Extension Basement

Extension basement slab & walls

When Beautiful Basements arrived on-site, the supporting kingpost system had been installed to ensure the team could work in a safe environment. Stage one of this project was preparing the site and creating the below basement slab and walls.

Beautiful Basements prepared the base of the basement by leveling the excavation and casting it in concrete, allowing the construction to begin. The team set out the basement, ensuring it was in the correct location for the extension above. Then, they erected shuttering for the 450mm slab around the perimeter and installed all the slab steel reinforcing, including the upright starter bars for the basement walls above.

Beautiful Basements’ team installed two ground water sumps within the slab for the waterproofing systems that would follow. They also placed four concrete pillars, equally spaced, that would link the basement slab and ground floor deck together. The team ensured the steel work for these columns was placed exactly, with just 50mm of space on all sides, which was their first major technical challenge.

As part of the slab, workers created an upstand of concrete, known as a kicker, around the perimeter to move the waterproofing weak point away from a horizontal surface. They finished stage two by casting the waterproof concrete slab.

Beautiful Basements’ built the walls for the basement in four stages as the engineers had designed. They treated the kicker with a waterproofing slurry and installed an expanding waterbar. The team erected the shuttering and steel reinforcing, ensuring the upright steels protruded from the concrete to allow the deck above to be tied in.  Finally for stage one, they cast each section in turn with waterproof concrete.

Extension basement deck

Beautiful Basements had completed the basement structure in stage 1. The next phase of the build was to construct the deck over the area. This presented the team with their biggest challenge of the project. The size of the deck (over 153m²) and the height (over 3.4m) at which they cast it was a physical challenge for the team. The main technical challenge was linking the four columns with the thickened sections of the deck.

The team placed shutters into place, one by one, supporting everything from below. They created thickenings throughout the deck, linking all four columns underneath. All the shuttering was placed around the perimeter, including the stairwell and steel laid all across the deck.

When complete, the whole deck was cast with waterproof concrete. Twenty-eight days later, with the concrete cured, the team stripped out the shuttering and formwork.

Garage retaining walls

After the deck concrete had cured fully and been stripped, the team built the ground floor retaining walls. The steel reinforcing links the basement slab, walls, deck, and the retaining walls, creating one monolithic structure. At the head of the retaining walls, they installed closers, finishing the structure. Beautiful Basements’ team shuttered the walls, fit the final reinforcing, and cast the walls in waterproof concrete.

Ground floor slab to garage

The final section of the build element of the project was the garage.

The Beautiful Basements’ team completed the main section of the basement and extension and then moved on to the four-car garage. They tied the slab into the main extension and then shuttered it. The joint between the slab was waterproofed with a cementitious slurry to ensure waterproofing integrity. Steel reinforcement was installed by the workers. As with the slab for the basement, they cast the slab with a vertical upstand known as a kicker to improve the waterproofing effectiveness.

Retaining walls

At this part of the site, the ground behind begins to step down, so Beautiful Basements’ team created the steps within the retaining walls. They shuttered the walls, installed all the steel with an expanding waterbar as part of the waterproofing.

They created the steps down in the walls as part of the shuttering and cast the walls with waterproof concrete.

Waterproofing

The project had to meet waterproofing requirements for a dual system as set forth by local authority building control (LABC) and 3rd party warranty provider regulations. Beautiful Basements’ Certified Surveyors in Structural Waterproofing (CSSW) designed all the waterproofing, and our dedicated waterproofing team installed it. The system meets BS8102:2022 standard for a grade 3, dry environment.

Basement primary waterproofing system

Slab & Walls Type B – Integral protection

Workers cast the basement slab and walls are cast with concrete including a waterproofing additive. A kicker in the slab moves the weak point up and away from a weak point.

Construction and day joints have a crystalising waterproofing slurry and an expanding waterbar applied to them to waterproof natural weak points.
The concrete deck over the basement was cast with the same waterproofing additive to prevent damp transfer across the ground floor.

Basement secondary waterproofing system

Type C – Drained cavity protection

At the base of the walls, the team created a recessed concrete channel within the slab. All walls and the floor are lined with a cavity drainage membrane (CDM). The membranes are linked to sump chambers via the recessed perimeter channels. Within the sump is two pumps and a high-level alarm. This system is designed to catch any water that is not stopped by the primary system and discharge it automatically. Flushing and maintenance points were installed to allow future servicing of the system.

Retaining walls & garage primary waterproofing

Garage Slab Type A – Barrier protection

Before the garage slab was cast, the team laid blinding concrete so they could then line it with the slab covering the external bentonite waterproofing membrane. They took great care around penetrations such as drainage pipes. The team laid the steel very carefully to avoid tears and defects and cast the concrete inside the membrane.

Retaining Walls Type A – Barrier protection

Beautiful Basement’s team cast the walls in waterproof concrete first. Once complete, they primed all external surfaces of the basement with a bitumen-based primer. They covered the walls from top to bottom with an external waterproofing membrane, overlapping it with the slab membrane at the bottom.

Retaining walls & garage secondary waterproofing

Slab & Walls Type B – Integral protection

Workers cast the garage slab and all retaining walls are cast with concrete including a waterproofing additive. A kicker in the slab moves the weak point up and away from a weak point.

The team treaded all construction and day joints with a crystalising waterproofing slurry and an expanding waterbar to waterproof natural weak points.

All waterproofing systems were designed by Structural Surveyors in Structural Waterproofing (CSSW) in accordance with BS8102:2009

All waterproofing materials were supplied by Triton Systems Ltd