The bottom of foundation elevation is an unobtrusive yet crucial detail of a structure’s foundation system. It defines where loads are transferred into the ground, where frost protection begins, and where special care is required during demolition, refurbishment, or conversion. In practice, this affects not only planning and geotechnics, but also controlled demolition using precise, low-vibration methods. In the area around the bottom of foundation elevation, low-vibration concrete crushers as well as controlled rock and concrete splitters from Darda GmbH have proven effective for selective exposure, removal, or underpinning.
Definition: What is meant by bottom of foundation elevation
The bottom of foundation elevation (also known as foundation base or foundation underside) is the lowest structural level of an isolated footing, strip foundation, or slab-on-grade foundation through which loads are introduced into the subsoil. It marks the governing foundation depth, separates structure and soil, influences load-bearing capacity and settlements, and forms the reference for frost protection, waterproofing, and drainage. In existing buildings, the exact position of the bottom of foundation elevation is a central criterion for interventions such as underpinning, partial demolition, and selective deconstruction.
Significance for design, load-bearing capacity, and subsoil
The height of the bottom of foundation elevation is set so that the subsoil under load is sufficiently bearing, frost damage is avoided, and structural movements remain within permissible limits. Geotechnical investigations, groundwater conditions, and the structure’s use determine the foundation depth. As a rule, recognized engineering standards and applicable codes are followed; specific verifications are project-dependent.
Foundation depth, frost protection, and water
In regions with frost, the bottom of foundation elevation typically lies below the governing frost depth. With a high groundwater table, buoyancy, erosion, and capillary water rise must be considered. A capillary-breaking layer, a blinding layer, and appropriate waterproofing measures are often coordinated with the foundation base.
- Soil and subsoil: load-bearing capacity, deformation properties, organic content, settlement behavior
- Water conditions: groundwater level, perched water, drainage concept
- Use: actions from column loads, shear walls, machine foundations
- Climate: frost depth and meltwater runoff
Influence on settlements and deformations
The position of the bottom of foundation elevation influences the distribution of contact stresses in the subsoil. Insufficient foundation depth or uneven bearing levels tends to cause differential settlements. As a result, the most uniform possible support, a sound formation level, and consideration of soil stratification are essential.
Foundation types and typical levels of the bottom
Depending on the foundation type, the bottom varies in function and configuration:
- Strip foundation: Continuous foundations beneath load-bearing walls; the bottom of foundation elevation usually runs at a constant level.
- Isolated footing: Point foundations beneath columns; the bottom of foundation elevation is often locally adapted to subsoil conditions.
- Slab-on-grade foundation: Areal foundation; the bottom of foundation elevation corresponds to the underside of the slab and acts over large areas.
- Foundation on rock: When founding on bedrock, the bottom of foundation elevation may follow the natural contour; careful profiling and smoothing of the rock formation level is important.
Execution: excavation, blinding layer, and concreting
Creating the bottom of foundation elevation begins with excavation down to the formation level, installation of any required blinding layer, and precise elevation control. A level, bearing, and non-softened base is a prerequisite for foundation quality.
Tolerances and control
Elevation and positional accuracy of the foundation base must be documented. Typical checks include visual inspection of the base, plate load tests where necessary, elevation measurement using level or laser, and verification of blinding layer thickness. Deviations should be corrected early to avoid subsequent settlement problems.
- Setting out and surveying the target elevation
- Excavation and creation of a bearing formation level
- Blinding layer for load distribution and as a working surface
- Formwork, reinforcement, and concrete cover coordinated with the bottom of foundation elevation
- Concreting, compaction, surface finishing
- Curing and documentation
Bottom of foundation elevation in existing structures: exposing, testing, rehabilitating
In existing structures, the exact position of the bottom of foundation elevation is not always known. Trial pits, careful exposure, and construction-phase testing provide information on material, layer sequence, and load paths. During exposure in the area of load-bearing elements, vibration minimization takes priority to avoid settlements.
