. The findings indicate that groundwater level in the sand layer trigger dampness, whereas the pebble layer exhibits dryness. Urban construction from 2006 to 2013 and 2014–2017 caused fluctuating groundwater levels, leading to repeated transitions between dryness and dampness at the site. Rainfall and river levels have negligible effect; even rainfall exceeding 100 mm (frequency <0.0056) only raises groundwater levels by approximately 1 m and increases water content by less than 2 % for a brief duration, typically less than half a month. The 2022 groundwater pumping strategy effectively maintained levels beneath the pebble layer (506 m), decreasing moisture by 12.71 % and conductivity by 0.27 dS/m. This study reflects on previous protection effortsand offers evidence-based strategies for heritage conservation amidst urban expansion.

A Phenomenological Model for Estimating the Wear of Horizontally Straight Slurry Discharge Pipes: A Case Study

When a slurry TBM advances in pebble and rock strata, large rock particles are carried in pipelines out of a tunnel by moving slurry. To estimate the wear of horizontally straight slurry discharge pipes, a phenomenological model was proposed that was mainly based on knowledge gained by means of direct and indirect in situ observations. The proposed model applies an equation composed of three variables, namely, the wear rate (λ), the central angle (2α), and the excavated tunnel length (L), to estimate the wear distribution along a pipe’s internal surface. The results indicated that wear mainly occurred on the bottoms of pipes. In addition, linear relationships between the maximum pipe wear amount (δmax) and the excavated tunnel length (L) were found for specific pipes and specified types of ground. The observed wear rates of different pipes in different types of ground had varied constants. The wear rates were higher for pipes in rock ground than for those in a pebble layer. For horizontally straight pipes, the observed wear rates were 0.0045 mm/m in a pebble layer and 0.0212 mm/m in rock ground. Lastly, to improve the proposed model, more field monitoring will be necessary to determine the pipe wear rates in different types of ground in the future.

Soils of the island bars of the Ola lagoon (Okhotsk sea)

The aim of the study. The purpose of the work is investigating soil formation on the island bars of the Ola Lagoon, as soil formation there has a special character due to the severe climate, dynamism and small age of the landscapes, and the influence of marine colonial birds. Location and time of the study. Field studies were conducted in June-July 2022 in the coastal zone of the Ola Lagoon of Magadan Region; laboratory and desk studies were conducted in 2022 and 2023. Methods. Methods included route observations with soil profile descriptions, mesomorphological studies; soil physical and chemical analyses: pH, electrical conductivity (TDS), loss on ignition (LOI); satellite photos interpretation and soil mapping. Results. The bar soils were qualified as dry-peat lithozems, differing in the thickness of the dry peat horizon TJ and the abundance of skeletal material (pebble). The dry-peat horizon is gradually merging into parent rock - dense marine pebbles, which are sometimes found in the dry-peat horizon due to storm "surges" from the sea. The TJ horizon consists of mesophilic plant residues and morphons of dark fine-disperse matter and a large number of semi-decomposed and living roots penetrating into the pebble layer. In the WRB system, soils correspond to Histic Leptosols (if shallow), or to Mawic Fibric Histosols. Conclusion. Formation of the dry-peat lithozems with the TJ-(TJ/С)-C profile is determined by the dry marine climate and low plant diversity of the meadows dominated by reed grass and wild rye. The main factors, shaping soil properties, are the island bars dynamics, specific of pebbles a parent rock material, salt impulverization and marine birds colonies presence. The research expands the knowledge on poorly studied soils of the region, and may contribute to substantiating their position in the classification of soils of Russia; in addition, the data obtained are useful within the framework of sustainable development, as it serves as nesting ground for colonial seabirds.

