Single Micropiles Research Articles

Parametric analysis of a strain state of a soil base strengthened with vertical elements

Purpose is to identify vertical displacements of the “soil basem strengthened with piles or micropiles based upon drilling-mixing technology” system relying upon parametric analysis of strain state of the mentioned system, and arrangement of the strengthening elements. Methods. Mathematical modeling took place for twelve finite-element models using the computing complex SCAD intended to analyze strength of structures by means of finite-element method. Numerical analysis was carried out with variation of elastic-strain modulus of vertical reinforcing elements and change in distance between them (3d and 6d of micropiles). Findings. Results of parametric analysis have been obtained for a model without a micropile (non reinforced soil base); a model with a single micropile; a model with two micropiles where distance between them is 1.5 m, i.e. 3d of micropiles; and a model with two micropiles where distance between them is 3.0 m, i.e. 6d of micropiles. The performed comparative analysis has made it possible to obtain the results proving the hypothesis by the authors as for the specific nature of vertical displacement formation of the “soil base strengthened with piles or micropiles based upon drill-mixing technology” system. Originality. It has been identified for the first time that basing upon the standardized document for the auger or displacement piles, it is impossible to decrease efficiently vertical displacements while approaching micropiles since distance between the micropiles is 3d for elements, developed on the basis of the drilling-mixing, is minimal. Practical implications. The obtained results of a strain state may become the keystone for the development of the genera-lized strain theory of the composite “soil base strengthened with piles or micropiles based upon drilling-mixing technology” system as a medium differing in minor changes of strain characteristics.

RESULTS OF EXPERIMENTAL TRAY STUDIES OF THE SOIL FOUNDATION BASE REINFORCED WITH MICROPILES

Purpose. Obtaining a change in the deformed state of models of unreinforced and reinforced micropile soil base to determine the influence of the reinforcement element. Methodology. The method of research in a flat tray was used, which makes it possible to solve a certain range of tasks related to the interaction of foundations with the soil base. The conditions for conducting experimental research and the features of creating models of unreinforced and micropile reinforced soil bases are given. The conditions for creating clay paste, laying it in a flat tray, preparing the model for loading, and taking measurements are given in detail. To assess the adequacy of experimental tray studies, modeling conditions were calculated based on the parameters of a real civil structure, its foundation and soil base. Findings. During the experimental studies of the soil foundation and its interaction with the base in a flat tray, two series of experiments were conducted, three experiments in each series. In series Nos. 1-3, an unreinforced base was studied, in series Nos. 4-6, a base reinforced with a single micropile. Vertical displacements were averaged, which were measured by three dial indicators installed in the middle and on both sides of the die that modeled the foundation. On the basis of the results obtained during the experimental studies, diagrams were plotted in σ – s coordinates (stress – settlement). Originality. The originality of the work consists of obtaining a change in the deformed state of the models of unreinforced and micropile reinforced soil base and determining the influence of the reinforcement element (the maximum vertical displacement of 14.7 mm in the model reinforced with micropile in contrast to 17.5 mm in the unreinforced one, i.e., the reduction of this component of the deformed state by 1.19 times). Practical value. The practical value of the conducted research lies in the proof of the positive impact of micropiles based on the results of experimental tray studies. Based on the obtained results, it is reasonable to apply micropiles created with the help of drilling-mixing technology to successfully reduce the vertical displacements of the foundations of civil structures on weak soil bases.

Application of Carbon-Fiber Composite Material in Micropile Structure

AbstractThe flexural capacity of a single micropile is small, and some additional materials need to be built into the steel pipe to increase the flexural and tensile capacity of the structure. This...

An Experimental Study of Horizontal Bearing Capacity of Vertical Steel Floral Tube Micropiles with Double Grouting

This paper examines the performance of a novel technology, vertical steel floral tube micropiles with double grouting. It is the combination of micropile technology and double grouting technology. A large-scale model tank was applied to impart horizontal bearing capacity, and the slope soil pressure and flexural performance of the micropile were investigated under four experimental conditions. The peak grouting pressure during the double grouting process was defined as the fracturing pressure of the double grouting, and it was positively correlated to the interval time between first grouting and secondary grouting. Compared with traditional grouting, double grouting increased the horizontal bearing capacity of the single micropile with the vertical steel floral tube by 24.42%. The horizontal bearing capacity was also 20.25% higher for the structure with three micropiles, compared with a 3-fold value of horizontal sliding resistance. In the test, the maximum bending moment acting on the pile above the sliding surface was located 2.0–2.5 m away from the pile top, and the largest negative bending moment acting on the pile below the slip surface was located 4.0 m away from the pile top. The ultimate bending moment of the single pile increased by 12.8 kN·m with double grouting, and the bending resistance increased by 96.2%. The experimental results showed that the double grouting technology significantly improved the horizontal bearing capacity of the micropile with the steel floral tube, and the soil reinforcement performance between piles was more pronounced. Also, the shear capacity and the flexural capacity were significantly improved compared with the original technology.

