Bottoms Up - What type of FOUNDATION is your property sitting on and how it affects your RENOVATION and BUILDING FINISHES in the LONG RUN

Do you actually know what foundations are beneath your home, shop or office? Have you recently purchased a property and thinking of fairly major renovations including additional extensions with 2 storeys or more? If your answers are NO and YES respectively, then it may be fruitful for you to spend 15 minutes to read the following.

Majority of property owners will answer 'NO' to the first question above. Indeed there it is usually not necessary for property owners to know what foundations their building is sitting on. Generally, property owners buy their properties from developers who in turn employ professional engineering consultants to carry out technical designs that fulfill the council, engineering and safety requirements. On top of that, all of the designs must be approved by the council engineering departments and the buildings inspected upon completion of construction, prior to the issuance of Certificate of Fitness (CF) by the local council for occupation. Given all of the above, why would any property owner need to know what foundations are below their building?

If your answer to the second question above is YES, then it is quite important that you do know. How you decide to renovate your property and the compatibility between the new foundations for your extension and the existing building is important in completing your renovations with less long term building defects, such as unsightly cracks and damage to expensive finishes due to building movements. 

It would be a shame to see expensive imported floor marble pieces or imported solid teak floor boards crack or twist due to inappropriate foundation considerations, which could have been avoided with a little more care and consideration during the renovation stages.

'EXPERIENCED' Renovation Contractors

In many instances, renovation contractors will claim to possess '20 over years of experience' or 'ZERO complaint record from previous customers'. However, building problems arising due to foundation issues tend to take some time before surfacing, unless there is a serious 'underlying' problem (pun intended) that was overlooked during design or construction. The problems may occur a few years after the renovation works have completed, after which the owners may not even realise that the defects are actually caused by the initial foundation problems and attribute them to normal wear and tear or aging of the property instead. It is a very common possibility that the original contractor may not even know that there were problems with what they did. No doubt, there are truly experienced contractors out there who have learnt from their past mistakes. The question is, 'Is your contractor one of them?'. Given that, is there any harm for you to better understand the building that you are in?

Types of Foundations

The 2 most common types of foundations for low rise buildings in Malaysia are PILES and SPREAD FOOTINGS. There are other different types of foundations for more complicated structures or poor ground conditions, which I will cover in a different post if the need arise. I believe information on the 2 foundation types above would be most relevant to majority of  property owners.

PILED foundations are used when the upper layers of the ground are too soft to provide proper support to the weight of the building, its contents and occupants. The piles are used to penetrate these 'soft' layers into the stronger layers beneath. The depth to which piles need to penetrate vary and depends very much on the strength of the underlying ground at the location of the building. Some locations have a hard rock layer, which the piles can rest upon. Other areas may not have such a hard layer but a thick layer of stiff soil that can support the piles by friction, similar to gripping a stick firmly to prevent it from sliding through your hands. In many instances, it is a combination of both. The actual engineering technicalities would be too much to digest, but I am sure the above gives you a good idea of what it is.

Below are some pictures of typical pile foundations, commonly seen in Malaysia. These are driven pre-casted concrete piles (using a machine with a drop hammer, similar to hitting a nail into the ground), which you see very often in low-rise housing or commercial developments. Other variations include piles that are drilled (in engineering we use the term 'bored') on site and casted with concrete on site, which is becoming increasingly common due to the fact that city areas with congested existing buildings will be damaged by the vibrations caused by pile driving. There is a range of different pile materials, but the actions in which how it supports the building are similar. When 1 pile is not strong enough to support the weight from the building at any point, groups of pile will be used and they are tied together using a 'cap' usually constructed using concrete, which can be seen in the last 2 pictures in the series below. The pile 'cap' distributes the weight from the building column to the group of piles.

SPREAD FOOTING foundations (also referred to as PAD FOOTINGS by some engineers) behaves as its name suggests. The weight from the building is spread over a larger area by enlarged pad bases. A good example would be snow skis. If you put your weight on your feet when standing on a few feet of snow, you will most likely have half your leg sink into the snow. In contrast, a person of the same weight wearing a pair of skis would be standing there laughing at your misdemeanour. The point is the pair of skis spreads your weight over a larger area, reducing the contact pressure with the snow below. The spread footing does exactly that.

In some locations, a relatively strong layer of ground exist below the ground surface (normally within 2 to 2.5m from the surface). In these instances, it is usually more economical to use concrete footings as foundation support for the building. The footing effectively spreads the weight from the building column over an area, so that the contact pressure is lower than what the ground can support. Below is a series of photos showing typical spread footings under construction.

 

So How Does The Foundation Type Affect Your Extension Renovation?

Incompatible Foundations

The main problem that usually arise is the difference in the amount of vertical movement of foundations. All foundations will move downwards, as the weight of the building increases during construction and subsequent occupancy, this is termed 'settlement'. The ground beneath the foundations will compress or in the case of piles slip slightly from the friction grip of the ground (in some special cases, foundations move upwards due to the expansion of the ground underneath caused by unloading of the foundations and the ground reabsorbing water content; similar to a spring action). The main question is how much do adjacent foundations move RELATIVE to each other.

In general, piled foundations settle less then spread footing foundations before reaching their full load support capacity. As such, problems usually arise by pairing up piled foundations next to footing foundations with a building structure above connected rigidly. The footing foundation will settle more than the piled foundation. If the building above is built as a solid structure without any allowance for such differential settlement, the structural components will have to crack to accomodate such differences in vertical movement.

