In foundation design, there are many practical construction problems and costs to be considered.
The main ones are:
- the foundation should be laid as shallow as possible, commensurate with the climatic effect and strength on the surface soil; Especially in waterlogged land. Digging in severely waterlogged land can be expensive and slow.
- Expensive and complicated shuttering details should be avoided, especially in tighter rafts. The focus should be on buildability.
- Reduction in the cost of deposits, improvements in soil remediation, advances in soil mechanics, etc. have substantially changed the economics of design, and many Standard The solutions are now out of date. Construction costs and techniques need to be constantly reviewed.
- Designers need to be more aware of the assumptions made in the design, the variability of ground conditions, the occasional inappropriateness of sophisticated soil analysis, and the practicality of construction.
- Reliability of soil testing by critical evaluation.
- Impact of construction on ground assets, i.e. vibration from piles, deterioration of exposed ground by excavation in adverse weather conditions, removal of overburden, seasonal changes in water level, compaction of land by construction plant.
- Effect of varying size, length and stiffness of foundations, and the need for movement and settlement joints.
- Full foundation effects of sulfate attack on concrete, ground movement due to frost, shrinkable soil and effects of trees; Also changes in the local environment, such as new construction, re-route of heavy traffic, plant installation in nearby factories cause impact and vibration.
- Rapid but costly construction can be more economical than low cost but slow construction for customers in need of a quick return on capital investment.
- Effect of new foundation loading on existing adjacent structures.
These practical considerations are illustrated by the following examples.
A simple example of excavation in waterlogged land is an example of the problems that may be encountered. At the commencement of the underpinning contract 1–2 m deep in the mass concrete, groundwater was rising much higher and faster than in the previous test pits. Circumstances were such that a minipiling contractor was quickly brought to the site, and rapidly installed, which, at face value, was a more expensive solution, but gradually struggled to build up the mass of concrete while pumping. proved to be less expensive overall. As many of our readers will know, some small site pumps are capable of running for more than two hours without malfunctioning!
At one site a separate earthen fill was placed on top of a soil of uniform soft to firm consistency to a depth of about 2 m.
Since a large industrial estate was to be developed on the site in several phases by various developers, a thorough site investigation was carried out. Yet, on more than one occasion, the project engineer found himself looking down a hole 2 meters or more deep, trying to decide whether to install a large concrete base in fill or virgin ground. to be done, and in any case will it be 100 kN/m . Receive2 Acceptable bearing pressure or not. It emphasizes the importance of engineers getting a first look at the ground by examining test pits rather than relying on site check reports from the relative comfort of their desks.
At one site a contractor cited a small diameter steel tube pile of length 5 m (to obtain a suitable set) on the basis of a site investigation report. In the event his stack achieved the set with an average of 22 m (!), there were obviously cost complications. Furthermore, one of the main difficulties was persuading the contractor to guarantee his piles at that depth, as he was quite concerned about their thinness.
An investigation of the authors’ practice of a particular failure comes to mind as an example. Part of a factory was demolished, which was a party wall, but the 20 m length of this wall was reduced by digging for a new service duct and one. Classic Failure occurred. The exposed excavation was then left open over a wet weekend, resulting in a softening of the face and a collapse early Monday.
So often the most catastrophic of failures result in these kinds of classic textbook examples, which can be prevented with the most basic of precautions.
A new storage silo was to be built within an existing mill, and the proposal was for it to be found on a filled basement, in the same manner as the adjacent silo had been 20 years earlier. The authors’ practice was called for their opinion quite late in the day, with steel silos already under construction.
After checking the fill, the customer was advised to move the new silo to the bedrock through the fill on a smaller diameter pile. This would thus avoid excess loading being put into the fill, and thus lead to the disposal of the existing silo.