• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.42 no.6
• /
• pp.825-836
• /
• 2022

Micropiles can be used to support additional load in extended building structures. However, their use brings about a risk of exceeding the bearing capacity of existing piles. In this study, pre-compression was applied to distribute the load of an existing building to micropiles, and an indoor loading test was performed to confirm the structural applicability of a wedge-type anchorage device designed to improve its capacity. According to the test results, the maximum strain of the anchorage device was 0.63 times that of the yield strain, and the amount of slip generated at the time of anchorage was 0.11 mm, satisfying structural standards. In addition, using MIDAS GTS, a geotechnical finite element analysis software, the effect of the size of the pre-compression, the thickness of the soil layer, and the ground conditions around the tip on the reaction force of the existing piles and micropiles were analyzed. From the numerical analysis, as the size of the pre-compression load increased, the reaction force of the existing pile decreased, resulting in a reduction rate of up to 36 %. In addition, as the soil layer increased by 5 m, the reduction rate decreased by 4 %, and when the ground condition at the tip of the micropile was weathered rock, the reduction rate increased by 14 % compared with that of weathered soil.

• Journal of the Korean Geotechnical Society
• /
• v.22 no.11
• /
• pp.89-99
• /
• 2006

The PCRW (Precast Concrete Retaining Wall) has many advantages compared with cast in place concrete retaining wall : shorter construction period, excellency of quality and minimum interference with the adjacent structure and traffics. However, shallow foundation type of PCRW, which has comparatively better ground condition, has some disadvantages such as difficulty in transportation and higher cost due to the size of PCRW being expanded by resisting only with self-weight if there is no other supplementary reinforcement. The presented study, in order to complement such disadvantages of PCRW, have applied the micropile method. The micropile method has advantages like low-cost and high-efficiency and does not require huge space, because it can be executed with small size equipment. However, the mechanical behavior characteristics of the PCRW reinforced by micropile, which is installed to improve the reinforcement effect, is not yet clearly identified and there is no suggested standard as to the length, diameter, install angle and install position of micropiles. Hence, this method is yet being designed depend on engineer's experience. In this study, various laboratory model tests as to sliding and overturning were performed in order to identify and present the optimum type of reinforcement and reinforcement effect of the PCRW reinforced by micropiles. In addition, it also executed numerical analysis for the purpose of verifying the optimum type of reinforcement for micropiles based on the results of laboratory model tests. The optimum reinforcement type of micropiles was estimated by model test and numerical analysis. The length of micropiles is 0.4 times wall height and the diameter is 0.04 times wall length.

• Geomechanics and Engineering
• /
• v.7 no.5
• /
• pp.477-493
• /
• 2014

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.

• Journal of the Korean Geotechnical Society
• /
• v.20 no.7
• /
• pp.25-32
• /
• 2004

This paper examines the results of full-scale field tests on micropiles and side resistance is evaluated with respect to axial displacements and soil properties. Both cohesive and cohesionless soils are included in this evaluation. For all practical purposes, the developed load-displacement relationship and the geotechnical soil properties for each micropile and soil type can be used to represent the available data well through normalized average values and empirical correlations. There is a significant difference in load-carrying capacity between micropiles and drilled shafts that results primarily from the micropile pressure-grouting installation effects on the state of stress in the ground. The results show that micropiles can have a significant increase of capacity over larger-diameter drilled shafts at shallower depths with D/B < 100 or so. In cohesive soils, the typical increase is on the order of 1.5 with values as high as 2.5. For cohesionless soils, the typical increases are in the range of 1.5 to 2.5 with values as high as 6.

• Journal of the Korean GEO-environmental Society
• /
• v.11 no.10
• /
• pp.25-31
• /
• 2010

Anchor, soil nail, micropile have been widely used for slope reinforcement and foundation. These all methods need grouting work after placing reinforcing member. In domestic case, gravity fill techniques and pressure grouting techniques are mainly used. In contrast the pressure re-injection grouting method is not commonly used because grouting equipment and lack of practical application example is short and the verification of reinforcing effect is difficult. Pressure re-injection grouting is a kind of post grouting which technique increases the radial stresses acting on the grout body and causes irregular surface to be developed around bond length that tends to interlock the grout and the ground. In this study, the field test was performed to evaluate the reinforcing effect with the variation of grouting methods and pullout characteristics of reinforcing member placed by pressure re-injection grouting method. The test results showed that the post-grouting methods were useful to increase the pullout capacity.

