Micropile System Research Articles

Thermo-mechanical behavior of energy micropiles

Since the first application of piles as energy geostructure in Austria in the 1980s [1], the use of geostructures has steadily increased worldwide. Energy geostructures are complex systems that combine structural, geotechnical, and thermal performance. They are based on the exploitation of the soil thermal energy at shallow depths to provide heating and cooling to buildings [1], allowing to reduce both the consumption of non-renewable energy and the harmful carbon emissions. Pile foundations are currently among the most used geostructures. They are particularly suitable for exploiting the ground thermal energy, as they reach depths where soil temperature is quite constant and independent from daily or seasonal variations [2–4] .
 Micropiles are small, drilled, and grouted-in-place piles having a diameter between 90-300 mm and a length up to 20 m [5–7]. Generally, they are constructed by drilling a borehole, placing reinforcement, and grouting the hole [6]. Micropiles can be loaded directly to transfer structural loads, both axial (compression and tension) and lateral, to a deeper stable stratum like in new foundations or they can reinforce the soil to theoretically make a reinforced soil composite (reticulated piles) that resists applied loads like in underpinning of existing foundations [6]. Due to the relatively small cross-sectional area of micropiles, load carrying capacity resulting from end bearing is generally considered to be negligible in soil or weak rock, thus micropiles mainly rely on shaft resistance for load bearing [6,7]. Innovative and vigorous drilling and grouting permit high grout-bond values to be generated along the micropile’s periphery. High-capacity steel elements, occupying upto 50% of the hole, can be used as the principal load bearing element with the surrounding grout serving only to transfer by friction the applied load between the soil and the steel [6]. The axial capacities of micropiles are in the range of 50–500 kN and up to 1000 kN when installed using pressure-grouting techniques [7]. In the Titan system, the hollow bar simultaneously acts as the drilling rod, injection tube and reinforcement for the micropile [8]. This system is fast, simple and flexible, can be used in restricted sites when access is difficult, can be installed with low vibration and noise in any type of soils including unstable soils. In the context of building rehabilitation, retrofitting, and underpinning of existing structures, the energy micropiles stand frequently as the best solution over energy piles [6]. The Titan 73/53 Energy micropile is a dual used micropile: as a geothermal energy pile with an equal extraction capacity as double U-pipe and as a foundation pile with characteristic load capacity of RM,K = 900KN [9]. Although energy micropile (EMP) systems are very similar to energy pile (EP) systems, their behavior cannot be assumed to be similar both from the mechanical point of view (axial load supported mainly by lateral resistance with almost null end bearing capacity) and the thermal point of view (potentially lower energetic efficiency, due to shorter primary circuit and smaller contact surface with the soil) [10]. For example, pile diameter and pipe diameter have been found to play respectively an important role and an insignificant role in energy piles [11] while the contrary was observed for energy micropiles [12]. Nevertheless, the thermal performance of micropiles in terms of specific heat flux (about 50W/m in fine grained soils) has been found encouraging to predict their use in heating, ventilation and air-conditioning systems[10]. Thermal response tests conducted on TITAN 73/53 energy micropiles show that their specific extraction capacity can be 111W/m in wet sediments, and between 60-80W/m depending on the soil’s properties[9]. To date, several studies are dealing with the thermo-mechanical behaviour of energy piles [13] and also the thermal performance of energy micropiles has been looked at to an extent by various
 researchers [10,12,14]. However, the coupled thermo-mechanical behaviour of energy micropiles is not well understood. In fact, the short and long terms effects of combined thermal and mechanical cyclic loading, the “group effects” of energy micropiles, among others, still need to be investigated. This study investigates the thermo-mechanical behavior of a TITAN 73/53 energy micropile, with the aim of identifying suitable evaluation criteria and design parameter for optimizing the structural, geotechnical, and thermal performance of energy micropiles. To achieve this objective, thermal response tests on instrumented TITAN 73/53 energy micropiles, and 3D Finite element analysis will be carried out. The results of the in-situ tests will be used to calibrate the numerical model of our simulation. Then the short and long terms performances of the system in various thermal, geotechnical, and structural conditions will be evaluated.

