Taking on a House With Underpinning Work : What to Know

Underpinning is a technique that is essential to ensure the stability of a building whose solidity is compromised due to a natural disaster, upper floor renovations with excessive added weight to support, or due to a foundation being poorly adapted to the ground where it is set.

This renovation project has complex ramifications. Thus, in order to demystify it, here’s a look at the important aspects to consider before embarking on this major transformation.

Taking on a house with underpinning work: what to know

Photo: Construction Keven Roy Inc

Underpinning a home may be necessary for a variety of situations. This renovation technique may be essential when the dimensions of the foundation are insufficient, as a result of the weight of the home being too heavy. This could be a result of an extension, the construction of an extra floor, or the addition of new furniture.

This technique may also be used for projects that require modifying load-bearing elements, such as the removal of a load-bearing column, as this type of transformation can cause vertical deformation of the building.

Preliminary studies before taking over a house with underpinning work

Photo: Gauthier Solution Inc

You may quickly notice that there are several different techniques when it comes to underpinning a home. However, it’s essential to carry out some pre-project studies in order to know the appropriate method to adopt for this type of project. If this study is carried out in compliance with the building code, then there is a good chance that the recovery will be successful.

The soil

Special attention needs to be paid to the soil in order to define the nature of the terrain. It’s recommended that you carry out an assessment of the soil via a study of samples. This analysis will need to be carried out by a geotechnical engineering office. They will be able to analyze the section of the ground and define how much resistance each layer of soil possesses. Additionally, it will be necessary to determine the positioning of the groundwater table in relation to the foundations. During bad weather, the water table is higher, which can cause serious flooding or water infiltration issues at the basement level.

The foundation

It’s important to understand that the recovery in this process is done at the level of the foundation. The condition of the foundation and its depth should be carefully checked.

The structure

Another element that relates to this project is the structure of the building, which must be studied meticulously. This study is largely intended to determine the supporting elements and guide the contractor when it comes to how to properly distribute the loads during the execution of this work

Techniques for underpinning a house

First of all, you should know that there are several house underpinning techniques. The main methods for this are the use of micropiles, solid injection, and jet grouting. The choice of the appropriate technique will strongly depend on the type of soil, the structure of the building, the condition of the foundation and the layout of the basement. 

It’s quite difficult to establish a price range, as it depends on the nature of the terrain, the obstacles that are encountered and which are specific to the condition of each stage of construction. For this reason, it’s important to ask for 3 or 4 quotes from different contractors in order to compare prices. 

The initial costs to consider for the underpinning of a corner of home are approximately $450 per linear foot. If you are underpinning a house as well as adding foundations, budget an amount of $1100 per linear foot. 

Micropiles in the case of flood or drought

If the ground is clay-like, any signs of drought will be likely to weaken it and to reduce the solidity of the foundation, causing the ground below to be quite unstable. Therefore, any movement in different parts of the foundation (often called differential settlement) will cause the appearance of cracks. 

The use of micropiles (140mm borehole and fitted with a metal tube that’s filled with cement grout) is the recommended solution for this situation. The drilling can be carried out directly into the foundation or into the surrounding pavement. Finally, note that the upper end of each micropile will be linked to the foundation. 

The dimensions (length and diameter) of the micropiles will depend on the nature of the soil and the loads that are in need of correction. Consequently, the soil should be studied beforehand in order to understand the structure of the foundations, while being able to identify the initial source of cracks on the structure if necessary.

Solid injection in the event of upper floor overload

The solid injection process aims to inject a low-flow non-liquid material into a metal rod drilled to an appropriate depth. The injection is stopped according to the predetermined pressure or volume. 

This technique makes it possible to substantially increase the solidity of the soil as well as its relative density, which makes it the ideal solution for the necessary reinforcement of the foundation generation by an excess of the load to be supported. Note that this technique greatly reduces the costs related to modifications to the foundation. 

Jet grouting for a lack of solidity

Jet grouting uses high capacity hydraulic pressure to create columns into which concrete will then be injected. The drilling (with a diameter of between 80 and 120 mm) initially creates a horizontal space at the end of the columns which will also be filled with concrete and serve as their basis. 

Note that jet grouting columns installed under or against piles provide additional support to foundations since they lack solidity due to their poor adaptation to the type of soil on which they were built.  

Precautions when choosing the right contractor

Given the risks of the house collapsing as a result of poor planning or execution, the choice of contractor is paramount. In this regard, the contractor you work with must have all the proper licences (every province is different) on top of liability insurance. This project requires the expertise of a certified contractor to ensure that the work is done without risking irreparable harm to the structure of the house. 

In addition, it’s important for the implementation of this type of project to do business with a specialized contractor who will be able to carry out the preliminary study on top of the implementation of the project. The contractor must present an elaborate plan for undertaking the work and carry out constant monitoring of its progress, as unforeseen events are common with this type of project. 

It’s strongly recommended that you ask for references before starting this type of work in order to obtain an additional guarantee concerning the quality and durability of the work carried out.