An underground terrace is distinguished from a ground-level terrace by a structural parameter: the excavation depth. This digging depth determines the stability of the structure, water management, and the type of foundation required. Setting it without prior study of the land is akin to constructing an unintentional retention basin.
Runoff Coefficient and Rainwater Management in Underground Terraces
Common content on excavation depth overlooks a technical point that changes the project’s sizing: the impermeable surface created by the terrace is included in the calculation of the overall runoff of the plot. The runoff coefficient of a solid terrace ranges from 0.7 to 0.9, meaning that almost all of the received water runs off rather than infiltrating.
To determine the ideal depth for an underground terrace, one must first quantify the volume of water likely to accumulate in the basin formed by the excavation. The deeper and more recessed the terrace, the greater the risk of concentrated runoff water.
Professionals now systematically integrate three devices to meet local requirements for non-aggravation of runoff:
- A peripheral drain placed at the base of the retaining walls, connected to an outlet or drainage network
- A base slope between 1.5 and 2% directed towards a low collection point, to evacuate water before it stagnates under the decking
- A retention basin or swale downstream, sized according to the impermeable surface and local rainfall, when direct connection to the network is not permitted
Without these devices, even a modestly deep underground terrace turns into a basin during a storm. The issue is not the depth itself, but the lack of a calibrated drainage plan based on the volume of captured water.
Frost Depth and Soil Bearing Capacity: Two Local Data Points to Check Before Excavation
The frost depth determines the minimum level to which the foundations of an underground structure must descend to avoid ground movements related to freeze-thaw cycles. This data varies by geographical area and altitude. In France, it can range from a few dozen centimeters in coastal areas to significantly higher values in mountainous regions.
For an underground terrace, the foundation footings of the retaining walls must extend below the frost depth. The excavation depth adds to this constraint. In practical terms, the total excavation depth always exceeds the visible height of the finished terrace.
Soil Bearing Capacity and Nature
A swelling clay soil does not excavate like a draining sandy soil. Clay retains water, swells in winter, and shrinks in summer. On this type of terrain, digging a basin without treating the walls amounts to creating a retention area that amplifies soil movements.
A geotechnical soil study before work allows for understanding the bearing capacity and hydric behavior of the land. It guides the choice between a stabilized gravel base, a concrete slab on a draining bed, or a structure on adjustable pedestals placed on concrete footings anchored below the frost line.
Formwork and Retaining Structure: Adapting Thickness to the Chosen Depth
The formwork for the vertical walls of an underground terrace is not just a simple cladding. These walls withstand the lateral pressure from the surrounding soil. The deeper the excavation, the greater the pressure exerted by the soil on the retaining walls.
For a shallow underground terrace, walls made of linked concrete blocks with standard reinforcement are usually sufficient. Beyond a certain depth, a cast-in-place reinforced concrete wall becomes necessary. The thickness of the retaining wall increases with the height of the retained earth, which consequently reduces the usable surface area of the terrace if the ground footprint was not increased from the design stage.
Base and Drainage Under the Decking
The bottom of the excavation receives a bed of compacted gravel (draining bed) that serves two functions: distributing loads and facilitating water evacuation towards the peripheral drain. The thickness of this bed depends on the nature of the soil. On clayey ground, a thicker layer should be planned with a contaminant-resistant geotextile between the natural soil and the gravel.
The choice of the final covering (concrete slab, joists on pedestals, composite boards) also influences the excavation depth. Installation on adjustable pedestals requires free space under the joists for ventilation and drainage, adding several centimeters to the total depth. A concrete slab poured directly on the bed reduces this additional thickness but requires drying time and careful peripheral formwork.
Guardrails and Safety Regulations for Lowered Terraces
As soon as an underground terrace creates a difference in level with the surrounding natural ground, the question of guardrails arises. French regulations require a fall protection device when the height of the fall exceeds a certain threshold. This threshold applies to both the interior side (access to the terrace from the garden) and the exterior side if the terrace adjoins a slope or wall.
Planning for the guardrail from the excavation plan avoids having to redo the foundations later. The fixings must be anchored in the retaining wall or in a dedicated beam, which means integrating reservations into the initial formwork.
This technical point is rarely anticipated by individuals embarking on a self-built underground terrace project. The cost and complexity of the guardrail increase with depth, and overlooking this at this stage can hinder the compliance of the structure during an inspection or resale.
The depth of an underground terrace is not chosen from a catalog. It results from the intersection of soil type, local frost depth, drainage plan, and safety obligations. Consulting the local urban planning document and conducting a geotechnical study remains the most reliable starting point before commencing excavation.