Before we get into the benefits of using micropiles to repair a foundation, we will first explain what micropiles are. A micropile is a drilled and grouted friction pile that consists of cement grout, centralized rebar, steel pipe pile, and a high-strength steel casing. Other names that are used for a micropile include a drilled micropile (DMP), root pile, pin pile, needle pile, and mini pile. Micropiles are an excellent foundation alternative that are used to strengthen deteriorating foundations on homes and commercial buildings in Florida or to repair foundations that are already compromised. Furthermore, micropiles are used for pre-construction of residential and commercial structures. They work extremely well when the project site has limited-access, a site with poor soil quality, and limited overhead constraints.
A Micropile is a Deep Foundation Friction Pile
A micropile is an amazingly durable, high-capacity foundation element that is used to strengthen a foundation. Mini piles are used for new construction projects as well as for addressing issues related to the foundations of existing buildings.
Micropiles derive their geotechnical capacity through transference. During this process, the mini pile transfers the physical load of the structure through its steel and cement grout, dispersing some of the pressure into the soil strata that is surrounding it. The process that is used to disperse this pressure is referred to as skin friction. This friction causes an adhesive effect at the points where the soil and pile connect.
In construction, mini piles are frequently used to:
- Transfer bearing loads to subsurface soils, thus, eliminating the need for deep foundations.
- Build large structures in limited-access areas.
- Strengthen existing foundations and foundation repair.
- Mini piles can even be used in areas with unfavorable ground conditions (e.g., karstic/limestone substrate).
- Underpin existing structures with foundation problems.
- Stabilize soil that is considered too loose for the construction of a large building.
- Strengthen new foundations to ensure they are solid.
- Stabilize a slope, a landslide, or an embankment.
Some Benefits of Using Micropiles in Florida
Mini piles do not require additional supports, even if they are being placed in an area with weak soil. Micropiles are smaller than traditional piles, the equipment used to install them is quite a bit smaller than that which is needed for the installation of traditional piling as well. Furthermore, using micropiles instead of traditional piles is typically less expensive. One of the cost-saving aspects of mini piles is that all you need to install them are ground anchors and grouting project equipment.
Since micropiles are smaller than many other piles options, underground utilities can usually be avoided. By being used in conjunction with other techniques, mini piles can meet complex and unique project requirements efficiently and economically. If being installed in weak soil, micropiling offers better quality assurance than a repair performed using cast-in-place piling. When dealing with ground conditions, such as natural or man-made obstructions, mini piles tend to be the best option. In many cases, micropiles are considered the perfect solution for replacing deteriorating foundation systems on existing buildings in Tampa, Orlando, and Naples, Florida. By using limited-access equipment throughout a micropiling project, the likelihood that nearby operations and utilities will have to shut down is reduced.
Micropiles for Foundation Repair
Underpinning refers to a process that is frequently used for foundation repair. Necessary repairs may include addressing cracks, buckled walls, heaved foundations, or cracked concrete floors. Also, underpinning can be used for strengthening or increasing the depth of a foundation.
The Micropile Process in Florida
Before beginning a micropile project in Florida, a geotechnical engineer may use soil assessments to determine how deep the micropiles can be placed or, when a geotechnical engineer is not used, the area next to the existing strip foundation can be excavated to determine its depth. The process involves the creation of small diameter friction piles that are drilled and grouted.
A foundation repair in Florida, using the micropiling process:
1. Using a micropiling rig, to install micropiles below the building’s existing foundation. The micropiles need to be installed at an angle or drilled through the existing slab or footer.
2. The soil in front of the micropile is excavated.
3. Once the top of the micropile becomes evident, the earth between the mini pile and the existing foundation is removed. This part of the foundation repair must be performed manually.
4. As the soil is removed from between the micropile and the foundation, a void in the shape of a cone is created.
5. Fill the cone-shaped void with concrete. This concrete creates a new foundation beneath the existing one.
6. After adding the cement, a hollow bar is installed inside the micropile. The bar is installed using the injection bored method.
7. After installing the hollow bar, it is filled with a cement grout mixture.
8. As the grout is being pumped into the bar, an anchor is added.
9. Once the concrete sets, the forces are transferred from the existing foundation to the micropile.
10. As each pin is completed and the concrete cures, these steps are repeated on each section of the foundation until the length of the entire wall is reinforced.
Excavation Walls
If excavation walls are required, wooden lagging can be used to span micropiles. Sometimes, post-grouting is used. In some instances, post-grouting is beneficial because this practice offers increased capacity as well as an increase in the friction of the soil surrounding the pile. Within the shear zones, mini piles can stitch soil together, a process that increases mass stability. Also, to follow the requirements associated with seismic design, micropiles can transfer loads to strata with the capability of providing support, as long as it is liquefiable.
Advantages of using the mini pile underpinning process include:
- Drilling and grouting are performed simultaneously, which makes the micropile underpinning process fast.
- No need for pre-drilling, which saves time and reduces cost.
- That micropiles are a corrosion-resistant solution for foundation repair.
- The possibly preventing the need to re-route utilities.
- Resisting compressive, lateral, and tension/uplift loads.
Micropiles Also Allow for the Construction of New Commercial Buildings in Some of the Most Challenging Florida Locations
Mini piles are specifically designed to sustain extremely heavy loads. Since the diameter of a mini pile ranges from 5- to 12-inches, these deep foundation elements are small enough to use while constructing a building in limited-access environments, including highly congested areas and areas with a low overhead clearance of just 8 feet. Mini piles can be inserted deeply into the ground (up to 200 feet). Furthermore, micropiles can achieve compression and tension capacities of up to 200 tons.
General Micropile Installation
The method used for mini pile installation varies and is based on the specific conditions of a project. The installation steps outlined below are for a general micropile installation.
General installation steps:
1. Using either compressed-air drilling methods or rotary wash, the steel casing is advanced to the predetermined design depth. This depth can be up to 200 feet.
2. Once the casing is in place, the drill rod is removed.
3. The casing is filled with grout.
4. The rebar is lowered into the casing.
5. Using pressure, the cementitious grout is pumped into the casing.
6. The casing is partially withdrawn. By withdrawing the steel casing, a bond zone is created between the surrounding bedrock or soil and the cementitious grout. Typically, some casing is left in the ground. By leaving some of the casing in the ground, structural connections are possible and various structural designs can be considered (e.g., seismic design, etc.).
7. The pile is cut and then capped.
8. A load test is performed.
Load-Testing Standards
The standards used for load testing mini piles include ASTM D 3966, ASTM D 1143, and ASTM D 3689. These standards assess lateral, uplift, and compressive load abilities.
Load Capacity of Micropiles in Florida
One of the last benefits of micropiles for foundation repair is the load capacity. The load capacity of a micropile depends on its size and subsurface profile. While the typical micropile has a compressive capacity of up to 200 tons, there are certain piling projects with compressive capacities of more than 500 tons.
Every foundation retrofit system case in Tampa, Orlando, and Naples, is unique, therefore, in some situations, traditional piling or another method may be a better option. Nonetheless, when suitable, micropiling offers a cost-effective solution to address a variety of construction projects.
Helicon is a top ground improvement contractor in the state of Florida. Helicon proudly serves Orlando, Naples, and Tampa. If you notice cracks in your foundation or if you would like to learn more about the underpinning process for foundation repair call 813-567-1065.