Micropile Groups Research Articles

Computational modeling of three-dimensional slope reinforcement by anchor cable micropile group considering coordinated deformation

The application of anchor cable micropile group (ACMPG) in small and medium-sized slope support is gradually widespread. However, the coordination deformation of the structure and the resistance of the soil in front of the piles are less considered in the existing calculation models. Therefore, based on the load transfer approach and according to the structural characteristics of the ACMPG, the calculation method was provided for the resistance of the soil in front of the pile and the tension of the anchor after deformation. And the equations for calculating the resistance of the ACMPG were established. Based on the method of upper bound limit analysis, the display equation had been established for the safety factor of reinforcing the 3D slope of the structure. And finite element models were developed to validate the theoretical model. Finally, a parametric study was conducted to investigate the influence of slope angle, soil friction angles, location, 3D effects, on the slope safety factor and the support effect of piles and anchor cables supporting the structure. The results showed that the theory proposed in this paper can accurately calculate the safety factor of slopes, which is helpful for the design and construction of slope reinforcement.

Research on the Pile–Soil Interaction Mechanism of Micropile Groups in Transparent Soil Model Experiments

Micropile groups (MPGs) are typical landslide resistant structures. To investigate the effects of these two factors on the micropile–soil interaction mechanism, seven sets of transparent soil model experiments were conducted on miniature cluster piles. The soil was scanned and photographed, and the particle image velocimetry (PIV) technique was used to obtain the deformation characteristics of the pile and soil during lateral loading. The spatial distribution information of the soil behind the pile was obtained by a 3D reconstruction program. The results showed that a sufficient roughness of the pile surface was a necessary condition for the formation of a soil arch. If the surface of the pile was smooth, stable arch foundation formation was difficult. When the roughness of the pile surface increases, the soil arch range behind the pile and the load-sharing ratio of the pile and soil will increase. After the roughness reaches a certain level, the above indicators hardly change. Pile spacing within the range of 5–7 d (pile diameters) was suitable. The support effect was poor when the pile spacing was too large. No stable soil arch can be formed, and the soil slips out from between the piles.

Predicting Micropile Group Capacity in Soft Cohesive Soil by Artificial Neural Network

Predicting Micropile Group Capacity in Soft Cohesive Soil by Artificial Neural Network

A Transparent Soil Experiment to Investigate the Influence of Arrangement and Connecting Beams on the Pile-Soil Interaction of Micropile Groups.

The use of a micropile group is an effective method for small and medium-sized slope management. However, there is limited research on the pile-soil interaction mechanism of micropile groups. Based on transparent soil and PIV technology, a test platform for the lateral load testing of slopes was constructed, and eight groups of transparent soil slope model experiments were performed. The changes in soil pressure and pile top displacement at the top of the piles during lateral loading were obtained. We scanned and photographed the slope, and obtained the deformation characteristics of the soil interior based on particle image velocimetry. A three-dimensional reconstruction program was developed to generate the displacement isosurface behind the pile. The impacts of various arrangement patterns and connecting beams on the deformation attributes and pile-soil interaction mechanism were explored, and the pile-soil interaction model of group piles was summarized. The results show that the front piles in a staggered arrangement bore more lateral thrust, and the distribution of soil pressure on each row of piles was more uniform. The connecting beams enhanced the overall stiffness of the pile group, reduced pile displacement, facilitated coordinated deformation of the pile group, and enhanced the anti-sliding effect of the pile-soil composite structure.

Ensemble Extreme Gradient Boosting based models to predict the bearing capacity of micropile group

In most cases in which non-allowable settlement or losing of bearing capacity has been encountered in geotechnical engineering, employing micropile usually leads to satisfactory outcomes. Considering the costly and time consuming of utilizing the experimental tests in order to determine the bearing capacity of micropile-cap footings, utilizing artificial intelligence based approaches is a reasonable process in this regard. Nevertheless, because the q/cu is almost complex, forecasting phenomena is challenable. In this regard, powerful approaches were developed employing the hybrid Extreme Gradient Boosting (XGBoost) model on the database to precise estimation of the output. For this purpose, 780 rows of data has been collected in order to construct a powerful dataset from various existing valid literatures. Besides, four recently published optimizing approaches containing Black Widow Optimization Algorithm (BWOA), Artificial Hummingbird Algorithm (AHA), Arithmetic Optimization Algorithm (AOA) and Fire Hawk Optimization (FHO) have been employed to reach the high fitness of internal parameters of the XGBoost model. By simulating XGBoost optimized models, it was revealed that the developed AHA−XGB model can be more accurate than that of the other models in predicting the q/cu. Scatter index, sensitivity and uncertainty analysis were conducted to identified the importance of various input parameters was created.

