Overburden Load Research Articles

Partitioning Control Mechanism and Engineering Practice of Rebuilding Bearing Arch in Surrounding Rock under High Ground Stress

The mining of coal seam has a significant influence on the stability of the roadway near it, especially under the condition of high ground stress. To study the control mechanism of the surrounding rock under the influence of high ground stress, a general idea for the partition control of the rebuilding bearing arch (RBA) was proposed in this paper. Based on the basic mechanical performance test of the bearing arch, this paper built a mechanical model of the RBA based on Protodyakonov’s pressure arch theory, analyzed the influence of the strength of the bearing arch on the surrounding rock failure, and obtained the ultimate thickness of the bearing arch failure under high ground stress. The results show that the RBA’s damage is closely related to the overburden load and RBA’s thickness. The tensile stress and shear stress of RBA increase linearly with the overburden load increase and increase sharply with the load‐bearing arch’s thickness, showing a nonlinear relationship. To maintain the surrounding rock’s stability, it is necessary to ensure that the RBA’s thickness is within a specific range. The results are applied to the Wantian coal mine. The theoretically determined load‐bearing thickness is 10 m, which can effectively control the surrounding rock deformation and significantly reduce the roadway’s repair rate.

Simulation Calculation on the Failure Mode of the Big-size Sandbag Cofferdam on Soft Foundation

The offshore engineering develop fast in recent years, a new method named “big-size sandbag cofferdam” is applied to many practical projects in China. The sandbag is made of geotextile with large sizes in length and width. Sandbags are filled with pumped sea sand and stacked together layer by layer. Many theoretical obstacles remain unsolved, which has restricted the application of this advanced technology. Based on an instability project located in Bohai Bay, the failure mode of the big-size sandbag cofferdam on soft foundation was studied. The results show that the deformation of cofferdam was concentrated on the bottom sandbag, under the action of insufficient foundation bearing capacity and overburden load, the stress of the geotextile at the bottom layer increase sharply during the fast filling, then exceed its ultimate tensile strength, and the cofferdam would be pulled apart from the bottom to top, lead to a continuous penetrating sliding surface failure of big-size sandbag cofferdam.

Overpressure evolution controlled by spatial and temporal changes in the sedimentation rate: Insights from a basin modelling study in offshore Suriname

AbstractThis study investigates overpressure evolution and its relationship to the spatial and temporal changes in the sedimentation rate in the passive continental margin of offshore Suriname. We analysed well data to estimate pore pressure at the well locations and to interpret relevant overpressure‐generating mechanisms. Three‐dimensional basin modelling was performed to reconstruct overpressure evolution in the area. The well data indicated disequilibrium compaction as the main overpressure‐forming mechanism. The results of the modelling indicated a temporal variation in the sedimentation rate caused cycles of overpressure formation in the region. Periods of high sedimentation lead to overpressure formation, and this overpressure dissipates during periods of quiescent sedimentation. The latest overpressure development since the Middle–Late Miocene was triggered by a notable increase in the siliciclastic supply to the basin, which reflects the Andean uplift and reorganization of the drainage system. Spatial changes in the sedimentation rate resulted in a lateral variation in the amount of overpressure. Disequilibrium compaction generated large overpressure in the area with a high‐sedimentation rate because of the fast increasing rate of overburden load. In addition, the high‐temperature condition beneath the rapid sedimentation area could cause additional mechanisms for overpressure formation in the deeper part of the basin. This study reveals that the complex overpressure history and its lateral variation in offshore Suriname were mainly caused by a single controlling factor (i.e. the sedimentation rate). This article gives an insight into overpressure evolution in sedimentary basins by considering the perspective of the spatio‐temporal variation in the sedimentation rate and its connection with hinterland evolution.

