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Retofit Foundation

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  DEVELOPMENT OF SEISMIC RETROFITTING TECHNOLOGIESFOR EXISTING FOUNDATIONS 1 2 3 Masahiro NISHITANI , Takeshi UMEBARA and Jiro FUKUI Abstract Seismic retrofitting and liquefaction prevention measures on an existing bridge foundation are performed under severe execution conditions. For example, incities, they are done close to other structures and in narrow spaces under girders.Model experiments, analyses and trial executions have been carried out to developseismic retrofitting technologies that are economical and can be used easily under such severe conditions. As a result, the authors have developed five rational seismicretrofitting technologies with superior execution properties under severe conditions. Introduction Since the Seismic Design Specifications for Highway Bridges were revisedafter the Hyogo-ken Nanbu Earthquake of 1995, the need for seismic retrofitting of existing structures has been increasing. For bridge piers, reinforcement methodsusing steel plate lining and so on had been established, but it is necessary to retrofit bridge foundations in order to improve the seismic performance of a whole bridge.When an existing foundation is retrofitted, it is difficult to obtain sufficient workingspace, because of restrictions by under-girder space and nearby structures.Consequently, it is difficult to apply usual methods such as increasing additional piles or ground improvement. For three years since 1999, Public Works ResearchInstitute has, with 13 private companies, carried out the joint research to developseismic retrofitting methods and liquefaction prevention measures that are notrestricted by site conditions, even when executed directly below an existing bridge.As a result, the authors have developed five rational seismic retrofittingtechnologies with superior execution properties under severe execution conditions.This report introduces these five seismic retrofitting technologies that wereestablished by the joint research. Outline of Seismic Retrofitting Technologies for Existing Foundations Through the joint research, design and execution methods for three methods,micropile methods, Small Pipe Drain method and SSP method, were developed asseismic retrofitting technologies for use in places with severe execution conditions.Senior Research Engineer, Foundation Engineering Team, PWRI 1 Research Engineer, Foundation Engineering Team, PWRI 2 Team Leader, Foundation Engineering Team, PWRI 3  The micropile methods includes High Capacity Micropile, ST Micropile andMulti-Helix Micropile methods. Micropile Methods A micropile is a pile with steel pipes that has a diameter of 300 mm or less.Figure 1 shows an image of retrofitting for an existing foundation by a micropilemethod. The followings are the features of the micropile methods developed by the joint research.1 Because the methods are executed by gradually lengthening the piles byconnecting short steel pipes with threaded joints, they can be executed in a locationwith under-girder height of only 3.5 m.2 Because the execution machinery is small and can be moved easily, themethods can be executed in a space with the width of a single vehicle lane about3 m .3 The methods produce little noise and vibration during execution.4 The piles can be driven at an angle up to about 15 degrees.5 The methods can be applied in liquefied ground.6 Because of the pressure injection of grout, using ground improvement together or blades, the methods can guarantee large bearing capacity regardless of a smalldiameter.7 Because the piles have a small diameter, the methods can minimize theexpansion of a footing width.The followings are the outlines of three micropile methods.1 High Capacity MicropileHigh Capacity Micropile method can be counted on to provide high skinfriction capacity because deformed bars and high strength steel pipes are used, andgrout is pressure injected into bearing layer. This method can be applied to variouskinds of the ground including soft ground, gravel ground and rock ground. Figure 2is the outline of High Capacity Micropile. High Capacity Micropile is used inUnited State as a retrofitting method for existing foundations.2 ST Micropile ST stands for Strong and Tube-fix . ST Micropile method is executed byimproving ground by pressure injecting and agitating cement milk, boring a newhole in the center of the improved ground, inserting steel pipes into the hole, andfinally injecting cement milk into the space of the hole. This method does notdisrupt the natural ground, because it is preceded by ground improvement. Figure 3shows the outline of ST Micropile.3 Multi-Helix Micropile  Multi-Helix Micropile method is executed by attaching four blades of differing diameters in a tapered pattern to the tip of a steel pipe at a interval, thenrotating the steel pipe to insert it directly into the ground. Because this method usesrotating thrust to insert the steel pipe, it can be executed without removing surplussoil. Figure 4 shows the outline of Multi-Helix Micropile. Screen Pipe Drain