Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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| 2.3.3 Constructability Considerations Constructability of deep foundations must be addressed during design. If the design is not constructible as designed (i.e., foundation cannot be installed to the required depth or size, then it may not be possible to achieve the required lateral load resistance. Therefore, constructability, and the associated cost to overcome potential construction challenges, must be considered when selecting the type of deep foundation. Some aspects influencing the constructability of deep foundations are briefly discussed below. General Constructability Site access. Is the site accessible for large cranes that would be necessary for pile driving or large drilled shaft construction If site accessibility is an issue for large foundations, then smaller elements maybe more appropriate. If the site is relatively small or if headroom is limited, micropiles maybe the only feasible foundation type. Available space and accessibility for the foundation units. Sufficient space must be available for the construction of a pile or shaft group and the cap otherwise a single large diameter drilled shaft maybe more appropriate. Local practice or precedent. Some DOTs prefer a certain type of foundation element for certain applications. Experience of local contractors. Some foundation types, notably micropiles and large diameter drilled shafts, require contractors experienced with these types of foundations. Subsurface Conditions Obstructions. The presence of hard layers or obstructions may make the use of driven piles impractical. Foundation that can penetrate obstructions, such as drilled shafts or micropiles, maybe more appropriate. Depth to rock. Laterally loaded deep foundations require a minimum depth of embedment that may require penetration of rock if rock is present and relatively shallow. Driven piles and CFA piles may not be able to reach the minimum lengths in such conditions. 28 Groundwater conditions. High groundwater, artesian conditions, or flowing or running sands can bean issue for drilled shaft construction. Other foundation types, such as driven piles, maybe more appropriate under these conditions. Contaminated soils. If contaminated soils are present, driven piles maybe preferable because they do not result in spoils. Drilled shafts, micropiles, and CFA piles would all require disposal of contaminated spoils. Installation Pile driving considerations. – The ability to drive a pile (drivability) to the required tip elevations must be evaluated during design. Piles must have sufficient capacity to overcome soil resistance during driving and reach specified minimum pile penetration. – A pile must have sufficient structural stiffness and strength to withstand driving forces without damage. A drivability assessment is recommended in the current AASHTO specifications (2014). Equipment tooling and changes. Drilled shaft construction may require changing equipment or drilling tools to penetrate hard layers, clear obstructions, or penetrate rock. The need for casing and the need for additional equipment, such as casing oscillators, should be assessed as well as the need for slurry and a slurry plant if the wet method of excavation is used. Time for installation. Installation times vary by foundation type and may impact the selection of the foundation type. For example, groups of CFA piles maybe preferred to groups of driven piles or small diameter drilled shafts owing to faster production for installation, assuming the axial load capacity is not an issue. Construction testing. The need for construction testing should be considered. Drilled shafts may require multiple types of testing, such as integrity testing (e.g., CSL testing, axial load tests, and lateral load tests. Lateral load testing maybe easier on smaller elements, such as an individual pile fora design based on pile groups, compared to a design requiring large diameter drilled shafts. The required equipment, space, setup, and performance of the testing should be considered. Construction inspection. Inspection requirements vary by foundation type and some require specialized inspection techniques or procedures. Pile driving inspection is relatively simple. Inspection of drilled shafts constructed with slurry may require more specialized techniques to verify the alignment, proper bottom clean out and concrete placement. Effects on Nearby Structures or Public Perception Noise and vibration. Noise or vibrations due to pile driving can bean important factor that may limit the use of driven piles. In urban areas, downhole hammers or drop chisels for excavating rock sockets for drilled shafts may produce relatively high noise and vibration from a public perception standpoint in an urban setting. Noise or vibrations may also have negative impacts on marine environments. Disturbance of the adjacent ground. – Driving piles can densify, displace, or heave the adjacent ground and nearby structures. Low- displacement piles (e.g., steel H-section piles) or pre-drilling can be used to reduce disturbance around the pile. – Post-driving consolidation of cohesive soils may occur as excess pore pressures built-up during pile driving dissipate. – Installation of casing for drilled shafts using vibratory hammers may also cause densification and settlement of the adjacent ground that may potentially damage nearby structures. 29 Additional details and considerations regarding constructability can be found in Hannigan et al (2016) for driven piles, Brown et al. (2010) for drilled shafts, Brown et al. (2007) for CFA piles, and Sabatini, et al. (2005) for micropiles. Download 6.03 Mb. Share with your friends:
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