Behavior Of Micropiles Research Articles

Seismic Behavior of Micropiles and Micropiled Structures Used for Increasing Resilience: A Literature Review

The aim of this paper is to present the relevant information and development available in the scientific literature regarding the seismic behavior of micropiles (MPs) and micropiled structures (MPed). The seismic behavior of MPs is not very well studied, but MPs are used in retrofitting of old buildings and for new resilient buildings, and in terms of seismic behavior they have a high potential. Additionally, their association with seismic dampers for improving the seismic behavior of buildings is not yet fully studied and it represents a major topic of interest for both new structures and historical monuments. After the introductory part, the paper describes all relevant information regarding MPs, as types and technology, seismic behavior, applications for increasing seismic resilience, and experimental and numerical modeling.

Numerical Analysis of Failure Mode of Micropile in Cohesive Soil

In this paper, fnite element was used to study the behavior of micropile. The model was developed using PLAXIS 3D foundation program. Micropiles are tested by the static axial load testing of individual piles. In the FE-analysis the pile is assumed to be linear elastic and for the subsoil is assumed as Mohr-Coulomb model. The model was developed to simulate the micropiles type A where the grouting is applied by gravity. This modeliscompared with the results of the pile load test. A good agreement was obtained with the experimental results. Finally the failure pattern was investigated.

Seismic Response of Multi Story Structure Strengthened with Micropiles

Micropiles are reinforced grouted piles that have small diameters commonly not higher than 30 cm. They are widely used for slope stabilization, controlling structural settlement, and in some cases, as retaining structures. Also, they are used for resisting dynamic uplift loads, seismic retrofit mainly in restrictive and low headroom areas, and retrofitting of historical monuments. The main goal of this research is to develop a finite element model that can capture the different aspects of seismic behavior of multi-story structure supported with deep foundation via using of micropiles. Also, a main target for the executing numerical modelling is to show the influence of the surrounding soil on this system and vice versa. Firstly, a representative two-dimensional finite element model is conducted to represent the soil-structure interaction system under seismic excitation supported with proper boundary conditions in PLAXIS 2D V20 for dynamic analysis based on previous recommendations considering the nonlinear soil behavior. The behavior of micropiles is studied and verified using previous results. Based on these models, the effect of lateral dynamic loads on the response of a structure with different foundation types is investigated. Also, a wide range of parametric studies, considering structure properties, earthquake magnitude, micropile diameter, micropile length, and the number of micropiles, have been carried out in order to investigate the actual interaction between soil, substructure, and superstructures. The study results showed that the seismic response of the structure is highly affected by the properties of the sub-surface soil layer. Consequently and similarly, analysis results established that underpinning using micropiles is an efficient technique for controlling the seismic response of existing structures.

Seismic response of micropiles-reinforced landslide based on shaking table test

A series of shaking table tests were conducted on unreinforced and micropiles-reinforced soil landslides to investigate the seismic response of micropiles-reinforced landslide and the dynamic mechanical behaviour of micropiles in landslide under earthquake. The test results indicate that: (1) The micropiles can raise the anti-seismic performance of landslide, and it can efficiently delay the development of earthquake landslide hazards. (2) After earthquake, the flexural failure of micropile mainly occurs in the area that 1.4–4 times pile diameter above the sliding surface and the area that 1.4–3.4 times pile diameter below the sliding surface. (3) After reinforced by micropiles, the acceleration response on the slope surface of landslide can be decreased, especially the slope toe. (4) Both thrust of sliding mass and resistance of sliding bed are mainly concentrated near the sliding surface. The dynamic bending moment distributions of the three rows of micropiles present as reverse ‘S’. The maximum bending moment of the pile above sliding surface is located at the point that 3.7 times pile diameter above sliding surface, and the maximum bending moment of the pile below sliding surface is located to the point that 1.43 times pile diameter below sliding surface.