Исследование динамики влажности почвы при капельном поливе

Purpose: to study the dynamics of moisture in soils on the drainage layer during drip irrigation to save water resources and ensure uniform watering of crops using moisture contours. Materials and methods. The study is based on the theory of moisture transfer under the influence of drip irrigation on soil. Moisture contours were studied in laboratory conditions using a soil lysimeter, which was used to model the geological cross-section of the irrigated area, represented by alluvial meadow soils formed on pebbles. The pebble layer thickness was assumed to be up to 3 m. A regression analysis of moisture movement was carried out, and a mathematical model describing the nature of its movement on the drainage layer was obtained. The estimated volume of wet soil was defined on the basis of moisture contours as a result of calculating the volume of the formed lathe object using cartograms for each hour of irrigation. Results. Data on the moisture distribution in soil profile according to irrigation hours, presented on cartograms constructed using a personal computer, were obtained. The contours with the same humidity – isolines on which the humidity was displayed: 5; 10; 40 % are highlighted on the cartograms. It has been determined that water at an emitter discharge of 2.5 l/h reaches the drainage layer 4 hours after the beginning of irrigation. The graph for moisture distribution in soil volume by hour, reflecting the dynamics of water distribution supplied from an emitter with a constant discharge over 10 hours of irrigation has been constructed. Conclusion. The dynamics of moisture distribution during drip irrigation in soils located on a drainage bed was studied. Under experimental conditions, when crop irrigation lasts for more than 8 hours, an ineffective excess of irrigation rate and an increase in the design capacity of the drip irrigation system are observed.

Influence of external insert angle on structural deformation of long pipe shed with shallow hole

The long pipe shed advanced support has the characteristics of large construction difficulty, wide support range, and easy deviation. The difference in construction dip angle will produce a different plastic zone of surrounding rock and supporting effects, and the rationality of advanced support design plays an important role in the safety of tunnel opening excavation. Based on the Tianshan Tunnel project, this paper aims at the problems of the loose pebble layer structure, poor cohesion, strong permeability, shallow excavation is not easy to form a confined arch, and easy to causes surrounding rock deformation and failure. Combined with the new method theory, the finite difference software FLAC3D was used to simulate seven excavation schemes of the shallow tunnel entrances. The mechanism and effect of advance support and the influence of construction angle on support effect are analyzed, and the simulation values are compared with the measured data. The results show that using long pipe shed advanced support can effectively reduce the disturbance of excavation to lose pebble rock mass and reduce the convergence value of surrounding rock. The plastic zone of surrounding rock produced by different angles can be divided into three parts. The structural stability difference of the C1 zone is small, and that of the C3 zone is large. The bending moment, shear force, and shape variables of the pipe shed decrease with the increase of the dip angle, while the axial force increases with the increase of the dip angle. According to the similarity between the simulated curve and the measured curve, the best supporting effect can be achieved when the dip angle of the pre-supporting pipe shed is set in the range of 0° ~ 3°.

Slurry Discharge Pipeline Damage and Wear Due to Transporting Rock Particles during Slurry Shield Tunneling: A Case Study Based on In Situ Observed Results

Rock particles in excavated materials can damage and wear down slurry discharge pipelines when slurry shield tunneling occurs in a pebble layer and rock ground. This pipeline damage and wear, if not properly dealt with, can lead to a broken-down tunneling machine. Based on a slurry shield tunneling project in China, damage and wear were thoroughly examined. The observed pipeline wear and leaks, transported rock particles, mechanical properties, and flow rate of the carrier slurry were presented. The measured results showed that the wear rates of a straight pipe, a pipe along a curved tunnel, a 60° inclined pipe, and a 90° elbow pipe in pebble ground were approximately 0.71 mm/100 rings, 1 mm/100 rings, 2.14 mm/100 rings, and 4 mm/100 rings, respectively. When the machine drove into rock ground, the wear rates increased by one to two times, which could be attributed to the sharper particle shapes. Countermeasures to address these issues, such as adjusting the pipeline layout, welding reinforcement plates in advance, and preparing additional pipes, were highlighted. The wear rates of different types of pipes, the effectiveness of new pipeline fixing methods, and the probability of pipeline leaks in different strata were discussed in detail.

