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SOIL EXPLORATIONPART-1(BORINGS)
AVINASH KUMAR GUPTAEXECUTIVE ENGINEER
CCW AIR NEW DELHIAPRIL 2017
Soil Exploration
• The process of collection soil data for the assessment soil properties at a site through series of laboratory and field investigation is collectively called Sub-soil Exploration
• Enables the engineers to draw soil profile indicating the sequence of soil strata and the properties of soil involved
Objective• To access the general suitability of the site.• To achieve safe and economical design of foundations and
temporary works. • To know the nature of each stratum and engineering properties of
the soil and rock, which may affect the design and mode of construction of proposed structure and foundation.
• To foresee and provide against difficulties that may arise during construction due to ground and other local conditions.
• To find out the sources of construction material and selection of sites for disposal of water or surplus material.
• To investigate the occurrence or causes of all natural and man made changes in conditions and the results arising from such changes. To ensure the safety of surrounding existing structures.
• To design for the failed structures or remedial measures for the structures deemed to be unsafe. To locate the ground water level and possible corrosive effect of soil and water on foundation material.
General
• Attempt at understanding the subsurface conditions such as:– Soil and rock profile– Geological features of the region– Position and variation of ground water table– Physical properties of soil and rock– Contamination, if any– General data of adjacent structures, hydrological data,
topography, soil maps, seismicity, etc.– Engineering properties of soil
General, Why soil Exploration
– To determine the type of foundation required for the proposed project at the site, i.e. shallow foundation or deep foundation.
– To make recommendations regarding the safe bearing capacity or pile load capacity.
– Ultimately, it is the subsoil that provides the ultimate support for the structures
PURPOSE OF SOIL EXPLORATION1. Selection of the type and the depth of foundation suitable for a given
structure.
2. Evaluation of the load-bearing capacity of the foundation.
3. Estimation of the probable settlement of a structure.
4. Determination of potential foundation problems (for example,
expansive soil, collapsible soil, sanitary landfill, and so on).
5. Establishment of ground water table.
6. Prediction of lateral earth pressure for structures like retaining walls,
sheet pile bulkheads, and braced cuts.
7. Establishment of construction methods for changing subsoil
conditions
Planning For Subsurface Exploration
• Fact finding and geological survey• Reconnaissance• Preliminary Exploration• Detailed Exploration
ASPECTS
• The three important aspect are planning, execution and report writing.
• PlanningTo minimize cost of explorations and yet give reliable
data.Decide on quantity and quality depending on type,
size and importance of project and whether investigation is preliminary or detailed.
Execution
Collection of disturbed and/or undisturbed samples of subsurface strata from field.
Conducting in-situ tests of subsurface material and obtaining properties directly or indirectly.
Study of ground water conditions and collection of sample for chemical analysis.
Geophysical exploration, if necessary.Laboratory testing on samples
EXECUTION
Report writing:Description of site conditions – topographic
features, hydraulic conditions, existing structures, etc. supplemented by plans/drawings.
Description of nature, type and importance of proposed construction
Description of field and lab tests carried out.Analysis and discussion of data collected
informationPreparation of charts, tables, graphs, etc.Calculations performedRecommendations
Leaning Tower of Pisaand Sinkholes
Methodology
A complete site investigation will consist of:– Preliminary work
Collecting general information and already existing data such as study of geologic , seismic maps, etc. at or near site.
Study site history – if previously used as quarry, agricultural land, industrial unit, etc.
– Site Reconnaissance: Actual site inspection.To judge general suitabilityDecide exploration techniques
Methodology
• Exploration
Preliminary Investigations: Exploratory borings or shallow test
pits, representative sampling, geophysical investigations, etc
Detailed Investigations: Deep boreholes, extensive sampling, in-
situ testing, lab testing, etc.
Depth and spacing: In general, depth of investigation should be
such that any/all strata that are likely to experience settlement or
failure due to loading. Spacing depends upon degree of variation
of surface topography and subsurface strata in horizontal
direction.
Methods of site exploration
• The various types of site investigation are: Open excavation Boring Subsurface Sounding Geophysical Methods • These available methods of exploration can be
broadly classified into two categories: Direct methods Indirect methods
Direct methods
• sinking a borehole at a predetermined location to the required depth by a method suitable for the site
• obtain fairly intact samples of soils from every stratum encountered or at suitably selected depths.
• information about the soil characteristics by means of laboratory tests
Semi Direct Method
Borings• Auger• Auger and shell• Wash Boring• Percussion drilling• Rotary Drilling
Indirect methods
• sounding In sounding methods, the variation in penetration
resistance of sample or cone is utilized to interpret some of the physical properties of the strata
• geophysical methods.. In geophysical methods, the change in subsoil strata
are identified by measuring certain physical characteristics, e.g. electrical conductance, wave velocity of subsurface deposits.
• projectiles, probes, and aerial photographs are also useful in interpreting the soil characteristics.
Classify the Site Investigation in another way
Investigation of site for new works Investigation of defects or failure of existing
works Investigations as the safety or stability of
existing works. Investigations relating to the suitability of
material for various constructional purposes.
Test/Trial pit
• Trial pits are the cheapest way of site exploration • do not require any specialized equipment.• A pit is manually excavated to get an indication
of the soil classification & obtain undisturbed & disturbed samples. Generally Gravelly soil.
• Trial pits allow visual inspection of any change of strata & facilitate in-situ testing.