Static analysis of prestressed micropile-raft foundation with varying lengths resting on sandy soil

The micropile-raft system is used in the foundation system of buildings due to the combination of the advantages of raft foundations and micropiles. In this paper, the numerical model was calibrated and validated with a field test, and then several parametric studies were performed. The overarching aim of the study is to determine the efficiency of different arrangements of micropiles and the effects of these parameters on the behavior of piled raft foundations in sandy soil. Parameters including diameter, length, and distance of micropiles from each other were considered in the analyses. Prestressed rebars have been used to increase the load-bearing capacity and prevent premature buckling under severe compressive load. Therefore, to compensate for the weak bearing capacity of the raft foundation, a micropile system equipped with prestressed rebars was considered to increase the bearing capacity of the foundation. The results revealed that using a set of micropiles with a diameter of 30 cm and a length of 10 m had the maximum efficiency among other models. Furthermore, an innovative arrangement of micropiles was proposed in which a set of micropiles with a length of 30 m was placed at the middle region of the foundation, and in the peripheral area of the foundation, several short micropiles were used.

Effect of Diameter of Micropile on the Minaret Behavior during Earthquake, Virtual study

This study aims to suggest a technique for soil properties improvement of AL- Kadhimin shrine Minaret and to support the foundation, which has a tilt of roughly 80 cm from the vertical axis. The shrine of the AL- Kadhimin is made up of four minarets with two domes set in a large courtyard. The four minarets have skewed to varying degrees due to uncontrolled dewatering inside the shrine in recent years. However, the northeast minaret was the most inclined due to its proximity to the well placed inside shrine courtyard. When the well near the minaret is operated, the water level drops, increasing the effective stresses of the soil and causing differential settling of the minaret foundation. To maintain the minaret's foundation from potential lateral stresses, a micropile system has been proposed around it. PLAXIS 3D is used to do a three-dimensional numerical analysis in this study. A micropile system of several diameters has been considered for the suggested technique. In the analysis, the modeling and verification findings revealed that the suggested micropile system plays a significant role in incrementing the minaret's lateral load resistance (earthquake).

Effectiveness of a vertical micropile system for mitigation of liquefaction induced lateral spreading effects on pile foundations: 1g large scale shake table tests

Liquefaction-induced lateral spreading has caused severe damages to pile foundations during past earthquakes. Micropiles can be used as a mitigation strategy against lateral spreading effects on pile foundations. However, the available knowledge regarding the possible efficiency of such a strategy is quite limited. In this paper, the effectiveness of a vertical micropile system as a lateral spreading countermeasure was evaluated using large scale 1g shake table tests on 3×3 pile groups. The results showed that the micropile system was not able to effectively reduce bending moments in piles while it reduced lateral soil pressures exerted on the upslope piles of the group by the upper non-liquefiable layer. The employed micropiles restricted lateral displacement of the upper non-liquefiable layer and partially that of the liquefiable layer, especially at upper depths. Solutions such as increasing the number of micropiles with a tighter pattern, using stiffer micropiles or fixing them in the underlying non-liquefiable layer can enhance their performance.

Vertical pullout tests of orchard trees for bio-inspired engineering of anchorage and foundation systems

Application of bio-inspired design in geotechnical engineering shows promise for improving the energy and material efficiency of several processes in infrastructure construction and site characterization. This project examines tree root systems for use in future bio-inspired design to improve the capacity of foundations used to support, for example, buildings and bridges. Foundation and anchorage elements used in industry are comprised almost solely of linear elements with a constant cross-sectional geometry. This functional form has remained the same for more than a century, primarily due to material availability and installation simplicity. Knowledge and understanding of the mechanisms that contribute to capacity development of natural nonlinear or branched foundation systems, such as tree root systems, could make foundation design more sustainable. The experiments described herein show that the root systems studied are 6–10 times as efficient as a conventional micropile system in developing tensile capacity on a per volume basis, with some systems displaying nearly 100 times efficiency in comparison to a conventional shallow footings. This paper explores the relationship between root system architecture and force–displacement behavior of tree root systems to better understand how to improve foundation capacity and demonstrates the potential for a more efficient use of materials and energy as compared to conventional pile and footing approaches.

