Soil Investigations PDF

# Building Construction ## Purpose of Soil Investigations - To evaluate the general suitability of the site for the proposed project - To enable an adequate and economical design to be made - To disclose and make provision for difficulties that may arise during construction due to ground and other...

# Building Construction ## Purpose of Soil Investigations - To evaluate the general suitability of the site for the proposed project - To enable an adequate and economical design to be made - To disclose and make provision for difficulties that may arise during construction due to ground and other local condition ## Phases of a Soil Investigation A site investigation must be developed in phases: ### Phase I: Collect available information such as: - Pile plan, type, size, the importance of the structure, loading conditions, previous geotechnical reports - Topographic maps, air photographs, geologic maps, and newspaper clippings ### Phase II: - Preliminary reconnaissance or site visit to provide general picture of the topography and geology of the site. - It is necessary that you take with you on the visit all the information gathered in Phase I to compared with the current conditions of the site. ### Phase III: Detailed Soils exploration, the objectives of detailed soil exploration are: - To determine the geological structure, which include the thickness, sequence, and extent of soil strata - To determine the groundwater conditions - To obtain the disturbed and undisturbed samples for laboratory tests - To conduct in situ tests ### Phase IV: - Write a report. - The report must be contain a clear description of the soils at the site, methods of exploration, soil profile, test methods and results, and the location of the groundwater. - You should include information and/or explanations of unusual soil, water-bearing stratum, and soil and groundwater conditions that may trouble-some during construction. ## Soil Exploration Program A soil exploration program usually involves test pits and/or soil borings (boreholes). During the site visit (Phase II). You should work out most of soil exploration program. A detailed soils exploration consists of: 1. Determining the need for and extent of geophysical exploration. 2. Preliminary location of each borehole and/or test pits. 3. Numbering of the boreholes or test pits. 4. Planned depth of each borehole or test pits. 5. Methods and Produces for advancing the boreholes. 6. Sampling instructions for at least the first borehole. The sampling instruction must include the number of sample and possible locations. Changes in the sampling instructions often occur after the first borehole. 7. Determining the need for and type of in situ tests. 8. Requirements for groundwater observations. Geophysical methods are more quick and cheaper. They provide thorough coverage of the entire area. The results of Geophysical testing however are less definitive and require subjective interpretation. Therefore both methods are important. In case geophysical testing in major in scope, few borings and sampling will be required for accurate determination of soil properties. If boring is major in scope then few geophysical lines will be required to know the conditions in-between the borings. ### Geophysical Techniques Indirect Methods: - Ground Penetrating Radar (GPR) - Electromagnetic (EM) - Magnetic - Utility Locating - Seismic - Electrical Resistivity - Gravity - Very Low Frequency (VLF). ## Advantages - Non-Destructive - Cost Effective - Provides Preliminary or Supplemental Information. ## Electrical Resistivity (Wenner Array) ## Test pits or trenches - The excavation of test pits is a simple and reliable method. - The depth is limited to 4-5m only. - The in-situ conditions are examined visually - It is easy to obtain disturbed and undisturbed samples - Block samples can be cut by hand tools and tube samples can be taken from the bottom of the pit. ## Auger Boring - Hand Auger. - Mechanical Auger. ### 1. Hand Auger - It is the simplest method of boring used for small projects in soft cohesive soils. - For hard soil and soil containing gravels boring with hand auger becomes difficult. - Hand-augered holes can be made up to about 20m depth, although depth greater than about 8-10m is usually not practical. - The length of the auger blade varies from 0.3-0.5m. - The auger is rotated until it is full of soil, then it is withdrawn to remove the soil and the soil type present at various depths is noted. - Repeated with drawl of auger for soil removal makes boring difficult below 8-10m depth. - The soil samples collected in this manner are disturbed samples and can be used for classification test. Auger boring may not be possible in very soft clay or coarse sand because the hole tends to collapse when auger is removed. ### 2. Mechanical Auger - Mechanical Auger means power operated augers. - The power required to rotate the auger depends on the type and size of auger and the type of soil. ## Wash Boring - Water with high pressure pumped through hallow boring rods is released from narrow holes in a chisel attach to the lower end of the rods. - The soil is loosened and broken by the water jet and the up-down moment of the chisel. - The soil particles are carried in suspension to the surface between the rock and the borehole sites. - The rods are raised and drop for chopping action of the chisel by means of winch. - Wash boring can be used in most type of soil but the progress is slow in coarse gravel strata. - The accurate identification of soil strata is difficult due to mixing