Study on Uplift Bearing Characteristics of Micropile Group in Gravel-Containing Silty Clay Regions

Study on Uplift Bearing Characteristics of Micropile Group in Gravel-Containing Silty Clay Regions

Nonlinear Response of Micropile Groups Stabilizing Landslides Considering Pile–Soil Interactions

Nonlinear Response of Micropile Groups Stabilizing Landslides Considering Pile–Soil Interactions

Numerical study on micropile group behavior supporting fixed bottom marine hydrokinetic devices in sandy seabed

This study aimed to investigate the behavior of micropile groups supporting fixed bottom marine hydrokinetic (MHK) devices in sandy seabed. A parametric study using PLAXIS 3D was conducted to examine the influence of key design parameters on the lateral load and moment capacity of the micropile group. The parameters considered were micropile diameter, spacing, micropile cap size, number of micropiles, micropile-soil interface properties, and sand packing state. Using the parametric study results, fitting regression techniques were employed to derive equations for predicting the MR (moment resistance in the absence of lateral load) and HR (Lateral resistance in the absence of moment loading), while meeting ultimate limit state (ULS). The equations were formulated based on soil relative density (DR), micropile group polar moment of inertia (J), and an interface reduction factor (Rint). A linear failure envelope is proposed in the Moment-Horizontal (M-H) load space by connecting MR and HR. The proposed framework was then applied to assess the load utilization ratio for various configurations, focusing on a load example of anchoring a 1 MW marine tidal turbine. The results of the study provide insights into the behavior and design of micropile groups supporting fixed bottom MHK devices in sandy seabed.

Horizontal and Uplift Bearing Characteristics of a Cast-In-Place Micropile Group Foundation in a Plateau Mountainous Area

Micropile groups have been progressively more frequently adopted in the construction of transmission tower bases due to their compact size and flexible construction advantages. However, the load-bearing characteristics and deformation mechanisms of micropile groups are complex, and the study of their coupling effects under combined loads remains unclear. Consequently, this paper presents a field static load test of micropile groups in a highland mountainous area. The analysis encompasses the axial force distribution and load-sharing ratio of micropiles. With a focus on micropile groups subjected to both uplift and horizontal combined loads, the coupled effects under different load combination ratios are examined using numerical simulation methods. The key findings are as follows: During the uplift loading process, the load distribution among individual piles is relatively uniform, with lower side friction resistance gradually coming into play to counterbalance the top load. The load–uplift displacement curve exhibits a steep characteristic, making it susceptible to sudden failure in practical engineering applications. Under the simultaneous action of uplift (V) and horizontal (H) loads, the unbalanced lateral frictional resistance on both sides of the pile segment induces additional bending moments, which is an important part affecting the load-coupling mechanisms. The uplift resistance capacity of micropile groups decreases with an increase in horizontal load, while the horizontal load-carrying capacity initially decreases and then increases with an increase in uplift load. The space enclosed by the yield envelope under combined load, and the vertical line of the ultimate load, is divided into a ‘failure zone’ and a ‘safety zone.’ In the design of the pile foundation, the uplift bearing capacity reduced by the ‘failure zone’ should be taken into account.

Cross-Scale Analysis on the Working Performance of Micropile Group and Talus Slope System

Micropile groups (MPGs), combined with the advantages of the anti-slip pile and anchor cable, offer an efficient support system that can be used as countermeasures for stabilizing the talus slopes. However, the performance of MPGs in stabilizing the talus slopes is rarely numerically investigated from the continuous-discontinuous viewpoints. To fulfil this knowledge gap, a numerical method coupled with the discrete element method (DEM) with the finite element method (FEM) is proposed first, and validated to be with good accuracy by the centrifuge model tests. A series of cross-scale analysis cases are then adopted to assess the behavior of MPG in the talus slopes, in which the influencing factors are also taken into account. The numerical results indicate that the MPGs reinforcement can significantly improve the stability of the talus slopes, avoiding the potential progressive shallow slip. For the MPGs with different pile spacing, the distribution laws of deformation and internal force are rather similar, but the one whose pile spacing is four times the pile diameter shows better performance. Moreover, the effective anchorage length of MPG is approximately 1/3 of the pile length, and the axial force distribution is influenced by the type of pile bottom constraint and the tangential contact between the micropile and the bedrock. Finally, the “bidirectional anchorage” attributed to the platform and the bedrock can greatly improve the performance of the MPG, which is a non-negligible part of the anti-slip mechanism of the MPG. This study is of great significance for facilitating the design of MPG in stabilizing the talus slopes.

