Thursday, December 13, 2012

Preliminary Site Reconnaissance and Site Walkabout.

With the above information, presented clearly in an easily digested report, the senior design engineer should visit the site and the immediate neighbourhood to develop a feel for the site. It is sometimes advisable for the senior engineer to visit the site before the ‘study of existing information’ so that assistants can be advised on important points, such as where there is a particular need for detailed study and the like in carrying out the investigation.

The senior engineer would note the soil type and condition in any adjoining cuttings (road, rail and stream banks), adjacent buildings showing signs of foundation distress, uneven ridge lines, tilting or settled boundary walls,  unstable or creeping slopes, depressions in the ground  and their possible cause, type and changes in vegetation  on  green  sites, previous use and ground behaviour of  abandoned sites and similar points.

Typical warning signs of possible foundation difficulties are:

(1) Unused sites in built-up pre-war housing estates which can indicate that local builders had encountered site problems.

(2) Flat, rubble-strewn derelict sites in inner-city housing areas which may be riddled with backfilled base-
ments, cellars and bomb craters (unexploded bombs remain a distinct possibility).

Dry, firm ground in summer which is sprouting marsh grass may be a quagmire in winter. Many cases of
landslip slope failure are caused by water, so the identification of possible sources of groundwater on both historic maps and on site is most important.

(4) Undeveloped areas around the outskirts of towns and not encroaching on green-belt boundaries which can indicate problem sites.

(5) Backfilled quarries; domestic refuse and industrial waste tips.

(6) Bumpy, irregular ground surface which can be indicative of glacial terminal moraine deposits.

(7) Evidence of ‘bell-working’ where mineral seams are near ground level.

(8) ‘Blow holes’ in chalk soils.

(9) Subsidence in areas of brine extraction.

(10) Evidence of erosion or deposition. Where structures are to be founded on coasts, estuaries or tidal rivers, then full hydrographic information on extremes of tides, velocity of currents, seasonal levels, flooding danger, etc., must be obtained.

(11) Warm soils in winter months or burnt shales indicating possible combustion.

Problems of confined access, overhead cables or steeply sloping sites should be noted since this can affect the soil investigation equipment and the contractor’s excavation and piling plant.

Knowledge of the position and type of the proposed structure is important so that particular attention can be given to areas where deep excavations for basements, heavy loads and the like are to be located.

It is useful for the senior engineer when visiting the site  to be assisted by a young engineer to make notes of any observations and to take photographs and soil samples.

This saves the senior engineer time and gives the young engineer valuable experience. The senior engineer should write up the notes and report any findings while they are fresh in the mind. Where possible the findings from the study and reconnaissance should be shown diagrammatically on the site survey plan. This enables a clearer image of site conditions and aids the planning of the soil survey.

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