Micropiles Specifications

Summary of Example Specifications

<h2>Summary of Example Specifications</h2>Three of the reviewed specifications have been selected for inclusion in this document. These specifications are representative of typical micropile specifications but should be tailored for project specific details and adjusted to include the recommended details found in the Design Guidance and QC/QA Procedures documents for this technology. <div class="grayed-title subsection">Specification Name/Number: CT DOT Micropiles Specification</div>Reference(s): Connecticut Department of Transportation (2009)The document provides a good discussion of construction, material, design, and QC/QA requirements but is not as comprehensive as other reviewed specifications. This specification is similar to the FHWA specification. The specification is easy to read and the sections are logically ordered. The majority of the risk lies with the contractor given the contractor is responsible for designing and installing capable micropiles. Because this is a design-build, performance based specification this is appropriate risk allocation. The specification allows the project to be fairly bid because it clearly describes the design procedures to be used and the QC/QA verifications that the contractor must comply with. The specifications do not require overly elaborate construction methods. QC/QA verifications include submittal of micropile installation records to the engineer, grout density/strength testing and verification/proof testing of the micropiles. A detailed description of the testing procedures and equipment is provided in the specification. The micropile testing section includes directions and actions that can be taken if verification/proof tests fail. The sections in this specification are relatively brief compared to other specifications that were reviewed. The specification lacks subsurface conditions and equipment sections. <div class="grayed-title subsection">Specification Name/Number: DFI-ADSC Micropile Model Specification</div>Reference(s): DFI-ADSC (2002)This specification can be modified to serve as a method or performance approach specification. This document is one the most complete specifications reviewed for this technology. However, it is written as a specification for private sector projects and it may need significant modification for application to DOT projects. The specification is intended for Case 1 micropiles where the micropiles accept the load directly and can be used for structural, slope stability, and earth retention applications. According to the document “groups or networks (Case 2) of micropiles as used for slope stabilization, ground improvement or support of excavation are not addressed, although much of the specification would equally apply.” The specification is easy to read and logically ordered. The risk allocation is a function of how the Model Specification is adapted. It can be adapted as a performance specification or a prescriptive (method) specification. The risk allocation appears to be fairly allocated for each of these cases. The specification is thorough and no additional information is necessary to develop a fair bid. There is a clear discussion of actions the contractor can take if the proof/verification tests fail. The specifications do not require overly elaborate construction methods. The QC/QA verifications include grout testing, installation records and pre-construction/production testing in full detail. Acceptance criteria are based on these performance tests. The specification also discusses the actions the contractor should take if the tests fail. If the specification is used as a prescriptive specification (method approach specification), the engineer will determine the reason for the failed load test and adjust the installation methods as needed to complete the job. The specification does not refer to a specific geotechnical design method or preferred design document to be used. This may be because the specification is not intended for DOT use. It should be noted that this is otherwise a very comprehensive specification and it includes helpful commentary throughout. The specification includes sections on determining the structural capacity of micropiles in axial and lateral loading. The specification includes discussions of load testing (both axially and laterally), but does not include geotechnical design guidance for either. <div class="grayed-title subsection">Specification Name/Number: FHWA Micropile Specification</div>Reference(s): Sabatini et al. (2005)The specification contains helpful commentary on the design, material, construction, and QC/QA requirements. Proof and verification testing are discussed in great detail. The owner is responsible for providing load combinations for static and seismic cases, geotechnical information, and identification of weak ground zones which cannot be relied on to contribute towards pile capacity. The contractor is responsible for providing the footing, pile structural, pile geotechnical, and the pile connection design. This specification is intended for structural support applications. This specification allows for a varying amount of design input from the owner-engineer and is therefore considered to be a performance/method approach specification. The specification is easy to read and logically ordered. According to the document “the selected micropile Contractor shall select the micropile type, size, pile top attachment, installation means and methods, estimate the ground-grout bond value and determine the required bond length and final micropile diameter. The micropile Contractor shall design and install micropiles that will develop the load capacities indicated on the contract plans. The micropile load capacities shall be verified by verification and proof load testing as required and must meet the test acceptance criteria specified herein.” As a result, the contractor assumes a majority of the risk, which is typical of micropile performance approach specifications. No additional information is required to make an informed bid. The specifications do not require overly elaborate construction methods. QC/QA verifications are clearly laid out and include material testing, micropile installation records, and load testing. These items are necessary for a micropile construction specification. The specification discusses the event of load test failure where the contractor must adjust the design/construction methods. The specification does not provide guidance for load test failure if the specification is adjusted to be a purely method approach specification. This specification is complete. It includes a helpful definitions section, commentary, and a list of ASTM and AASHTO testing references in the document. This specification can easily be adapted to different levels of owner-design. Recommendations for these modifications are provided in the commentary.