MICRO-PILE: RECENT ADVANCES AND FUTURE TRENDS

Transcription

1 MICRO-PILE: RECENT ADVANCES AND FUTURE TRENDS Jaydip V Chaudhari 1, Jayeshkumar Pitroda 2, J.J.Bhavsar 3 1 Student of first year M.E. (C.E M), B.V.M Engineering College, Vallabh Vidyanagar 2 Assistant Professor and Research Scholar, Civil Engineering Department, B.V.M. Engineering College, Vallabh Vidyanagar-Gujarat-India 3 Associate Professor, Civil Engineering Department, BVM Engineering College, Vallabh Vidyanagar, Anand, Gujarat India. Abstract: Micro piles have been used effectively in many applications of ground improvement to increase the bearing capacity and reduce the settlements particularly in strengthening the existing foundations. Advantages of Micro piles are high carrying capacity, less site constraint problem and self sustained operation. This piling system is therefore attractive to both the client and foundation designer. Apart from this the light and compact drilling rigs other ancillary equipment, like grout mixer and grout pumps, is very compact size. Micro installations can penetrate to hundreds of feet in depth; each of the piles can support many tons of loads. Micro piles transfer loads through liquefiable soil to competent bearing strata to conform to the design requirements. This paper deals with a case study in which micro piles of 100 mm diameter and 4 m long have been used to improve the bearing capacity of foundation soil and in the rehabilitation of the total building foundation system. The micro piles were inserted around the individual footings at inclination of 70 with the horizontal. Micro pile technology is a reliable pile system that can withstand large capacity axial or lateral loads with minimal disturbance to the existing structures. They became very popular due to their ability to transfer loads efficiently through skin friction and due to their many installation advantages over a conventional pile system. Micro piles have the capability of combining micro pile technology with one or more of the other ground improvement techniques to meet unique or complex project requirements cost effectively and efficiently. This paper explores these recent advances and looks beyond to the newest developments and future advances envisioned for micropiles. Keywords: Micro piles, ground improvement, technology, construction 11th April, 2015, S.N. Patel Institute of Technology & Research Centre, Umrakh, Bardoli 1

2 INTRODUCTION Micropiles are smalldiameter piles that are sometimes called minipiles, root piles, pin piles or needle piles. The conceptual idea behind this important technological development was to create a type of pile that would be able to carry large loads while causing minimal vibration or disturbance to in situ materials at the time of installation. The rigs required to install them are often relatively small. Because of these important advantages, micro piles have been widely used in seismic retrofitting, in the rehabilitation of foundations of structures that are very sensitive, and in locations with low headroom and severely restricted access conditions. Furthermore, micropiles have been increasingly used, not only as underpinning foundation elements, but also as foundations for new structures. Prevalent design methods for micropiles are adaptations of methods originally developed for drilled shafts. However, installation of micropiles differs considerably from that of drilled shafts, and micropiles have higher pile length to diameter ratios than those of drilled shafts. Improved understanding of the load-transfer characteristics of micropiles and the development of pile settlement estimation tools consistent with the load-transfer response of these foundation elements is needed Micropiles, are often used for underpinning. They are also used to create foundations for a variety of project types, including highway, bridge and transmission tower project. They are especially useful at sites with difficult or restricted access, or with environmental sensitivity. Micropiles are made of steel with diameters of 60 to 200 mm. installation of micropiles through top soil sand and cobblestones overburden and into soil rock can be achiveved using air Rotary or Mud Rotary or Mud drilling, impact driving,jacking, vibrating or screwing machinery. Strengthening of foundations of existing buildings in earthquake prone megacities and addition of new floors to existing buildings due to sky- high land values in big cities, usually need excavation and temporary support system below the foundation level. This is difficult not only due to limited head room and access in congested area but also for the risk of collapse of the structure during the excavation process itself. Moreover, underground spaces need to be utilized for alternate modes of transport. Micropiles may be used economically in such situations. Micropile are small diameter (less than 300 mm) and 20 to 30 m in length, grouted drilled pile. The grout is either placed or injected under pressure (grouting pressures above 0.8 to 1 Mpa). It consists of a continuously threaded hollow 11th April, 2015, S.N. Patel Institute of Technology & Research Centre, Umrakh, Bardoli 2