Bearing behavior of rock socketed pile in limestone stratum embedded with a karst cavity beneath pile tip

This paper investigates the bearing behavior of axially loaded rock socketed piles in limestone stratum embedded with a karst cavity beneath the pile. The pile foundation supporting a cross-river bridge located in a karst area in China is considered. Based on this project, three-dimensional finite element analyses are performed to capture the effects of pile length, pile diameter, limestone cohesion, and cavity roof thickness. Their impact on the bearing characteristics of the pile is comprehensively outlined through analysis of load-settlement curves, variations in pile bearing capacity, distributions of axial pile force and pile side friction, as well as contours of vertical ground and cavity roof displacement. It is found that the load-settlement curves exhibit the saltation type, based on which the pile-bearing capacity can be conveniently obtained. The pile bearing capacity increases nonlinearly with increasing the pile length, whereas almost linearly with an increase in the pile diameter, limestone cohesion, and cavity roof thickness. For the considered conditions, larger magnitudes of vertical ground displacements occur primarily surrounding the pile side in the pebble layer and at the pile bottom. This is attributed to the weakening effect of the karst cavity on the bearing capacity of the cavity roof. The achievements from this study are beneficial for the design optimization of rock socketed piles in karst areas.

Performance of geotechnical seismic isolation using stone pebble - geogrid layer: Experimental investigation

The effect of geogrid on the efficiency of geotechnical seismic isolation using a layer of stone pebbles is experimentally investigated in this study. Four different foundation support cases (rigid base, thin layer of stone pebble, thin layer of stone pebble with geogrid at the layer top, and thick layer of stone pebble with geogrid at the layer top) were tested on four models of different stiffness’s (from very stiff to flexible). The models were exposed to the action of four different accelerogram. Test results confirmed that the geogrid significantly improved the seismic isolation performance of the stone pebble layer. Tested seismic isolation is highly promising for low-rise stiff buildings (T < 0.4 s) resting on solid ground.

Safety level assessment of shield tunneling in water rich sandy pebble strata with large particle size

Shield tunneling method is widely used in urban metro construction. The construction stability is closely related to the engineering geological conditions. Sandy pebble strata have a loose structure and low cohesion, resulting in great engineering-induced stratigraphic disturbance. Meanwhile, the high water-abundance and strong permeability are extremely detrimental to construction safety. It is of great significance to evaluate the dangerousness of shield tunneling in water-rich pebble strata with large particle size. In this paper, risk assessment of engineering practice is carried through with Chengdu metro project in China as a case study. Referring to the special engineering situations and assessment workload, seven evaluation indices, including compressive strength of pebble layer, boulder volume content, permeability coefficient, groundwater depth, grouting pressure, tunneling speed and tunnel buried depth are selected to establish an evaluation system. A complete risk assessment framework is established based on the cloud model, AHP and entropy weight method. Further, the measured surface settlement is taken as the risk degree characterization to verify the results. This study can provide reference for method selection and evaluation system establishment in the risk assessment of shield tunnel construction in water-rich sandy pebble strata, and contribute to proposing safety management in similar engineering projects.

Study on the Seismic Effect of the Pebble Soil Site in the Zhongwei Basin

Based on a large amount of drilling and geophysical exploration work in the Zhongwei Basin, and combined with the collected borehole data of a seismic safety assessment, this paper summarizes and builds 16 typical pebble soil layer calculation models. The effects of the thickness of the pebble layer, the thickness of the overlying silty clay, the top interface of the pebble layer on the peak acceleration and the response spectrum of the site seismic response were analyzed using the equivalent linearization method of the one-dimensional soil layer seismic response. The analysis results showed that the variation in pebble layer thickness had no obvious effect on the peak acceleration of the ground surface under different inputs; the influence of the pebble layer thickness on the ground acceleration response spectrum was primarily concentrated in the middle/high-frequency band of 0.2–0.6 s. Within this range, the acceleration response spectrum of the site with a 30 m pebble layer thickness was small, and the response spectrum curve showed a “trough” shape with a certain “weak isolation” effect. Under the same pebble layer thickness, the upper ground surface peak acceleration with a silty clay layer thickness increased with the increase in the central basin pebble soil field, where a short cycle of the seismic wave amplification effect was more obvious. The response spectrum peak period points were within 0.1–0.2 s and were influenced by the action of rare earthquakes. Moreover, the response spectrum curve showed a more obvious phenomenon of “twin peaks”, and the second peak point appeared in the period of 0.5–0.7 s. With the increase in the input intensity, the PGA amplification ratio of the pebble-top interface was significantly smaller than that of the site surface; under different intensities of input, the acceleration response spectrum of the pebble-top interface showed a “trough” phenomenon that was lower than the bedrock input at approximately 0.1 s. Under the action of rare ground motion, the acceleration response spectrum curve of the pebble-top interface showed a “double peak” phenomenon, and within 0.24–0.4 s, the spectrum value was lower than the bedrock input, showing an obvious shock absorption and isolation effect. Under the action of an earthquake, the energy of the pebble-top interface was concentrated in the low-frequency range of 1.1–2.2 Hz, and the amplification effect was obvious. In the range of 8–10 Hz, the amplitude was lower than the bedrock input, and the seismic isolation effect was obvious.