Gentle exposure in the context of concrete demolition and specialist deconstruction
For exposing and selective removal down to the bottom of foundation elevation, low-vibration, controlled methods have proven themselves:
- Concrete crushers: Precise nibbling of concrete along defined edges, suitable for stepwise removal down to the foundation base with low risk of cracking in adjacent components.
- Rock and concrete splitters: Hydraulic splitting enables controlled crack propagation in concrete or in the underlying rock. Stresses are introduced in a targeted manner, which is advantageous in sensitive environments.
- Rock splitting cylinders: Compact splitting technology for confined workspaces beneath foundation projections and in shafts.
Hydraulic power units supply these tools with the required energy without generating additional combustion emissions at the component. This is particularly helpful in basements, shafts, or when underpinning.
Bottom of foundation elevation and adjacent components
At the transition to the bottom of foundation elevation, several trades meet: drainage lines, waterproofing, frost aprons, foundation plinths, and, if applicable, the foundation earth electrode. Interventions in existing structures should consider these interfaces to avoid leaks, corrosion, or thermal bridges. If reinforcing steel is exposed, corrosion protection and proper restoration must be taken into account. During selective deconstruction, reinforcement can be cut with steel shears or multi cutters without unnecessarily vibrating the foundation base.
Work at the bottom of foundation elevation during conversion, underpinning, and partial demolition
Underpinning and excavations at the bottom of foundation elevation are carried out in stages and with structural securing. The subsoil must not be unloaded abruptly. In practice, a combination of small-scale removal and controlled splitting technology has proven effective for keeping load redistribution manageable.
Low-vibration methods in sensitive environments
- Concrete crushers for controlled edge removal down to the target level of the bottom of foundation elevation
- Rock and concrete splitters for coarse separation of massive foundations or when lowering individual blocks
- Combination shears and multi cutters for hybrid components with reinforcement, embedded parts, and structural steel
- Steel shears for reinforcement bundles and connected steel components
In industrial buildings, foundations for tanks and machinery can pose special requirements. When dismantling installations with connected foundations, tank cutters may be used depending on the situation before controlled concrete removal at the bottom of foundation elevation follows.
Safety, permits, and documentation
Interventions at the bottom of foundation elevation affect structural stability. Preliminary structural assessments, a staged approach, and settlement monitoring are recommended. Depending on scope and project, permits and notifications may be required. Statements on this must always be checked on a project-specific basis. Complete documentation of elevations, ground conditions, and executed safeguarding measures facilitates subsequent quality assurance.
Measurement, elevation reference, and quality assurance
Fixed elevation references and control measurements are used to determine the exact position of the bottom of foundation elevation. Lasers, levels, and measuring staffs enable robust control in underground construction environments. Tolerances for flatness and elevation are to be defined contractually and in accordance with current standards. Photo documentation, measurement logs, and, if applicable, inspections of the base prior to concreting ensure execution quality.
Typical errors and how to avoid them
- Not extending below frost depth: risk of heave and damage at the foundation base
- Uneven or softened formation level: non-uniform load transfer and increased settlements
- Missing or insufficient drainage: moisture ingress, erosion, and loss of soil parameters
- Uncontrolled demolition down to the bottom of foundation elevation: cracking due to vibrations; concrete crushers and splitters are advantageous here
- Ignored reinforcement and embedded items: poor cut quality, corrosion paths; orderly separation with steel shears or multi cutters helps
- Incomplete elevation control: deviations that can only be corrected later with significant effort
Bottom of foundation elevation in the context of application areas
In concrete demolition and specialist deconstruction, the bottom of foundation elevation is often defined as the boundary level down to which selective demolition is carried out. During strip-out and cutting, it serves as a reference for cuts, separation joints, and underpinning sections. In rock demolition and tunnel construction, the rock base forms the natural counterpart to the foundation base; splitting technology allows controlled adjustments there. In natural stone extraction, plane surfaces at the removal base are comparable to the foundation base. Special operations with restricted access benefit from compact hydraulic tools that work precisely and with low vibration.