Method Screen Pipe Drain method is a method of preventing liquefaction bydispersing excess porewater pressure generated during an earthquake. This methodis executed by using a small machinery, and placing small diameter steel pipes 50to 100 mm diameters with water drainage functions Screen Pipe, Figure 5 atintervals between 500 and 1,500 mm in saturated sandy ground. But depending onthe level of seismic force, it is necessary to use additional piles together. Thefollowings are the strong points of the method.1 The method can be executed at a height of about 3 m under girders.2 Because the execution machinery is small and easily moved, the method can beused in a space with a width of 2 m.3 The execution can be performed little noise or vibration.4 Screen Pipe can be inserted at any angle from vertical to horizontal.5 The method can be executed without removing any soil.6 Screen Pipe has high drainage performance and can be counted on to providecountermeasure effects equal or superior to those of a gravel drain method. SSP Method SSP stands for Super Strengthening Pile Bents . SSP method is a methodof retrofitting existing pile bent foundations. This method is executed by placingnew steel plates around an existing pile and continuously pressure inserting them tothe stipulated position, then injecting grout between the existing pile and new steel plates. Figure 6 shows the outline of this method. The followings are the strong points of the method.1 The method can be executed if, when executed over water, the height from the bottom of girders to water surface is 2 m, and when executed over land, there isapproximately 2.5 m of a space under girders.2 The execution generates little noise or vibration because of just inserting steel plates.3 The method can be applied in liquefied ground.4 The method hardly obstructs the cross section area of a river. Results of Experiments and Design Method for Micropile This part presents the results of experiments and analyses performed todevelop micropile methods, and a design method proposed based on these results.  Static Horizontal Loading Tests on Models of Group Piles 1 Outline of Loading TestsTo retrofit an existing foundation with micropiles, many micropiles areinstalled around existing piles and linked to a footing. But the seismic retrofittingeffects of a pile foundation that is retrofitted by micropiles are not fully confirmed.So to clarify the behavior of a group piles with differing diameter piles, statichorizontal loading tests on models of group piles had been performed. Andsimulation analyses based on the ductility design method had been performed todevelop a design method that can be used to rationally perform retrofitting designof a pile foundation retrofitted by micropiles.The model used for the loading tests is approximately 1/5 of the size of anactual pile foundation. The diameter and thickness of a steel pipe used to representan existing pile were 114.3 mm and 3.5 mm respectively, and the diameter andthickness of that used to represent a micropile were 34.0 mm and 2.3 mmrespectively. Strain gauges were attached to the surface of each pile to measure itsstrain. The top of each pile was rigidly connected to a steel footing. And the tips of the model piles were fixed to the bottom of the test pit. The loading tests weredone for seven cases shown in Table 1. Figure 7 shows the loading test for Case-3.The test ground was prepared by firstly setting models, then inserting sand into thetest pit by a grab bucket every 300 mm of thickness and compacting the sand by atamper. The loading tests, which were carried out by a multi-cycle load controlmethod, were performed by pulling a PC steel bar fixed to the footing with a center hole jack.2 Results of Loading TestsFigure 8 shows the load and displacement curves for every case. Comparingthe results of Case-4 to Case-6 with the result of Case-3 with no micropiles, it isconfirmed that the micropiles provide retrofitting effects. A comparison of theresults of Case-4 and Case-5, which were performed under the differing intervals between the existing piles and micropiles, reveals no any marked difference in theretrofitting effects by the pile interval. Case-6 was a case that the micropiles wereinstalled incliningly. By comparing the result of Case-6 with the results of Case-4or Case-5, it is confirmed that the retrofitting effects by the micropiles in Case-6are clearly greater than those in Case-4 or Case-5.The simulation analyses were performed based on the ductility design methodstipulated in the Design Specifications for Highway Bridges. In the analyses,compensation factors were set to account for the effects of group piles as the testresults were reproduced. Figure 9 compares the load and displacement curves of thetest results with those of the analytical results for Case-3 to Case-6. As shown inFigure 9, by appropriately setting the compensation factors in Case-3, the load anddisplacement curve were reproduced with good precision. It was also reproduced in

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Jul 23, 2017
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