Micropile Groups Research Articles

A Simplified Analytical Method for Stabilizing Micropile Groups in Slope Engineering

Stabilizing micropile groups is a light retaining structure constructed quickly and safely for slope reinforcement in practice. To carry out engineering design of any structure, a simplified analytical procedure for a micropile group with consideration of stability of the piled slope is presented. According to the upper bound theorem of kinematical limit analysis, an analytical method is proposed to evaluate the net thrust force on a micropile group with 3 × 3 layout of piles and the slip surface of the piled slope for a specified factor of safety. Then, internal forces of the micropile group can be computed using plane rigid frame model for the part of the structure above the slip surface under the net thrust force and beam-on-elastic-foundation model for the rest part. A laboratory model test and corresponding 3D-numerical simulation are conducted to verify the proposed method. Moreover, analysis of a practical slope shows that flexural rigidity of a micropile and micropile numbers of a group have a great effect on internal forces of micropiles. In particular, the internal forces are relatively sensitive to pile numbers in a group. However, micropile length and spacing in plane in a group have little effect on the internal forces, which is rather different from traditional stabilizing piles with a large cross section.

Dynamic Analysis of Embankments Reinforced with Micro-Piles

Application of micro-piles to stabilize unstable slopes has been widely considered in recent years. A micro-pile is a small-diameter (typically less than 300 mm), drilled, and grouted as non-displacement pile that is typically reinforced. The main objective of this study is to evaluate the influence of the micro-pile group on increasing safety factor as well as finding the optimum position of micro-piles across the embankments slopes. In this study, numerical analyses were performed to investigate the efficiency of micro piles on the behavior of embankments using ABAQUS Finite Element code. The analyses have been carried for plane strain conditions and under earthquake loads. Based on acquired results, it was implied that using micro pile groups on the lower part of embankment slope is the optimal approach to reinforce embankments. This can increase the embankment safety factor by 33% in static mode and by 34% in dynamic mode. The impacts of the parameters of micro pile inclination angle, diameter, length, micro-pile spacing, and the type of soil on safety factor in dynamic mode have been investigated. Results show that increasing the length and diameter of the micro piles increase the safety factor. By increasing the micro pile angle in respect to vertical axis up to 30°, safety factor increases but any additional increase of the angle decreases the factor. Furthermore, by increasing the spacing of micro piles up to 4 times the diameter, causes improving the safety factor, but more space has a negative impact. By strengthening the type of the adhesive soil (simultaneous increase of embankment adhesion from 8 KPa to 16 KPa and elasticity module from 30 MPa to 60 MPa), embankment safety factor increases by 40%. Furthermore, by strengthening the sandy soil (simultaneous increase of internal friction angle from 32 to 40 and soil elasticity module from 20 MPa to 45 MPa), the safety factor of embankment increases by 41 percent.

Micropile group behaviour subjected to lateral loading

Micropiles are suitable for underpinning and seismic retrofitting of structures. It is specially suitable for low head room area and can easily access congested areas where it is difficult to construct conventional piles. Its two basic functions are structural support and soil reinforcement. Literature review showed contradictory group effects on micropile behaviour subjected to vertical and lateral loading. This study is to investigate the response of micropile groups subjected to lateral load installed in loose sand (relative density 30%), in medium dense sand (relative density 50%) and in dense sand (relative density 80%). The group behaviour is found to be a function of the state of the sand, spacing of the micropile groups and length-to-diameter (L/D) ratio. Ultimate lateral resistances of micropile groups are maximum at 2D spacing in loose sand and in medium dense sand. In dense sand, maximum lateral resistance is observed in 6D spacing. Overall, it was observed that groups with higher L/D ratio had positive group effects. Modes of failure of the micropile groups are found to be a function of the length-to-diameter ratio and relative density of the sand.