Behaviour of micropiles in heterogeneous coarse soils

This paper reports on an experimental investigation on the behaviour of small-diameter piles, commonly referred to as micropiles. This particular type of deep foundation is frequently used in many barely accessible Italian mountainous areas, often characterised by complex ground profiles composed of mixtures of coarse soils with some fine matrix elements including cobbles and large-diameter boulders. In such ground conditions, the lack of reliable site and laboratory geotechnical investigations providing an accurate soil mechanical description and conservative approaches for micropile design often lead to significant underestimation of the vertical ultimate load. In order to improve micropile design in such geological contexts, a new field trial investigation involving tension and compression load tests on micropiles up to failure was set up in a selected test site located in the Italian alpine region. From interpretation of the load tests carried out so far, the reliability of commonly used calculation methods for estimating bearing capacity is discussed. As a result, a new approach for a more suitable calculation of the mobilised shaft and base resistance of micropiles bored in highly coarse soils is tentatively proposed.

Static skin friction behavior of a single micropile in sand

Numerical pile segment analyses were conducted using an advanced soil model to investigate the development of side resistance and the soil behavior around a micropile installed in sand and subjected to static axial loading. A series of static pile segment analyses were performed for micropiles and Conventional Drilled Cast-In-Place (CDCIP) piles considering effects of pile installation, relative density of sand and pile diameter. Results of analyses were used to determine the effects of pile installation, pile diameter, and relative density of sand on static shear behavior of a micropile and changes in effective stresses and volumetric strains in the soil adjacent to the interface. Relatively high side resistance and stiff t-z response were characteristic features of a micropile compared with CDCIP piles. Results of static numerical pile segment analyses for micropiles were compared and found to exhibit good agreement with pile loading tests and design methods.

Behavior of Micropiles in Soft Soil under Vertical Loading

The behavior of micropiles in soft clay, under vertical compression and tension loading, was examined by field tests at a site in Shanghai. The soil profile consists of topsoil, silty clay, sandy silt, muddy clay, and clay soil. Two compression and three tension loading tests were conducted on five single micropiles. The piles were instrumented with vibrating wire force sensors, and they were monitored during the process of loading to investigate the mechanisms of load transfer. Both the ultimate vertical load capacity and the deflection at applied loads were examined. The results indicate that the pile load–displacement response under vertical compression or tension loadings was nonlinear. Both the compression and the tension load carrying capacity basically increased with a linear trend. But, the ultimate load capacities under tension were about 50-60% of those under compression. Tip resistance was about 10-15% of the applied compression load, not existing in the micropiles under tension. The average skin friction for micropiles under compression loading was about 50% higher than that for piles under tension loading.

A field study on the behavior of micropiles in clay under compression or tension

Micropiles have been used for underpinning existing foundations founded on problematic soil, disturbed by the intrusion of underground activities (such as tunneling), or subjected to additional loads (such as stories added to existing buildings). This paper presents a field study on the behavior of single micropiles in soft clay subjected to compression or tension. Two compression and two tension loading tests were conducted on four single micropiles. All the micropiles were instrumented with rebar strain gauges, and they were monitored during the progress of loading. To set up a baseline for underpinned loading tests presented in a companion paper, a full-scale loading test was performed on natural soil. The results from all these full-scale loading tests are presented and analyzed in this paper. The paper discusses load–displacement responses, elastic moduli, axial forces, tip resistance, and skin friction relating to micropiles and their interactions with soft clay. Theoretical solutions for bearing capacity of soil, load capacity of piles, and percentage of tip resistance to the total load were adopted for examination of the test results.Key words: micropile, load–displacement, load capacity, skin friction, tip resistance.

A field study on the behavior of a foundation underpinned by micropiles

Micropiles have been used for many applications, including underpinning existing foundations to support structures subjected to additional loads, such as stories added to buildings. A field study was conducted to simulate a footing initially constructed on a natural soil to support a structure and later subjected to additional loads. This study investigated load transfer mechanisms during the connection of micropiles to the concrete plate under the initial load and during the additional loading. Earth pressure cells and rebar strain gauges were installed underneath the loading plate and in the micropiles, respectively, to monitor variation of stresses during the loading and connection processes. The behavior of micropiles in the underpinned foundation under the additional loading was analyzed and compared with that in the single-micropile loading tests presented in a companion paper. Theoretical solutions were adopted for examining the results obtained from this field study, including the overall stiffness of the underpinned foundation, the tip resistance, and the ratio of the load carried by the plate (raft). Guidelines are proposed for designing underpinned foundations using micropiles subjected to additional loads.Key words: underpinning, micropile, load transfer, skin friction, tip resistance, piled raft.