Q.
Foundation is the most essential part of a building structure which transmits the loads of the structure safely to the ground. For the scenarios given below suggest suitable foundations to be used using sketches.
a) This site designated for a high-rise building is situated in a limestone formation area with the existences of cavities at various levels and also sinkholes. (13 marks)
b) This site is located in a weak bearing soil area and the owner wants to build a double storey terrace house. (12 marks)
(25 marks, 2018 Q1)
A.
a) High-rise in a limestone formation with cavities and sinkholes.
Refer to Part 1 BT1 postings:
The high-rise condominium is a huge load and thus, deep foundation is required. Furthermore, the soil type of the ground is limestone, and although some area might be hard, risk of cavities and sinkholes can weaken the support. Thus, the foundation type should ideally be deep foundation with piles.
There are two major classification of piles - the displacement and non-displacement piles.
What we see normally in the piling site are the displacement piles. Displacement piles comprise of solid-section piles or hollow-section piles driven or jacked into the ground. The soil inside the ground is disturbed and displaced. All
types of driven piles are displacement piles. Timber, steel, concrete, precast concrete, composite and micro piles are some of the displacement piles.
The non-displacement piles are formed by first removing the soil by boring method. Concrete is then used to fill the hole. Bored and augered piles belong to this category of pile.
Since the site is located on limestone bedrock which is known for its karstic features, cavities problem caused by acidic solution, exist in certain location. Presence of large cavities can endanger the stability of the building structure, therefore cavity probing is recommended to determine the extent of the cavities. Cavities problem
can be treated by using compaction grouting, pressure grouting or filling with aggregates and then grout, depending on the size of the cavity.
Generally, grouting is done by replacing the slump or soft materials in the infill cavities with stiffer materials such as cement mortar, grout and aggregates so that the imposed load at the rock roof of the cavity can be transmitted through the stiffer materials and avoid the collapse of thin roof. Mortar or grout mix or neat cement can be used as filling materials where voids presence in the cavities.
At cavity areas, micropiles are used instead of the proposed bored piles. Micropiles, which are 300mm in diameter, are bored through the cavity and further into the limestone bedrock where no cavities are detected by the micropile boring. Reinforcement is lowered into the bored hole and followed by grouting.
Since the capacity of micropile is much lower than the bored pile, large numbers of pile and larger pile cap are needed. In addition, if cavity is detected by the micropile boring, cavity treatment is still required to prevent buckling of the reinforcement within the cavity. Compaction grouting is recommended over micropiles due to its cost effectiveness and ease of construction. The selection of foundation is mainly due to the soil condition at that site.
The presence of cavities at certain areas must be treated because that may affect the stability of the building.
Ref:
Jia Hui, Leow. 2005. A Study of Building Foundations in Malaysia. Bachelor of Engineering (Civil Engineering) dissertation. Uni of Southern Queensland, Australia. Case 2.
b) Double Storey Terrace on weak bearing soil area
The foundation type should be sufficient with Shallow Foundation and Strip type for a double storey terrace house. As it is on weak bearing soil, strips could be wider. The load bearing walls should be supported with more strip foundations.
Wide Strip Foundations
Where the soil being built on is especially soft or has a low load-bearing capacity, Wide Strip Foundations may be used. These help to spread the load over a larger area, and are usually reinforced with steel bars.
The width of a concrete strip foundation depends on the bearing capacity of the subsoil and the load on the foundations. The greater the bearing capacity of the subsoil the less the width of the foundation for the same load.
Strip foundations on subsoils with poor bearing capacity, such as soft sandy clays, may need to be considerably wider than the wall they support to spread the load to a sufficient area of subsoil for stability.
The concrete strip could be as thick as the projection of the strip each side of the wall which would result in concrete of considerable uneconomic thickness to avoid the danger of failure by shear.
The reason for the use of reinforcement of steel in concrete is that concrete is strong in compression but weak in tension. The effect of the downward pressure of the wall above and the supporting pressure of the soil below is to make the concrete strip bend upwards at the edges, creating tensile stress in the bottom and compressive stress under the wall. These opposing pressures will tend to cause the shear cracking illustrated in diagram below. It is to reinforce and strengthen concrete in tension that steel reinforcing bars are cast in the lower edge because steel is strong in tension. There has to be a sufficient cover of concrete below the steel reinforcing rods to protect them from rusting and losing strength.
