Micropiles offer cost-effective and efficient geotechnical solutions

By Doug Horvath

Building settlement and cracked concrete are simple facts of life in most construction projects. However, given that all structures experience settling in one fashion or another, the level or degree of the settlement determines whether or not problems will result. This occurrence is also a reality for sites slated for redevelopment, especially if they are located in close proximity to other structures. In fact, in this situation the tight site may deem many geotechnical services unrealistic because of the harmful results of vibration or noise. However, one method that is growing in popularity as a means to reduce the effects of settlement for both existing and new construction is micropiles.

In recent years, especially in urban areas experiencing a huge growth in redevelopment - either by tearing down an existing building and building something new in its place or by upgrading an existing structure - cost-effective stabilization of structures has become paramount. Caused by poorly compacted fill, improper drainage, development of sinkholes, or even improper design, settlement can have drastic effects on a structure. If a structure experiences loads for which it was not designed, severe cracking in the floor or walls is likely and doors or windows may become difficult to operate.

Further, drainage and plumbing pipes may be compromised. In extreme cases, the building can actually collapse. As such, it is important to begin with a thorough examination of the subsoil, including its condition and compaction, as the subsoil greatly affects the longterm settlement of any structure. Once identified, measures can be taken to improve the stability of the ground conditions and thus stabilize the structure.

However, geotechnical solutions must not only address many variables and complexities that make each project unique, but also solve underground challenges.

An extensive knowledge of soils, structures, and geotechnics is required.

The right solution balances knowledge with specialized skills and equipment to improve ground conditions and control settlement, as well as to stabilize and strengthen foundations and slopes.

Micropiles defined

Micropiles - also commonly referred to as minipiles and pin piles - are small-diameter, reinforced piles that are drilled and grouted to support structures in all ground conditions. These piles may reach working loads up to 300 tons, can be installed to depths of approximately 200 feet, and usually utilize some type of steel bar or bars and/or steel casing pipe. The bars are grouted into the ground or the casing pipe is filled with grout.While conventional piles - steel or reinforced concrete, either driven or cast-in-place - are generally quite large and require heavy equipment and large staging areas for installation, micropiles can be used in applications where conventional piling is not convenient or possible, such as for underpinning or retrofitting existing buildings or structures. Micropiles have proven effective in many ground improvement applications by both increasing bearing capacity and reducing settlements, particularly in strengthening existing foundations.

The names minipile or micropile depict the respective size of the pile: minipiles are 6 to 12 inches in diameter, while micropiles measure 2 to 5 inches.

Depending on pile diameter and soil conditions, micropiles can extend to depths of 200 feet and exceed design loads of 400 kips. The pipes used for micro or minipile installations are manufactured in segments and feature threaded lengths with male and female ends that allow them to be fitted together. Because the pipes are inserted one at a time in lengths of 3 to 4 feet, they are suited for low-access and retrofit applications. Another benefit is that the drilled installation methods can be used for new construction applications where surrounding structures are sensitive to vibration. Micropiles using large diameter, threaded bars of high-strength steel (75 kips per square inch) also can be used to attain specific design loads when traditional “H” piles (a common foundation structure) cannot be used because of overhead physical constraints.

Another method of overcoming the challenge of micropile installations in areas with restricted headroom is the use of the simultaneous micropile casing and drill rod method.

For installation of this type of micropile, a special rig capable of handling duplex drilling is used to drill into the bedrock. Next, the pile casings are advanced into the bedrock. The drill pipe is removed, leaving the piles in the rock socket, and reinforcement bars are lowered into the steel casings. Grout is pumped or pressure-fed into the casings and the piles are lifted to the mouth of the sockets to allow the piles to bond to the rock. Finally, the micropile tops are cut to elevation and capped for foundation rebar. The last step involves loadtesting the piles to prove the design.