• Geomechanics and Engineering
• /
• v.24 no.1
• /
• pp.29-42
• /
• 2021

The utilization of buildings can be improved by extending them vertically. However, the added load of the extension might require building foundations to be underpinned; otherwise, the loads on the foundations might exceed their bearing capacity. In this study, a preloading method was presented aiming at transferring partial loads from existing piles to underpinning piles. A pneumatic-type model preloading device was developed and used to carry out centrifuge experiments to evaluate the load-displacement behavior of piles, the pile-soil interaction during preloading, and the additional loading caused by vertical extension. The results showed that the preloading devices effectively transfer load from existing piles to underpinning piles. In the additional loading test of group piles, the load-sharing ratio of a pile increased with its stiffness. The load-sharing ratio of a preloaded micropile was less than that of a non-preloaded micropile as a result of the reduction in axial stiffness caused by preloading before additional loading. Therefore, a slight reduction of the load-sharing capacity of an underpinning pile should be considered if the preloading method is applied. Further, two full scale preloading devices was developed. The devices preload underpinning piles and thereby produce reaction forces on a reaction frame to jack existing piles upward, thus transferring load from the existing piles to the underpinning piles. Specifically, screw-type and hydraulic-jack type devices were developed for the practical application of foundation underpinning during vertical extension, and their operability and load transfer effect verified via full-scale structural experiments.

• Journal of the Korean Geotechnical Society
• /
• v.23 no.5
• /
• pp.5-14
• /
• 2007

The purpose of this study is to evaluate the normalized behaviors of existing bridge foundations reinforced by micropiles. In order to do numerical method a finite element program was used to predict the micropile behavior and quantify their reinforcing effects on existing bridge foundations. In addition, the installation effects of battered micropiles on existing foundations were compared with vertically reinforced bridge foundations. Based on the study performed, it was found that the use of battered micropiles more efficiently reduces displacement of existing foundations than vertically installed micropiles under vertical and horizontal loadings, respectively. The batter angle of micropiles was also found to be most effective at about $15^{\circ}{\sim}20^{\circ}$ in reducing the vertical displacement. The horizontal reinforcing effect continues to be larger with an increase in batter angles. So, it is believed that the results presented could give an idea to enhance In-service performance of existing bridge foundations reinforced by micropiles.

• Journal of the Korean Geotechnical Society
• /
• v.39 no.11
• /
• pp.41-51
• /
• 2023

This study employs three-dimensional simulation through FLAC3D to investigate the impact of installation angles on the dynamic characteristics of Triaxial Micropiles. The numerical model is validated against centrifuge test results to ensure accuracy. The findings reveal significant influences of the installation angle on the dynamic behavior of Triaxial Micropiles. Specifically, under seismic conditions such as the Capetown and San Fernando earthquakes, the lowest recorded values for peak bending moment and settlement occurred at an installation angle of 15 degrees. In contrast, when subjected to an artificial earthquake with a frequency of 2 Hz (Sine 2 Hz), Micropiles installed at 0 degrees exhibited the lowest peak bending moment, maximum axial load, and settlement values.

• Proceedings of the Korean Institute of Building Construction Conference
• /
• 2014.05a
• /
• pp.74-75
• /
• 2014

In remodeling business or construction of both new strucures and existing structures, Case that pile foundation was set is often. Micro pile, holding compressive force and tensile force by spherical friction, is supported by skin friction rather than end bearing capacity. but, This is weak in tension. Active area of micro pile's skin friction is narrow and micro pile don't do unification behavior hence. So bearing capacity was not fully mobilized in existing researching. In this study, in order to compensate for this method, micro pile to install Resisting Fixture is proposed.

• Proceedings of the Korean Geotechical Society Conference
• /
• 2001.03a
• /
• pp.355-362
• /
• 2001

Since micropiles were conceived in Italy in the early 1950s, which have been widely used for In-situ reinforcement, bearing pile or the concept of combination in the world-wide. The meaning of micropiles usually differs from that of a general deep foundation. Because the load capacity of it was mainly affected by skin friction. Also, it could be obtained the improvement effects of load capacity or ground's rigidity by the unitary behavior of ground and micropiles. In this study, The model tests were peformed on the dense sand where micropiles are set to the vertical direction. Strip footing was used in it. Steel bars of dia. 2 and 4㎜ were used in model tests of which the sand was attached on the surface, and the length of it was changed as 2B to 6B(where, B is width of strip footing) Through this process, the load capacity were analyzed from the test results in the relationship between load and displacement.