Providing structural
solutions for differential settlement is a challenge that face by many structural Engineers during their career.
Generally ,allowable differential settlement is 20mm ,maximum, between two adjacent columns. This is based on the 75% of the maximum allowable
cumulative settlement for shallow
foundations of 25mm.
Below are few general solutions that widely used to cope the
differential settlement problems;
11.)Provision of
thick /rigid raft foundation
22.)Provision of deep basement to reduce the Net Bearing Pressures
33.)Adopt deep foundation solutions such as Piles
4.)Pre Loading of lightly loaded Columns
55.)Structural Isolation between differently loaded
areas with a Joint
if i get a core sample of concrete in a newly built existing structure and found out based on laboratory test that the compresive strength of the sample is below 3000psi, does it conclude that the structure is unsafe or structurally unsound.
Are there other factors needed to be considered in determining structural soundness of the structure?
Answer ( Eng. Yasela)
Well , whether structure is unsafe or not depends on several factors.
first issue to be resolved , what is the design strength of the concrete that used in design of the structure. Presuming it is 3000 psi, as you have mentioned, below the par values of 3000 psi required further inquire . Further it is important to evaluate by what margin it get failed.
innitially, you can carry out a non destructive test such as hammer bound test though the particular tests are not widely accepted . then you can carry out core drilling and test the sample. by means of these tests, you can evaluate the actual strength of the concrete of the structure.
with this figure you can check the design calculations. many distinguish concrete practice worldwide use factor of safety for the material and strength. if the actual concrete strength figures are higher than these values which is after the applying of factor of safety, the strength of the structure should not be in question
however durability aspects of the structure may compromised due to serviceability conditions such as width of the crack, fatigue etc. these issues are to be examined by a experienced structural engineer.
In bored pile construction , there are few widely spread methods in stabilizing the bored hole. This post discusses these methods in summary.
The most common and economical solution is to use bentonite
and the main idea behind using bentonite is to stabilize the bored hole. Owing to
the high density, it prevents water inflow to the hole. However, if it is a floating
pile or depend significantly on skin friction to resist load then the use of
bentonite is controversial.
If bentonite is not the solution then the next economical
solution is to use temporary casing. Steel
casings are widely used in the industry. However extraction of the casing
during the construction is challenging explicitly in deeper depths.
Inserting a temporary steel casing- checking for versatility of the steel casing using the buble
In that case you can decide for permanent casing and it has
to be ensure that you have considered
the cost allocation for that. This can be decided with the specialized pile
contractor.
The mix design of the
concrete using for piles also play a pivotal role and there are many
specification on this subject. BS 6235,ACI 357,Dnv are few of them…
Generally cement water ratio is around 0.4 for the submerged and minimum cement content
would be 360 kN/m3. Minimum 28 days characteristic strength would be 35 N/mm2. Of
course these are general specification and designer has the authority to adopt
tailor made solution. The Portland cement is preferable and low content of tri calcium aluminate is preferable to reduce
the risk of sulfates attacks, if it is offshore construction explicitly.