3 reinforcing tendon as a load carrying steel member together with a grout body of furnace (Portland) cement which allows transfer of tension and compression forces mainly from the friction of the threaded tendon via the grout body into the surrounding soil. Due to high pressure grouting there is insignificant shrinkage between the pile and the soil. The penetration of the fluid part of the cement mix into the surrounding soil creates a transitory zone between the body of the micropile and the soil leading to a strong grout/ground bond. Due to this reason ultimate load carrying capacity of micropile is higher than anticipated capacity based on conventional bearing capacity theory. Micropiles are typically used for structural support. It is usually installed for bridge and building foundations supports and seismic retrofits. Another main application of micropile is soil reinforcement e.g. slope stabilization/earth retention projects. The basic philosophy of micropile design differs little from that required for any other type of pile. The system must be capable of sustaining the anticipated loading conditions with the pile components operating at safe stress level and with resulting displacements falling within acceptable limits. For conventional piling systems, the large cross sectional area results in high structural capacity and stiffness, hence the design is normally governed by the geotechnical load carrying capacity. Due to the micropiles small cross sectional area the micropile design is more frequently governed by structural and stiffness considerations. The emphasis on structural pile design is further increased by the high grout to ground bond capacities that can be attained using pressure grouting techniques. HISTORY OF MICROPILE - The first use of micropiles dates back to the early 1950 s in italy - the mid 1970 s a number of US specialty foundation contractors previously engaged in drilling and grouted anchor work had developed their own variants of the technology. - There has been a rapid growth in the in the specification and use of micropile in the US since the mid 1980 s to early 1900 s partly as a result of FHWA research efforts. -In the early 2006 and 2007 respectively the international Building Code and the AASHTLRFD Design Specification incorporated design code section for micropiles thus making way for further expansion of applications in both building and highway construction. -Micro pile currently are widely specified and used in all construction sector worldwide. 11th April, 2015, S.N. Patel Institute of Technology & Research Centre, Umrakh, Bardoli 3

4 ADVANCES IN CONSTRUCTION PRACTICE Specialists in the use of cement additives and admixtures are also coming into favor. Their expertise is being called upon to help design grouts of superior fluid and set properties. Care should be taken, however, to ensure that certain of the rheological properties, which can be imparted by such components, are not detrimental to the in situ performance of the grout. For example, an extremely thixotropic grout may not be able to penetrate into fissures or pore spaces or may not have good adhesive properties to the steel reinforcement. All such innovative mix designs must be fully evaluated prior to routine use. Manufacturers of threaded casin are also increasingly valuable sources of technical information, especially regarding the behavior of their threaded sections when subjected to tensile or bending stress. Likewise the suppliers of self drilling bars have conducted extensive in-house research to show prospective customers that fears of corrosion, for example, can be allayed in practice, given the special conditions of the installation process and the configuration of the bar deformations. FUTURE ADVANCEMENTS UNDERWAY/ENVISIONED The commercial advancement of micropiles has been an impetus to the material suppliers developing new pile materials, drilling methods and monitoring equipment. Advancements such as improved duplex drilling tools, rotary-sonic drilling, and hollow core bars, appear to be commercially viable. We are no longer seeing micropiles as having limited application to restricted access sites. We are now seeing very high capacity micropiles competing successfully with more traditional driven pile and drilled shaft systems. When comparisons are made based on cost per kn, micropiles are a very attractive solution for even new construction on open sites. New contractors are obtaining the tooling to construct micropiles. As such, we will continue to see this commercial acceptance expand. 11th April, 2015, S.N. Patel Institute of Technology & Research Centre, Umrakh, Bardoli 4

5 CONSTRUCTION OF MICROPILES Drilling Techniques The drilling method is selected on the basis of causing minimal disturbance. The ground and nearby sensitive. Structures and able to achieve the required drilling performance. In all drilling methods, drilling fluid is used as a coolant for the drill bit and as a flushing medium to remove the drill cuttings. Water is the most common drilling fluid compared to other drilling fluid such as drill slurries, polymer, foam and bentonite. Another type of flushing medium is using compressed air, which is commonly used in Malaysia. Single tube advancement: By this method, the toe of the drill casing is fitted with an open crown or bit, and the casing is advanced into the ground by rotation of the drill head. Water flush is pumped continuously through the casing, which washes debris out and away from the crown. Rotary duplex: With the rotary duplex technique, drill rod with a suitable drill bit is placed inside the drill casing. It is attached to the same rotary head as the casing, allowing simultaneous rotation and advancement of the combined drill and casing string. Rotary percussive duplex: Rotary percussive duplex systems are a development of rotary duplex methods, whereby the drill rods and casings are simultaneously per cussed, rotated, and advanced. The percussion is provided by a top-drive rotary percussive drill head. This method requires a drill head of substantial rotary and percussive energy. (Shown in figure 1) Fig 1 Schematic representation of the six generic overburden drilling methods 11th April, 2015, S.N. Patel Institute of Technology & Research Centre, Umrakh, Bardoli 5

