Thursday, December 13, 2012

The Need for Site Investigation: The designer’s need, The contractor’s need, The client’s need.

Site investigations can determine the soil properties and behaviour which will affect the choice and design of the foundations, the method of construction, and can also affect the design of the superstructure as an economic and viable proposition. So the designer, the contractor and the client all have a ‘need to know’.

Site investigations are also necessary prior to carrying out remedial measures to a failed existing foundation.

1 The designer’s need
The following information does not cover all of the designer’s needs but it may assist in producing the most
economical design:

(1) Is the site suitable for the proposed structure, i.e. can it be built economically on the soil or should an alternative location be investigated or has the right price been paid for the land in the first instance?

(2) The load-bearing capacity, settlement and behaviour
characteristics of the soil.

(3) The effect of the new foundation loading on adjoining structures and sub-structures.

(4) The presence of aggressive chemicals in the soil, e.g. high sulfate content which could attack concrete.

(5) Possible changes in settlement behaviour, i.e. future and past mineral extraction, changes in permeability
and moisture content, danger of running sand.

(6) Shrinkage and swelling characteristics, frost heave susceptibility and vibration sensitivity of the soils.

(7) Water-table fluctuations, tidal effects, sub-surface erosion, seasonal and possible long-term variations.

(8) Change in behaviour of the soils due to exposure during foundation construction.

(9) The advisability and economy of ground treatment.

2 The contractor’s need
Similarly the following information assists the contractor in producing the most economical construction:

(1) The stability of the soil during excavation and foundation construction, i.e. soft mud and similar material
will not support heavy piling frames without matting.

(2) The amount of timbering and shoring necessary to support the sides of excavations.

(3) The need for geotechnical processes such as dewatering, freezing and chemical injection.

(4) The presence of any fill material which must be treated or removed, including health and safety implications inherent in dealing with contaminated ground.

(5) The presence of useful excavated material such as broken rock for hardcore, sand for concreting or suitable backfill material.

(6) The suitability of the ground at excavation inverts as a base for poured concrete.

(7) The need for special plant such as rippers and drills for
decomposed rock, or draglines and grabs where the
ground is too weak to support scrapers.

(8) The ground levels relative to a known datum. (This is particularly important for piling operations where pile cut-off levels are specified.)

(9) The need for any special health and safety precautions due to ground conditions, e.g. dangerous shafts, running sand, etc.

(10) The position and other details of existing services, old foundations, etc.

3 The client’s need
The client needs to know:

(1) If it is worth buying the site.

(2) If the foundations will be slow and expensive to construct.

(3) If the soil conditions are such that there are planning constraints on the proposed building.

(4) If the site contains contaminants for which he is legally

(5) If the soils on the site are combustible.

(6) If methane gas or other dangerous gases exist beneath the site.

(7) If the site is subject to flooding, subsidence or landslides.

(8) If the developable area is likely to be restricted by mineshafts or other sterilized zones.

4 Site investigation for failed, or failing, existing foundations
Failures of existing foundations are often due to changes in local environment such as re-routing of heavy traffic, leaking drains and water mains, new adjoining construction work (e.g. piling, inadequately shored excavations), new fast-growing tree planting, extra load on sub-soil from new buildings and similar. Before carrying out a soil investigation it is usually worthwhile examining such possible causes of failure in the same way that a desk study and a site walkabout should precede any other soil investigation.

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