The Comparison of Laboratory Tests and Numerical Analysis of Pressure and Tension Bearing Capacities of the New System of Microbulb and Micropile System on Clay in Shiraz, Iran

Abstract In recent years, the implementations of Micropiles have been increased extensively in large project constructions due to the unique advantages of this method comparing to concrete piles. The results of numerical analysis and centrifuge tests show that the efficiency and importance of micropiles can be considered as an effective solution. This research is designed to compare the newly designed system of microbulb with micropile applying a numerical modeling with finite element software Plaxis 3D foundation v1.6. We also used the FHWA code for a comparative study between micropiles and microbulbs. Then the effective factors were analyzed in order to decide which method is better between these two methods of micropile and microbulb. The results showed that the new microbulb system is an appropriate solution for improvement of mechanical properties of the soil, increase of bearing capacity and settlement decrease.

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.

Improved Load Carrying Behavior of Perfobond Rib-Installed Partially Expanded Micropiles

Abstract This study proposed an advanced type of a micropile system. The proposed micropile system consists of perfobond ribs, installed on steel rod to improve shear resistance between the thread and the grout, and partially expanded drill holes to increase shear resistance between the grout and the ground. This study contained experimental evaluations on the proposed micropile system to verify the increase in the shear capacity of perfobond rib installed on the steel rod and the load-carrying capacity of shear key created by the partially expanded drill hole. Push-out tests were conducted on a rolled screw thread and perfobond rib-installed steel rods in order to compare their load-carrying capacities and load carrying behavior. It was confirmed that the proposed micropile system with perfobond-rib installed steel rods show much superior load carrying behavior in terms of initial stiffness, ultimate load, and ductile behavior. Through on-site pull-out tests, it was found that the load carrying capacity of the proposed micropile system increased significantly due to the effect of shear key created by partially expanded drill hole.

확공형 마이크로 파일의 강도 평가에 관한 연구

This study proposes an advanced type of a micropile system. The proposed micropile system consists of perfobond ribs installed steel rod to improve shear capacity between the thread and the grout, and partially expanded drill holes to increase resistance capacity between the grout and the ground. This study contains experimental evaluations on the proposed micropile system to verify the shear capacity of perfobond rib installed on the steel rod and the load-carrying capacity of shear key created by the partially expanded drill hole. Push-out tests were conducted on a rolled screw thread and steel rods which perfobond ribs are installed instead of rolled screw, in order to compare their load-carrying capacity and behavioral characteristics. As a result, it was confirmed that the perfobond-rib steel rods show much superior structural behavior in terms of initial stiffness, ultimate load, and ductile behavior.

확공형 마이크로 파일의 지지력 해석

기존 마이크로 파일의 경우 전조나사 강봉과 그라우트와의 부착력 및 그라우트와 지반과의 주면마찰력에 의해 내하성능이 결정된다. 그러나 강봉과 그라우트 간의 부착성능이 가지는 취성파괴 특성과 낮은 내하성능에 의해 천공 깊이가 깊어지는 문제점을 갖고 있다. 따라서 본 연구에서는 기존 마이크로 파일의 지지성능을 개선하고 경제성을 향상시키기 위하여 perfobond rib를 구비한 강봉과 그라우트와 지반간의 주면마찰력을 증가시키기 위해 부분확공부를 설치하는 마이크로 파일을 제안하고, 거동 및 파괴특성에 대한 평가를 수행하였다. 현장 인발시험 결과 부분 확공하여 시공된 지반부 전단키에 의한 내하성능은 상당히 증가된 것으로 평가되었다. The load-carrying capacity of a conventional micropile is determined by the adhesion between the rolled screw thread and the grout, and the skin friction force between the grout and the ground. One of the disadvantage of the conventional micropiles is that the boring depth can be deeper because of low load-carrying capacity and the brittle failure behavior between rolled screw thread and grout. Therefore, it is necessary to develop an advanced feature of a micropile system which efficiently increases load-carrying capacity with a reasonable cost. This study proposes an advanced type of a micropile system. The proposed micropile system consists of perfobond ribs installed steel rod to improve shear capacity between the thread and the grout, and partially expanded drill holes to increase resistance capacity between the grout and the ground. Through on-site pullout tests, it was found that load-carrying capacity increased significantly in the proposed micropile system, due to the shear keys created by the partially expanded drill hole. The proposed micropile system was also evaluated to be much more economical in a case study.

