Micropiles Micropiles are small diameter piles (up to 300 mm), with the capability of sustaining high loads (compressive loads of over 5000 kN). The drilling equipment and methods allows micropiles to be drilled through virtually every ground conditions, natural and artificial, with minimal vibration, disturbance and noise, at any angle below horizontal. The equipment can be further adapted to operate in locations with low headroom and severely restricted access. The load is mainly accepted by the steel and transferred via the grout to the surrounding rock or soil by high values of interfacial friction with minimal end bearing components, as in the case for ground anchors and soil nails. The majority of micropiles are between 100 and 250 mm in diameter, 15 to 30 m in length and 250 to 1000 kN in compressive or tensile service load, although far greater depths and much higher loads are also being use. Applications For Structural Support Foundation for new structures Seismic retrofitting Underpinning of existing foundation Repair / Replacement of existing foundations Arresting / Prevention of movement Upgrading of foundation capacity B. For In Situ Reinforcement Embankment, slope and landslide stabilization Soil strengthening and protection Settlement reduction Structural stability. From Hayward Baker (A Keller Company) Micropiles Types/Classification Classification based on Behavior CASE 1 – The pile resists directly the applied loads. This is usually true for cases when individual piles or groups of piles are used. In this context a group is defined as a tight collection of piles, each of which is subjected to direct loading. When axially-loaded piles of this type are designed to transfer their load only within a remote founding stratum, pile head movements will occur during loading, in proportion to the length and composition of the pile shaft between structure and founding stratum. CASE 2 – Reticulated root pile structure. The concept of it is to support and stabilize by an interlocking, three-dimensional network of reticulated piles. This concept involves the creation of laterally confined soil/pile composite structure that can work for underpinning, stabilization and earth retention. Classification based on Method of Grouting Type A – Grout is placed in the pile under gravity head only. Since the grout column is not pressurized, sand-cement mortars, as well as neat cement grout, may be used. The pile drill hole may have an under-reamed base (to aid performance in tension). Type B – Neat cement grout is injected into the drilled hole as the temporary steel drill casing or auger is withdrawn. Pressure are typically in the range of 0.3 to 1 Mpa, and are limited by the ability of the soil to maintain a grout-tight seal around the casing during its withdrawal, and the need to avoid hydrofracture pressures and/or excessive grout consumptions. Type C – Neat cement grout is placed in the hole as for Type A. Between 15 and 25 minutes later, and so before hardening of this primary grout, similar grout is injected, once, via a preplaced sleeved grout pipe at a pressure of at least 1 Mpa. Type D – Neat cement grout is placed in the hole as for Type A. some hours later, when this primary grout has hardened, similar grout in injected via a preplaced sleeved grout pipe. In this case, however, a packer is used inside the sleeved pipe so that specific horizons can be treated, if necessary, several times, at pressures of 2 to 8 Mpa. Relationship between micropile application, design concept, and construction type Application Structural Support In Situ Earth Reinforcement Sub-application Underpinning of existing foundation. New foundations. Seismic retrofitting. Slope stabilization and excavation support. Soil strengthening Settlement reduction Structural stability Design concept Case 1 Case 1 and 2 Case 2 with minor Case 1 Case 2 Case 2 Construction Type Type A (bond zones in rock or stiff clays) Type B and D in soil (Type C only in France) Type A (Case 1 and 2) and Type B (Case 1) in soil Type A and B in soil Type A in soil Type A in soil Estimate of relative application Probably 95 % of total world application 0 to 5 % Less than 1 % -- Less then 1 % Design Aspects Analytically viewpoint: settlement, bursting, buckling, cracking, and interface considerations; Practical viewpoint: corrosion protection and compatibility with the existing ground and structure; Economical viability. Mode of Load Transfer The system must be capable of sustaining the anticipated loading requirements within acceptable settlement limits. Micropiles typically transfer load to the ground through skin