Effect of input frequency on the seismic response of deep circular tunnels

Underground structures must be able to support both static overburden and seismic loads. Previous work has found that the dynamic amplification of stress waves impinging on a tunnel is negligible when the wave length (λ) of peak velocities is at least eight times larger than the width (B) or diameter (D) of the opening. This condition is applicable to tunnels located far from the seismic source, where the predominant frequencies range between 0.1 and 10 Hz. While such statement has been used and verified for underground structures placed in linear-elastic ground under dry or drained conditions, the effect of input frequency (f) on the seismic response of tunnels placed in nonlinear ground has not been well investigated, especially when undrained conditions apply. Two-dimensional dynamic numerical analyses are conducted, using FLAC 7.0, to evaluate the effect of frequency on the seismic response of deep circular tunnels placed in nonlinear ground under drained and undrained loading. It is assumed that the liner remains elastic, and that plane strain conditions apply. For the ground, an elastoplastic constitutive model is implemented in FLAC. It is found that the effect of input frequency is negligible for λ/D ratios larger than eight to ten, which, given the geometry of the tunnels investigated, correspond to frequencies f≤ 5 Hz. Pseudo-static numerical analyses are also conducted for a much smaller and less expensive model, and the results are compared with those of the dynamic analyses. Differences smaller than 2% are found, which suggests that pseudo-static analyses may be sufficient to evaluate the drained or undrained seismic response of deep tunnels placed in nonlinear ground far from the seismic source.

Effect of surcharge loading on horseshoe-shaped tunnels excavated in saturated soft rocks

Underground facilities are usually constructed under existing buildings, or buildings are constructed over existing underground structures. It is then imperative to account for the current overburden loads and future surface loadings in the design of tunnels. In addition, tunnels are often constructed beneath the groundwater level, such as cross-river tunnels. Therefore, it is also important to consider the water pressure impact on the tunnel lining behaviour. Tunnels excavated by a conventional tunnelling method are considered in this paper. The hyperstatic reaction method (HRM) is adopted in this study to investigate the effect of surcharge loading on a horseshoe-shaped tunnel behaviour excavated in saturated soft rocks. The results obtained from the HRM and numerical modelling are in good agreement. Parametric studies were then performed to show the effects of the water pressure, surcharge loading value and its width, and groundwater level on the behaviour of the horseshoe-shaped tunnel lining, in terms of internal forces and displacements. It displays that the bending moment, normal forces and radial displacements are more sensitive to the water pressure, surcharge loading and groundwater level.

Evaluation of soil arching effect due to partially mobilized shear stress in piled and geosynthetic-reinforced embankment

In piled and geosynthetic-reinforced (PGR) embankment, the arching behavior determines the overburden load on piles and subsoils. Placement of geosynthetic is effective in reducing the relative displacement between pile and subsoil. When the mobilized shear stress is less than the shear strength, partially developed arching will occur. Consequently, existing analytical methods, adopting the ultimate shear strength failure criterion, need to be improved. This study developed a simplified 2D analytical method, which is based on the developing arching effect, to evaluate the load redistribution of the PGR embankment. Then, the influences of embankment height and internal friction angle, subsoil depth, ratio of pile cap width to pile clear spacing (RPC) and geosynthetic tensile stiffness on the critical height ratio, stress concentration ratio, soil arching ratio, geosynthetic tension and axial strain were investigated. This study suggests that a RPC of 1:1.0 and a one-way of single-layer geosynthetic tensile stiffness of 2000 kN/m should be considered as the sensitivity thresholds for the PGR embankment.

Experimental Investigation on Suffusion Characteristics of Cohesionless Soils Along Horizontal Seepage Flow Under Controlled Vertical Stress

