Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
DESIGN CONSIDERATIONS AND CHANGES IN CONSTRUCTION
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| 13.3 DESIGN CONSIDERATIONS AND CHANGES IN CONSTRUCTION As construction occurs, considerations, impacts, or changes to design may include 1. Proposed design changes based on data available before construction value engineering or alternative design proposals by the contractor. This may require an entirely new design which would follow the design process above, or this maybe a change in one aspect of the design (i.e., pile type, drilled shaft size, etc. The requirements for lateral load design, such as design specifications, Strength and Service Limit State deflections, should be clearly communicated to the contractor to ensure that the proposed design will meet project criteria. Also, this should include required or suggested resistance factors (in some cases resistance factors maybe required by the agency or design specification, in other cases it will be a matter of judgment of the designer. 2. Consideration of means and methods of construction review and approval of means and methods of construction is typically part of the construction process, especially if aspects of the foundation performance maybe impacted by the means and methods that are selected. This maybe more of an issue for axial loading, but the potential impacts of the lateral loading capacity should be considered as well. For example, pre-excavation for removal of obstructions may reduce the lateral resistance of the soils within the depth of pre-excavation. 3. Design changes due to data received in construction data regarding subsurface conditions is received in construction through additional explorations as well as actual construction operations including installation of deep foundations, excavation, and groundwater control. a. In some cases, additional field investigation and testing are performed in construction. i. Ina design-bid-build approach, this maybe a requirement for the construction contractor, such as performing borings at certain locations that were inaccessible prior to construction. For example, performing a boring at every drilled shaft location in order to verify the quality of rock within the socket depths. It is worth noting that deferring part of the subsurface investigation until the construction phase in a design-bid-build contract introduces a significant risk element into the construction contract. If conditions in the construction phase borings are found to be significantly different than assumed in the design, the design may need to be revised, which will likely adversely impact the construction schedule and costs. 199 Inclusion of means to pay for construction phase investigations and a means to adjust the payment of the foundations should be included in the contract terms for this scenario. ii. Borings may also be performed by the contractor to investigate conditions for design of temporary works, a value engineering design, or for data regarding a possible differing site condition. iii. Fora design-build approach, additional investigations are usually performed to develop a design, to finalize a preliminary design shown in the bid documents, and to investigate opportunities to optimize the design, in addition to the purposes stated above. Typically, in design-build contracts, the final subsurface investigation program must be deferred until the locations of the bridge foundations are determined by the design-build contractor’s design consultant. b. Deep foundation design is often impacted by data received during construction of the foundations. Deep foundation design inherently includes some aspect of the observational method because the ground conditions at each particular foundation site may vary from the conditions or the behavior expected, and such variations are typically not apparent until construction inspection and/or verification testing is performed. i. An example of this would be where the ground is weaker than anticipated. The lateral capacity may need to be reviewed, reevaluated, and the design possibly adjusted. Construction testing, including lateral load tests, maybe needed. These conditions may include conditions where weak strata are thicker than anticipated or geotechnical resistance is less than anticipated, as evidenced by investigations, testing, or by construction inspection records (pile driving, etc. ii. Another example could be where rock is higher or lower than anticipated. If rock is higher than the anticipated tip depth of deep foundations that were not designed to be socketed into rock (driven piles, CFA piles, or drilled shafts in soil, then a change in the design or construction means and methods maybe necessary. A higher rock elevation would result in a shorter pile or shaft length which may not satisfy the design requirements for lateral loading. A change in pile size or a requirement to add rock sockets or pre-drilling into rock maybe required. Where rock is lower than expected and the foundations were to be socketed into rock, the condition is similar to that described above where the weaker material overlying rock is thicker than anticipated and may impact the lateral capacity of the foundations. c. Design changes maybe made as a result of construction load testing. If lateral load testing is performed, either as a requirement of the contractor at the election of the contractor to optimize the design (more common for design-build but could also be part of a value engineer design or for claim resolution for design-bid-build), the results of the lateral load test will provide data that can be used for verification or updating the design of the foundations. Design updates maybe to accommodate actual performance that is better than or worse than the anticipated performance. Download 6.03 Mb. Share with your friends:
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