Construction Technology B (CON4313)

Transcription

1 2 - Foundation The British Standard Code of Practice BS8004 defines a foundation as 'that part of the structure designed and constructed to be in direct contact with and transmitting loads to the ground' 1 Shallow Foundation (quick revision) 1.1 Pad footing 1.2 Strip footing 1.3 Raft footing Solid Slab Raft Beam and Slab Raft Cellular Raft 1.4 Combined footing 2. Piled Foundation 2.1 Classification of Piles Classification by Method of Load Transmission End bearing piles Friction piles Classification by Method of Installation Displacement Piles Replacement Piles - 1/26

2 3. Displacement Piles Displacement piles are preformed piles made in a factory, transported to the site, and then driven into the ground. They are also called preformed piles or driven piles. Preformed piles may be made of reinforced concrete or steel. (There was a pile type called driven cast-in-situ pile. It was formed by driving a shell or a casing with a closed end into the ground and then casting the annular space with concrete. This type of pile is now obsolete) 3.1. Piling equipment for displacement piles Displacement piles are generally driven into the ground by one of the following methods: 1. Percussion 2. Vibration Percussive driving Piles are driven into the ground by the impact of heavy pile hammers. There are several types of pile hammers: a. Drop hammer (Fig. 2.1) b. Diesel hammer (Fig. 2.2) (These two types of piles are now obsolete.) c. Hydraulic Hammer A hydraulic hammer unit is mounted on the pile while operating. It is similar to a drop hammer but the weight is raised by hydraulic jacks. The weight then falls freely under gravity on to the pile head. Hydraulic jacks are so powerful that the weight can be up to 18 tonnes. The stroke height is about 1-2 m. The blow rate depends on the stroke height and ranges from 40 to 100 blows per minutes. 2/26

3 Most hydraulic hammers are equipped with silencers nowadays. They produce less noise than diesel hammers and they do not emit exhaust fumes. They are suitable for hard drive Vibratory driving vibration hammer (Fig. 2.3) Vibration hammer is comparatively silent. It is used in driving small displacement piles such as H-piles and sheet piles. The equipment consists of a vibrating unit mounted on the pile head transmitting vibrations down the length of the pile shaft. These vibrations are in turn transmitted to the surrounding soil, reducing its shear strength enabling the pile to sink into the subsoil under its own weight and also that of the vibration hammer Piling rigs A piling rig has the function of guiding the pile at its correct alignment from the stage of first pitching in position to its final penetration. It also carries the hammer and maintains it in position co-axially with the pile. (Fig. 2.4) Helmets The head of displacement piles must be protected from damage during driving. The protection consists of resilient packing which is held in position by a steel helmet. The resilient packing distributes the impact load from the piling hammer evenly on to the pile head. (Fig. 2.5) The helmet should fit loosely around the pile, so that the pile may rotate slightly without inducing torsion on the pile. 3/26

4 3.2 Precast prestressed concrete tubular pile In the past, most of precast concrete piles were square in cross sections. They can be made of either of normal reinforced concrete or prestressed concrete. Their bearing capacity ranged between KN. In 1980s, precast concrete piles in H.K. became tubular because of the development of a manufacturing technology. This type of pile is prestressed and the concrete is compacted by centrifugal spinning process. The spinning process effectively compacts the zero slump concrete and produces a hollow tube without the need of a former. They are then steam cured in an autoclave under high steam pressure and high temperature in order to obtain a minimum concrete compressive strength of about 80 MPa within three days. The outside diameters of the piles are about mm and in lengths of up to 12 m. They have a nominal bearing capacity in the range up to 3500 KN. (Fig. 2.6) Each pile length has integral steel joint plates (end plates) cast onto ends of the pile. Lengthening is done by a full penetration butt welding of the end plates. The disadvantages of these piles are that they produce great noise and vibration during the installation of these piles. Moreover, most Hong Kong soils are coarse, containing boulders and rocks which are not too suitable for large diameter displacement piles. 3.3 Preformed Steel piles Hollow steel piles Two main types of hollow steel piles: tubular and box sections. Box piles are normally driven with closed ends (now obsolete) with internal drop hammers. Open ended tubes can be driven with vibration hammers. Hollow Steel piles have a high resistance to lateral loading and buckling and good energy-absorbing properties. 4/26

