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GEOTECHNICAL ENGINEERING - II. UNIT – I GEOTECHNICAL EXPLORATION. Importance and objective of field exploration.
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GEOTECHNICAL ENGINEERING - II UNIT – I GEOTECHNICAL EXPLORATION
Importance and objective of field exploration The stability of the foundation of a building, a bridge, an embankment or any other structure built on soil depends on the strength and compressibility characteristics of the subsoil. The field and laboratory investigations required to obtain the essential information on the subsoil is called Soil Exploration or Soil Investigation.
Soil exploration involves broadly the following:- Planning of a program for soil exploration. To determine bearing capacity of the soil. To select the type and depth of foundation for a givrn structure. To investigate the safety of the existing structures and to suggest the remedial measures. Collection of disturbed and undisturbed soil or rock samples from the holes drilled in the field. The number and depths of holes depend upon the project. Conducting all the necessary in-situ tests for obtaining the strength and compressibility characteristics of the soil or rock directly or indirectly.
Soil exploration involves broadly the following:- Study of ground – water conditions and collection of water samples for chemical analysis. Geophysical exploration, if required. Conducting all the necessary tests on the samples of soil / rock and water collected. Preparation of drawings, charts, etc. Analysis of the data collected. Preparation of report.
OPEN EXCAVATIONS. BORINGS. SUB-SURFACE SOUNDINGS. GEOPHYSICAL METHODS. PRINCIPLE METHODS OF SUBSURFACE EXPLORATION
OPEN EXCAVATIONS:- PITS
OPEN EXCAVATIONS:- b) TRENCHES
2. DRIFTS AND SHAFTS:- a) DRIFTS
2. DRIFTS AND SHAFTS:- b) SHAFTS
3. BORINGS FOR EXPLORATION:- Auger Boring Auger borings are used in cohesive and other soft soils above water table. Hand augers are used for depths=6m. Mechanically operated are used for greater depths. Samples recovered from the soil brought up by augers are badly disturbed and are useful for identification purposes only. Auger boring is for highway explorations at shallow depths and for exploring borrow pits.
Auger Boring Hand operated augers Power driven augers
Wash Boring Wash boring is very convenient method provided the soil is sand, silt or clay. It is not suitable for gravel or boulders. When an undisturbed sample is required at a particular depth, the boring is stopped, and the chopping bit is replaced by a sampler. The sample is pushed into the soil at the bottom of the hole and the sample is withdrawn.
Wash Boring. Diamond Drill Bit Tricone drill bit
3. Rotary Boring In the rotary drilling method a cutter bit or a core barrel with a coring bit attached to the end of a string of drill rods is rotated by a power rig. When boring is over in soil, the drilling bit is removed and replaced by a sampler
Percussion drilling Grinding the soil by repeated lifting and dropping of heavy chisels or drilling bits. Water is added to form slurry of cuttings. Slurry removed by bailers or pumps. In general, a machine used to drill holes is called a drill rig (generally power driven, but may be hand driven). A winch is provided to raise and lower the drilling tools into the hole.
SOIL SAMPLES Two types of soil samples can be obtained during sampling disturbed and undisturbed. The most important engineering properties required for foundation design are strength, compressibility, and permeability. Reasonably good estimates of these properties for cohesive soils can be made by laboratory tests on undisturbed samples which can be obtained with moderate difficulty. It is nearly impossible to obtain a truly undisturbed sample of soil; so in general usage the term "undisturbed" means a sample where some precautions have been taken to minimize disturbance or remolding effects. In this context, the quality of an "undisturbed" sample varies widely between soil laboratories.
SOIL SAMPLES Soil samples are obtained during sub-surface exploration to determine the engineering properties of the soils and rocks. Soil samples are generally classified into two categories : Good quality samples necessary. AR<10% soil area ratio sampling tube Thicker the wall, greater the disturbance.
SOIL SAMPLERSdesign features affecting the sample disturbance • Area ratio:- The area ratio is defined as, Area ratio where, D1 = inner diameter of cutting edge. D2 = outer diameter of cutting edge. For obtaining good quality undisturbed samples, the area ratio should be 10 percent or less.
