British Columbia Carpenter Apprenticeship Program

Transcription

1 British Columbia Carpenter Apprenticeship Program Level 2 Line G Competency G-8 Use Pile Foundations and Shoring

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4 Competency G-8 Use Pile Foundations and Shoring Contents Objectives... 2 Learning Task 1: Describe Piles... 3 Learning Task 1 Self-Test... 9 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 1

5 Competency G-8 Use Pile Foundations and Shoring Pile foundations are common where the bearing capacity of the surface soil is insufficient to support the building or to extend the foundation below the frost penetration. Shoring and underpinning are used in heavy construction to temporarily and permanently support excavations and buildings. Carpenters working in areas of the province where there is significant frost need to understand the principals of pile foundations. All carpenters working in heavy construction must understand the concepts of piled foundations, shoring and underpinning. Objectives When you have completed the Learning Tasks in this Competency, you will be able to: identify different types of piles describe how piles are used Competencies Written: Describe Pile Foundations and Shoring You will be tested on your knowledge of piles and methods of constructing them. 2 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

6 Competency G-8 Learning Task 1 Learning Task 1 Describe Piles Notes Pile Foundations Bearing and friction piles are used to support the weight of a building or large structure. They are used when the surface soil cannot support the load on conventional spread footings. Bearing and friction piles act like underground columns and are made of wood, concrete or steel or a combination of these materials. Pile foundations are used when the soil bearing capacity is insufficient to carry the required loads. Instead of the usual foundation walls and footings, deep foundations that may have piles or caissons going down to bedrock are used. Piles are driven into the ground until they are able to support the building. Piles can be used in groupings or singly at each location depending on the load conditions. If piles are driven into the ground vertically, they will only be able to carry vertical loads. If, however, they are driven in groupings and some of the piles are driven at an angle, then lateral loads can also be resisted. Angled piles can also resist movement caused by wind or seismic loads on very soft soils. Caissons consisting of large hollow concrete sections may be driven with pile driving equipment to great depth. The Three Gorges Dam project on the Yangtze River is an example of extremely large caissons being used. Bearing and Friction Piles Bearing piles consisting of a variety of materials such as wood, steel or concrete extend down to bedrock or other solid soils which increase the ability to stabilize and support a structure. Bearing piles transfer a vertical load to solid ground below and they bear directly on either solid soil or rock. Usually they are driven in groupings in a specific number and spacing which then is topped with a pile cap (see Figure 1) the shape of which is determined by the engineer. Piles can be driven in the group on an angle to resist lateral loads on the building. On top of pile caps a grade beam (see Figure 2) is often placed below the finish grade to allow larger spacing of the pile groups, this is a massive concrete beam poured on the pile caps or directly on the piles on the which the building is erected. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 3

7 Learning Task 1 Competency G-8 Notes Column Pile Cap GRADE BEAM PILE Piles ELEVATION VIEW PILE PLAN VIEW Figure 1. Pile cap Figure 2. Grade beam These bearing piles are driven down to the point of refusal which may be bedrock or soil that is able to withstand the loads placed upon it by the structure. The point of refusal is a measure of resistance experienced by the pile driving equipment to drive the pile further in to the ground, or penetration resistance. The engineer will specify the criteria for when to terminate the driving of the pile, for example, 25 blows for one inch. When wood piles are used as bearing piles, it is important that they are reasonably straight. Wood piles, because they are part of a tree, will often be curved and if the pile sweep is too pronounced, will prevent them from being driven straight into the ground. A measure of that straightness is the pile sweep (Figure 3) which determines the curvature of the pile from head to foot as a deviation from a straight line. 4 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

8 Competency G-8 Learning Task 1 Notes PILE SWEEP Figure 3. Pile sweep Friction piles are generally made of wood due to the increased skin resistance between wood and soil as well as the tapered shape of the wood. Friction piles transfer a vertical load to the soil around the pile. They depend on the total surface area of the pile and the shear strength of the surrounding soil (known as skin friction). As friction piles are driven into the ground, the pressure of soil against the sides of the pile, as well as the end bearing, support the load. These piles are driven into softer soils that extend down to great depths and are driven in the ground to the point of refusal as per engineer design. When driven in groups, the piles share the load evenly from the cap above. In areas such as Richmond, where deep peat soils are predominant, wood friction piles are commonplace. Wood piles made of Pine or Western Red Cedar is generally preferred by the industry and is treated with creosote or other preservative for increased resistance to decay and insect damage. All piles driven into the ground will cause the soil around them to heave up, this is known as pile swell. It is the displacement of the soil by the pile that causes this. Most modern piles have a built-in resistance to deterioration unlike the wood pile which must be protected from the elements. This is done by treating them with creosote or CCA compounds as well as keeping them covered with a concrete cap. The caps will isolate the wood pile top from moisture and light where they are most likely to rot. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 5

9 Learning Task 1 Competency G-8 Notes The various parts of piles are named and are important to know for future reference. Pile Ring or Driving Head Head Butt Pile Cutoff Foot Tip Pile Shoe Figure 4. Parts of a pile The corresponding terminology of pile parts originates with wood piles as shown in Figure 4 and is self explanatory. There are numerous methods used to create and insert or drive piles into the soil and the following are some examples of piles in use. 6 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

10 Competency G-8 Learning Task 1 Thin steel shell Permanent plain or corrugated steel shell Button foot Compressed base Button-Bottom Cased Concrete Pile Cased Pile with Compressor Base Cased Pedestaled Concrete Pile Bedrock Belled Pile Shell-Less Pile Swaged Pile Step-Taper Pile Figure 5. Various piles BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 7

11 Learning Task 1 Competency G-8 Notes In sandy soils, piles may be jetted into position with the use of water where precast piles have a hole in the centre where water is forced through under high pressure and is forced back up, taking the sand with it, through the slots around the perimeter. The resulting cavity created allows the pile to be lowered by gravity until it reaches solid bearing as in Figure 6. A Water in Water out Section AA A Figure 6. Jet piles Other methods of improving soilbearing capacity include the use of high frequency vibration. In this method, the vibration probe causes the sand to compact in a columnlike structure which can be done at regular intervals as required. The recent Palm projects in Saudi Arabia as well as the airport in Japan are examples of such a method (See Figure 7). Vibratory driver extractor Soil to be compacted Vibratory probe Soil being compacted Compacted soil Figure 7. Vibro-float Now complete Learning Task 1 Self-Test. 8 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

12 Competency G-8 Learning Task 1 Learning Task 1 Self-Test 1. Label the parts of the pile in Figure 1. a b c d e f g Figure What type of pile depends on the resistance developed between the soil and the surface area of the pile? 3. Name the piles shown in Figure 2. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 9

13 Learning Task 1 Competency G-8 Figure Describe the difference between a sheet pile and a bearing pile. 5. Why are friction piles driven in groups? 6. What is a caisson? 7. When is the driving of bearing piles stopped? What identifies the stopping point by name? 10 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2