Suffusion is one of the main modes of internal erosion leading to selective erosion and gradual migration of fine soil particles through the voids of soil skeleton formed by coarse soil particles under seepage flow. Suffusion may induce deterioration and even failure of hydraulic geo-structures, such as embankment dams, dikes, levees, as well as landslide dams, if they are formed by gap-graded cohesionless soils. Soil mass in the hydraulic geo-structures is always subjected to overburden load and seepage through the soil mass generally has a large component along the horizontal direction. In the literature, however, little attention has been paid to the horizontal seepage-induced suffusion under controlled vertical stress. In this study, horizontal seepage-induced suffusion tests were carried out using a testing apparatus with controllable vertical stress and hydraulic gradient. The seepage flux and eroded fine particle mass, as well as local hydraulic gradients can be measured. The effects of initial dry density and initial fines content on suffusion characteristics of a cohesionless soil were investigated. Test results showed that after the initiation of suffusion, alternate decrease and increase in the coefficient of permeability occurred possibly due to alternate clogging and dredging of soil pores by the movable fine particles. Uneven distribution of local hydraulic gradients along the flow path was observed and this could be regarded as a sign of the initiation of suffusion. Both initiation and failure hydraulic gradients increased with initial dry density and initial fines content.

Performance Comparison between Polyurethane Foam and Cement Grouting Slab Replacement for Soft Ground Improvement at Shallow Depth Using Finite Element Model

Two methods of ground improvement have been proposed to overcome excessive and differential settlement problem of soft ground foundation for infrastructure such as road, highway and parking space namely polyurethane (PU) foam and cement grouting slab. It has been executed by excavating and replacing the soft soil at shallow depth with the proposed ground improvement methods. The ground improvement methods able to minimise the excessive and differential settlement as the shallow depth of soft soil is removed and replaced by the stiff materials, thus the load can be distributed evenly to the underlying soil. The comparison of performance between both methods are evaluated in this study by carried out finite element analysis for soft ground problem namely PLAXIS. The results show that the settlement can be reduced significantly to the tolerable amount by applying PU foam instead of cement grouting slab as the increase in thickness of cement grouting slab cause the increase in settlement. On the other hand, the increase in PU foam thickness has not contributed to further settlement as the PU foam is lightweight, however, the soil may experience upward displacement due to insufficient overburden load to counter uplift.

Stability analysis of underground pillar in the presence of overlying dump: a case study

The stability of pillar and immediate overlying strata is important in the Bord and pillar method of mining. Stable pillars are required to provide support against overlying overburden and dump load. This paper discusses the stability of pillar existent below a dump formed due to opencast mining. The stability of a pillar is determined in terms of factor of safety evaluated from empirical and numerical formulations based on different variants viz. dump dimension, rock quality, and depth of cover. The input parameters for empirical and numerical simulation are taken from two mines named Phularitand mine and Kuya mine which are located in Dhanbad. Subsequently, the parameters (RMR, depth of cover, dump dimension) used in the numerical model are varied to observe the effects of parameters on the stability of the pillar (factor of safety). A comparison between numerical and analytical methods is performed based on the state of stress on a pillar, determined from the numerical and analytical model. The values of the factor of safety from the numerical model were found to be 1.9 and 2.1 while 3.47 and 5.69 from empirical methods for Phularitand and Kuya mine respectively. It is observed that the factor of safety was decreased from 4 to 0.5 with a decrease in the quality of rock (Rock mass rating) from 70 to 40. The factor of safety is also found to be decreased with an increase in the dimension of dump and depth of overlying rock cover due to an increase in load on the pillar.

Shear Resistance of Rock Joint under Nonuniform Normal Stress

Many factors influence the shear resistance of rock joints. Among them, the above overburden load is the most important factor. The uneven thickness of the overburden causes the joints to be subjected to the nonuniform distribution load. While the peak shear strength shows nonlinear relationship with normal stress, linear superposition cannot be used to calculate the overall shear resistance of joint under nonuniform normal stress distribution. In this paper, the nonlinear shear strength model, JRC‐JCS model, is applied to study the overall shear resistance of the joint under four nonuniform distribution patterns of normal stress. The results show that when the normal stress is distributed in a nonuniform way, the shear resistance provided by rock joint as a whole decreases with the increase of the normal stress distribution interval. Given the nonuniform distribution of normal stress along the joint, the shear resistance obtained by the Mohr‐Coulomb linear model is overestimated. In order to give full play to the overall shear performance of the joint, the shear strength at different positions on the joint should be as close as possible. Then, the shear strength of joint parts can enter peak state condition simultaneously, at which time the shear strength is fully exerted.