• They are suitable for exploration of shallow depth only(18-20 feet)
Test Pits
Soil Profiles
Boring
Drilling a hole into the soil strata upto specified depth is known as boring
• 1. Displacement boring.• 2. Wash boring.• 3. Auger boring.• 4. Rotary drilling.• 5. Percussion drilling.• 6. Continuous sampling.
Displacement Boring
• It is combined method of sampling & boring operation. • Closed bottom sampler, slit cup, or piston type is
forced in to the ground up to the desired depth. Then the sampler is detached from soil below it, by rotating
the piston, & finally the piston is released or withdrawn.
• The sampler is then again forced further down & sample is taken. • After withdrawal of sampler & removal of sample from
sampler, the sampler is kept in closed condition & again used for another depth
Displacement Boring
• Simple and economic method if excessive caving does not occur. Therefore not suitable for loose sand.
• Major changes of soil character can be detected by means of penetration resistance.
• These are 25mm to 75mm holes.• It requires fairly continuous sampling in stiff and
dense soil, either to protect the sampler from damage or to avoid objectionably heavy construction pit.
Wash Boring
• Initially, the hole is advanced for a short depth by using an auger.
• Then a casing pipe is pushed in and driven with a drop weight. The driving may be with the aid of power.
• A hollow drill bit is screwed to a hollow drill rod connected to a rope passing over a pulley and supported by a tripod.
• Water jet under pressure is forced through the rod and the bit into the hole.
• This loosens the soil at the lower end and forces the soil-water suspension upwards along the annular surface between the rod and the side of the hole
Wash Boring Contd
• This suspension is collected in a settling tank. • Soil particles are allowed to settle down and water is
allowed to overflow into a sump which is then recalculated
• Very disturbed sample is obtained. Hence cannot be used for determining engineering properties.
• whenever a soil sample is required, the chopping bit is to be replaced by a sampler.
• The change of the rate of progress and change of colour of wash water indicate changes in soil strata.
• Below GWT. May not be used for soils mixed with gravel and boulders
Wash Boring
Auger Boring
• Drilling is made using a device called Soil Auger• Power driven (upto 3 to 5m) and Hand operated
(Greater than 5m)• Advancement is made by drilling the auger by
simultaneous rotating and pressing it into the soil• Dry and unsupported bore holes• When the auger gets filled with soil same, it is
taken out and the soil sample collected
Auger Boring
• It is very suitable for soft to stiff cohesive soils and also can be used to determine ground water table.
• Soil removed by this is disturbed but it is better than wash boring, percussion or rotary drilling.
• It is not suitable for very hard or cemented soils, very soft soils, as then the flow into the hole can occur and also for fully saturated cohesionless soil.
Auger Boring
COMPARISION OF BORING
Rotary Drilling• Rotary drilling method of boring is useful in case of highly
resistant strata. It is related to finding out the rock strata and also to access the quality of rocks from cracks, fissures and joints.
• It can conveniently be used in sands and silts also. • Here, the bore holes are advanced in depth by rotary
percussion method which is similar to wash boring technique.
• A heavy string of the drill rod is used for choking action. The broken rock or soil fragments are removed by circulating water or drilling mud pumped through the drill rods and bit up through the bore hole from which it is collected in a settling tank for recirculation.
• If the depth is small and the soil stable, water alone can be used. However, drilling fluids are useful as they serve to stabilize the bore hole.
Rotary Drilling
• Drilling mud is slurry of bentonite in water. The drilling fluid causes stabilizing effect to the bore hole partly due to higher specific gravity as compared with water and partly due to formation of mud cake on the sides of the hole
• As the stabilizing effect is imparted by these drilling fluids no casing is required if drilling fluid is used.
• This method is suitable for boring holes of diameter 10cm, or more preferably 15 to20cm in most of the rocks.
• It is uneconomical for holes less than 10cm diameter.• The depth of various strata can be detected by
inspection of cuttings
Rotary drilling
Percussion drilling• In case of hard soils or soft rock, auger boring or wash
boring cannot be employed. For such strata, percussion drilling is usually adopted.
• Here advancement of hole is done by alternatively lifting and dropping a heavy drilling bit which is attached to the lower end of the drilling bit which is attached to the cable.
• Addition of sand increases the cutting action of the drilling bit in clays.
• When coarse cohesion less soil is encountered, clay might have to be added to increase the carrying capacity of slurry.
• After the carrying capacity of the soil is reached, churn bit is removed and the slurry is removed using bailers and sand pumps.
Percussion Drilling
• Change in soil character is identified by the composition of the outgoing slurry. The stroke of bit varies according to the ground condition.
• Generally, it is 45-100cm in depth with rate of 35-60 drops/min. It is not economical for hole of diameter less than 10cm. It can be used in most of the soils and rocks and can drill any material.
• One main disadvantage of this process is that the material at the bottom of the hole is disturbed by heavy blows of the chisel and hence it is not possible to get good quality undisturbed samples.
• It cannot detect thin strata as well.
Percussion Drilling
Continuous sampling• The sampling operation advances the borehole and the boring
is accomplished entirely by taking samples continuously.
• The casing is used to prevent the caving in soils. It provides more reliable and detail information on soil condition than the other methods. Therefore it is used extensively in detailed and special foundation exploration for important structures.
• It is slower method and more expensive than intermittent sampling.
• When modern rotary drilling rigs or power driven augers are not available, continuous sampling may be used to advantage for advancing larger diameter borings in stiff and tough strata of clay and mixed soil.
CONTINOUS SAMPLING