Case study on viability of using head-separated micropiles as foundation system for check dams

Check dams constructed in steep mountainous areas require the rationalization of the dam body and foundation system. In general, soil cement replacement or caissons are often adopted for foundations. In such cases, reducing the construction effort is a critical issue. To address this, the authors studied the viability of a new type of check dam foundation consisting of a group of micropiles whose heads are structurally disconnected from the dam body. The system, coined a head-separated micropile group (HMG) foundation, enables the saving of labor and a reduction in the cross-sectional forces applied to the micropiles. Firstly, a full-scale loading test of the HMG was conducted. Then, a finite element model was formulated and its parameters fitted to make it suitable for reproducing the experimental results. Finally, using the FE model, the performance of a typical rigidly connected micropile foundation and that of the HMG system were compared in terms of the bearing capacity and displacement of the check dam body. The results confirmed that, although its displacement was 1.25 times larger than that of the rigidly connected foundation, the HMG system led to a factor of safety of 3.5 against micropile buckling.

Study on Interaction Mechanism of Natural Gas Pipe-Landslide System Reinforced by Micropile Groups Based on Model Test

Natural gas pipeline projects in mountainous areas are inevitably affected by geological disasters such as landslides, which pose a serious threat to the safe operation of pipelines along the routes crossing landslide areas. In this paper, based on a pipe-landslide project in a mountainous area in southwest China, the interaction mechanism and failure evolution process of the landslide-pipeline system reinforced by two kinds of micropiles are studied through indoor large-scale physical model tests, and some suggestions on the support work of the pipe-landslide project are put forward according to the test results. It was found that the deformation process of the engineering system composed of landslide, micropile, and pipeline presents a high degree of synergy under the external force and mainly experiences four stages: initial deformation period, uniform deformation period, accelerated deformation period, and residual deformation period. The bending deformation of the perforated pipe micropile is large at the 1/4 position of the pile top from the pile bottom, and the deformation of the screw micropile near the sliding surface is serious. The pipeline welding port is the weak position of the pipeline; after the failure of the pile, the pipeline interface is first cracked, along the interface position along the two ends of the tear, and finally completely broken. The screw micropile cannot effectively resist the landslide thrust at a large load level, so the risk of pipeline damage is greater. The yield strength and ultimate strength of the perforated pipe micropile are greater than those of the screw micropile, and the perforated pipe micropile can still exert a certain residual resistance after reaching the ultimate bearing capacity, which has a beneficial effect on the reinforcement of the pipeline crossing the landslide system. The research results provide important reference value for landslide-pipeline treatment engineering.

Model Test of Micro-Pile Group Reinforcing High Steep Landslide

High steep landslides are a major concern for infrastructure construction in the mountainous areas of Western China. The micro-pile technique has been gradually used to prevent landslides, due to convenient construction and good performance. However, the application of the micro-pile technique on landslide prevention was generally implemented on the front edge of landslides, which is not applicable for the high steep landslides due to the limited operation space. In this study, a large-scale model test on the performance of a micro steep pile group on the prevention of high steep landsides was conducted in order to implement the micro-pile on the top of landslides. The force-deformation characteristics and failure modes of the steel pipe micro-pile group reinforcing high steep landslides were investigated. The test results showed that the landslide thrusts acting on the micro-pile group showed a triangle distribution. The maximum soil earth pressure was observed near the slip surface during landsides. The resistance of the micro pole group was distributed in an inverted triangle, mainly in the upper half of the loaded section. The sliding bed resistance is unevenly distributed along the height direction, and is larger near the slip surface. Once the landslide occurred, the force distribution of each row of steel pipe micro-piles was basically the same. The bending moment of the loaded section of the steel pipe micro-pile was mostly negative, with a larger bending moment in the range of eight times the pile diameter above the slip surface. The largest bending moment value is located at two times the pile diameter on the slip surface. On the other hand, the bending moment of the embedded section of the steel pipe micro-pile is mostly positive, showing a tension state with a maximum value at four times the pile diameter under the slip surface. This implies that the role of loaded and embedded sections of the micro-pile group on the landsides is different. The failure mode of the micro-pile group was mainly attributable to the bending failure within eight times the pile diameter above and below the slip surface.