Effects of Corn Stalk Fly Ash (CSFA) on the Mechanical and Deformation Properties of Cemented Coal Gangue Backfill

To reduce the amount of cement used in cemented coal gangue backfill (CCGB, a mixture of coal gangue, cement, fly ash, and water), mechanical and deformation properties of CCGB in which CSFA partially replaces the cement (0, 10, 20, 30, and 40 wt%) were studied. Compressive strength, acoustic emission during uniaxial loading, shear strength, and drying shrinkage were analysed. The compressive strength, shear strength, and drying shrinkage tests were performed at different curing times. The results showed that cemented coal gangue and corn stalk fly ash backfill (CGCAB) presented better performance, and the CGCAB with a 20% substitution rate had the best performance at day 28. Despite having the largest drying shrinkage value, 20% is the best choice for the substitution rate of CSFA. A 20% CSFA addition can enhance the bearing capacity of CGCAB and improve its failure mode, which is of great significance to support the upper overburden load and maintain the surface stability of the goaf.

Laboratory investigation of the mechanical properties of coal-rock combined body

Understanding the behavior of the top-coal caving mining face and immediate roof can be used to enhance buffering effects. The mechanical properties of the coal-rock combined body (CRCB) play a vital role in the performance of overburden load transmittance and support resistance design. We define and derive the relative physical and mechanical parameters of CRCB to illustrate and analyze the influence of coal-rock height ratio (CRHR), coal and rock mass behavior, and interface parameters on CRCB mechanical properties. We conducted uniaxial compression tests to obtain uniaxial compressive strength (UCS), elastic modulus (EM), and the full range of stress–strain curves. Our results show that UCS is positively correlated with EM. However, CRCB EM and UCS decrease with increasing CRHR or effective coal-rock height ratio (ECRHR) and the slope of the curves gradually decreases. CRCB mechanical parameters increase linearly with EM of the coal or rock mass. Although increased coal-rock interface angles (IA) lead to increased CRCB mechanical parameters, the incremental value can be ignored. Sensitive analysis shows that the rank of influential factors on CRCB properties is CRHR/ECRHR > coal strength > rock strength > IA.

Solution of the ultimate bearing capacity at the tip of a pile in inclined rocks based on the Hoek-Brown criterion

To investigate the bearing mechanism of piles in inclined slope, this paper proposed an analytical method through geometric transformation to calculate the ultimate bearing capacity at the tip of a pile in inclined rocks based on the characteristic line method. It was found that there were five failure modes for piles in inclined rocks depending on the embedment ratios, slope angles, average overburden load and tensile strength parameter of the rock mass. When the pile failure mode was under the modes of deep pile with minor overburden (DL) and deep pile with major overburden (DH), the ultimate bearing capacity had no change as the slope angle and the pile embedment ratio changed. When the pile was under the failure modes of semi-deep pile with minor overburden (SL), semi-deep pile with major overburden (SH) or shallow pile (SS), the ultimate bearing capacity decreased with an increasing rate as the slope angles increased; and to get the same ultimate bearing capacity at the pile tip, the pile embedment ratio should increase. The proposed analytical method can be served as an efficient method to estimate the bearing capacity of piles in inclined slope with small slope angle (typically less than 40°).

Determination of fracture location of double-sided directional fracturing pressure relief for hard roof of large upper goaf-side coal pillars

After mining the upper-goaf side, large coal pillars and part of hard roof exposed above the pillars remain. The hard roof can significantly deform the roadway by transferring high stress through coal pillars to the roadway. This paper reports the use of hydraulic fracturing technology to cut the hard roof on both sides (i.e. the broken roof slides to the goaf) to relieve the pressure. The position of the roof fracture is the key to controlling the pressure relief. The bearing characteristics of the large coal pillars and hard roof are analyzed to establish a mechanical model of the broken-roof sliding instability after directional fracturing and determine the width of the coal pillars that can collapse under maximum overburden load on coal pillars as a reasonable hydraulic fracturing position. The results show that the distance from the mine gateway to the fracture location increases with increasing hard-roof length, coal pillar depth, coal pillar thickness (mining height), and goaf width. In addition, the distance to the mine gateway decreases with increasing coal strength, support of the coal pillar by the anchor rod, cohesive force, and internal friction angle of the coal–rock interface. Engineering tests were applied in coal roadway 5107 of coal seam 5# of the Baidong Coal Mine of the Datong Coal Mine Group. Given the site conditions, a reasonable fracturing length of 8.8 m was obtained. Next, directional hydraulic fracturing was implemented. The comparison of the roof deformation before and after fracturing suggests that this method reduces the local stress concentration in coal pillars, which allows the surrounding rock deformation in roadway 5107 to be controlled.