6 Grouting Micro pile grouting equipment consists at a minimum of a colloidal high speed, high shear mixer, holding tank grout pump capable of reaching pressures of 300 psi, pressure gauges, recirculation lines. The colloidal mixer is a high-shear grout plant that is capable of rapidly mixing neat cement based grout in a few minutes, with a thorough wetting of the individual cement grains. A thorough wetting allows a low water-cement ratio grout to be pumped easily through the grout lines that run from the plant to the pile. Without a colloidal plant, clumps of cement will cling together, clogging injection lines, and ultimately yielding a lower strength grout, because significant amounts of the cement grains are not hydrated. Following a thorough mixing, the grout must be stored in an agitation tank with agitation blades that constantly stir the mixed grout, prolonging separation of the cement from the mix water. With proper admixtures, grout life may be extended easily to a working time of 6 hours, and in some cases, may be suspended indefinitely until the reaction is re-initiated on demand. A Marsh Cone simplifies optimization of the grout mix design with respect to retarders and fluidifiers. A Marsh Cone time of seconds is typically considered ideal for pumping into micro pile packer systems Installation T h e micro pile contractor must keep accurate and contemporaneous logs showing detailed installation information. Boring logs are used for planning the pile installation, but in karst conditions, rock head may vary greatly within a few feet of the boring. In karst, it is conceivable that each pile will be unique in overall length, unbonded length, depth to rock head and sequencing of adequate bearing stratum.the pile installation log is the only record of the verification of the pile design dimensions and compositions having been achieved. The pile installation log is also generates the pay items for the project, such as drilled footage in soil as well as rock, and amount of grout consumed. The contractor must be able to verify through his logs that the design requirement of a given amount of rock has been encountered in order to substantiate that a pile of the design capacity has been installed. 11th April, 2015, S.N. Patel Institute of Technology & Research Centre, Umrakh, Bardoli 6

7 CASE STUDY Micropile under pinning Mandalay bay hotal (shown in figure 2) and casino Micropiles are small diameter high capacity drilled and grouted piles that are ideal for building foundations on sites with poor ground conditions, sensitive surroundings, re- stricted vertical clearance, or difficult access. They are also well suited for foundation under- pinning, arresting ground movements, and increasing the capacity of existing foundations. This paper provides a case history of the emergency installation of over 500 micropiles to underpin the 43 story Mandalay Bay Resort and Casino in Las Vegas, NV prior to completion of final construction. In all, inch diameter micropiles, were drilled and grouted over their full 200 foot length both outside and inside of the pipe Due to the full bonding, the piles acted as both ground reinforcement and structural supports to significantly reduce the rate of settlement of this major structure. Each pile was tested to 600kips = 1.5 times design load. The major construction feat was the installation of about 110,000 lineal feet of high capacity piles in a very small plan area, with only about 20 feet of overhead clearance, all in about 2-1/2 months. All of the 536 load tests and attachment frames were completed only about 4 weeks after the last pile was drilled, at which time the nominal load of 50 kips was applied to all the piles. This entire time, crews worked together in one space performing Figure : 2 Mandalay bay hotal Advantages of micropile [1] Micropile pile rigs are small and able to get into places other deep foundation rigs can t and can be used in soil or bedrock. 11th April, 2015, S.N. Patel Institute of Technology & Research Centre, Umrakh, Bardoli 7

8 [2] Micro piles are installed in threaded sections of any length. This makes micro piles ideal for low headroom installations. [3] They can be installed for new construction or for existing foundation remediation or enhancement. [4] Micro piles cause minimal vibration and noise cause minimal disturbance to structures. [5]Micro piles produce minimal amounts of spoil. The typical micro pile is five, seven or nine inches in diameter - much smaller than other drilled foundations. [6] Micropiles are often used to underpin the existing need of minimal vibration or noise is structure where prime importance [7] Micropiles can be easily laid where low head room is a constraint [8] Micropiles can be easily installed at any angle below the horizontal using the same equipment used for ground anchors and grouting projects. [9] Offer a practical and cost-effective solution to costly alternative pile systems as well as a solution to job sites with difficult access. Disadvantages: [1] Underground objects or boulders can block or deviate the pile [2] Corrosion can occur [3] Relatively expensive [4] The hammer is noisy and the vibrations can disrupt or damage adjacent structures [5] Requires a lot of headroom CONCLUSIONS The following conclusion comes through the study of Micropile technology is gaining rapid acceptance. Although significant advances continue to come as a product of contractor innovation, significant efforts have been expended and are underway by 11th April, 2015, S.N. Patel Institute of Technology & Research Centre, Umrakh, Bardoli 8

