If you own an older commercial building, it’s likely sitting on top of an extremely old foundation. This may not always be a problem, but in cases where a new building is constructed nearby, it can lead to issues. Nearby or adjoining structures with new foundations may be placed deeper into the soil. This can result in a situation where the foundation of your commercial building isn’t getting the support that it needs.
For example, let’s say you have a building that is around 100 years old. It’s made out of masonry, dimension stone, or cobblestone, and the neighboring lot is only a few feet (or even inches) away. Someone comes in and decides to develop that lot by your building and chooses to construct one or even two stories below grade. You’re then put in a position where you need to make changes to your building’s existing foundation to prevent serious problems.
Foundation Underpinning Choices
The top solution in this kind of situation is foundation underpinning. Foundation underpinning typically refers to transferring the foundation load of an existing structure deeper into the ground to a depth below the elevation of adjacent new construction. This is a specialized retrofit solution used on commercial buildings and expensive older homes. Underpinning techniques include:
- Micropiles (or other deep foundation elements)
- Soil nails, tiebacks, and shotcrete (or other anchor and earth retention systems)
- Hand-dug underpinning piers
- Needle beams (used in conjunction with micropiles)
- Sister walls (used in conjunction with micropiles)
Each of these solutions has its own set of pros and cons, and every project requires consideration about which will work the best. For example, hand-dug piers are a fantastic option for areas that are tight to get into, while sister walls offer a considerable amount of stability for an old commercial building, so it lasts for many more years.
Below, we offer more insight into each of these options so you can get an idea of your choices. We’ll share how each of the solutions works and talk about what situations some of them are the best choice for. Then you can work with a professional to make sure your foundation underpinning meets the needs of your high-end older home or your commercial business.
Micropiles as a Solution
The underpinning technique(s) employed will depend upon access, clearance between the existing structure and planned structure, overhead clearance, existing foundation estimated load, construction material type and condition, soil conditions anticipated, and the maximum allowable movement of the structure to be underpinned. Working with an expert is the only way to be sure that all of these things are considered, so you get the results that you desire.
Micropiles are used to extend the load of an existing foundation below the elevation of planned construction. Micropiles are drilled adjacent to the existing foundation to a depth below the new construction and are attached to the existing foundation. If the existing foundation is a strip footing or spread footing that is wide enough, micropiles can be drilled through core holes that penetrate the existing foundation.
Micropiles can be ‘pre-compressed’ once attached to the foundation. Pre-compression can significantly reduce the amount of anticipated settlement by mobilizing inelastic strain due to load transfer of the system and to mobilize elastic strain of the pile and is performed prior to excavating below the footing. This is a reliable option for creating better stability in an older building.
Soil Nails, Shotcrete, and Tieback Options
Soil nails and shotcrete (or other anchor and earth retention systems) retain the soil beneath the existing foundation once the micropiles (or another deep foundation system) are in place. With soil nailing, steel tendons are placed into drilled holes to ensure a stable block. Shotcrete, which is used with reinforcing steel or wire mesh, is applied to the face of the excavation to secure soil between nails. These two things work together to preserve the integrity of your foundation.
The shotcrete and soil nails system is versatile and has the potential to bond to many types of soil, which means it can be applied in many different situations. It works well in various conditions that might include sand, silt, clay, cobble, and gravel. This is part of what makes it a common choice for underpinning old foundations.
A tieback system is another option to underpin commercial buildings and old homes. Helical anchors are first screwed into the ground until they reach the foundation. After that, the tieback anchor is attached and mounted on the surface for stabilization. The anchors are designed to handle pressure from water, soil, and rock, so they work well when underpinning an old commercial building near a new construction.
Hand-Dug Underpinning Results
Hand-dug underpinning piers are as large as 5 ft long by 3 ft wide. Hand-dug pits beneath the foundation are dug and lined with wood lagging to temporarily retain the soil. Once the underpinning pit is dug below the planned structure, concrete is then poured into the pit. Load transfer between the existing foundation and underpinning pier is achieved by placing dry packing between the bottom of the existing foundation and the top of the concrete pier once poured and cured.
This is another standard option for the underpinning of an old commercial business where the area is tight and getting large equipment in can be a challenge. Since there is less space required to get the job done, it may be the top solution when other methods will not work well or may take additional time and labor.
Sometimes this type of underpinning is also known as traditional underpinning, mass concrete underpinning, or pit underpinning. Regardless of the name associated with it, the process is going to be the same. This is a method that has been around for about 100 years and hasn’t undergone much change in that time since it works exactly as it always has.
The Use of Needle Beams
Needle beams are used when eccentric loads cannot be resolved with micropiles only on the exterior foundation wall. Needle beams are structural steel elements oriented perpendicular to and placed beneath the existing foundation so that the ends of the needle beam extend beyond the interior and exterior wall face. The needle beam straddles and is attached to the micropiles at each end. Small access drilling equipment is used to drill micropiles on the interior of the existing structure.
There are many benefits to choosing a needle beam solution for your foundation. It tends to be one of the quicker methods of underpinning and it requires access to only one side of the foundation. In addition, this method has a massive load carrying capacity, so further work is unlikely to be needed. Going with this solution is best for situations where the foundation needs to be extended on a single side.
This option is based on both hand-dug underpinning and micropiles underpinning but offers more versatility than either in situations that warrant it. The design of the beams can be explicitly based on the building’s configuration and the load it applies to ensure the project comes off without a hitch.
Placing Sister Walls
Sister walls are foundation walls that are constructed with rebar doweled into the existing foundation wall and covered with shotcrete. Sister walls are installed as a means to augment masonry or stone walls that lack the shear and bending strength of reinforced concrete walls. Micropiles are then attached to the sister wall and offer extra support for the existing foundation.
There are many benefits to choosing the sister wall solution, but it’s most apparent when a home was created with a brick or stone foundation. This type of foundation is not as durable and reliable as the foundations made today of concrete. When a professional installs a sister wall, it addresses the fact that the foundation may be unstable so that it will last for many more years.
A True Craft and Expertise
GeoCraft Builders has underpinned structures dating from the 19th century built on cobblestone foundations and century-old masonry foundations. Underpinning structures of stone and masonry footings require strict adherence to construction sequencing and attentiveness to ground conditions and ground behavior. Having the right team to handle the process quickly and efficiently is something we pride ourselves on.
We offer a selection of foundation underpinning solutions based on your specific needs. Placing sister walls, introducing needle beams, hand digging underpinning, and using soil nails and shotcrete are all within our wheelhouse. We’ll work with you to determine which method is right for your building and create a plan that works with your budget and schedule.
GeoCraft installs movement monitoring systems using instruments such as tiltmeters, electronic water levels, inclinometers, load cells and/or strain gauges. Movement is monitored and recorded via data loggers at a set frequency 24 hrs./day, seven days a week. All project parties (owner, engineer, etc.) can remotely access dataloggers and download data.
Contact Our Team Today
Are you in a situation where foundation underpinning is needed due to a new construction near your older commercial business? Ensuring the professionals who work on this job are experienced and knowledgeable is a top priority. Getting it right the first time so you can go about your business without worries is something any good team will make possible. In the Denver area, GeoCraft Builders works to be the right company for your foundation underpinning needs.
Our team has the experience, expertise, and technical knowledge to successfully and safely construct foundation underpinning systems on all foundation types and in the most challenging ground conditions. We are here to help answer any questions you may have. Engage our pre-construction services today to mitigate risk, improve constructability and safety, and maximize economy.