Micropiles are small diameter piles, typically between 3-12 inches, that are used for foundation underpinning and new construction. They can extend up to 200 feet, and provide deep foundation support for a variety of structures.
Micropiles can be useful in areas where space limitations exist or where soil conditions are not practical for the installation of helical piers or piles. They can be used in a variety of difficult soil and ground conditions such as clays, sands, silts, gravels and cobbles—even rocks and boulders.
With their versatility and bonding capability to soils, micropiles are useful in many applications including:
Foundation support in low strength or otherwise unsuitable soils
Stabilizing foundations for tall structures such as towers or wind turbines
Seismic upgrades and retrofitting
What Are Soil Nails?
Soil nails are usually installed on an angle to stabilize and reinforce the face of a slope or a wall, and they are often designed in a grid pattern. Similar to micropiles, they are drilled and grouted into place. Some soil nail applications also include a reinforced shotcrete surface for additional stability.
Which One Is Right For My Project?
Micropiles and soil nails are essentially the same thing; the difference is in their application. Micropiles are used in tension and compression as load bearing tendons. They are made of high-strength steel casing, rebar and grout. Soil nails are typically used in steep slope or wall stabilization where the nails are installed beyond the soils’ failure plane (angle of internal friction). They use grouted, tension-resistant steel nails to reinforce the soil and create a retaining wall during excavation.
Both applications utilize a hollow bar steel tendon with a sacrificial cutting bit that is installed with a rotary-percussion hammer. During installation, a neat cement and water mixture is pressure injected down the hollow bar and through the cutting bit into the soils surrounding the hollow bar/tendon. This method of installation is called Injection BOring (IBO). IBO piles and soil nails develop their strength, in both compression and tension, by the summation of the steel tendon plus the strength of the annular soil-cement bond with its surrounding soils, referred to as the area of influence.
The area of influence is the annular area in which the soils are cemented together. Micropiles and soil nails can be installed in almost any soil condition with excellent load bearing characteristics which makes them an ideal alternative in rocky or high blow count (high density) soils where helical pile installations are not conducive.
For more information or to speak to a Reno contractor, give Versa Grade a call at 775-284-1964!
J & J Nevada was a project involving a seismic retrofit and tenant improvement to an existing warehouse in Sparks, Nevada. Additional truck docks were required to be installed, changing the structural characteristics of the tilt up concrete building. The installation of a moment frame on the interior of the structure required some relatively high loads to be addressed in tension and compression at specific locations. VersaGrade was contacted early in the design phase to help determine the best foundation system to address these loads.
Originally built in 1979, the warehouse was constructed for General Motors Corporation as a parts distribution center. The 634,000 Square Foot facility included dozens of truck dock openings and an interior railroad spur for the unloading of train cars inside the building. The soils are predominately Alluvium of the Truckee Meadows, consisting of pebble to cobble sand, silty sand, and sandy silts overlain by scattered, well rounded volcanic & granitic cobbles. Due to the rocky nature of the site and limited soils investigations, VersaGrade concluded that the use of Micro Piles would be the best system to meet the required loads. Pile groups of 3-4 would be necessary to achieve the 140 Kip allowable loads at specific points along the perimeter foundation.
VersaGrade, Inc. was awarded the subcontract to install 40mm Contech Systems Micro Piles along with a custom Ram Jack bracketing system. K2 Structural Design & Ram Jack Engineering were contacted to help design the bracket that would carry a 35 kip allowable capacity (70 Kip Ultimate) in tension and compression for each pile. Each bracket was designed to work on shear strength alone, and attach directly to the tilt up panels themselves as there were no footings to attach to in some locations. Six each 7/8” diameter epoxy doweled bolts (per bracket) were used to secure the brackets to the face of the concrete tilt-up walls. A 5 ½” diameter x 8’ long steel guide sleeve was then inserted through the brackets and set to elevation. After bracket installations were complete, VersaGrade installed Contech 40/16 Micro Piles with a 4” carbide cutting bit down through each guide sleeve. The Micro Piles were installed to an average depth of 21’. One of the challenges of the project was higher than expected grout usage during the Micro Pile installation due to unconsolidated material and fissures. Some piles took in excess of 35 cubic feet of grout to complete.
VersaGrade used a Komatsu PC-160 Track-mounted hydraulic excavator with an Excavator Mount TEI 350 Rock Drill and ChemGrout CG600 Batching Plant for the Micro Pile installations.
Total No. of Contech Micro Piles and Ram Jack Custom Brackets installed: 14
Step 2 - New Construction Pile Fixing Foundation 12/23/2015
STEP 2 is a private non-profit organization that has been serving Northern Nevada since 1986. Its mission is to provide comprehensive, coordinated services related to the treatment and recovery of chemically dependent women and their families. The 12,000 square foot expansion project adds a 20 bed residential housing component and storage facility to the already existing Mathewson Family Counseling Center and Transitional Cottages. The design vision of the project is to accommodate the everyday living needs of women and children in distress.
The project site is located north of the Reno / Sparks Area in Nevada on the foothills of Peavine Mountain. The area is predominately made up of Alta Formation soils components that contain hard volcanic rock, highly expansive clay minerals, and corrosive soil properties. The soils investigation confirmed the project site would not be conducive for conventional foundation construction. A deep foundation system was decided to be the best course of action, but due to the soil properties and its inconsistent nature, no one foundation system type could be utilized. VersaGrade was contacted early in the design process to help determine the best and most economical approach for this problem site to prevent fixing the foundation if issues later arose. It was determined that a mix of deep foundation types that included Ram Jack Helical Piles and Contech Micro Piles would be necessary to overcome these problem soils.
HOW WAS RAM JACK INVOLVED?
VersaGrade, Inc. was awarded the subcontract to install Ram Jack Helical Piles and Contech Systems Micro Piles on two adjacent project sites. One site was “The Living Center” and the other was “The Storage Facility”. Both building foundations were similarly designed as a pile and grade beam system. Since the soils were highly expansive, the entire foundation system was designed with a 6” void space under it to allow the clay soils to expand and contract without contacting the foundation system. 6” thick by 10” wide Sure Void was installed along the bottom of all grade beam trenches to provide the separation between soil and grade beam. Lateral stability which is normally achieved by the weight of the building on the underlying soils, was alternately achieved by the use of angled (Battered) piles installed at load points throughout the buildings footprint.
The Living Center foundation plan consisted of 95 vertical piles and 22 angled piles with working loads of 34,000 pounds and 10,000 pounds respectively. Installation depths varied wildly with helical piles ranging from 10’ - 60’ to achieve adequate load carrying capacities. Where helical piles could not be installed due to solid and fractured rock obstructions, Injection Bore (IBO) Micro Piles were installed to a depth of 20’ with a 4’ unbonded length. The Storage Facility foundation plan consisted of 62 vertical piles and 14 angled piles. The grade beam design on both buildings was only 10” wide, which made layout and installation techniques critical. Pile migration during installation of 1.5” was unacceptable.
VersaGrade used a Komatsu PC-160 Track-mounted hydraulic excavator with a Pro-Dig two stage 12K drive motor for the Helical installations and an Excavator Mount TEI 350 Rock Drill and ChemGrout CG600 Batching Plant for the Micro Pile installations.
Total No. of Ram Jack Helical Piles installed: 150