Excavation and Basements: Planning and Protection

A basement adds floor area to a house, but if excavation, wall stability and damp protection are poorly planned it becomes the most troublesome part of the building.

A basement gives a private house extra space for a plant room, storage, a gym, a home cinema or a garage, without taking any area from the plot above ground. But every square metre below ground comes with requirements that differ from those above it. Planning for safe excavation, walls that resist earth pressure and a space that stays dry all year must begin at the earliest stage of the project.

Before deciding on a basement

The first step is to look at the geotechnical investigation results. The soil type shows how the sides of the excavation will stand, and the groundwater level shows how much waterproofing the basement will need. If the water table is above the basement floor, the job becomes much more complex, and the basement effectively has to be designed as a concrete tank sitting in water.

The second question is what the basement is for. Simple ventilation may be enough for storage. Living or exercise space needs daylight, natural ventilation, thermal insulation and a safe means of escape. These requirements determine the depth of the basement, light wells and the position of the stairs.

It also needs deciding which part of the house the basement will sit under. A basement under the whole house means more excavation and walling but can simplify the foundation design. A basement under only part of the house creates foundations at different depths, and the junction between them needs its own structural solution.

Planning the excavation

Before digging, underground services on the site are located precisely. Damaging a cable or pipe is both dangerous and a reason for work to stop. Once the footprint is marked on site, the dig is made wider than the house itself so that crews have room to build the foundation and apply waterproofing.

Where the excavated soil will be stored is planned too. Soil needed for backfilling stays on site, and the surplus is removed. Placing a spoil heap too close to the edge adds load to the excavation side and increases the risk of collapse.

Supporting the excavation sides

In a shallow excavation, soil can be dug back to a safe slope, with the angle depending on the soil type. When the site is tight or a neighbouring building is close, battering the sides is impossible, and the excavation walls are supported by temporary or permanent structures.

Excavating near a neighbouring foundation needs special care, because it can weaken the ground beneath that foundation. In such cases the condition of the neighbouring building is recorded before work starts, excavation proceeds in stages and the support method specified by the structural engineer is used.

Worker safety is part of excavation too. A safe ladder for access, barriers along the edges and inspection of the sides after rain are mandatory measures.

Basement walls and waterproofing

Basement walls carry not only the building load but also lateral earth pressure. They are therefore usually built in reinforced concrete based on calculation. The junction between wall and floor slab and the construction joints between pours are the weakest points; special waterstop strips or profiles are used there.

Waterproofing is usually applied on the outside, on the surface in contact with the soil, because water pressure then pushes the membrane against the wall. This layer is covered with protection boards so that it is not damaged during backfilling. Since repairing external waterproofing later means excavating again, its quality must be checked as concealed work.

Drainage, backfill and ventilation

A drainage pipe is laid along the basement walls or below the floor so that groundwater does not build up against the wall and is carried to a lower level. Backfilling is done with free-draining material, compacted in layers; this prevents the paving around the house from settling later.

Ventilation is the main defence against damp and mould in a basement. Where natural airflow is not enough, mechanical ventilation or a dehumidifier is provided. If a plant room is to be located in the basement, the ventilation and safety requirements for the boiler room are considered separately.

A short checklist

• What groundwater level does the geotechnical report show?
• How will the excavation sides be supported?
• Are neighbouring buildings and services at risk?
• Are the waterproofing system and junction details shown in the design?
• Have drainage, ventilation and a safe exit been planned?

Zaferoğlu İnşaat carries out work across the full range, from topographic and geological mapping of the site to excavation and roofing. Tell us early what the basement is for, a plant room, gym or storage, because excavation depth and waterproofing follow from it.