Micropile Foundation Design MinimizesEnvironmental Impact for Southern California EdisonProject by Chih-Hung Chen, P.E., Burns & McDonnell,

and Nick Salisbury, Crux Subsurface, Inc.

Micropiles are one of the deep foundation systemsthat fall under the ADSC Technical product umbrella.The following article which describes the use of mi-cropiles as a deep foundation solution in a very remoteenviroment comes to Foundation Drilling magazinefrom ADSC Contractor Member, Crux Subsurface,Inc., Spokane, Washington.(Editor)

For construction within the Angeles National Forest, the U.S.Forest Service requires minimal disturbance of the land. An un-conventional design and construction solution helped South-ern California Edison (SCE) gain approval from the U.S. ForestService.

Background

According to a report published by Southern California Edi-son, entitled “The Future of Transmission,” California has astrong appetite for electricity. And its residents increasinglyfavor a supply produced through renewable resources such aswind.

(SCE) is one of the utilities that leads the pack toward meet-ing state mandates for renewable sources of power, having put

contracts in place for more than 20 percent of delivered powerto come from renewable sources. A big part of SCE’s effort is theTehachapi Renewable Transmission Project (TRTP), which will

deliver renewable energy to customers in southern California.The Tehachapi Wind Resource Area is in Kern County, north

of Los Angeles. The route from the wind farm area to the elec-trical grid traverses a range of land uses from urban residentialto rural farmland. The terrain varies from flat, high desert torugged mountains. Much of the construction for the TRTP takesplace in remote, inaccessible locations. Critical segments crossthe Angeles National Forest.

To construct a 500-kV transmission line in California andacross land managed by the U.S. Forest Service, SCE and itsowner’s agent, Burns & McDonnell, faced a level of scrutiny notexperienced when most existing lines were constructed twentyor more years ago. The environmental measures required to per-mit and begin construction on the line may be a harbinger ofthings the electrical transmission industry must be prepared tomanage as upgrades to the grid are made across the country inthe coming decades.

Not only in this instance but in general, transmission line pro-jects are facing higher levels of scrutiny than ever before, and theTehachapi Renewable Transmission Project in Kern Countynorth of Los Angeles is no exception. The new 500-kV linecrosses the Angeles National Forest, where the U.S. Forest Ser -vice has strict requirements. For example, for construction pro-jects within the Angeles National Forest, the Forest Servicerequires minimal disturbance of the land. The Southern Califor-nia Edison Report states “when working in the Angeles National

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The environmental measures required to per-mit and begin construction on the line may bea harbinger of things the electrical transmis-sion industry must be prepared to manage asupgrades to the grid are made across the coun-try in the coming decades.

(Continued on page 17)

Completed transmission power lines in Angeles National Forest.

Forest, nothing goes in or out on the ground. This includesequipment, supplies, crew, inspectors, and portable toilets.” As aresponse an unconventional design and construction solutionhelped SCE gain approval from the U.S. Forest Service.

In the initial design phase drilled shaft foundations were con-sidered to be the most appropriate foundation system for thisproject. However, due to certain environmental restrictions and

the very remote construction site, in this instance, it was nec-essary to consider other viable drilled foundation options. Hel-icopter-supported micropile foundations were selected as theoptimal alternative. This article will discuss the foundation de-sign selection process, as well as technical challenges encoun-tered during implementation.

Micropile foundations, a high-capacity version of foundationsolutions sometimes known as pin piles or mini piles because oftheir small diameter, provided cost and schedule advantagesover other designs due to restrictions imposed by workingwithin the forest.

Design Selection

The initial design for the TRTP included a drilled shaft foun-dation solution, which would have required constructing milesof access roads for drilling and construction equipment. Withinthe forest, access roads were not approved. In this case it was de-

termined that due to the remoteness of the location drilledshafts were not a viable option, and that micropiles would pro-vide an acceptable alternative.

Other alternatives, including prestressed or post-tensioned rockanchors and micropile foundations, were considered. Rock anchorfoundations, which use anchors to resist uplift and utilize bearingbetween concrete cap and rock to resist foundation rotation, com-pression, and shear loads, were deemed impractical because of thehighly variable near-surface rock and soil conditions.

Micropile foundations were proposed by the general con-tractor, PAR Electrical Contractors, as the foundation alterna-tive. Foundations were designed by Crux Subsurface and its

subconsultant and reviewed and approved byBurns & McDonnell and SCE. The foundationsystem was constructed by Crux and PAR.

