6._Bearing Capacity tests.pptx shallow f
- 1. FOUNDATION ENGINEERING Lecture# 6 Civil Engg. Dept. - PTUT Lahore Email: shahidameen682@gmail.com
- 4. Several in-situ tests define the stratigraphy and obtain direct measurements of soil properties and geotechnical parameters. The common tests include: Standard penetration test (SPT) Cone penetration test(CPT) Vane shear test(VST) Borehole Shear Test (BST) Piezocone test (CPTu) Flat dilatometer test (DMT) Pressuremeter test (PMT) Plate Load test (PLT)
- 6. Standard Penetration Test, SPT SPT is the most widely used test procedure to determine the properties of in-situ soils 63.5 kg Drill rod 0.15 m 0.15 m 0.15 m Number of blows = N1 Number of blows = N2 Number of blows = N3 Standard penetration resistance (SPT N) = N2 + N3 Number of blows for the first 150 mm penetration is disregarded due to the disturbance likely to exist at the bottom of the drill hole The test can be conducted at every 1 0.76 m vertical intervals Various correlations have been developed to determine soilm strength parameters (c, q) from N
- 7. Standard Penetration Test, SPT SPT (Manual operation)
- 8. Standard Penetration Test (SPT) 140 lb (63.5 kg) Hammer 30in (76 cm) free fall Drive sampler over 18 inches Record no. of blows per each 6 inch penetration SPT blow count=blows for 2nd 6 inch penetration + blows for 3rd 6inch penetration
- 9. Standard Split Spoon Sampler (SPT) Thick wall (0.25in) cylinder Sampling tube is split along the length Hammered into the ground
- 12. Types of SPT Hammers
- 20. 1. Points to note If number of blows for blows for 150 mm drive exceeds 50, it is taken as refusal and the test is discontinued
- 21. Standardization of SPT The validity of the SPT has been the subject of study and research by many authors for the last many years. The basic conclusion is that the best results are difficult to reproduce. Some of the important factors that affect reproducibility are:
- 22. Variation in the height of fall of the drop weight (hammer) during the test The number of turns of rope around the cathead, and the condition of the manila rope Length and diameter of drill rod Diameter of bore hole Overburden pressure
- 23. Corrections to SPTBlow Counts Factors affecting SPT blow count: Hammer Efficiency Borehole diameter Type of sampler Rod length
- 24. qu for cohesionless soil from SPT After knowing value of angle of internal friction, use bearing capacity equation
- 25. Peck et al. 1974
- 26. is a proportionality factor. Bowles (1996) suggested the value of 12 for this Peck et al. 1974
- 27. qu for cohesive soil from SPT After knowing value of qu (Unconfined compressive strength), use Skempton Equation.
- 28. qnu CNc qu Df Skempton’s Bearing Capacity Factor NC Skempton (1951) for Saturated clay soils and strip foundations suggested qu CNc Df C is half of unconfined compressive strength (Recall unconfined compression test)
- 29. Notes on SPT The standard penetration test (SPT) is performed during the advancement of a soil boring to obtain an approximate measure of the dynamic soil resistance, as well as a disturbed drive sample (split barrel type). The test was introduced by the Raymond Pile Company in 1902 and remains today as the most common in-situ test worldwide. The procedures for the SPT are detailed in ASTM D 1586 and AASHTO T-206.
- 30. The SPT involves the driving of a hollow thick- walled tube into the ground and measuring the number of blows to advance the split-barrel sampler a vertical distance of 300 mm (1 foot). A drop weight system is used for the pounding where a 63.5-kg (140-lb) hammer repeatedly falls from 0.76 m (30 inches) to achieve three successive increments of 150-mm (6-inches) each.
- 31. The first increment is recorded as a seating, while the number of blows to advance the second and third increments are summed to give the N-value ("blow count") or SPT- resistance (reported in blows/0.3 m or blows per foot). If the sampler cannot be driven 450 mm, the number of blows per each 150-mm increment and per each partial increment is recorded on the boring log.
- 32. For partial increments, the depth of penetration is recorded in addition to the number of blows. The test can be performed in a wide variety of soil types, as well as weak rocks, yet is not particularly useful in the characterization of gravel deposits nor soft clays. The fact that the test provides both a sample and a number is useful, yet problematic, as one cannot do two things well at the same time.
- 33. The SPT is conducted at the bottom of a soil boring that has been prepared using either flight augers or rotary wash drilling methods. At regular depth intervals, the drilling process is interrupted to perform the SPT. Generally, tests are taken every 0.76 m (2.5 feet) at depths shallower than 3 meters (10 feet) and at intervals of 1.5 m (5.0 feet) thereafter.
- 34. The head of water in the borehole must be maintained at or above the ambient groundwater level to avoid inflow of water and borehole instability. In current U.S. practice, three types of drop hammers (donut, safety, and automatic) and four types of drill rods (N, NW, A, and AW) are used in the conduct of the SPT. The test in fact is highly- dependent upon the equipment used and operator performing the test. Most important factor is the energy efficiency of the system.
- 35. The theoretical energy of a free-fall system with the specified mass and drop height is 48 kg-m (350 ft-lb), but the actual energy is less due to frictional losses and eccentric loading. A rotating cathead and rope system is commonly used, and their efficiency depends on numerous factors including type of hammer, number of rope turns, conditions of the
- 36. sheaves and rotating cathead (e.g., lubricated, rusted, bent, new, old), age of the rope, actual drop height, vertical plumbness, weather and moisture conditions (e.g., wet, dry, freezing), and other variables. Trends in recent times are towards the use of automated systems for lifting and dropping the mass in order to minimize these factors.
- 38. Cone Penetration Test (CPT) (ASTM D 3441 and D 5778: ) Originally Developed in Netherlands 1930s Further developments in 1950s “Dutch Cone” Types of CPT devices –mechanical cone –electric cone –piezocone
- 39. Cones 60 degree apex cone Standard cone φ = 35.7 mm Area = 10 cm2 Measures: Tip resistance (qc) Sleeve friction (fs) Pore pressure (pw) Shear wave
- 42. CPT Truck
- 43. Crawler Type CPT Truck
- 45. Mechanical Cone Penetration Test. The idea for the mechanical cone penetration test is similar to the SPT except that instead of driving a thick-walled sampler into the soil, a steel cone is pushed into the soil. The most common type of mechanical penetrometer is the Dutch mantle cone, This test is often referred to as the Dutch cone test or the cone penetration test
- 47. The cone is first pushed into the soil to the desired depth (initial position) and then force is applied to the inner rod, which moves the cone downward into the extended position. The cone is pushed into the soil at a rate of about 2 to 4 ft/min (10 to 20 mm/sec).
- 49. Static Cone Penetrometer test penetrometers Cone with pore water pressure measurement capability are known as piezocones 40 mm 40 mm 40 mm 40 mm
- 52. Cone 60o Base Area10cm2 For obtaining cone resistance, the cone is pushed down at a steady rate of 10 mm/sec through a depth of 40 mm each time Area of Sleeves: 150cm2