A 1-g shaking table investigation on response of a micropile system to earthquake excitation

Considering the increasing applications of micropile systems in seismically active areas, a better understanding of their seismic performance and the key controlling factors is of high significance. This paper presents experimental investigations of the seismic performance of a small-scale physical model of micropiles under seismic ground motions. Shaking table tests were performed on a 4 × 4 group of vertical micropiles embedded in loose sand. Horizontal acceleration time histories recorded from the 1940 El Centro earthquake and 1995 Kobe earthquake were applied to the base of the soil container. The response of the physical model was monitored in terms of horizontal accelerations at different levels and bending moments induced in the micropiles. The experimental results indicate that in all shaking events, the magnitudes of horizontal accelerations on the soil surface and the pile cap were amplified with respect to the input motion. Bending moments were induced in the micropiles and peaked at the mid-depth. It was also found that inclining the micropiles led to improvement in their seismic performance, reducing both the acceleration response on soil surface and pile cap, and the induced bending moments in the micropiles. Finite element simulations of shaking table tests have been performed, and the results are in reasonable agreement with observations from the experiments.

Experimental Study of Micropile Lateral Resistance Effectiveness

Abstract The range of soil around the micropile when the pile is mobilized by force and the sensitivity of design parameters variation to the effect of micropile was studied by the prototype test and numerical calculation of the effect of lateral resistance of micropile. During the experiment, the variation of soil pressure around the micropile was monitored under the step loading, and the influence distance of lateral resistance on the pile is analyzed, and the high, middle, and low impact radius is defined according to the degree of influence. The distribution of vertical displacement of soil around pile when the pile diameter, pile length, and soil physical properties change was analyzed using numerical calculation, as well as the effects of normalized parameters on the influence of lateral resistance. Results showed that to better utilize the anti-slide coupling effect between piles where the soil overlying on the slope is formed by silty clay, pile spacing should be within the mid to high impact radius. Compared to other parameter changes, optimizing the pile length had the greatest effect on increasing pile lateral resistance and impact radius. Results provided a basis for designing effective pile spacing in anti-slide micropile groups.

Experimental Investigation on Load Carrying Capacity of Micropiles in Soft Clay

Use of high capacity micropiles is in great demand today because of their ability to solve some typical foundation problem like the construction of low- to medium-rise buildings over a soft to the very soft clayey soil of large depth. The normal practice of construction of long cast-in-situ concrete piles is costly and time consuming for this type of soil. In the present study, the group behavior of cast-in-situ grouted micropiles in highly plastic clayey soil is investigated. The paper discusses the experimental observations on the behavior of micropile groups under static axial vertical compressive load. The micropiles were constructed in the clayey soil of very soft consistency in a test pit of size 2.0 m $$\times $$ 4.0 m $$\times $$ 3.0 m. Load-settlement behaviors of two different sets of micropile groups were studied. In one set, micropile caps were rested on the ground surface and in another set, they were constructed sufficiently above the ground surface. The variables involved in this study were diameter, length, number and spacing of micropiles in a group. The ultimate load carrying capacity of a micropile group and the group settlement under the safe load were determined from experimental observations. The group efficiency and the resistance offered by the micropile cap alone were also determined from the present study. It is observed that the load carrying capacity of a micropile group increases with the increase in diameter, length, number, and spacing of micropiles. A nonlinear equation is established from the experimental data to determine the ultimate load carrying capacity of a micropile group.

Mathematical Modelling for Micropiles Embedded in Salt Rock

Abstract This study presents the results of the mathematical modelling for the micropiles foundation of an investement objective located in Slanic, Prahova county. Three computing models were created and analyzed with software, based on Finite Element Method. With Plaxis 2D model was analyzed the isolated micropile and the three-dimensional analysis was made with Plaxis 3D model, for group of micropiles. For the micropiles foundation was used Midas GTS-NX model. The mathematical models were calibrated based with the in-situ tests results for axially loaded micropiles, embedded in salt rock. The paper presents the results obtained with the three software, the calibration and validation models.

