Micropile Construction Case History

Micropiles with 3/1 batter

Piles constructed with RevDrill tools and load test to 272 kips

Bridge Replacement: Ensor Rd. over Third Mine Branch, Northern Baltimore County.

A long needed bridge replacement is underway on Ensor Rd over Little Mine Branch in Northern Baltimore County, MD. Hillis-Carnes Engineering Associates, Inc. (HCEA), a Mid-Atlantic based consulting, engineering firm, was awarded the foundation job. HCEA gave careful consideration when designing the micro pile foundations to assure that environmentally non-destructive methods are used to install the required 38 micro piles for the new bridge abutments. A skilled and experienced crew of technicians led by Mike Shanaberger was deployed to perform the challenging foundation installation.

Job requirements: 38 Micro Piles; 7” OD X 38 ft. deep including a 7 ft. rock socket, installed on a 3/1 batter, load tested to 272,000 lbs.

Problem: Choosing the proper equipment and tooling for the installation of the micro piles was critical to minimize the impact to the surrounding environment. The drill used for the installation needs to be capable of reaching out approx. 12 ft. and down approx. 10 ft. to the pile locations at subgrade and be capable of drilling on a 3/1 batter. 19 micro piles on each side of Little Mine Branch are to be installed 8” diameter through 19 ft. of overburden and then 7 ft. into solid rock. Typical methods to drill in these conditions utilize high volumes of pressurized air to power the rock drilling equipment. Many times the air escapes into the surrounding strata through fissures in the soil and rock and can enter the stream contaminating the waterway with silts and clays. Escaping air can also cause the surrounding areas of ground to heave and potentially lift nearby structures.

Design Solution: Hillis-Carnes decided to use their new REV Drill Model 24HDT drill, mounted on their CAT 336E excavator to tackle the job. REV Drill engaged with HCEA’s design engineers and Project Manager to review the job requirements and equipped the drill with a hydraulic clamp and breaker system to assist in adding additional drill rod and casing pipe to achieve the desired tip elevation. The REV Drill is easily maneuverable, capable of reaching out to the required pile locations, has a hydraulic tilting mount for easy alignment of each pile and can utilize many types of tooling.

Elemex drilling system

An Atlas Copco “Elemex” system is being utilized to install the 7” casing pipe, in conjunction with a 900 CFM air compressor running at 120 PSI. Conventional systems push air through the hammer and directly into the ground, but the Elemex system minimizes the air escaping to the surrounding ground. The air is diverted up through the annulus between the casing pipe and the drill rod and escapes out the top of the casing pipe. Air is injected through a top mounted air swivel above the rotary motor and runs down through the drill rod to actuate a DTH hammer. 7” casing pipe is loaded onto the drill, over the drill rod and hammer.

A “ring bit” is welded to the bottom of the casing pipe and the center drill rod and hammer has a “pilot bit” that engages and turns the ring bit and the casing pipe follows the bit as it drills. Once the casing is drilled and seated into the top of the rock, the inner rod and hammer continue to drill the 7 ft. rock socket. The drill rod is retracted and the casing pipe remains in the ground. The casing is then grouted and a threaded bar is inserted to complete the installation.

HCEA is making great progress with the new bridge foundation with nearly half of the required micro piles successfully installed and tested to their required loads, and with little to no impact to the environment.

micropile construction site

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