Performance investigation of micropile groups in stabilizing unstable talus slopes via centrifuge model tests

Micropile groups (MPGs) are an effective means of geological disaster prevention for small- and medium-sized landslides, with the advantages of light structures and convenient construction. However, the mechanical and deformation characteristics of MPGs are complex, and their practical application is ahead of theoretical research, which greatly limits the popularization and application of MPGs. This paper conducts a series of centrifuge model tests to investigate the mechanical and deformation characteristics and the antislip mechanism of MPGs, then compare them with those of conventional piles (CPs). In particular, MPGs with and without the platform in the strengthening process of the talus slope are compared, and monitoring the reinforcement effect of MPGs subjected to gravity loading. The results suggest that the soil pressure shows a triangular distribution pattern and is influenced by the position of the potential slip zone and the geometry of the bedrock surface. The compatibility deformation of the pile–soil leads to the stress release of the soil behind the pile, which is an important part affecting the antislip mechanism of the MPG. The platform limits 75% of the pile top displacement of the MPG and simultaneously redistributes the stress of the piles, providing a better overall antislip effect of the pile–soil composite.

Large‐Scale Model Test of a Micropile Group for Landslide Control

A large‐scale model test on the interaction between a micropile group and a landslide was conducted, to investigate the effect of micropiles on the landsides prevention. The bearing mechanism, force condition, and failure mode of a micropile group for reinforcing landslide were analyzed in detail. The results showed that the thrust force over micropiles induced by landslide showed a trapezoidal distribution, with a higher Earth pressure near the sliding surface. The resistance from the sliding body behind the pile behaved in a parabolically trend. Meanwhile, the resistance force from the sliding bed was distributed unevenly along the height direction, with a higher resistance force near the sliding surface behind the pile. When a landslide occurred, micropiles were subjected to an increase in loading and displacement, eventually to the failure state. The load‐bearing sections of the micropiles were all subjected to negative bending moments, with larger bending moments within the half length of pile range near the sliding surface. The maximum negative bending moment occurred at the height of seven times the diameter of the pile above the sliding surface. The damage mode along each row of micropiles was almost the same, showing a damage area within the range of three times the diameter of the pile above and below the sliding surface. The failure of micropile induced by landslides was mainly due to a combination effect of bending and shearing near the sliding surface.

Numerical Study on Micropile Stabilized Foundation in Flyash

Because of the scarcity of land for urbanization, multistoried construction projects are becoming a preferred choice. Such projects more often require soil reinforcement or ground improvement to strengthen the soil and reduce the foundation settlement of structure to meet the project demanding standards. Micropiles have been used as one of the simplest ground improvement methods for a long time. The micropile also finds its applicability in the retrofitting of old structural foundations. The present study has been carried out for laboratory scaled model to investigate the efficiency of the micropiles to strengthen the soil and reduce the foundation settlement of structure constructed on Indian flyash. The efficiency of micropiles has been studied when used as a group, for stress versus displacement characteristic of the surface footing. Next, a 2D finite element analysis of the same model has also been carried out in GeoStudio 2016. The efficiency of micropile groups has been investigated for the slenderness ratio of micropile with value as 60, 80, 100 and 120. The study concluded that the slenderness ratio of micropile has a substantial effect on the bearing capacity of flyash. Moreover, quantitative improvement in the bearing capacity of the flyash has been determined in terms of a factor known as bearing capacity ratio (BCR). The BCR value is found to be greater than 1 in each studied case of the micropile group having a different slenderness ratio.

Numerical Simulation Study of the Load Sharing of an Arched Micropile Group in the Tizicao High-Position Landslide, China

This study presents a combined anti-sliding structural system of micropile groups, with an arched shape on the plane and cement paste between the micropiles, in the Tizicao high-position landslide study conducted in the Wenchuan seismic area, China. Improvements in the landslide’s stability as a result of the embedded micropile groups alone and in combination with long prestressed anchor cables are studied using FD-SRM and FOS contour analysis methods, respectively. The soil arching effect and the load sharing of the arched micropile group are analyzed using the numerical simulation method. Findings show that the arching effect is due to arc-shaped arrangements of the micropile groups. The design scheme of the embedded micropile groups improves the stability of this kind of thickly layered, high-position landslide. Moreover, its construction is convenient and the mechanism is clear, making it a good engineering reference for the treatment of similar landslides. The improvement in the landslide’s stability is even better than that achieved by the combination of micropile groups and long prestressed anchor cables. Furthermore, the arching effect of multiple rows of micropile groups is significant and can effectively block the movement of the sliding body. The load transfer involved in the micropile–soil interaction process is a new type of two-part load sharing pattern. The soil arching behind the micropile groups takes more than 80% of the total load. A tangential tension zone appeared along the top of the pile–group arc crown when the arch protruded upward, and a related structural performance design should be considered in the future.