Uplift and Settlement Prediction Model of Marine Clay Soil e Integrated with Polyurethane Foam

Abstract Polyurethane (PU) foam is a lightweight material that can be used efficiently as a ground improvement method in solving excessive and differential settlement of soil foundation mainly for infrastructures such as road, highway and parking spaces. The ground improvement method is done by excavation and removal of soft soil at shallow depth and replacement with lightweight PU foam slab. This study is done to simulate the model of marine clay soil integrated with polyurethane foam using finite element method (FEM) PLAXIS 2D for prediction of settlement behavior and uplift effect due to polyurethane foam mitigation method. Model of soft clay foundation stabilized with PU foam slab with variation in thickness and overburden loads were analyzed. Results from FEM exhibited the same trend as the results of the analytical method whereby PU foam has successfully reduced the amount of settlement significantly. With the increase in PU foam thickness, the settlement is reduced, nonetheless the uplift pressure starts to increase beyond the line of effective thickness. PU foam design chart has been produced for practical application in order to adopt the effective thickness of PU foam within tolerable settlement value and uplift pressure with respect to different overburden loads for ground improvement works.

The study of buoyancy behaviour of polyurethane foam as a ground improvement by constant rate strain test

The buoyancy of lightweight polyurethane (PU) foam as a ground improvement is of important aspect to be investigated to ensure the stability of the founded structure. Constant Rate Strain test (CRS) was carried out to evaluate the buoyancy behavior of PU foam in water and to determine the effect of different water content of the marine clay to the settlement and uplift in short term undrained condition. The percentage thickness of PU foam to soft clay layer and water varies from 10% to 80%. The buoyant force of PU foam to immerse at the water surface depends on the thickness of PU foam that displaces the water. On top of that, water content in the soil has a crucial effect on the buoyancy of PU foam. For PU foam partial replacement on water, the downward displacement is very less compared to the settlement for PU foam partial replacement on marine clay soil due to higher buoyant force produced in the earlier condition. However, the settlement for improved condition of marine clay reduces significantly compared to the condition of existing condition. In addition, the reduction in soil weight as part of the soil is replaced with lightweight PU foam assist to reduce the overburden load imposed on the soil. Therefore, settlement reduces significantly with the increase in PU foam thickness. The verification of the buoyancy test results was done using finite element model PLAXIS 2D.

Method to Predict the Height of the Water Conducting Fractured Zone Based on Bearing Structures in the Overlying Strata

Numerous mining-induced water inrush hazards have been recorded in northern and eastern China, wherein landforms comprise thick unconsolidated layers. To prevent such hazards, the height of the water conducting fractured zone (WCFZ) must be determined in advance. This study presents a method to predict the height of this zone by determining the location of the key strata (KS) and considering the overburden load transfer under an arch structure in the unconsolidated layers. When the distance between the primary key stratum (PKS) and coal seam is more than 7–10 × M, where M refers to the mining height, the WCFZ tends to extend up to the floor level of the first KS, approximately the same distance (7–10 × M) above the mining seam, thereby making the height of this zone equal to the distance between the KS and coal seam. When the vertical distance between the PKS and coal seam is less than 7–10 × M, the height of this zone tends to be equal to or greater than the thickness of the bedrock strata. The theoretical results were verified by field observations.