Have a look at the simple sketch below. When the footing foundation on the right settles downward more than the piled foundations on the left, the structural concrete beam above will have to rotate clockwise to accomodate such movement. This results in cracks forming on the beam, since the beam was built rigidly as  a straight beam. As such, any finishes on floors and walls within these zones would be susceptible to cracking, spalling (pieces falling off) and tearing. If it is exposed to rain, severe rain water penetration and leakage may occur. In a more severe state, the rain that penetrates the structure can cause corrosion and eventually structural problems.

Settlement Behaviour and Its Effect

Another important aspect of settlement is that there are 2 main periods of settlement. Short term and long term. When a foundation is loaded, there will be an immediate settlement due to the weight from the foundation causing the ground beneath to compress and water to be squeezed out of the foundation zone. In areas where the ground are mainly sandy or granular (which means the ground is relatively full of  tiny holes, termed by engineers as 'porous') the water is squeezed out relatively quickly and the foundation settles quickly with low long term settlement.

However, in certain areas where the ground comprises of less porous material such as clay, the water is squeezed from beneath the foundation very slowly. In many instances taking years before the foundation reaches a balanced state. Under these conditions, it is not uncommon for foundations to have larger long term settlements over many years.

Most cases would be a mixture of the 2 scenarios above, due to the fact that the ground underneath usually comprises a mixture of layers of different materials.

This brings us to the important point of how this affects your property?

In the previous scenario, we looked at how piled foundations are generally incompatible with footing foundations in terms of settlement. However, if an extension is sitting on newly built footing foundations and connected to an old building, the same problems may arise. Since the old building have experienced majority of its long term settlement under the building's own weight for so many years, the new extension sitting on newly built footing foundations have yet to experience majority of its long term settlement. As a result, there may not be any noticeable problems in the first few years, but cracks may start to appear soon after and worsen over time. In fact, even if the cracks are repaired, they may open up again due to the fact the new foundations have not reached the stable period of its settlement regime. Therefore, one should always be careful whenever a new extension is built and CONNECTED to an EXISTING building.

Additional Weight or Altering the Weight Distribution In Existing Building Foundations

One common problem with renovations carried out by property owners are to revise the layout of the spaces within the building. This generally requires the removal of existing partition walls (commonly constructed using bricks in Malaysia) and the building of new ones in various locations. This effectively changes the weight that each set of foundations are designed to carry. This also assumes that the building structural beams and slabs can carry the weight of the walls at the new locations, which I will write a seperate post to cover this topic.

Engineers design foundations with a certain amount of safety factors to account for unforeseen additional loads. As such, the likelihood of exceeding the maximum capacity of the foundations causing failure due to the relocation of a few partition walls is not very high. However, the increased permanent load on any foundation will cause it to settle further over time. Cracks similar to the previously discussed scenarios will appear, when the overloaded foundation settles more than the other foundations surrounding it. Again, such problems do not manifest until a certain period of time after the changes have been made. By which time, expensive finishes would have been installed and susceptible to cracking and damage when the foundation settles slowly.

So How Can We Reduce or Prevent Such Problems?

The fact is, settlement WILL OCCUR and there is no stopping it. It is only natural. As such, the best decision any property owner can do would be to work along side this phenomenon and not try to fight against it.

I will list some of the possible considerations that can be used to reduce problems arising from settlement of foundations.

Increased Footing Size

One option, if there is sufficient land space, would be to increase the size of the new footing foundations. As explained earlier, the larger the area you spread the load the lower the pressure acts on the ground and hence resulting in reduced settlement movement. The foundations will still settle but less due to the reduced pressure. This will assist in reducing movement cracks due to the difference in settlement between new and old foundations.

Avoid Brittle Finishes At Joint Location Between New and Old Building

Care should be taken to avoid placing any expensive, hard and brittle finishes directly over the link joint between the old and new building. Finishes placed here have a high chance of cracking and damage. Avoid having a wall crossing perpendicular at 90 degrees to the joint location. Severe cracks may result when the new building settles more than the old building.

It would be a good idea to architecturally hide this joint location without tying across the joint between the old and new buildings.

Floor finishes can stop short on either side of the joint location. A wall parallel along the joint line can be placed on one side of the joint and a skirting or architectural piece installed on one side of the joint used to aesthetically cover the joint line.

Finishes that link between the new and old building should be avoided.

Maintain Weight or Weight Pattern Acting on Foundations

When altering  the layout of your property rooms and spaces, it would be useful to try and maintain the overall weight acting in the same zone or area. This will reduce excessive further settlement of existing foundations. For examples, since brick walls are built  from the floor up to the ceiling they are similar in weight wherever your put them. If you remove 3m length of wall in one area and reposition it to a different direction, then it would be  ideal for the new wall to be approximately 3m in length as well. This needs to be considered with the knowledge of where the existing columns and foundations are in your property. 

Therefore, it is necessary to obtain a copy of the structural plans from the developer or previous owner whenever you purchase a property. Otherwise you will always be taking a risk with your renovations if you do not.

To Conclude

There are many possibilities how foundations can give rise to problems. Ideally, all changes that affect the weight or layout of the building should be referred to a civil and structural (C&S) engineer for advice. The primary objective of this post is to generate the awareness among property owners on the importance on knowing what foundations their property are founded on, so that you can make better informed decisions and start posing the right questions to your renovation contractor and consultants. Many 'professionals' often forget some of the problems I highlight above, as they rush to complete their work, design and drawings.