Axial performance of micropile groups in cohesionless soil from full-scale tests

Hollow bar micropile (HBMP) groups are used for supporting large loads as an alternative foundation option to large diameter drilled shafts. In such cases, it may be necessary to increase the micropile’s diameter by increasing the drill bit diameter (Db). This paper investigates experimentally and numerically the effect of increasing Dband micropile spacing on the group performance. A field load testing program was conducted on four groups of HBMPs installed in sand; each group comprised four micropiles arranged in a square configuration. All micropiles were constructed with the same size hollow bar, Dh= 51 mm; two groups comprised micropiles constructed with drill bit, Db= 115 mm, and two groups comprised micropiles constructed with drill bit, Db= 152 mm. One group of each set was installed with spacing to micropile diameter ratio, S/Db= 3 and the other group with S/Db= 5. In addition, full 3D finite element model (FEM) was developed and calibrated to simulate the behaviour of micropile groups and to evaluate the failure load for groups that were not loaded to failure. The results demonstrated that micropile groups constructed with the large diameter drill bits displayed higher stiffness and load carrying capacity than the groups constructed with small diameter bits, which confirms the effectiveness of using a larger drill bit. In addition, the group efficiency ratio values at both working load and ultimate capacity were found to be close to unity for all groups. The ultimate skin friction values of grouted micropiles obtained from this study were higher than the values suggested by the US Federal Highway Administration for medium to very dense sand. It was also found that the settlement of the 4-HBMP group increased by 25% to 33% over that of a single HBMP due to group effect.

Bridge Bed Strengthening, Disaster Prevention due to Scouring

One of the major factors in deliberating the depth of foundations in structures adjacent to the water flow is the scouring phenomenon; the scouring is a phenomenon caused by the interactions between water flow and erodible bed materials, which causes the removal of sediments where hydraulic structures are located, including bridge piers. Every year, a great number of bridges are damaged as a result of local scoring of their piers and foundations. In this paper, the geotechnical study of Malahide viaduct failure due to scouring was carried out applying Plaxis 2D software. For this purpose, the Malahide viaduct, which was damaged in 2009 due to bed scouring of one of its piers, was selected and the necessary simulations were carried out in consonance with the bridge specifications, and the conditions of the bridge underlying bed was investigated. Simulations results revealed that the cause of scouring in the bed of collapsed pier was the high shear strains of the bed, bed shear strength parameters (i.e. angle of internal friction and cohesion) reduction and as a result, reducing the bed resistance to the scouring. Moreover, it was found that by using the micropile group below the foundation of bridge pier as a solution to reduce the scouring effect, the bed maximum scour depth is significantly reduced compared to the shallow foundations without micropiles; Furthermore, by using the micropile group, the shallow foundation thickness can be reduced, provided that after foundation thickness reduction and micropiles application, the structure safety factor remains in the stable range.

Dynamic performance of a full-scale micropile group: Relevance of nonlinear behaviour of the soil adjacent to micropiles

An extensive experimental in-situ campaign of dynamic tests carried out on a full-scale 2 × 2 group of inclined injected micropiles in alluvial soils is described in this paper. The campaign includes ambient vibration and impact load tests to investigate the dynamic behaviour of the soil-micropile system in the small strain range, and forced vibration tests to investigate the performance of the system in the nonlinear field. Accelerometers and geophones are adopted to acquire the dynamic response on the cap and the soil nerby the cap, while strain gauges are installed along the shaft of one micropile to capture the local curvatures and deflections. The influence of the micropile inclination on the translational and rotational behaviour of the group is evaluated. Moreover, the evolution of degradation phenomena such as the gap opening and slippage at the soil-micropile interface and the radial cracking of soil surrounding micropiles, are recognized, critically commenting the relevant effects on the fundamental frequencies and damping ratios of the system.