Building a better foundation

After determining the cause of settlement through a geotechnical boring survey, a structural engineer and geotechnical engineer can evaluate if micropiles are the best solution for the specific project. For projects requiring structural support, micropiles serve as an efficient means to underpin an existing or new structure, reduce/prevent settlement, upgrade foundation capacity, repair or replace deteriorated foundations, and also can serve as a seismic retrofit. In contrast, for scenarios requiring in-situ reinforcement, micropiles provide settlement reduction, structural stability, slope stabilization, and soil mass strengthening.

In cases of strengthening an existing structure because of the effects of settlement or a desired change in use that requires an increase in loads, micropiles provide an excellent alternative because of their cost-effectiveness.

In fact, using drilled shafts for cases in which piles are installed through existing floors and foundations can give the building additional support. Such was the case for Michie Stadium in West Point, N.Y. Constructed in 1924, the football stadium for the United States Military Academy required many construction projects to meet the new demands on this old structure. These included a new athletic center and press box, the Hall of Army Sports, the 150,000-square-foot addition of a new strength and development center, an auditorium, and other amenities. These new structures were to be constructed outside and immediately adjacent to the existing stands. During the excavations for the new facilities, however, the team discovered that the footings of the columns and bearing walls of the stands were bearing on soils above the proposed bottom of excavation. This scenario caused great concern about the potential instability of the stadium structure. Underpinning the footings was deemed the only option; however, there also was concern about significant impact to the construction schedule.

Working in concert with Schnabel Engineering, Structural Preservation Systems' GeoStructural Division provided a turnkey solution to this problem that included use of micropiles for underpinning the footings prior to the start of the excavations. The micropiles were conceived as selfdrilling, self-grouting bars embedded 5 feet into the rock. For the column footings, Schnabel Engineering designed a reinforced concrete cap attached to the existing pedestal. The bars were provided with a top plate that was embedded in the pile cap concrete.

Then, for the wall footing, a bracket was used to transfer the load from the wall to eccentric micropiles. This efficient design facilitated an accelerated micropile installation time with reduced or no impact to the project schedule. In fact, the entire underpinning system was in place in one week.

Being a good neighbor

Although micropiles are certainly a proven solution for existing structures, the method also is a growing solution for new construction because of its minimal impact to surrounding structures as compared with driven piles that typically cause extensive vibration. And, as compared with caissons or drilled shafts, micropiles are typically more cost-effective.

The owner of a building torn down between two other structures on 42nd Street in New York City wanted to construct a new 18-story condominium facility, but the site only provided inches between the two existing structures, requiring a solution with minimal vibration and noise effects. Driving piles using conventional methods was thought to be too risky (because of the proximity to the existing buildings) and drilled caissons were cost-prohibitive.

Further, the soil conditions were very soft and the borings located rock 30 feet below the surface. Because of these factors, the geotechnical and structural engineers selected the micropile technique.

In January 2005, Structural Preservation Systems began installing 7-inch-diameter, 35-foot-long piles. By February, the settling issues were resolved.

A similar project recently constructed in a Harlem neighborhood in New York City involved construction of a new, 16- story condominium facility sandwiched between two existing structures. This site had no piling - the project's original retaining wall was constructed from built-up boulders. Because of these conditions and the fact that other methods would create vibration harmful to the existing buildings, micropiles were chosen as the ideal solution. Structural Preservation Systems installed the 9- 5/8-inch-diameter, 25-foot-long micropiles at a rate of about four per day.

Pilling up success

In addition to its applications in commercial structures, the micropile method has gained interest from the public sector as a viable solution for bridges and roads. Regardless of the structure, a solution using micropiles can meet underground challenges in a cost effective, efficient manner when it is developed by a team comprised of skilled geotechnical and structural engineers and an experienced geotechnical contractor.

Doug Horvath is a project manager with Structural Preservation Systems' GeoStructural Division. He can be reached at 410-850-7000 or dhorvath@structural.net.