Advanced Foundation Engineering. Soil Exploration

Transcription

1 Shahrood University of Technology Department of Geotechnical Engineering Advanced Foundation Engineering Soil Exploration Mohsen Keramati, Ph.D. Assistant Professor 1

2 - Introduction The field and laboratory investigations required to obtain the necessary data for the soils for proper design and successful construction of any structure at the site are collectively called soil exploration. The choice of the foundation and its depth, the bearing capacity, settlement analysis depend very much upon the various engineering properties of the foundation soils. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 2

3 - Purpose of Subsurface Exploration 1. Selecting the type and depth of foundation suitable for a given structure. 2. Evaluating the load-bearing capacity of the foundation. 3. Estimating the probable settlement of a structure. 4. Determining potential foundation problems (e.g., expansive soil, collapsible soil, sanitary landfill, and so on). 5. Determining the location of the water table. 6. Predicting the lateral earth pressure for structures such as retaining walls, sheet pile, bulkheads, and braced cuts. 7. Establishing construction methods for changing subsoil conditions. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 3

4 - Soil data required Soil profile - layer thickness and soil identification Index properties - Water content, Atterberg limits, etc Strength & compressibility characteristics - C, Cu,, Cc, OCR Others (e.g., water table depth) Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 4

5 First stage of site investigation, Negligible cost, Look for currently available information Aerial photographs Topographical maps Existing site investigation reports (for nearby sites) Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 5

6 - Site Reconnaissance The engineer should always make a visual inspection of the site to obtain information 1. The general topography of the site, the possible existence of drainage ditches, abandoned dumps of debris, and other materials present at the site. 2. Also, evidence of creep of slopes and deep, wide shrinkage cracks at regularly spaced intervals may be indicative of expansive soils. 2. Soil stratification from deep cuts, such as those made for the construction of nearby highways and railroads. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 6

7 - Site Reconnaissance 3. The type of vegetation at the site, which may indicate the nature of the soil. For example, a Mesquite cover in central Texas may indicate the existence of expansive clays that can cause foundation problems. 4. High-water marks on nearby buildings and bridge abutments. 5. Groundwater levels, which can be determined by checking nearby wells. 6. The types of construction nearby and the existence of any cracks in walls or other problems. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 7

8 - The methods available for soil exploration may be classified as follows Direct methods Test pits, trial pits or trenches Semi-direct methods Borings Indirect methods Soundings or penetration tests and geophysical methods Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 8

9 - Direct Methods Test Pits Test pits or trenches are open type or accessible exploratory methods. Soils can be inspected in their natural condition. The necessary soils samples may be obtained by sampling techniques and used for finding strength and other engineering properties by appropriate laboratory tests. Test pits are considered suitable only for small depths- up to 3 m; the cost of these increasing rapidly with depth. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 9

10 - Direct Methods Test Pits For greater depths, lateral supports or bracing of the excavations will be necessary. Tests pits are usually made for supplementing other methods or for minor structures. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 10

11 - Semi Direct Methods Boring Boring: making or drilling bore holes into the ground with a view to obtaining soil or rock samples from specified or known depths is called boring The common methods of advancing bore holes are: Auger Boring Wash Boring Rotary Drilling Percussion Drilling Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 11

12 Auger Boring Soil Auger is a device that is used for advancing a bore hole into the ground. Augers may be hand-operated for relatively small depth less than 3 to 5 m and power-driven for greater depths up to 60 or 70m in the case of continuous-flight augers The soil auger is advanced by rotating it while pressing it into the soil. The soil samples obtained from this type of borings are highly disturbed. Auger boring is convenient in case of partially saturated sands, silts and medium to stiff cohesive soils. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 12

13 Auger Boring Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 13

14 Auger Boring Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 14

15 Wash Boring Wash boring is commonly used for exploration below ground water table for which the auger method is unsuitable. This method may be used in all kinds of soil except those mixed with gravel and boulder. A casing pipe is pushed in and driven with a drop weight. 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. This suspension is led to a settling tank where the soil particles settle while the water overflows into a sump. The water collected in the sump is used for circulation again. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 15

16 Wash Boring Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 16

17 Wash Boring The soil particles collected represent a very disturbed sample and is not very useful for the evaluation of the engineering properties. Wash boring are primarily used for advancing bore holes; whenever a soil sample is required, the chopping bit is to be replaced by sampler. The change of the rate of progress and change of color of wash water indicate changes in soil strata. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 17