An Experimental Study for Reinforcing the Ground Underneath a Footing Using Micropiles

Abstract The in situ application of micropiles has gradually increased in confined spaces of complex cities because the micropile has many advantages, such as low noise and vibration and compact machine size. Using the results of experimental analyses, this study purposes to improve the reinforcement efficiency of micropiles by recommending the most effective pattern and design method for installing them. In this study, model tests were carried out to understand the ground reinforcement effect induced by the mechanical interaction between the micropile and soil. The micropiles were installed in the soil underneath footings. Factors such as micropile diameter (D), micropile length (L), and number (n) of micropiles were considered as variables in the model tests. The positive effect of grouping micropiles and the confining effect of the ground increased the bearing capacity of a single micropile by about 2.27–2.60 times and 1.10–1.33 times, respectively.

Behaviour of reinforced polyurethane resin micropiles

This paper deals with the field behaviour, under tension and compression, of an innovative kind of micropile made from reinforced expanded polyurethane resin. Micropiles were installed in various sample sites characterised by the presence of silty–clayey soils. In order to analyse the frictional resistance of a single micropile and to determine the bearing capacity in different conditions, a series of field load tests was carried out. The method of construction of the micropiles and the execution of field tests are briefly described. On the basis of the field tests, a method to evaluate the ultimate load was defined. Moreover, the micropile–soil mechanical interaction was modelled by applying a simplified analytical solution of the load–displacement behaviour. The obtained results provide a useful support in the evaluation of both the serviceability and the ultimate design capacity of the investigated micropiles. The most innovative aspect of this technology lies in using less costly equipment than required for other pile types, with machines of reduced dimension, which can be very useful for underpinning works in restricted spaces under existing buildings.

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.

Behaviour of micropiles in collapsible loess under tension or compression load

This study examines the behaviour of single micropiles subjected to axial tension or compression load in collapsible loess under in-situ moisture content and saturated condition. Five tension loading tests and five compression loading tests on single micropiles were carried out at a typical loess site of the Loess Plateau in Northwest China. A series of laboratory tests, including grain size distribution, specific gravity, moisture content, Atterberg limits, density, granular components, shear strength, and collapse index, were carried out during the micropile loading tests to determine the values of soil parameters. The loess at the test site poses a severe collapse risk upon wetting. The tension or compression load-displacement curves of the micropiles in loess, under in-situ moisture content or saturated condition, can generally be simplified into three distinct regions: an initial linear, a curvilinear transition, and a final linear region, and the bearing capacity or failure load can be interpreted by the L1-L2 method as done in other studies. Micropiles in loess should be considered as frictional pile foundations though the tip resistances are about 10%-15% of the applied loads. Both the tension and compression capacities increase linearly with the ratio of the pile length to the shaft diameter, L/d. For micropiles in loess under in-situ moisture content, the interpreted failure loads or capacities under tension are 66%-87% of those under compression. However, the prewetting of the loess can lead to the reductions of 50% in the tensile bearing capacity and 70% in the compressive bearing capacity.

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.

Three‐dimensional analysis of the seismic behaviour of micropiles used in the reinforcement of saturated soil

AbstractThis paper includes a numerical study of the behaviour of micropiles used for the reinforcement of saturated soil. Analysis is carried out using the (u–p) formulation (displacement for the solid phase and pore‐pressure for the fluid phase) implemented in a three‐dimensional finite element program. The soil behaviour is described by means of a cyclic elastoplastic constitutive relation which was developed within the framework of the bounding surface concept. The paper is composed of three parts. The first one is concerned with a presentation of the numerical model; the second includes analysis of the seismic behaviour of a single micropile; the last part deals with the group effect under seismic loading. Copyright © 2001 John Wiley & Sons, Ltd.

Seismic Behavior of Micropiles: Used as Foundation Support Elements: Three-Dimensional Finite Element Analysis

Presented is a three-dimensional finite element analysis of the seismic behavior of micropiles used in engineering applications, such as construction in earthquake areas, and seismic retrofitting of structure foundation and retaining systems. It is composed of two parts. The first is concerned with analysis of the seismic behavior of a single micropile supporting a superstructure; results obtained with three-dimensional finite element modeling are compared to those obtained from a simplified model based on the Winkler approach. The second part includes a study of the seismic behavior of groups of micropiles. Numerical simulations are conducted to study the variation of the group effect with the variation of micropile spacing, number of micropiles, and the position of the micropile in the group. Analysis of results of numerical simulations shows that the presence of the superstructure induces significant inertial effect, which causes a high increase in the bending moment in the upper part of micropiles; the inertial effect depends mainly on the loading frequency with respect to the fundamental frequency of the soil layer and that of the superstructure. A group effect is observed in groups of micropiles connected by a rigid cap. The group effect depends mainly on micropile spacing, number of micropiles, and position of micropiles in the group. Results show an increase in moment at the head of micropiles with the increase of the micropile spacing (presence of inertial effect) and a concentration of seismic loading on corner micropiles.

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.