Identification of Primary Failure Modes of Tunnel System and Influence of Supporting Structures on Tunnel System Reliability using Multiple Response Surfaces

A method of calculating the failure probability of the tunnel system combining finite element numerical simulation with multiple response surface method (MRSM) and Monte-Carlo simulation (MCS) is presented. The applicability of the proposed methodology is verified through a subway interval soft rock tunnel in Qingdao, China. The sensitivity of Monte-Carlo sampling number and coefficients of variation for rock masses on the failure probability of tunnel system is conducted. The primary failure modes in tunnel system are identified by reanalyzing the failure samples. The simulation results demonstrate that the failure probability of a tunnel system within soft upper and hard lower surrounding rock mass is mainly attributed to the soft upper part of the surrounding rock. The coefficients of variation of the elastic modulus E1 and the internal friction angle ϕ1 of the pebble layer (soft upper part) have significant effect on the failure probability. The failure probability of tunnel system increases as the coefficients of variation of E1 and ϕ1; increase. Two primary failure modes are found to contribute to the tunnel system reliability. The effect of rock bolt length L and pipe-roof thickness H on tunnel system reliability and two primary failure modes as well are investigated. The simulation results indicate that both the enhancements in L and H tend to be more effective than the enhancement in either L or H if a small target failure probability of tunnel system is expected. The supporting structures design can be performed based on the potential sets of (L, H) satisfying target failure probability.

Age and causes of the Yellow River dissecting the Zoige Basin in the eastern Tibetan Plateau, China

Through extensive field investigation in the Zoige Basin of the eastern Tibetan Plateau, a typical sedimentary profile was found on the front scarp of the second terrace of Maqu reach of the Yellow River at the outlet of the Zoige Basin. This profile contains the stratigraphic sequence of paleolake facies and paleo-riverbed floodplain facies. Field observation and the analysis of particle size characteristics and optically stimulated luminescence dating showed the following results: (1) The profile includes a pale blue-grey paleolake deposit, which formed in a deep-water anoxic environment. This deposit was overlain by a mottled pebble layer with pale yellow-orange lenticular sand, which represents typical paleo-riverbed floodplain deposits, indicating a strong hydrodynamic process. The typical unconformity contact relationship between the two deposits shows that the profile recorded the sedimentary information of the transformation of the Zoige paleolake environment into the Yellow River paleo-river environment, which provides powerful sedimentary evidence for the Yellow River dissecting the Zoige paleolake. (2) The optically stimulated luminescence dating results showed the Zoige paleolake was dissected by the Yellow River at 37 ka BP, resulting in the discharge of lake water. At 35 ka BP, the lake water gradually became shallow and disappeared, and the Yellow River subsequently connected the water system of the Zoige Basin. (3) The East Kunlun fault induced strong tectonic movement in the late Pleistocene, and the Tibetan Plateau had a warm and humid climate at 37 ka BP. Under these conditions, the headward erosion of the Yellow River was aggravated in the faulted wide valley grassland between the Anyemaqen mountains and Xiqing mountains, and the Zoige paleolake was dissected by the Yellow River from west to east in the Maqu bottleneck reach. The results of this study have important scientific significance for in-depth understanding of the evolution of river-lake water systems and the formation of the Yellow River source water system in the eastern Tibetan Plateau.