Micropiles typically range from 4 to 12 inches indiameter. They combine the uplift resistance of arock anchor foundation with the compression andlateral bending resistance of a drilled shaft founda-tion. To install these small but high-capacity deepfoundation members, Crux developed lightweight,componentized drill rigs that can be transported byhelicopter and assembled at each site.

Beyond environmental advantages realized bythe elimination of road building, the lightweightmaterials and construction equipment create benefits including a significant reduction inspoils, elimination of fluids, reduced emissionscompared to conventional equipment, and asmaller foundation footprint. These combine for

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(Continued on page 19)

Micropile foundations, a high-capacity versionof foundation solutions sometimes known as pinpiles or mini piles because of their small diam-eter, provided cost and schedule advantagesover other designs due to restrictions imposedby working within the forest.

Micropiles being installed.

...Finished product.

an overall reduced impact on the environment, which con-tributed to the approval of the Forest Service.

On the TRTP, micropile diameters range from 5.5 inches to8.625 inches at depths between 25 and 51 feet. Groups of 3 to12 micropiles per tower leg were constructed, depending on thetower type and soil condition. A geotechnical report for the pro-ject served as a basis for identifying soil type and condition. The

Federal Highway Administration’s Micropile Design and Con-struction Guidelines Implementation Manual, which was devel-oped with the assistance of members of the ADSC’s MicropileCommittee, and the Post Tensioning Institute’s Recommenda-tions for Prestressed Rock and Soil Anchors were used for initialassumptions of soil strength.

Several sacrificial preproduction micropiles were tested toevaluate the ultimate grout-to-ground bond stress before con-struction began. A licensed geologist characterized the soil on-site for each tower footing during construction.

The micropile design was considered to be advantageous be-cause it could be adapted to accommodate individual site con-ditions by varying the pile length and/or adding additional pilesto address anomalies that were not identified in the more gen-eral geotechnical investigation. The 224 micropile foundations

(four per tower) installed were grouped into three tower fami-lies. The number of piles and length of each upper cased sectionand lower bond section were installed to meet a minimum cri-teria for a variety of soil and rock conditions. After micropileinstallation, proof load tests were conducted at each tower siteto confirm that piles met factored design loads.

Technical Challenges

Micropile foundations utilize the complex interaction of nu-merous components including rock or soil, steel micropile re-inforcement and casing, cast-in-place concrete and steel latticestub angles. Comprehensive design of the entire foundation sys-tem is essential to ensure long-term tower performance.

Micropiles are slender, flexible members. SCE required thatthe foundations meet stringent deflection criteria and havebuilt-in safety factors to minimize stress buildup in the stubangle transferred up the tower leg and adjacent bracing member.Compliance with the desired deflection criteria was achieved bythe arrangement of the piles within the group.

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MICROPILES Contd.

The Federal Highway Administration’s Mi-cropile Design and Construction GuidelinesImplementation Manual, which was developedwith the assistance of members of the ADSC’sMicropile Committee

The remoteness of the Angeles National Forest presented chal-lenges.

Micropiles being installed in a batter.

Environmental factors such as wind, fog, and dry, hot weatherthat increased fire risk all had impacts on this helicopter sup-

ported project. Scheduling of critical items such as placement ofconcrete and micropile grout, each within a specified time of

batching, enhanced the need for project planning and execu-tion.

In the end, helicopter-supported micropile installations pro-vided foundations that could meet strict design criteria, mini-mize ground disturbance and environmental impact, andprovide scalability to adapt to varying ground conditions with-out downtime for further design or agency review.

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In the end, helicopter-supported micropile in-stallations provided foundations that couldmeet strict design criteria, minimize grounddisturbance and environmental impact, andprovide scalability to adapt to varying groundconditions without downtime for further designor agency review.

Project Team

Project Owner: Southern California Edison

Contractor: PAR Electrical Contractors

Subcontractors: Crux Subsurface, Inc.Summit HelicoptersSwanson Group HelicoptersMountain Air Helicopters

Consultants: Burns & McDonnellDCI EngineersTerraconArroyo Engineering Consultants, Inc.

Agencies: U.S. National Forest ServiceCalifornia Public Utilities Commission

Getting equipment to the sites required the use of helicopters.

The ultimate finished foundation.