Engineering and Construction Matters
With Innocent Chatikobo
Often times, people lose their hard-earned cash in trying to repair their buildings which would have cracked. One of the major causes of buildings developing cracks is because of fundamentals that would have been ignored at foundation level.
In engineering, a foundation is the element of a structure which connects it to the ground (or water as in floating structures), transferring loads from the structure to the ground.
Types of foundations
There are two main categories of foundations namely Shallow and Deep foundations.
Shallow foundations — It’s a foundation that transfers structural load to the earth very near to the surface. They are less technical and more economical and most widely used for light structures. The footings are always wider than the members they support. A structure is not limited to one footing — depending with structural design and soil conditions —multiple types of footings may be used in a construction project.
Examples of shallow foundations include:
i. Wall / strip footing: A continuous strip that supports structural and non-structural load bearing walls
i. Isolated /single column footing: Square, rectangular or circular slab supporting structural members individually.
These are used when structural load is relatively low; columns widely spaced and soil’s bearing capacity is adequate at a shallow depth.
iii. Combined footing: When more than one column shares the same footing. This is used when spacing columns. Its is used when spacing of columns is too restricted.
iv. Strap footing: When individual columns are connected to one another with the use of a strap beam
v. Mat/raft foundation: A single continuous slab that covers the entire base of a building.
They support all the loads of a structure and transmit to the ground evenly. Due to its nature, this foundation type is favoured when individual footings are not possible due to the low bearing capacity of the soil. Its suitable for structures such as silos, chimneys and large machinery.
In my years of practice, I have also witnessed this being used in in WASH programmes to support tank stands. This foundation type can also be called a floating foundation.
Deep foundations —It’s a foundation that transfers building loads to the earth further down from the surface to a subsurface layer or a range of depths. This is the type used for skyscrapers.
A deep foundation has structural element known as a pile or piling which is driven or drilled into the ground in situ at the building site. Reason for recommending a deep foundation over a shallow foundation include: very large design loads; poor soil at shallow depth and site constraints.
What determines the choice of a foundation
There are several factors that determines the selection of a suitable foundation for buildings whose details we shall not get into:
i. Economic feasibility
ii. Climate and weather conditions for example rainfall patterns
iii. Building size and height
iv.Soil characteristics – type/nature of soil; depth of the water table; depth of the different layers of soil; and soil bearing capacity at different levels (for large structures)
v Type of structures in neighbourhood and the success of foundations used there
Foundations solutions in swelling soils
Foundations for any structure require stable geological conditions. Special measures have to be adopted for the foundations including ground improvement, eg through blinding.
There are two main categories of soils namely : expanding clays and non-expanding clays
It is important to identify the soil type to design the suitable foundation. Non expanding clays are not problematic in construction therefore we will not delve much into them.
The problem of expansive or swelling soils is linked to
i.The presence of smectite minerals
ii. Changes in moisture content of the soil — cyclical water content changes
In semi arid areas, the ground loses water through evaporation during drought/dry periods, and this produces volumetric contraction of the soil thus creating fissures in the soil.
When dry spell is over, rainwater penetrates the fissures and causes saturation of the clays, thereby swelling back towards its original value.
These movements happen over the depth known as the active layer which happens to be the depth affected by climatic changes.
Volumetric changes due to contraction and / or expansion can also be caused by changes in moisture content below buildings due to leaking of pipes or by the presence of vegetation and trees especially roots.
Swelling and shrinkage of soil causes damage through those changes in volume. It is therefore imperative that thorough ground clearance is carried out prior to commencement of construction works.
Therefore, when building foundations in areas with unstable soils and areas with high water content there are various remedial measures that can be used to solve these problems and Engineers are always there to assist in these processes.
Examples of remedies include but are not limited to the following and all must be done with a competent Engineer’s guidance:
a)Removal of unstable soils and replacement with stable ones
b)Use of special foundations
c)Use of reinforced concrete footings
d)Use of reinforced floating foundations
e)Use of pile foundations that can be replaced by jet grouting techniques
f)Use of shaft foundations where trench is filled with concrete
This week’s questions
1.What is underpinning and what purpose does it serve?
2 In foundation concrete works we have witnessed two ways of mixing being employed by builders
a. Full mixing of cement and aggregates prior to pouring
b. How does quality of aggregates affect strength and quality of concrete?
c. A method whereby they put sand and stone layers in the trench then they spread cement on top.
Mixing is done using a pick or a hoe as they apply water
Is this method acceptable and structurally sound?
Innocent Chatikobo is an Entrepereneur and Engineer by profession with AtroServe Engineering Zimbabwe. He has extensive knowledge and experience in Structural Engineering and Construction. For your comments, views, questions and feedback he can be contacted on the following platforms
Cell : +263 777 950 224 ; +263 712 376 037 ; +263 782 502 732
Email : [email protected] : AtroServe Engineering Zimbabwe