Study on Hole-forming Construction Technology of Large-diameter Pile Foundation in Water-rich Pebble Layer

Study on Hole-forming Construction Technology of Large-diameter Pile Foundation in Water-rich Pebble Layer

Analysis and Prediction of Subway Tunnel Surface Subsidence Based on Internet of Things Monitoring and BP Neural Network.

With the acceleration of the urban development process and the rapid growth of China's population, the subway has become the first choice for people to travel, and the urban underground space has been continuously improved. The subway construction has become the focus of urban underground space development in the 21st century. During the construction of subway tunnels, the problem of surface settlement will inevitably be caused, and the problem of surface settlement will have a certain safety impact on the safe use of surface buildings. The impact of surface construction is predicted, so as to select the best construction technology and avoid the problem of surface subsidence to the greatest extent. On the basis of analyzing the principle of surface subsidence, this paper studies the optimal control strategy and process of subsidence in subway tunnel engineering. The research results of the article show the following. (1) The two sections of the pebble soil layer have basically the same subsidence trend. Among them, the first section has a larger settlement amplitude and both sides are steeper. The second section is mainly cobble clay soil. The pebble layer has good mechanical properties. If it can be well filled, its stability will be improved. The comparative analysis of the two sections shows that with the increase of the soil cover thickness, the maximum subsidence at the surface gradually decreases. The reason is that when the stratum loss is the same, the greater the soil cover thickness, the greater the settlement width. Sections 2 and 3 of a single silty clay have relatively close settlement laws, and the settlement changes on both sides of the tunnel are similar. (2) The surface subsidence caused by the excavation of the side hole accounts for more than 50% of the total surface subsidence, and the width of the settlement tank after the excavation of the side hole is increased by 8–10 meters compared with the excavation of the middle hole. (3) The prediction error of the BP neural network model proposed in this paper is the lowest among the four models, whether it is the prediction of the cumulative maximum surface subsidence or the location of the cumulative maximum surface subsidence, and the average relative error of the cumulative maximum surface subsidence is 3.27%, the root mean square error is 3.87, the average relative error of the location of the cumulative maximum surface subsidence is 7.96%, and the root mean square error is 21.06. In the prediction process of the cumulative maximum surface subsidence, the prediction error value of the Elman neural network is relatively large, and the GRNN generalized neural network and RBF neural network have no significant changes; in the process of predicting the position where the cumulative maximum surface subsidence occurs, the prediction error value of RBF neural network is maximum.

Hydrological Performance of Green Roof Systems: A Numerical Investigation

Green roof systems could help reduce peak discharge and retain rainwater in urban areas. The objective of this study was to investigate the hydrological behavior of a green roof system by using the SEEP/W model. The rainfall-runoff relationship within the green roof system was simulated and the results were compared with actual data from a test bed for green roof systems to verify the applicability of SEEP/W. Then, the verified SEEP/W model was used to simulate the green roof system by varying four factors (soil type, rainfall intensity, substrate depth, and green roof slope) to explore the hydrological performance through the peak discharge to rainfall intensity (PD/RI) ratio and the rain water retention rate. The results show that the model presents slightly faster and greater peak time and peak discharge values, respectively, as compared to the observational data. This is attributed to the vegetation conditions in the real green roof system. However, it is also shown that the SEEP/W model can be used to design green roof systems and evaluate their hydrological behavior because of its modeling efficiency. Thus, the SEEP/W model can be used to reliably design and manage green roof systems by further considering the vegetation conditions and water flow dynamics. Furthermore, it would be desirable to consider additional factors, such as vegetation and an insulating pebble layer, in the design and management of green roofs in future work.