SOIL SAMPLERSdesign features affecting the sample disturbance • 2. Inside clearance :- The inside clearance is defined as Where, D3 = inner diameter of the sampling tube. For an undisturbed sample, the inside clearance should be between0.5 and 3 percent.
SOIL SAMPLERSdesign features affecting the sample disturbance 3. Outside clearance :- The outside clearance is defined as where, D4 = outer diameter of the sampling tube. For reducing the driving force, the outside clearance should be as small as possible. Normally, it lies between zero to 2 percent.
2. SCRAPER BUCKET SAMPLER • A scraper bucket sampler consists of a driving point which is attached to its bottom end. There is a vertical slit in the upper portion of the sampler. As the sampler is rotated, the scrapings of the soil enter the sampler through the slit. • When the sampler is filled with the scrapings, it is lifted. Although the sample is quite disturbed, it is still representative. • A scraper bucket sampler can also be used for obtaining the samples of cohesionless soils below the water table.
3. SHELBY TUBES AND THIN-WALLED SAMPLERS Shelby tubes are thin wall tube samplers made of seamless steel. The outside diameter of the tube may be between 40 to 125 mm. The commonly used samplers have the outside diameter of either 50.8 mm or 76.2 mm. The length of the tube is 5 to 10 times the diameter for sandy soils and 10 to 15 times the diameter for clayey soils. The diameter generally varies between 40 and 125mm, and the thickness varies from 1.25 to 3.15mm. The sampler tube is attached to the drilling rod and lowered to the bottom of the bore hole. It is then pushed into the soil. Care shall be taken to push the tube into the soil by a continuous rapid motion without impact or twisting. The tube should be pushed to the length provided for the sample. At least 5 minutes after pushing the tube into its final position, the bottom before it is withdrawn. The tube is taken out and its ends are sealed before transportation. Shelby tubes are used for obtaining undisturbed samples of clay.
DEPTH OF FOUNDATION Depth of exploration. Depth of exploration for friction piles. Depth of exploration for closely –spaced footings.
STANDARD PENETRATION TEST The standard penetration test is the most commonly used in-situ test, especially for cohesionless soils which cannot be easily sampled. The test is extremely useful for determining the relative density and the angle of shearing resistance of cohesionless soils. It can also be used to determine the unconfined compressive strength of cohesive soils.
Dilatancy Correction: - Silty fine sands below the water table develop pore pressure which is not easily dissipated. The pore pressure increases the resistance of the soil and hence the penetration number (N). • Terzaghi and Peck (1967) recommend the following correction in the case of silty fine sands when the observed value of N exceeds 15. The corrected penetration number,
Where NR is the recorded value, and NC is the corrected value. If • Overburden pressure Correction: - In granular soils, the overburden pressure affects the penetration resistance. Gibbs and Holtz (1957) recommend the use of the following equation for dry or moist clean sand. • …….. (i) where , NR = observed N-value, Nc =corrected N-value, = effective overburden pressure (kN/m2). Equation (i) is applicable for Overburden Correction Diagram.
GEOPHYSICAL METHODS 1. Seismic Method: -The seismic methods are based on the principle that the elastic shock waves have different velocities in different materials. Seismic methods of subsurface explorations generally utilize the refracted waves. The shock wave is created by a hammer blow or by a small explosive charge at a point P. The shock wave travels through the top layer of the soil (or rock) with a velocity V1, depending upon the type of material in layer – I. The observation of the first arrival of the waves is recorded by geophones located at various points, such as A, B, C. The geophones convert the ground vibration into electrical impulses and transmit them to a recording apparatus. It is assumed that V3 > V2 > V1 . At geophones located close to the point of impact, such as point A, the direct waves with velocity V1 reach first.
SUB – SOIL INVESTIGATION REPORT A report is the final document of the whole exercise of soil exploration. A good report should normally comprise the following: 1. A general description of the nature of the project and its importance. 2. A general description of the topographical features and hydraulic conditions of the site. 3. A brief description of the various field and laboratory tests carried out. 4. Analysis and discussion of the test results 5. Recommendations 6. Calculations for determining safe bearing pressures, pile loads, etc. 7. Tables containing bore logs, and other field and laboratory test results 8. Drawings which include an index plan, a site-plan, test results plotted in the form of charts and graphs, soil profiles, etc.
Questions ? GT - II Unit 1
