Site Investigation PDF

Building Construction Lec.1&2 Site investigation The first step in the foundation investigation is to obtain preliminary information on the project and to plan the work. The next step is typically to perform the subsurface explor...

Building Construction Lec.1&2 Site investigation The first step in the foundation investigation is to obtain preliminary information on the project and to plan the work. The next step is typically to perform the subsurface exploration. The goal of the subsurface investigation is to obtain a detailed understanding of the engineering and geologic properties of the soil and rock strata and groundwater conditions that could impact the foundation. Specific items that will be discussed are as follows: 1. Document review. 2. Purpose of subsurface exploration. 3. Borings, including a discussion of soil samplers, sample disturbance, field tests, boring layout, and depth of subsurface exploration 4. Test pits and trenches (Sec. 2.5) 5. Preparation of logs (Sec. 2.6) 6. Geophysical techniques (Sec. 2.7) 7. Subsurface exploration for geotechnical earthquake engineering. 8. Subsoil profile (Sec. 2.9) Prior Development: If the site had prior development, it is important to obtain information on the history of the site. The site could contain old deposits of fill, abandoned septic systems and leach fields, buried storage tanks, seepage pits, cisterns, mining shafts, tunnels, and other man-made surface and subsurface works that could impact the new proposed development. There may also be information concerning on-site utilities and underground pipelines, which may need to be capped or rerouted around the project. Aerial Photographs and Geologic Maps: During the course of the work, it may be necessary for the engineering geologist to check reference materials, such as aerial photographs or geologic maps. Aerial photographs are taken from an aircraft flying at prescribed altitude along pre-established lines. Interpretation of aerial photographs takes considerable judgment and because they have more training and experience, it is usually the engineering geologist who interprets the aerial photographs. By viewing a pair of aerial photographs, with the aid of a stereoscope, a three- dimensional view of the DR. Ghusoon S. Alqaisee 1 Building Construction Lec.1&2 Purpose of soil Investigations 1. To evaluate the general suitability of the site for the proposed project. 2. To enable an adequate and economical design to be made. 3. 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: 1. To determine the geological structure, which include the thickness, sequence, and extent of soil strata. 2. To determine the groundwater conditions. 3. To obtain the disturbed and undisturbed samples for laboratory tests. 4. 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 ground-water. You should include information and/or explanations of any unusual soil, water-bearing stratum, and soil and groundwater conditions that may troublesome during construction. DR. Ghusoon S. Alqaisee 2 Building Construction Lec.1&2 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. DR. Ghusoon S. Alqaisee 3 Building Construction Lec.1&2 Methods of Soil Investigation Subsurface Investigation:  Geophysical methods: Seismic or electrical-variations in the speed of sound waves or electrical resistivity of soil formations  Test pits or trenches: shallow depths only.  Augers: shallow depths only  Wash Boring.  Rotary rigs. Boring test holes and sampling with drill rigs, principal method for detailed soil investigations. Boring or Drilling Boring refers to advancing a hole in the ground. Boring is required for the following: To obtain representative soil and rock samples for laboratory tests. To identify the groundwater conditions. Performance of in-situ tests to assess appropriate soil characteristics. Some of the common types of boring are as follows:  Auger boring  Wash boring  Percussion boring  Rotary drilling Geophysical methods Nondestructive technique used to provide information on soil ,rock, hydrological and environmental condition.  Although boring and test pits provide definite results but they are time consuming and expensive.  Subsurface conditions are known only at the bore or test pit location. DR. Ghusoon S. Alqaisee 4 Building Construction Lec.1&2  The subsurface conditions between the boring need to be interpolated or estimated.  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). DR. Ghusoon S. Alqaisee 5 Building Construction Lec.1&2 Advantages  Non-Destructive  Cost Effective  Provides Preliminary or Supplemental Information. 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. DR. Ghusoon S. Alqaisee 6 Building Construction Lec.1&2 DR. Ghusoon S. Alqaisee 7 Building Construction Lec.1&2 Auger Boring 1. Hand Auger. 2. 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. 1. Helical (worm types) Augers b. Short flight Auger c. Iwan (posthole) Auger DR. Ghusoon S. Alqaisee 8 Building Construction Lec.1&2 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.  Downwards pressure can be applied hydraulically, mechanically or by dead weight.  The diameter of the flight auger usually is between 75 to 300mm, although diameters up to 1m and bucket augers up to2m are available.  Borehole depths up to 50m are possible with continuous-flight augers.  The most common method is to use continuous flight augers. Continuous flight augers can be solid stem or hollow stem with internal diameter of 75-150mm.  Hollow stem augers are used when undisturbed samples are required. Plug is withdrawn and sampler is lowered down and driven in to the soil below the auger.  If bed rock is reached drilling can also take place through the hollow stem.  As the auger acts as a casing it can be used in sand below water table. The possibility of rising sand in to the stem by hydrostatic pressure can be avoided by filling the stem with water up to the water table  The soil rises to the surface along the helical blades, obviating the necessity of withdrawal.  They are not suitable for soil bore that require casing, which demand removal of auger for driving the casing.  The presence of cobbles and boulders create problems with small- sized augers.  There is a possibility that different soil types may become mixed as they rise to the surface and it may be difficult to determine the depths of changes of strata. Experienced driller can however detect the change of strata by the change of speed and the sound of drilling. DR. Ghusoon S. Alqaisee 9 Building Construction Lec.1&2 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. DR. Ghusoon S. Alqaisee 10 Building Construction Lec.1&2 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 carries 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 sides of the hole if no casing is used  There are two forms of rotary drilling, open-hole drilling and core DR. Ghusoon S. Alqaisee 11 Building Construction Lec.1&2 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 be 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 intact 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. DR. Ghusoon S. Alqaisee 12 Building Construction Lec.1&2 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 hard-to-sample soil and weak rocks. DR. Ghusoon S. Alqaisee 13 Building Construction Lec.1&2 Test methods and standard a requirement to drill for at least 1 m before carrying out an SPT a return to a 63.5-kg (140 lb) drive weight the specification of an anvil mass between 15 kg and 20 kg a specified maximum overall weight for the drive assembly (or trip hammer) of 115 kg the introduction of a maximum rod weight of 10 kg/m the specification of a maximum permitted curvature for bent rods, in the form of a relative deflection of 1/1000 maximum blow counts for the seating drive and test drive: In seating drive In test drive In HK (GEO 1995), a SPT test should be terminated when any increment of 75 mm penetration cannot be achieved after 100 blows or where the total number of blows, excluding the seating drive, reaches 200. where the penetration under the dead weight of the rods and hammer exceeds 450 mm, the N value is recorded as zero. DR. Ghusoon S. Alqaisee 14 Building Construction Lec.1&2 DR. Ghusoon S. Alqaisee 15 Building Construction Lec.1&2 2. Cone penetration Tests (CPT)  Electronic Steel Probes with 60° Apex Tip  ASTM D 5778 Procedures  Hydraulic Push at 20 mm/s  No Boring, No Samples, No Cuttings, No Spoil  Continuous readings of stress, friction, pressure DR. Ghusoon S. Alqaisee 16 Building Construction Lec.1&2 DR. Ghusoon S. Alqaisee 17 Building Construction Lec.1&2 DR. Ghusoon S. Alqaisee 18