friction as opposed to end bearing: a pile 200 mm in diameter with a 5-m-long bond zone has a peripheral area 100 times greater then the cross sectional area. This mode of load transfer directly impacts performance in that the pile movements needed to mobilize lateral friction resistance are of the order of 20 to 40 times less then those needed to mobilize end bearing. Micropile Components (Internal design) Reinforcing steel – the reinforcement to be use is dictated by the purpose of the micropile, its working load, and its permitted elastic deflection. For example, relatively small-capacity micropile designed to act only in compression usually comprise either a "cage" of high yield rebars supported by helical reinforcement, or very limited number of high strength bars. When such micropile have to act in tension, the reinforcement will consists upon high strength bars. Yet, there is no standard approach nationwide, as a consequence of the absence of national code, and the presence of various and differing local regulations and contractors preference. Deeper discussion presented by D. A Bruce (1994). Grout – most commonly the grout consists of simply cement and water. The type of cement will be dictated by the nature of the groundwater or by the strength/time requirements. Water should always be fresh and potable but with a chloride ion content of less then 500 mg/liter. Cement grout should be sufficiently fluid to allow efficient pumping and injection, and sufficiently stable to resist displacement and erosion after injection. The principal variable affecting the properties of cement grout is the water cement ratio (w:c). the amount of water determines the rate of bleeding, subsequent plasticity, and ultimate strength of grout. Typically w:c values of 0.45 to 0.55 are used. Fine sands can be added to neat cement-water suspensions to form an economical grout. Adixtures can be added in relatively small quantities to modify grout properties, especially to prevent shrinkage, to allow reduction in w, to accelerate or retard setting, and to prevent bleeding. Grout-Steel Bond – this interface is fundamental in that it is a mechanism of load transfer from steel to ground. Bond stresses are assumed to be uniformly distributed along the element. Bond values have typically been generated in tension testing and can be regarded as at least equally valid for the compressional sense. In the majority of cases, the grout-steel bond consideration does not govern the design: internal load capacity, or grout-ground capacity are the principal controls. Load transfer (external design) Grout-Ground Bond – Drilling and grouting methods used for micropiles act to produce excellent bond characteristics in the load-transfer zone. Codes or regulations are available (FIP, 1982; PTI, 1986; BS, 1989), and published data (Littlejohn and Bruce, 1977; Littlejohn, 1990; Xanthakos, 1991). Deeper discussion about the design of Single, Groups, and Network of micropiles are presented in the FHWA–RD–96 Volume II report. Design Parameters Construction Micropiles are installed mainly by two methods – drilling and grouting, or displacement. Piles which are driven are termed ‘Displacement Piles’ because their installation methods displace laterally the soils through which they are introduced. Conversely, piles that are formed by creating a borehole into which the pile is then cast or placed, are referred to as ‘Replacement Piles’ because existing material, usually soil, is removed as part of the process. Jet grouting and post grouting have recently been used to produce high- capacity piles. Drilling – the drilling method is chosen to impart minimal disturbance or upheaval to the structure or soil, while being the most efficient, economic, and reliable means of penetration. Micropiles holes must often be drilled through an overlying weak material to reach a more competent bearing stratum. They therefore typically require the use of overburden drilling techniques to penetrate and support weak and unconsolidated soils and fills. In addition, unless the bearing stratum is rock or a self-supporting material such as a stiff clay or marl, the drill hole may need temporary support for its full length. Grouting – Details of each type of grouting vary somewhat throughout the world, depending on the origins of the practice and the quality of the local resources. However, as general observation, it may be noted that: Grouts are designed to provide high strength and stability, but must also be pumpable. This implies water/cement ratios in the range of 0.45 to 0.5 by weight for micropile grout. Grout are produced with fresh