18 Rotary drilling Can be used in sand, clay and rock (unless badly fissured) A drill bit, fixed to the lower end of a drill rod, is rotated by power while being kept in firm contact with the hole. Drilling fluid or bentonite slurry is forced under pressure through the drill rod and it comes up bringing the cutting to the surface. Even rock cores may be obtained by using suitable diamond drill bits. When soil samples are required, the drilling rod raised and drilling bit is replaced by a sampler. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 18

19 Rotary drilling Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 19

20 Percussion Drilling A heavy drill bit is suspended from a drill rod or a cable and is driven by repeated blows. Water is added to facilitate the breaking of stiff soil or rock. The slurry of the pulverized material is bailed out at internals. The method cannot be used in loose sand and is slow in plastic clay. The formation gets badly disturbed by impact. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 20

21 Percussion Drilling Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 21

22 Types of samples Samples of soil taken out of natural deposits for testing may be classified as: Disturbed sample Undisturbed sample A disturbed sample is that in which the natural structure of the soil gets modified partly or fully during sampling. An undisturbed sample is that in which the natural structure and other physical properties remain preserved. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 22

23 Types of samples Disturbed, but representative, samples can generally be used for the following types of laboratory test: 1. Grain-size analysis 2. Determination of liquid and plastic limits 3. Specific gravity of soil solids 4. Determination of organic content 5. Classification of soil Undisturbed soil samples must be used for: 1. Consolidation test 2. Hydraulic conductivity test 3. Shear strength test Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 23

24 Undisturbed samples Required for triaxial, consolidation tests in the lab. Good quality samples necessary. (A R <10%). When the area ratio is 10% or less, the sample generally is considered to be undisturbed. Thicker the wall, greater disturbance. Take good care in transport and handling. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 24

25 Types of samplers Soil samplers are classified as: Thick wall Samplers (Split spoon sampler) Thin wall samplers (Shelby tubes) Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 25

26 - Split spoon sampler The standard size of the spoon sampler is of 35 mm internal and 50.8 mm external diameter. The sampler is lowered to the bottom of the bore hole by attaching it to the drill rod. The sampler is then driven by forcing it into the soil by blows from a hammer. The assembly of the sampler is then extracted from the hole and the cutting edge and coupling at the top are unscrewed. The two shalves of the barrel are separated and the sample is thus exposed. Samples are generally taken at intervals of about 1.5 m (5 ft). Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 26

27 - Split spoon sampler Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 27

28 - Split spoon sampler For a standard split-spoon sampler Hence the samples are highly disturbed When the material encountered in the field is sand (particulary fine sand below the water table), a device such as a spring core catcher is placed inside the split spoon. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 28

29 Shelby sampler (Thin walled sampler) Commonly used to obtain undisturbed clayey samples. Outside diameter: 50.8 mm (2 in) and 76.3 mm (3 in) (common) Sampler with a 50.8 mm outside diameter has an inside diameter of about mm. the area ratio is: The sampler pushed into the soil continuously. The two ends are sealed and sent to the laboratory. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 29

30 Shelby sampler (Thin walled sampler) Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 30

31 How many bore holes? The number of bore holes depends on: Type and size of the project Budget for site investigation Soil variability Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 31

32 Spacing of boring Type of project Spacing (m) Multistory buildings One-story industrial plants Highways Residential subdivision Dams and dikes Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 32

33 Minimum depth of boring (ASCE, 1972) 1. Determine the net increase of stress,, under the foundation. 2. Estimate the variation of the vertical effective stress with depth. v 3. Determine the depth D=D 1, at which stress increase =q/10, where q= estimated net stress on the foundation. 4. Determine the depth D=D 2, at which / v = Unless bedrock is encountered, the smaller of two depths, D 1 and D 2 will be the approximate minimum depth of boring required. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 33

34 Ground water table A correct indication of the general ground water level is found by allowing the water in the boring to reach an equilibrium level. In sandy soils, the level gets stabilized very quickly-within a few hours at the most. In clayey soils it it will take many days for this purpose. Hence, standpipes or piezometers are used in clays and silt. Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 34

35 Ground water table Casagrande-type piezometer Mohsen Keramati, Ph.D., Department of Geotechnical Engineering, Shahrood University of Technology, 35