Thursday, December 13, 2012

Design Foundations: Practical Considerations.

There are, in foundation design, a number of practical construction problems and costs to be considered. The chief ones are:

(1) The foundations should be kept as shallow as possible, commensurate with climatic effects on, and strength of, the surface soil; particularly in waterlogged ground.

Excavation in seriously waterlogged ground can be expensive and slow.

(2) Expensive and complex shuttering details should be avoided, particularly in stiffened rafts. Attention should be paid to buildability.

(3) Reduction in the costs of piling, improvements in ground treatment, advances in soil mechanics, etc.
have considerably altered the economics of design, and many standard solutions are now out-of-date. There is a need to constantly review construction costs and techniques.

(4) Designers need to be more aware of the assumptions made in design, the variability of ground conditions,
the occasional inapplicability of refined soil analyses and the practicality of construction.

(5) The reliability of the soil investigation, by critical assessment.

(6) Effect of construction on ground properties, i.e. vibration from piling, deterioration of ground exposed by
excavation in adverse weather conditions, removal of overburden, seasonal variation in the water-table, compaction of the ground by construction plant.

(7) Effect of varying shape, length and rigidity of the foundation, and the need for movement and settlement joints.

(8) After-effects on completed foundations of sulfate attack on concrete, ground movements due to frost
heave, shrinkable clays, and the effects of trees; also changes in local environment, e.g. new construction, re-routing of heavy traffic, installation of plant in adjoining factories causing impact and vibration.

(9) Fast but expensive construction may be more economic than low-cost but slow construction to clients needing quick return on capital investment.

(10) Effect of new foundation loading on existing adjoining structures.

These practical considerations are illustrated by the following examples.

Example 1: Excavation in waterlogged ground
A simple example of excavation in waterlogged ground exemplifies the problems which may be encountered. At the commencement of a 1–2 m deep underpinning contract in mass concrete, groundwater was found to be rising much higher and faster than previous trial pits had  indicated. The circumstances were such that a minipiling  contractor was quickly brought onto site, and speedily installed what was, at face value, a more costly solution, but proved far less expensive overall than slowly struggling to construct with mass concrete while pumping. As will be well-known to many of our readers, few small site pumps are capable of running for longer than two hours without malfunctioning!

Example 2: Variability of ground conditions
On one site a varying clay fill had been placed to a depth of roughly 2 m over clay of a similar soft to firm consistency.

Since a large industrial estate was to be developed on the site in numerous phases by different developers, a thorough site investigation had been undertaken. Nevertheless, on more than one occasion, the project engineer found him-self looking down a hole of depth 2 m or greater, trying to decide if a mass concrete base was about to be founded in fill or virgin ground, and in either case whether it would achieve 100 kN/m2 allowable bearing pressure or not. This emphasizes the importance of engineers looking at the ground first-hand by examining the trial pits rather than relying on the site investigation report from the relative comfort of their desk.

Example 3: Reliability of the soils investigation
On one site a contractor quoted a small diameter steel tube pile length of 5 m (to achieve a suitable set), based upon  a site investigation report. In the event his piles achieved the set at an average of 22 m (!), so obviously cost complications ensued. In addition to this, one of the main difficulties  was convincing the contractor to guarantee his piles at that depth, as he was understandably concerned about their slenderness.

Example 4: Deterioration of ground exposed by excavation
An investigation by the authors’ practice of one particular failure springs to mind as an example. Part of a factory had been demolished exposing what had been a party wall, but a 20 m length of this wall was undermined by an excavation for a new service duct and a  classic  failure ensued. The exposed excavation was then left open over a wet weekend, resulting in softening of the face and a collapse occurred early on the Monday.

So often the most catastrophic of failures are as a result of these types of classic textbook examples, which could be prevented by the most basic precautions.

Example 5: Effect of new foundation on existing structure
A new storage silo was to be constructed within an existing mill, and the proposal was to found it on a filled basement, in the same way that the adjacent silo had been 20 years before. The authors’ practice was called in for their opinion fairly late in the day, with the steel silo already under  fabrication.

After investigation of the fill, the client was advised to carry the new silo on small diameter piles through the fill down to bedrock. This would thereby avoid placing additional loading into the fill, and thus causing settlement of the existing silo.

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