A new expression for determining the bending stiffness of circular micropile groups

Increased bending stiffness and decreased foundation rotation are two main factors which reduce the rocking motion of foundation. A micropile group in circular arrangement is an innovative technique for reducing the rocking motion of the foundation. In this paper, the effects of several parameters were numerically investigated on the rocking stiffness of circular micropile group foundations. The finite difference software FLAC3D was used to model the foundation, soil, and the structure. The micropiles used in this study varied in the diameter, but had similar length. A total of seven records were selected to cover a wide range of frequency content, duration and amplitude. The results from the numerical simulation were compared and verified against those obtained from an alternative numerical method as well as a set of experimental test results. The model was then used for parametric studies and the effects of relevant parameters were investigated. The results showed that slenderness, inclination angle and distance ratio of a micropile group are main factors which increase the soil stiffness and control the seismic motion of high-rise buildings. Based on the results, a new expression was proposed that can be used to determine the optimal moment of inertia of circular micropile groups. The proposed expression revealed that the primary factors which reduce the effective period of the buildings are the number, diameter, and injection pressure of micropiles in a circular micropile group. Using the proposed relationship, it was found that using circular micropile groups can reduce the destructive seismic effects (i.e. drift demand) in high-rise buildings by up to 60%.

Group behaviour of hollow-bar micropiles in cohesive soils

The use of hollow-core bars in micropiles has greatly increased over the past 10 years. In this study, the group behaviour of hollow-bar micropiles is investigated. The research methodology encompasses a full-scale field study on single hollow-bar micropiles and pairs of micropiles installed in cohesive soils and numerical investigations using three-dimensional (3D) finite element analysis. Four hollow-bar micropiles were installed in stiff silty clay deposit using large drilling carbide bits. Four axial monotonic tests on pairs of hollow-bar micropiles were conducted as part of the field study. The field test results were utilized to calibrate–verify a 3D finite element model (FEM). A parametric study was performed using the verified FEM to characterize capacity and performance of hollow-bar micropile groups. Group efficiency factors and interaction factors were established to evaluate the capacity and response of hollow-bar micropile groups under vertical loading. The results from the field tests and numerical investigations revealed that for hollow-bar micropiles, group efficiency factor, GE, can be taken equal to 1. A method for calculating the group settlement utilizing the interaction factors approach is proposed and a family of interaction factor diagrams is established to evaluate the group settlement.

Impact of Coupling Beams on Shearing Force of Micro-Pile Group in Landslide

In recent years, micro-pile has been widely applied to landslide treatment engineering due to its advantages in application and construction, with quite evident engineering effect. Different geological experts have focused on sundry research direction on its resistance mechanism. Based on results from numerical simulation, shearing forces of micro-pile in landslide treatment engineering with and without coupling beams were analyzed, all of which are hoped to reveal the impact of coupling beams between the top of the piles on shearing force of micro-pile group in landslide. Results from contradistinction showed that coupling beam played greater role with shearing force to segment piles, comparing with less to the force near the sliding surface. Without coupling beams, shearing force of each pile from each row suffered quite different, comparing to that of small differences with beams. With no coupling beams, first damage points were located under top of the piles at about six times the pile diameter.

Behaviour of Micropile Groups under Oblique Pull Out Loads in Sand

Micropiles have been installed on many projects in many parts of the world for different purposes like underpinning, seismic retrofitting, slope stabilization, resisting uplift dynamic loads. However information on the behaviour of micropiles when subjected to vertical and oblique pull is rather limited in literature. A model testing programme was designed and carried out to investigate behaviour of micropile groups under vertical and oblique pull out loads. Micropile groups having different length to diameter ratio and different spacings were grouted in sand having a relative density of 50 %. The groups were subjected to 0°, 30°, 60° and 90° pull. Ultimate pull out loads and efficiency were found to be a function of length to diameter ratio of the piles and angle of inclination of the pull out loads. Groups subjected to vertical pull failed by soil failure and groups subjected to oblique pull failed by structural failure.

