What is preliminary investigation? and what are the purpose of this investigation?
The purpose of preliminary investigation is to provide an engineer a rough idea on the soil conditions at a given site. A preliminary investigation generally consists of:
- Fact finding survey
- Trial boring
Fact Finding Survey:-
In fact finding survey, all the available information on the soil condition near the site and behavior of other structures in the vicinity is digested. The desired information can be obtained from technical journals, published reports, geological and topographic maps, aerial photographs and hydrological and meteorological records. Furthermore a consultation to the local authority would be of much help.
Topography – The general topography of the site affects the design and construction of the foundation. This includes surface configuration, adjacent property, and presence of watercourses, ponds, hedges, trees, rocky outcrop and access to construction equipment and plants.
Geology – The general geology of the area with particular reference to the main geological formation underlying the site and possibility of subsidence from mineral extraction and other causes shall be explored. Old geological maps help to disclose the previous history and use of the site. Particularly the maps help while investigating the back filled area.
Aerial Photographs – Aerial photographs are helpful in very extensive sites where no existing information is available. The photographs if interpreted carefully can disclose much of the geology and topography of the site. Preparation of geological maps from aerial photography is a well-established science. The aerial photographs can provide information on landform, soil color, erosion, surface drainage, Vegetable cover, slopes and land use.
Seismicity – The seismicity can provide the records on past earthquakes in the area. The records could be of much help in the earthquake resistant design of the foundation.
Hydrological and Meteorological Data – These data provide information on high and low tides for marine structures, river levels and discharges, velocity of river and tidal currents, Wind velocities, intensity of rain fall and temperature variation etc.
Building Codes – Building code if available can provide useful information regarding the carrying capacity of soils.
Reconnaissance survey is the actual visit to the site. The information obtained from the fact finding survey is checked in reconnaissance survey for its authenticity. A close inspection by walking over the site is helpful in obtaining information on subsurface features. For example, random depression in area indicates the presence of swallow holes. Leaning trees on a slope indicate creep of soils. Old shafts and heap of mineral wastes show abandoned mine workings. The surface indication of ground Water is the presence of springs, wells and marshy lands. Riverbanks, existing excavations, quarries and railway or road cuttings can yield valuable information regarding nature of the soil strata and ground water conditions. On the basis of the fact finding survey and the reconnaissance the further method of preliminary investigation is decided.
In most of the soil deposits the trial boring is carried out by augurs, Wash boring, percussion drilling, rotary drilling and extracting the samples for identification and in some instances, for testing. Field tests such as Standard Penetration Test and Dynamic Cone Penetration teats are usually conducted. Standard Penetration Test is conducted in boreholes while Dynamic Cone Penetration Test does not require borehole. Other methods of exploration are not usually considered in preliminary investigation unless underlying materials consist of bedrock, soft clay, silts or highly organic matters. For small structures, where structures can be founded on bedrock, or soil of high bearing capacity or in an area where much practical experience has been summarized in the form of building codes, no further investigation after the preliminary investigation is necessary.