Three-Dimensional Stability Analysis of a Homogeneous Slope Reinforced with Micropiles

Micropiles are widely used to reinforce slopes due to their successful performance and fast construction. In this study, a simple nonlinear method is proposed to analyze the stability of a homogeneous slope reinforced with micropiles. This method is based on shear strength reduction technique, in which the soil behavior is described using the nonassociated Mohr-Coulomb criterion and micropiles are modeled as 3D pile elements. A series of slope stability analyses is performed to investigate the coupled mechanism of micropile system, and the optimum of pile position, depth of embedment, and length of truncation are analyzed. Results show that the position of micropile system plays an important role not only in the calculation of the safety factor, but also in locating the failure surface, which demonstrates the dominating coupled effect exists between micropiles and slope. The critical embedment depth of the micropile is about 2 times the length of micropile above the critical slip surface, and the micropiles flexure rather than rotation becomes increasingly prevalent as the depth of micropiles embedment increases. Truncation of micropiles may improve the capacity of the micropile system, and the largest truncation length of micropile is about 1/4 depth of critical slip surface in this study.

Design aspects of micropiles

Micropiles were invented in Italy in the early 1950’s in response to the demand for innovative techniques for underpinning historic buildings and monuments that have sustained damage with time. The micropile systems used today are evolution from the basic small diameter cast-in-place pile. A typical micropile construction involves drilling a pile shaft to the required depth, placing the steel reinforcement, initial grouting by tremie and additional grouting under pressure where applicable. Advantages of micropiles are high bearing capacity, less site limitation problems and self-sustained operation. This piling system is therefore attractive to both the client and the foundation designer.

Discussion: Seismic behaviour of micropile systems

Discussion: Seismic behaviour of micropile systems

Seismic behaviour of micropile systems

This paper includes analysis of the performance of micropile systems under seismic loading. It presents results of both centrifuge model testing and three-dimensional finite element modelling carried out within the FOREVER research programme. The experimental and numerical results were analysed to study the effect of the main controlling parameters of the micropile-soil-structure systemsuch as the kinematic interaction, the group effect, the influence of micropile inclination, and performance assessment of micropile systems in liquefiable soils. Analysis shows that micropile systems offer effective engineering solutions for construction in seismic areas. Cette étude est constituée d'analyses de performances pour des systèmes à micropiles soumis à des charges sismiques. Elle présente les résultats d'essais sur modéle centrifuge ainsi que les résultats d'une modélisation en trois dimensions d'éléments finis, le tout effectué dans lecadre du programme de recherche FOREVER. Nous avons analysé les résultats expérimentaux et numériques afin d'étudier l'effet des principaux paramétres de contrôle du système micropile-sol-structure, comme l'interaction cinématique, l'effet de groupe, l'influence de l'inclinaison des micropiles et afin d'évaluer la performance des systémes à micropiles dans des sols liquéfiables. Les analyses montrent que les systémes à micropiles offrent des solutions d'ingénierie efficaces pour la construction dans des zones sujettes aux séismes.

Seismic behaviour of micropile systems

This paper includes analysis of the performance of micropile systems under seismic loading. It presents results of both centrifuge model testing and three-dimensional finite element modelling carried out within the FOREVER research programme. The experimental and numerical results were analysed to study the effect of the main controlling parameters of the micropile-soil-structure systemsuch as the kinematic interaction, the group effect, the influence of micropile inclination, and performance assessment of micropile systems in liquefiable soils. Analysis shows that micropile systems offer effective engineering solutions for construction in seismic areas. Cette étude est constituée d'analyses de performances pour des systèmes à micropiles soumis à des charges sismiques. Elle présente les résultats d'essais sur modéle centrifuge ainsi que les résultats d'une modélisation en trois dimensions d'éléments finis, le tout effectué dans lecadre du programme de recherche FOREVER. Nous avons analysé les résultats expérimentaux et numériques afin d'étudier l'effet des principaux paramétres de contrôle du système micropile-sol-structure, comme l'interaction cinématique, l'effet de groupe, l'influence de l'inclinaison des micropiles et afin d'évaluer la performance des systémes à micropiles dans des sols liquéfiables. Les analyses montrent que les systémes à micropiles offrent des solutions d'ingénierie efficaces pour la construction dans des zones sujettes aux séismes.

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.