Foundation Underpinning – Preconstruction Investigation
Foundation underpinning means transferring the foundation load of an existing structure deeper into the ground. Foundation underpinning is needed if planned structures or development adjacent to an existing structure is close enough and deep enough such that:
The depth of the excavation required for the new structure/development from below the bottom of existing footing exceeds the horizontal distance from the outside edge of existing foundation to the limits of excavation. This is termed the 45 degree or 1:1 rule of thumb and it is a rule of thumb only. An excavation outside of 45 degrees from the bottom of existing foundation can still negatively impact an existing foundation given the right ground conditions.
There are many underpinning techniques each with a unique advantage or purpose. Selection of the most appropriate underpinning technique and pricing will depend upon:
- Existing construction material type and condition which influences:
- Structure’s sensitivity to differential settlement and length foundation can free span between underpin elements.
- How to best attach the underpinning system to existing foundation to provide load transfer.
- The estimated foundation loads, access and overhead clearance, anticipated soil conditions and depth to bearing layer which influences:
- Underpinning element spacing, size, depth and material.
- Means & methods and equipment capabilities.
- Distance and clearance between existing structure and planned structure and corresponding space available for underpinning system.
Planning early is key. The older the structure, the more likely surprises will present themselves during construction which will lead to redesign and project delays. It cannot be stressed enough to plan early by investigating the existing conditions by the best available means:
- Expose existing foundation:
- Test pits on exterior in multiple places to assess foundation depth, geometry, material type, material condition and any variability.
- Investigation of crawl space.
- Investigation of original design drawings (if available).
- Accurate assessment of estimated load based on structural analysis of existing structure.
- Structural review of drawings (if available), or field investigation to reconstruct the building design and assessing loads.
A reduction in 1” to 2” of available space can significantly increase the underpinning system price. It is therefore important to note and accurately measure existing foundation geometry and if the footing steps out from the plane of wall directly above.
Figure 1. Preconstruction investigation of 100+ yr structure exposing masonry atop cobblestone foundation underlain with bulk concrete leveling pad.
Figure 2. Same structure as Fig 1, cobblestone basement wall atop bulk concrete leveling pad.
Figure 3. Same structure as Fig 1&2, shallow masonry step footing adjacent to basement.
Let Geocraft bring value to your project
Geocraft Builders can help guide the necessary preconstruction investigation to assess existing foundation conditions and optimize the foundation underpinning design to eliminate unnecessary surprises that result in costly redesign and construction delays.