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