9 different entities to accommodate the ever changing applications of micropiles. Advances have been realized in the form of increased capacities, and introduction of new materials and design methods for integrating micropiles into existing projects. As demonstrated by FOREVER, JAMP, and numerous other commercial projects completed to date, micropiles provide viable solutions to difficult problems. Although sometimes designers wish to accommodate every conceivable loading condition, it is usually more appropriate to follow design procedures that balance safety and economic concerns.further research is necessary on several aspects of micropile behavior. The success of these efforts will be fundamental for future development of micropile technology and to create new areas of application. micropile technique is now widely used in rehabilitation works and is likely to be used even more in the future for strengthening and foundations. ACKNOWLEDGMENT The Authors thankfully acknowledge Dr. C. L. Patel, Chairman, Charutar Vidya Mandal, Er.V.M.Patel, Hon. Jt. Secretary, Charutar Vidya Mandal, Dr. F.S.Umrigar, Principal, BVM Engineering College, Dr. L. B. Zala, Professor and Head, Civil Engineering Department, BVM Engineering College, Prof. J. J. Bhavsar, Associate Professor and P.G. Coordinator (Construction Engineering and Management), B.V.M. Engineering College, Mr. Yatinbhai Desai, Jay Maharaj construction, Vallabh Vidyanagar, Gujarat, India for their motivations, infrastructural support and cooperation to carry out this research. REFERENCES [1] FHWA-SA Micropile Design and Construction Guidelines [2] Lizzi F (1988) The pali radice (root piles) symposium on soil and rock improvement techniques including geotextiles reinforced earth and modem piling Methods Bangkok D3 [3] Marek, A.R., Muhunthan, B., FHWA Supported Structures Research Seismic Behavior of Micropiles, Research Report, 2005, Washington State Transportation Center (TRAC), Washington, USA [4] plumelle c.(1984) improvement of the bearing capacity of the soil by inserts of group and reticulated micropiles int emotional symposium on in-situ-reinforcement of soil and rocks, paris, [5] Sridharam A,and Murthy, B.R.S (1993). Remedial measures to building settlement problem proceeding of third Internationl conference on case histories in Geotechnical Engineering, st.louis, Missouri [6] Venice Charter (1964), II nd International Congress of Architects and Technicians [7] 11th April, 2015, S.N. Patel Institute of Technology & Research Centre, Umrakh, Bardoli 9

10 AUTHOR S BIOGRAPHY Chaudhari jaydip was born in 1988 in Vyara City. He received his Bachelor of Engineering degree in Civil Engineering from the L D Collage of Engineering, Gujarat University, in At present, he is First year student of Master's Degree in Construction Engineering & Management from Birla Vishvakarma Mahavidyalaya, Gujarat Technological University. He has published papers in National Conferences. Prof. Jayeshkumar R. Pitroda received his Bachelor of Engineering degree in Civil Engineering from the Birla Vishvakarma Mahavidyalaya, Sardar Patel University in In 2009 he received his Master's Degree in Construction Engineering and Management from Birla Vishvakarma Mahavidyalaya, Sardar Patel University. He joined Birla Vishvakarma Mahavidyalaya Engineering College as a faculty in 2009, where he is Assistant Professor of Civil Engineering Department with a total experience of 14 years in the field of Research, Designing and education. He is guiding M.E. (Construction Engineering & Management) Thesis work in the field of Civil/Construction Engineering. He has published many papers in National Conferences and International Journals. Prof. Jaydevbhai J. Bhavsar received his Bachelor of Engineering degree in Civil Engineering from the Birla Vishvakarma Mahavidyalaya, Sardar Patel University in In 1986 he received his Master's Degree in Building Science and Technology from University of Roorkee. He joined Birla Vishvakarma Mahavidyalaya Engineering College as a faculty where he is an Assistant lecturer of Civil Engineering Department with a total experience of 32 years in the field of Research, Designing and education. He is guiding M.E. (Construction Engineering & Management) Thesis work in the field of Civil/ Construction Engineering. He has published many papers in National Conferences and International Journals. 11th April, 2015, S.N. Patel Institute of Technology & Research Centre, Umrakh, Bardoli 10