Information required from a site investigation - Foundation.

Wednesday, February 5, 2020

For geotechnical categories 2 and 3 the following information should be obtained in the course of a site investigation for foundation engineering purposes
(a) The general topography of the site as it affects foundation design and construction, e g surface configuration, adjacent property, the presence of watercourses, ponds, hedges, trees, rock outcrops, etc, and the available access for construction vehicles and plant
(b) The location of buried services such as electric power, television and telephone cables, water mains, and sewers.
(c) The general geology of the area with particular reference to the main geological formations underlying the site and the possibility of subsidence from mineral extraction or other causes
(d) The previous history and use of the site including information on any defects or failures of existing or former buildings attributable to foundation conditions, and the possibility of contamination of the site by toxic waste materials
(e) Any special features such as the possibility of earth-quakes or climatic factors such as flooding, seasonal swelling and shrinkage, permafrost, or soil erosion
(f) The availability and quality of local constructional materials such as concrete aggregates, building and road stone, and water for constructional purposes
(g) For maritime or river structures information on normal spring and neap tide ranges, extreme high and low tidal ranges and river levels, seasonal river levels and discharges, velocity of tidal and river currents, wave action, and other hydrographic and meteorological data
(h) A detailed record of the soil and rock strata and ground-water conditions within the zones affected by foundation bearing pressures and construction operations, or of any deeper strata affecting the site conditions in any way
(j) Results of field and laboratory tests on soil and rock samples appropriate to the particular foundation design or constructional problems
(k) Results of chemical analyses on soil, fill materials, and ground water to determine possible deleterious effects on foundation structures
(1) Results of chemical and bacteriological analyses on contaminated soils, fill materials, and gas emissions to determine health hazard risks
Items (a)—(g) above can be obtained from a general reconnaissance of the site (the 'walk-over' survey), and from a study of geological memoirs and maps and other published records A close inspection given by walking over the site area will often show significant indications of subsurface features. For example, concealed swallow holes (sink holes) in chalk or limestone formations are often revealed by random depressions and marked irregularity in the ground surface, soil creep is indicated by wrinkling of the surface on a hillside slope, or leaning trees, abandoned mine workings are shown by old shafts or heaps of mineral waste, glacial deposits may be indicated by mounds or hummocks (drumlins) in a generally flat topography, and river or lake deposits by flat low-lymg areas in valleys The surface indications of ground water are the presence of springs or wells, and marshy ground with reeds (indicating the presence of a high water table with poor drainage and the possibility of peat) Professional geological advice should be sought in the case of large projects covering extensive areas
Information should be sought on possible long-term changes in ground-water levels, cessation of abstraction of ground water for industrial purposes from bored wells, or pumping from deep mine-shafts can cause a slow rise in ground water over a wide area.

On extensive sites, aerial photography is a valuable aid in site investigations Photographs can be taken from model aircraft or balloons Skilled interpretations of aerial photographs can reveal much of the geomorphology and topography of a site

Geological mapping from aerial photographs as practised by specialist firms is a well-established science Old maps as well as up-to-date publications should be studied, since these may show the previous use of the site and are particularly valuable when investigating backfilled areas Museums or libraries in the locality often provide much information in the form of maps, memoirs, and pictures or photographs of a site in past times Local authorities should be consulted for details of buried services, and in Britain the Geological Survey for information on coal-mine workings Some parts of Britain were worked for coal long before records of workings were kept, but it is sometimes possible to obtain information on these from museums and libraries

If particular information on the history of a site has an important bearing on foundation design, for example the location of buried pits or quarries, every endeavour should be made to cross-check sources of information especially if they are based on memory or hearsay
People's memories are notoriously unreliable on these matters

Items (h), (j), and (k) of the list are obtained from boreholes or other methods of subsurface exploration, together with field and laboratory testing of soils or rocks It is important to describe the type and consistency of soils in the standard manner laid down in standard codes of practice In Britain the standard descriptions and classifications of soils are set out in the British

Standard Code of Practice Site Investigations, BS 5930 Rocks should be similarly classified in accordance with the standard procedure of codes of practice, BS 5930 requires rocks to be described in the following sequence

Grain size (the grain size of the mineral or rock fragments comprising the rock)
Texture (e g crystalline, amorphous, etc)
Structure (a descnption of discontinuities, eg laminated, foliated, etc)
State of weathering

Strength (based on the uniaxial compression test)
Other characteristics and properties
Of the above descnptions, the four properties of particular relevance to foundation engineering are the structure, state of weathering, discontinuity spacing, and uniaxial compression strength

The discontinuity spacing is defined in two ways
(1) The rock quality designation (RQD) which is the percentage of rock recovered as sound lengths which are 100 mm or more in length
(2) The fracture index which is the number of natural fractures present over an arbitrary length (usually 1 m)

The structure is of significance from the aspect of ease of excavation by mechanical plant, and also the frequency and type of discontinuity affects the compressibility of the rock mass The state of weathering, discontinuity spacing, and uniaxial compression strength can be correlated with the deformation characteristics of the rock mass and also with the skin friction and end-bearing of piles

In stating the description of rock strength in borehole records the classification adopted in BS 5930 should be followed Thus