Design method for stabilization of earth slopes with micropiles

As one of the measures for slope fast reinforcement, micropiles are always designed as a group. In this paper, an analytic model for the ultimate resistance of micropile is proposed, based on a beam–column equation and an existing p–y curve method. As such, an iterative process to find the bending moment and shear capacity of the micropile section has been developed. The formulation for calculating the inner force and deflection of the micropile using the finite difference method is derived. Special attention is given to determine the spacing of micropiles with the aim of achieving the ultimate shear capacity of the micropile group. Thus, a new design method for micropiles for earth slope stabilization is proposed that includes details about choosing a location for the micropiles within the existing slope, selecting micropile cross section, estimating the length of the micropile, evaluating the shear capacity of the micropiles group, calculating the spacing required to provide force to stabilize the slope and the design of the concrete cap beam. The application of the method to an embankment landslide in Qinghai province, China, is described, and monitoring data indicated that slope movement had effectively ceased as a result of the slope stabilization measure, which verified the effectiveness of the design method.

Experimental Research on Failure Mode of Micro-Pile in Landslide Reinforcement

In recent years, micro-pile has been widely applied to landslide treatment engineering due to its advantages in application and construction, and the engineering effect is very evident. Large-scale physical model test was made for studying failure mode of micro-pile group in landslide, which indicated that numerical magnitudes between the displacement and the stress of the piles are better consistent along the load transfer direction. Concentrated destruction points locate on about three times the pile diameter up and down the slip surface. Failure mode of micro-pile in landslide treatment engineering is: pile of the loaded segment usually breaks for bending moment and shear force, and back of sliding side mainly exposes to the tension role, compared to role of tension of anchored segment in front of the micro-piles and compression behind of the piles.

Mechanism of Bearing Capacity of Spread Footings Reinforced with Micropiles

ABSTRACT The Hyogoken-Nambu Earthquake in 1995 caused extensive damages to the foundations of bridges. Ever since, methods to improve the bearing capacity of existing foundations have become an important aspect of foundation engineering in Japan. Micropiles are considered to provide promising solutions. The mechanism which enhances the bearing capacity of surface footings reinforced with micropiles is the subject of investigation in this study. As an initial phase, model tests were conducted to understand the load-displacement behavior of surface footings with and without micropiles on loose, medium dense, and dense layers of sands. Salient factors which influence the behavior of the footings were selected and their influence on bearing capacity was examined through a comprehensive series of model tests. Notable improvements in the bearing capacity of surface footings reinforced with vertical micropile groups were observed in the case of dense sand which is dilative during shear. To assess quantitatively the degree of improvement in the bearing capacity of surface footings reinforced with micropiles, an index R called “Network Effect Index” was introduced in this study. The index R of unity means that the bearing capacity of footings reinforced with micropiles is simply equal to the summation of the individual value of the surface footing and that of the micropile group. An index R of more than two is achieved in this study where surface footings reinforced with a group of vertical micropiles bear on a dense layer of dilative sand. By contrast, with loose and medium dense sand, which are contractive in nature, the index R is found to be less than unity.

Three-dimensional finite element analysis of the seismic behavior of inclined micropiles

This paper presents a thorough study of the behavior of inclined micropiles under seismic loading. Analysis is carried out using a full three-dimensional finite element modeling. The soil media is assumed to be elastic with Rayleigh damping, while micropiles are modeled as 3D elastic beam elements. The structure is described by a single degree of freedom system composed of a concentrated mass and a column. The paper is composed of four parts. The first part includes a literature survey on the behavior of inclined micropiles. The second part presents the numerical model used in this study. The third part concerns analysis related to the influence of micropiles inclination on the seismic behavior of a group of micropiles embedded in a homogeneous soil with a uniform stiffness. The last part deals with the seismic behavior of inclined micropiles embedded in a soil layer with a depth-based increasing stiffness. The results of this study provide valuable information about the influence of micropiles inclination on dynamic amplification and on the seismic-induced internal forces in micropiles.