Research on the Stress Variation Effect of Overburden Loads in Utility Tunnel Roof

As an underground structure, the utility tunnel is subjected to the pressure of overburden soil. Calculation of the overburden pressure soil are different when the burial depth changes. In this paper, the calculation methods of soil loads are summarized. In view of utility tunnel, the ANSYS finite element analysis is adopted to analyze the pressure of the roof. When calculating the structural stress, the working condition is considered as the backfill after the excavation of the soil. Considering the stress variation of the roof edge of the structure with different burial depth, it can be found that compressive stress at 1/2 span is about 1.7-2.2 times of the tensile stress at the corner. With the increase of burial depth, the rate of increasing roof stress value decreases. When the burial depth reaches a certain depth, soil arching effect plays a role and the roof stress value tends to be stable. Some suggestions are given to the designers of the utility tunnel which can reduce the waste of building materials.

Engineering Characteristics and Reinforcement Program of Inclined Pre-stressed Concrete Pipe Piles

When it comes to the deep soft soil foundation, improper construction, preloading, and excavation may result in the inclination of the pre-stressed concrete pipe (PCP) piles. Lack of understanding on deformation characteristics may lead to inaccurate reinforcement plan, thereby causing new engineering accidents. This study analyzes an engineering accident induced by the pile inclination, based on which the finite element method model is built. Deformation characteristics of the foundation incorporating inclined PCP piles are investigated, and the reinforcement program involving placement of the pile with opposite inclination is proposed and compared with the conventional reinforcement program using vertical piles. Findings of this study are two-folds: 1) A critical inclination angle affecting the vertical settlement exists, below which the pile inclination has limited effects upon the vertical load bearing capacity of the foundation. 2) The vertical subsidence, the lateral displacement and the bending moment of the reinforcement program based on the reversely inclined pile are all decreased, which helps bear the overburden load and improve stability. In the process of engineering accident treatment, the bearing capacity of PCP piles with small inclination angles can be first reduced according to their inclination angles, followed by judgement that whether these PCP piles meet the requirements on bearing capacity. The PCP piles with large inclination angles must be strengthened, and the reverse inclined PCP piles are recommended.

Numerical Modeling Approach on Mining-Induced Strata Structural Behavior by Considering the Fracture-Weakening Effect on Rock Mass

By employing the longwall mining method, a series of intensive strata structure responses and activities will be induced including stress redistribution, fracture extension and strata movement. Due to the geological stratification feature of coal mine strata, tensile failure and tension-induced fracturing play dominant roles in the strata of the fractured zone. These responses induced in the strata require the consideration of the weakening effect on the rock mass behavior due to failure and fracturing in tension. In this study, a numerical modeling approach on mining-induced strata structural behaviors was proposed by considering the mechanical behaviors of the caved zone consolidation and tension-induced weakening in the fractured zone. Based on a numerical model built according to a study site, a parametric study with respect to different fracturing intensity parameters was performed to investigate the fracturing weakening effect on the mining-induced stress redistribution and strata movement. The numerical results showed that the tensile fracture intensity had a notable effect on the mining-induced stress distribution in two aspects: (1) Increase in peak and area of the front abutment stress; (2) variation in the patterns of stress recovery in the goaf. The stress data obtained from numerical simulation represent and help to back-analyze the structural behaviors (failure, movement) of the overlying strata. The high stress on the coal seam indicated that the strata lay on and transferred loads to the seam, while the low stress indicated the detachment between the seam and the suspending strata. With the increase in fracture intensity, the roof strata were more prone to breaking and caving, and the suspending length of the roof beam decreased, which made the strata sufficiently break, cave and transfer the overburden load to loose rock in the goaf; caving along the strike direction of the panel became the dominant overlying strata structure movement, while the dominant movement caved along the dip direction in the case of strong and intact overlying strata with few tensile fractures. Thus, the tensile fracturing intensity should not be ignored in studies related to the behaviors of the overlying strata. Validated by analytical studies, this study presents a novel numerical modeling approach for this topic and can be utilized for multiple studies based on proper roof fracturing estimation or back analysis.