water, to reduce the danger of reinforcement corrosion. The best quality grouts, in terms of both fluid and set properties, are produced by high-speed, high-shear mixers, as opposed to low-speed, low-energy mixers, such as those that depend on paddles. Lower pressure injection(to 1 Mpa) is usually effected via constant pressure, rotary-screw-type pumps; while higher pressure grouting, such as for Type C or D micropiles, usually requires a fluctuating pressure piston or ram pump. Deeper discussion about the construction process of micropiles presented in the FHWA–RD–96 Volume III report. Investigation before Construction Detailed knowledge of soil conditions, especially in the bond zone (e.g., the presence of thin partings of silt or sand in clay). Classification of the soil with respect to shear strength, compressibility, density, groundwater conditions, and chemical analysis. Consideration of the lateral variability as well as vertical changes. Location of key boreholes at the site extremities to permit interpolation as opposed to extrapolation. Other intermediate holes should have a maximum spacing of 20 m. Investigation during Construction Especially when dealing with potentially variable glacial soils, as much information as is reasonably and economically possible should be recorded during production drilling and grouting. Any other construction activities on the site that may impact on the micropiles performance (e.g., dewatering, blasting, superstructure changes).should be recorded and considered in the design. RESEARCH & DEVELOPMENT A five-year National Project termed 'FOREVER' (Fondations Renforcees Verticalement) has been undertaken by a French consortium under the aegis of the Institute for Applied Research and Experimentation in Civil Engineering (IREX) in cooperation with the Federal Highway Administration (Bruce et al., 1997). The project conducted under the technical direction of Professor Francois Schlosser and Dr. Roger Frank of the National Civil Engineering School (ENPC) involves research institutes, contractors, and governmental agencies. FOREVER includes desk studies, numerical modeling, laboratory testing (centrifuge) and Full scale field testing. Its chief objective is to promote the use of micropiles in all fields: deep foundations of new buildings and structures, stabilization of slopes and embankments, underpinning of existing foundations, and seismic retrofitting of retaining walls, and shallow foundations. Selected Minipile Case Studies  From Hayward Baker (A Keller Company) RECOMMENDED READING A. Benslimane, I. Juran and D. A. Bruce (1997). " Group and Network Effect in Micropile Design Practice". XIV International Conference on Soil Mechanics and Foundation Engineering. . Hamburg, Volume 2. pp. 767-770 Bruce, D.A., (1994), "Small Diameter Cast-in-Place Elements for Load Bearing and In Situ Earth Reinforcement", Ground Control and Improvement, John Wiley and Sons Inc., New York, NY, pp. 406-492 Barley, A.D. and Woodward, M.A., (1992), "High Loading of Long Slender Minipiles", Piling: European Practice and Worldwide Trends, Institution of Civil Engineers, London, England. Bruce, D.A. & Juran, I., (1997), "Drilled and Grouted Micropiles: State of Practice Review", FHWA-RD-96, Volume I-IV, NTIS, Springfield, VA 22161. Bruce, D.A. and Nicholson, P.J., (1 989), "The Practice and Application of Pin Piling", Foundation Engineering: Current Principles and Practice, Proceedings of the 1989 ASCE Foundation Engineering Congress, pp 272-290. D. A. Bruce, A. F. Dimillio, and I. Juran. (1997), "Micropiles: the state of practice Part I: Characteristics, definitions and classifications". Ground Improvement Journal Volume 1 Number 1, January 1997. Gularte, F.B. et al., (1996), 'Seismic Retrofit of the Fourth Street Viaduct with High Capacity Small Diameter Tiedown Piles", Proceedings of Deep Foundations Institute Annual Meeting and Conference, San Francisco, CA. Hayward Baker publication, Minipile, 1996. I. Juran, A. Benslimane and D. A. Bruce (1996). Slope Stabilization by Micropile Ground Reinforcement ". International Symposium on Landslides, Trondheim, Norway. Roberts, N.R., Darnell, D.E., Henry, J.F. and Hussin, J..D., (1989), 'Ground Modification Techniques Applied to Sinkhole Remediation", Proceedings of the 12th International Conference on Soil Mechanics and Foundation Engineering, Rio de Janeiro. Mitchell, J.K., et al. (1987), NSHRP Report No. 290, Transportation Research Board, 1987. ASCE Geotechnical Special Publication No. 69 (1997). "Ground Improvement, Ground Reinforcement, Ground Treatment Development 1987 - 1997". Edited by R. Schaefer.