of the material has they are carried to the surface. - The method is unacceptable for obtaining soil samples. - It is only used for advancing the borehole to enable tube sample to be taken or field test to be carried at the hole bottom. - The advantage is that the soil immediately below the hole remains relatively un-disturbed. ## Rotary Drilling - The rig consists of a derrick, power unit, winch, pump and a drill head to apply high-speed rotary drive and downward thrust to the drilling rods. - Primarily intended for investigation in rock, but also used in soils. - The drilling tool, (cutting bit or a coring bit) is attached to the lower end of hollow drilling rods - The coring bit is fixed to the lower end of a core - Water or drilling fluid is pumped down the hollow rods and passes under pressure through narrow holes in the bit or barrel - The drilling fluid cools and lubricates the drilling tool and carrie the loose debris to the surface between the rods and the side of the hole. - The fluid (bentonite slurry) also provides some support to the side of the hole if no casing is used - There are two forms of rotary drilling, open-hole drilling and core drilling. - Open- hole drilling, which is generally used in soils and weak rock just for advancing the hole - The drilling rods can then be removed to allow tube samples to l taken or in-situ tests to be carried out. - In core drilling, which is used in rocks and hard clays, the diamond or tungsten carbide bit cuts an annular hole in the material and an inner core enters the barrel, to be removed as a sample. Typical core diameters are 41, 54 and 76mm, but can range up to 165 mm. ## Field Tests 1. Standard penetration Tests (SPT) 2. Cone penetration Tests (CPT) 3. Vane Shear test (VST) 4. Pressure meter test (PMT). ### 1. Standard penetration Tests (SPT) - The Standard Penetration Test, or SPT, is the most widely used in-situ test, in Hong Kong and throughout the world, as an indicator of the density and compressibility of granular soils. - It is also commonly used to check the consistency of stiff or stony cohesive soils and weak rocks. - Estimation of the liquefaction potential of saturated granular soils for earthquake design is often based on these tests. - Available design methods for both shallow and deep foundations rely heavily on SPT results. - The test consists of driving a standard 50-mm outside diameter thick- walled sampler into soil at the bottom of a borehole, using repeated blows of a 63.5-kg hammer falling through 760 mm. - The SPT N value is the number of blows required to achieve a penetration of 300 mm, after an initial seating drive of 150 mm. - One of the advantages of the test is that it is carried out in routine exploration boreholes of varying diameters, so that (in contrast with other in-situ tests, such as the Cone Penetration Test) there is no need to bring special equipment to site. ### Advantage of SPT - The Standard Penetration Test was introduced in 1947, and is now in widespread use because of its low cost, simplicity and versatility. - The SPT procedure initially arose from a desire to obtain cheap additional information during small-diameter sampler driving. - The test is not yet fully standardised, either nationally or internationally. - Differences in boring, equipment and test procedure are likely to influence the SPT N value. - The SPT provides a simple, universally applicable, testing method. - No sophisticated boring or testing rig is required. - The test has sufficient versatility that it can provide information on soil and weak rocks. ### CPT - Continuous sampling, 1cm vertical resolution. - Conservatively, 5 times faster than traditional drilling. - $6 to $9 per foot (NHCRP findings). - Superior accuracy and precision compared to typical drilling and testing. - Predicts many design parameters normally obtained by traditional drilling and sample testing. - Laboratory sampling requirements are greatly reduced for added cost savings. - No drilling spoils are generated. - Does not eliminate the need for drilling and testing, but can greatly reduce number of borings/samples. - Can collect additional data such as soil resistivity and shear wave velocity with little added cost. ### Disadvantage - physical soil sample is generally not collected. Only used in unconsolidated sediments. ## Brinch Hansen's Bearing Capacity equation - The bearing capacity depends on many factors and Terzaghi's bearing capacity equation does not take in to consideration all the factors. - Hansen and several other researchers have provided a comprehensive equation for the determination bearing capacity called generalised Bearing Capacity equation considering the almost all the factors mentioned above. - Equation for ultimate bearing capacity is as follows from the comprehensive theory. $q_r = cN_{s,d,i} +qN_{q,s,q,d,i,q}+ 0.5yBN_{s,d,i}$ - Equations are available for shape factors (Sc, Sq, Sy), depth factors (de, de) and load inclination factors (ic, iq, iy). - The effects of these factors is to reduce the bearing capacity. - The bearing capacity of soil can be determined by the following methods: 1. Plate load test 2. Penetration test 3. Safe bearing capacity values given in codes ### Major advantages of field tests are - Sampling not required - Soil disturbance minimum ### Major disadvantages of field tests are - Difficult - Time consuming - Heavy equipment to be carried to field - Short duration behavior ### Plate load test - This is a field test for determining the ultimate bearing capacity soil and a likely settlement due to given loading.