Foundation Damage due to Fungus Rots

Dear reader, still there have many foundation constructed of wood. The main draw-back of timber foundations materials is deterioration under many biological and chemical action; sources are obviously from ambient environment.

Dear reader here we will learn about rotting and we have already discussed about insect attack on wood foundation. Dear reader we like to include a great palace that were founded over 13569 piles and these piles were all timber pile. You may be surprised to hear that this palace was built in 1648 and still it is standing quite good condition. Dear reader we are talking about Royal Palace of Amsterdam. So why are we discussing about rotting of timber foundations?

The palace was renovated in 2005 and left opened for visitor in 2009. But the renovation was only removing asbestos. We know health hazard associated with asbestos.

Timber is locally available foundation material in many regions of the world and historically this has been good and reliable material for constructing home and foundation. Still now it is more affordable material than steel or concrete.

As piling material, it is a popular choice but have low load bearing capacity than steel and concrete. Another consideration that should include in designing timber pile is rotting and subsequent insect attack.

Rotting is synonyms to decomposition which defines a process which break down organic substances into new simpler form. Now necessary supply of following elements is required to commence rotting:

a. Water

b. Oxygen

The royal palace of Amsterdam is still surviving today as these numerous piles remain below groundwater table. These piles have sufficient water but lack of dissolved oxygen.

Fungus grow vigorously using nutrient available in wood. But these have also a controlling factor; temperature. The temperature within (0-40)0C is essential for such growth. The wooden foundation elements must have not less than 20% water (in respect of their dry weight) to grow fungus with the wood.

Sometimes a foundation elements that is designed to have below ground water table, may suffer fungus rotting, as ground water is lowered with seasonal or other many man made changes in ambient environment. So if a foundation is suspected to subject water table fluctuation, the wood/timber has to be preserved by pressure treatments. With pile foundation, rafts are also subjected to rotting when water table drops below top of foundation.

We will learn about these preservation method in our upcoming posts

What is the Interference Effect in Closely Spaced Foundation?

We know raft foundation is adopted when bearing area required to support superstructure loading is equal or close to available floor area. But let’s consider-required bearing area is very close to available floor area and we want to avoid raft; what should be foundation design consideration?

This means there are very small gaps between footings or footings have to be placed in very close spacing. Sometimes footing may even touch each other. Now the question arises about interference effect. We will uncover the answer of the doubt about their structural behavior.

This effect of interaction between footings has been considered in many studies of researchers. They found a parameter that defines whether the interference should be considered seriously or not.

The parameter is angle of shearing resistance. When the value of angle of shearing resistance is low, the interference is too low to be considered negligible. In case of high values, this effect is significant. When a foundation surrounded in both sides by foundation, this effect is very significant.

Another parameter that influences interference effect is length to width ratio of foundation. When the length to width ratio tends to unity, this effect is found reduced.

In analyzing punching shear failure, interference effect needs not to be considered. For these reasons, it is recommended not to consider in design and analysis. However, a design professional should be careful about possibility of existence of them in special circumstances.

Why are Timber Piles Substituted by Concrete Piles?

Once timber piles were very popular as deep foundation; still they have importance in foundation construction and many building foundations are also supported by this type of deep foundation, especially light buildings. Now concrete pile is more popular and it becomes a versatile deep foundation element.

The main issue is durability and goes against timber pile, but the economy goes in favor of timber pile. Here we will discuss about a comparison between timber and concrete piles to explain why concrete piles are becoming popular day by day. These are:

a. The main problem with timber pile is durability and durability is also related to availability of water. For both concrete and timber, water is the vital issue about durability.

Except water-logged area, the water found around deep foundation is at or below ground water table. Concrete can be made waterproofed, porous, resistant against harmful chemical, in one word, concrete can be modified to have any degree of corrosion/deterioration protection. Thus durability of concrete piles is not a function of ground water table. Whereas the cut-off level of timber pile must be below ground water table.

Where ground water table means level of permanent ground water level as the level may vary due to seasonal change in ambient environment.

b. The concrete piles can be constructed to any size and shape and large size of these piles offer greater bearing capacity which limits number of piles in a pile group.

c. The embedment required for pile group, known as pile cap, becomes smaller in size as less piles requires for each group, which save concrete and reinforcement.

d. Any length of pile can be constructed with concrete. Pile length depends on bearing stratum and available skin friction. Now piles of more than 30 m length are very common.

e. Concrete piles can be applied in marine environment without having any treatment. The mix design is done carefully to have resistance against exposure to water, here also permanent exposure level to sea water is important as above these oxygen is available to emphasize corrosive environment.

f. Another important fact is connection with pile cap. Concrete piles usually cast monolithically with pile cap. The rigidity of connection is dependent on detailing of pile cap. We will learn these in our upcoming posts. Except pile cap, concrete pile can be corrected by grillage foundation.

In case of timber pile, the connection cannot be done perfectly.

Some advantage of timber piles cannot be ignored but considering durability, constructability and imperfection in connection they become less popular than concrete pile.

Requirements for Steel Grillage Foundation (IBC)

Dear reader in our previous post we have discussed about many types of foundation, but so far grillage foundation has not been discussed. Dear reader we will just discuss here about IBC requirements for steel grillage foundation. At first we like define grillage foundation in few words.

This foundation consists of sleeper with steel framework or some sleepers with some steel beams which will transfer loads to ground from support.

The sleepers should be made of wood, concrete or steel.

Generally they are used to support steel columns having heavy imposed load of superstructure on soil poor bearing capacity. This type of foundation is light and economical too; at the same time they can be placed at shallow depth.

To achieve attachments, grillage foundation should be constructed with sleeper support that transfers load (horizontal component) to sleepers. Sleeper supports are fixed rigidly to foundation beams.

International building code provided some requirements for steel grillage foundation; only steel grillage foundation is included in code, as follows:

-Grillage foundation of the structural steel shapes should be kept separate by using steel spacer approved by building officials.

-The spaces left between shapes as discussed above should be entirely filled with cement grout or concrete

-The encasement of concrete should not be less than 152 mm (6”) at bottom portion and no less than 102 mm (4”) at any points.