A three dimensional embedded beam element for reinforced geomaterials

AbstractThis paper presents the formulation and verification of a 3D embedded beam element, which is intended for numerical modelling of three dimensional problems concerned by reinforced geomaterials. This element permits analysis of reinforced geomaterial structures with simplified meshes, that do not need to account for reinforcement orientation. The paper is composed of four sections. Section 1 discusses the need for the development of a particular beam element for soil reinforcement, which can be easily used in practical applications. Section 2 describes the mathematical formulation of this element, while Section 3 deals with its verification on various examples. Section 4 illustrates an application of this element by analysing the behaviour of a group of micropiles containing inclined elements and subjected to lateral loading. Copyright © 2004 John Wiley & Sons, Ltd.

Three-dimensional finite element analysis of the seismic behavior of inclined micropiles

This paper presents a thorough study of the behavior of inclined micropiles under seismic loading. Analysis is carried out using a full three-dimensional finite element modeling. The soil media is assumed to be elastic with Rayleigh damping, while micropiles are modeled as 3D elastic beam elements. The structure is described by a single degree of freedom system composed of a concentrated mass and a column. The paper is composed of four parts. The first part includes a literature survey on the behavior of inclined micropiles. The second part presents the numerical model used in this study. The third part concerns analysis related to the influence of micropiles inclination on the seismic behavior of a group of micropiles embedded in a homogeneous soil with a uniform stiffness. The last part deals with the seismic behavior of inclined micropiles embedded in a soil layer with a depth-based increasing stiffness. The results of this study provide valuable information about the influence of micropiles inclination on dynamic amplification and on the seismic-induced internal forces in micropiles.

Engineering Analysis of Dynamic Behavior of Micropile Systems

A series of centrifugal tests was conducted on micropile groups and network systems to investigate the system response to earthquake loading and the superstructure soil-micropile interaction behavior. Model tests on vertical and batter micropile systems embedded in loose to medium-dry sand under different levels of shaking are described. Group and network effects were investigated for different configurations and at different levels of loading. The experimental test results were analyzed parametrically to study the effect of the main controlling parameters. For the model testing conditions, the experimental and parametric results indicated a positive group effect increasing with the number of piles and the batter angle. Dynamic p-y curves were found to be nonlinear with low damping and to be much softer than the published data. For inclinations of 10° and 30°, a substantial improvement is observed in the superstructure response with acceleration reduction to 40 percent of the values obtained in the case of vertical piles. The computer programs LPILE and GROUP were used in a pseudostatic analysis approach to simulate the representative centrifugal model tests. Comparisons between the method predictions and the experimental results illustrate that these methods could be used as engineering tools to assess the seismic behavior of micropile groups and network systems.

Micropiles: the state of practice. Part II: design of single micropiles and groups and networks of micropiles

This series of papers summarizes the research project initiated in 1993 by the US Federal Highway Administration to review the state of practice, case studies and design methods of micropile group systems. Following a brief description of the recently adopted classification system for micropile design previously outlined by Bruce et al. (1997) (part 1), this paper (part 2) presents a summary of recommendations for the design of single micropiles and groups and networks of micropiles for selected engineering applications, including direct structural support and in situ soil reinforcement. Preliminary estimates of the ultimate axial and lateral capacity of micropiles, as outlined by different authors, are presented first. Design guidelines which have been developed generally through observations on full-scale testing and field experiences are discussed with regard to cohesionless soils, cohesive soils and rocks. Group and network effects are investigated and preliminary conclusions are presented along with proposed design guidelines for micropile groups. Existing analytical approaches are evaluated through comparisons with experimental data obtained by different investigators on the engineering behaviour of micropile groups and reticulated micropile networks under different loading conditions.

Foundations with micropiles in tamped trenches on construction projects in belarus

A procedure is presented for the installation of foundations with micropiles a distinguishing characteristic feature of which is the tamping of a trench and its concreting with a group of micropiles under an embedded grillage. A set of field investigations under different soil conditions, which has indicated the reliability of these foundations and their effectiveness for the construction of any buildings and structures, is described.