Feasibility Analysis of Freezing Method in Strong Permeability Strata

If the underground engineering meets the strong permeable strata, such as sand and pebble strata, it will face huge challenges. Effective sealing of water has become a prerequisite for determining the safety of the project. Freezing method is currently an effective method for dealing with complex permeable strata, however, the influence of the groundwater velocity on the freezing engineering is huge. If the groundwater flow rate is too high, the freezing cannot be completed normally. Therefore, for the freezing project of strong permeable formation, the speed and flow direction of groundwater is a focus of our research. The connecting channel at the bottom of the river of Fuzhou Metro is planned to be constructed by freezing method. The main soil layers are the strong permeable strata represented by the coarse medium sand layer and the pebble layer. Therefore, the measurement of the velocity and direction of the groundwater at the bottom of the river was carried out, and the influence of the groundwater on the freezing project was judged based on the measurement results. If the groundwater velocity is too high, normal freezing cannot be achieved. Referring to the form of the cofferdam, a composite reinforcement method is proposed.

Testing the Jeans, Toomre, and Bonnor–Ebert Concepts for Planetesimal Formation: 3D Streaming-instability Simulations of Diffusion-regulated Formation of Planetesimals

Abstract We perform streaming-instability simulations at Hill density and beyond to demonstrate that planetesimal formation is not completed when pebble accumulations exceed the local Hill density. We find that Hill density is not a sufficient criterion for further gravitational collapse of a pebble cloud into a planetesimal, but that additionally the accumulated mass has to be large enough to overcome turbulent diffusion. A Toomre analysis of the system indicates that linear self-gravity modes play no role on the scale of our numerical simulation. We nevertheless find that self-gravity, by vertically contracting the pebble layer, increases the strength of turbulence, which is either an indication of Kelvin–Helmholtz instability or a boost of the streaming instability. We furthermore determine the Bonnor–Ebert central density to which a pebble cloud of a given mass has to be compressed before it would be able to continue contraction against internal diffusion. As the equivalent “solid body” size of the pebble cloud scales with the central density to the power of −1/6, it is much easier to have a pebble cloud of 100 km equivalent size to collapse than one of 10 km for the same level of turbulent diffusion. This can explain the lack of small bodies in the solar system and predicts small objects will form at large pebble-to-gas ratios, so either in the outskirts of the solar nebula or at late times of generally reduced gas mass.

Soil Conditioning Tests on Sandy and Cobbly Soil for Shield Tunneling

Soil conditioning is an important stage in shield tunnel construction that can effectively improve muck flow plasticity, reduce cutter head wear and improve tunneling efficiency, especially in sandy and cobbly soils. The aim of this paper is to obtain the optimal ratio of residue modifiers in sandy and cobbly soil and to improve the equipment used in residue-related shield machines. Residual soil in a sandy pebble layer was selected, and particles with sizes over 50 mm were removed to form soil samples. Through laboratory tests, the optimal ratio of the modifier was obtained, and the shield tunneling machine equipment was optimized. The research results for the laboratory tests and equipment optimization were verified and adjusted appropriately through field tunneling tests. The following results were obtained. 1) A 3% foaming agent C was selected for the soil conditioning tests. The final mass ratio of the bentonite mixed solution of water, bentonite, soda ash (Na2CO3), and carboxymethyl cellulose (CMC) was 6:1:0.028:0.035. 2) Improving earth pressure balance (EPB) shield machine equipment can improve the effects of soil conditioning. 3) Compared with the volume of residual soil, the optimal ratio of soil conditioning in sandy and cobbly layer was 13% water, 4–5% bentonite mixed solution and 12–15% foam. In addition, the maximum ratio shall be taken for the strata with poor grain composition. These results can be used to provide excavation guidelines for using an EPB machine under these soil conditions.

Study on the Influence of Micro Parameters of Numeric Model Using Particle Flow Code on Macroscopic Mechanical Properties in Cobble Stratum

The widely distributed cobble in the subsurface brings about lots of technical problems to underground projects. Based on the physical and mechanical characteristics of the pebble layer, PFC particle flow software was used to establish a triaxial numerical test model. Considering multifarious situations, triaxial numerical tests were carried out with different mesoscopic parameters, and the influence of mesoscopic parameters of different types of sand pebbles on macroscopic mechanical properties was analyzed. Studying the influence of both contact stiffness and friction coefficient on the macroscopic response of pebble, the conclusions could provide references for the numerical simulation of cobble tunnel excavation.