In most cases, a new basement construction will need extensive underpinning works carried out. As a result, in the below we will cover what you need to know and how this works.  

Image of newly installed reinforcing bars to a shallow RC Underpin

What is Basement Underpinning ?

Underpinning is a method used to increase the existing foundations depth by extending it depth wise. For this reason, this is suitable for the construction of new basements beneath existing properties. In addition, the installation of the Underpins can be achieved from one side of the wall which makes the construction process practical.

Factors and conditions which can dictate a certain design

There are a few site condition related elements which the structural engineer will need to confirm. This will enable him to carry out his final design. Such design elements can be:

1. Are there any adjacent properties with basements developments ? A site survey, consulting the planning registry of your local council or visual inspection can confirm if this is the case. If so, the engineer will incorporate the depth and width of the installed underpins in his design.

2. Land Stability – design and construction phase. The Basement Impact Assesment will determine the appropriate measures. These measures may include: the control of ground water during the construction of the basement; the monitoring of structures before and during the basement development.

3. Soil conditions and ground water – carrying out a borehole site investigation will assess the soil conditions and ground water. Consequently, it will also reveal the underlying soil strata, in-situ soil parameters as well as soil make up.

Please Note: Underpinning works is a major and extensive undertaking to a property. As a result, a qualified and experienced structural engineer should carry out the design. Furthermore, your appointed contractor will need to draft a construction methodology. The proposed methodology is than reviewed and approved by the appointed structural engineer. This needs to demonstrate that the underpinning works and new basement construction will not have any detrimental impact on the stability of the existing or the neighbouring properties. 

Underpinning types

The most commonly used methods of basement construction in London as well as throughout the UK are: Mass Concrete Underpins and Reinforced Concrete Underpins (RC Underpins).

Mass Concrete Underpins (made up of concrete only) – the installation of Mass Concrete Underpins will be less costly. Thus, you will need to bear in mind that typically these underpins will be wider. This will result in the loss of internal space. Moreover, during design, the structural engineer might request the installation of a further retaining wall in front. This will carry the lateral soil loads and hydrostatic pressure.

Reinforced Concrete Underpins (contain steel reinforcing bars or prefabricated mesh elements) – these types of underpins will have a steel reinforcement installed. Usually two layers, one to the outer face and one to the inner face. The advantage of these foundations is that the width is greatly reduced. For this reason, most designers as well as owners will opt for these. For this type of underpin, the owners of the adjoining properties will need to agree and sign a ‘Special Foundations Consent’.

Piling

Piling – it may be necessary to install deeper foundations which can be in the form of Contiguous or Secant piles. Design of these piles will depend on the results of the site specific soil investigations. This will be assessed once the underlaying soil conditions are confirmed. Hence, piles can form part of the permanent structure and usually a further retaining wall is to be installed to the front. Depending on the design, restricted access piling rigs are available which can even fit thru a standard doorway.

Note: underpinning of a foundation as well as piling is notifiable work under the Building Regulations. Therefore, you must ensure an application is made to either your local building authority or a private building control. They will carry out regular site visits to ascertain that the works are completed according to the design.

Installation

The installation of underpins is a specialised process. Consequently, companies who have extensive experience of working within the basement industry should be carrying out such works. For a better understanding, please see below typical sequence for the installation of an underpin:

Basement Underpinning sequence designed by structural engineer

1. Commence localised excavation. Install underpins measuring no more than 1 meter in width. Construct underpins in a hit and miss sequence to avoid undermining the existing foundations.


Underpinning installation with trench sheets

2. Prop the 3 sides of the excavation with interconnected trench sheeting. Install a further cementitious board to the face of the excavation. This will form back of the underpin. Installation of all the above elements will provide safety to the site operatives as well as result in the limitation of settlement.


Pouring of toe of underpin in a basement construction

3. Once the excavation reached the designed formation level, in case of a RC underpins, the installation of the shuttering which will retain the concrete will follow. Typically following sequence will be the pouring of the toe (heel) of the underpins.


Completed underpin in a typical basement construction in London

4. The removal of the shuttering will follow Once the the concrete has gained sufficient strength. The installation of temporary shoring will follow. On completion, dry pack is rammed in between the newly poured underpin and underside of the existing foundation.

Monitoring

A requirement which can be imposed by the appointed engineers is the use of a monitoring system during structural works. In effect, this system will detect any structural movements to your or any adjoining properties. In summary, this involves the installation of target points throughout the surveyed buildings by an appointed Movement Monitoring Company . At the agreed intervals, the company will carry out readings of movement levels. The appointed engineers will review and sign off the circulated reports. This report will also include base readings and measurements taken during each site visit. As a result, the appropriate actions can be taken if movements are detected,

Note: It is not unusual to have settlement (hairline) cracks during a basement development. In fact, these usually require no more than minor decorative intervention only.

Target location for monitoring during underpinning and basement excavation in London

Typical set out for target locations during basement underpinning works. The installation of this particular system will safeguard the existing structures during the underpinning and basement excavation process.