5 They are particular suitable for structures subjected to lateral and impact loading such as jetties and dolphins Steel H-pile Steel H-pile is strong and tough. The displacement of the soil is little compared with other types of displacement piles. It is widely used in HK. Driving steel piles generally do not require pile shoes. In hard conditions, the toe of the pile may be strengthened. Strengthening may be carried out by welding steel plates or angles to the toe of the pile (Fig. 2.7). Preformed pile shoes for H-piles are also available (Fig. 2.8). In ground conditions where boulders exist, Oslo point may be used (Fig. 2.9) Lengthening Lengthening of steel H-pile is done by butt-welding. Splicers may be used to help splicing Corrosion Protection Steel preformed piles may deteriorate as a result rusting. Corrosion protection is of particular importance for steel piles because they are embedded in the ground and become inaccessible for maintenance. A protective coating shall be applied on the steel piles before being driven into the ground. The surfaces of steel piles to which protective coatings will be applied shall be prepared by blast cleaning. (Source: General Specification for Civil Engineering Works) 5/26

6 3.4 Procedure of driving a displacement pile 1. Prepare a piling rig and a piling hammer. 2. Put marks on the pile length at 500 mm interval to indicate the length being driven. 3. Pitch the pile in the piling rig. Check its alignment and verticality. 4. Place a suitable pile helmet on the pile head. Mount the piling hammer on top of it and then start driving. 5. If the whole length of the pile has been driven into the ground, splice a new length on it, then continue the driving. 6. Repeat step 5 until it renders significant resistance against driving. 7. Perform the set measurement. If the predetermined set is attained, the piling process is completed. Otherwise repeat from step 5 until it does. 3.5 Measurement of set Set related to the ultimate bearing capacity of the pile can be determined by pile driving formulae by considering the parameters of the pile, the soil and the ground condition. There are many pile driving formulae, among which the Hiley formula is the most commonly used one. The final set shall be measured as: 1. penetration per 10 blows, or 2. the number of blows required to produce 25 mm penetration. 6/26

7 3.6 Advantages and disadvantages of displacement piles Advantages 1. Where large numbers of piles are to be installed in easy driving conditions, it can be more economic. 2. Projection above ground level advantageous to marine structures. 3. The pile can be inspected for quality and soundness before driving. 4. Construction operations not affected by ground water. 5. Not liable to squeezing or necking Disadvantages of displacement piles 1. Transporting the complete length of pile through narrow and/or congested streets may be difficult. 2. The driving process, which is generally percussion, can cause unacceptable noise and vibrations. 3. Pile driving hour is limited by environmental regulations. 4. May break during driving, causing delays and replacement charges. 5. Displacement of soil during driving may damage adjacent structure or cause up lifting of adjacent piles. 6. Cannot be driven in very large diameters. 7. Cannot be driven in conditions of low headroom. 7/26

8 4. Replacement Pile Replacement pile is also called 'non-displacement pile'. It is formed by boring a pile shaft and replacing the soil with in situ concrete. In Hong Kong, it is commonly called bored pile. 4.1 Methods and equipment for boring pile shafts Rotary boring Rotary boring involves a drilling rig which may be mounted on a mobile crane or a truck. The drilling rig consists of a telescopic or extendable kelly bar on which a boring tool is attached. There are various types of boring tools: 1. Cheshire auger The auger has only a few helix turns. It has to go up and down repeatly to cut the soil and bring the spoil to the ground surface. 2. Continuous auger (flight auger) It has a long helix. The spoil is continuously brought to the ground surface by the spiral motion. However, the drilling rig must provide sufficient torque to overcome the great friction (Fig. 2.10). 3. Drilling bucket It is designed to withstand the high torque forces developed during penetration of dense strata. The spoil is temporarily stored in the bucket to reduce the friction created. The frequency of up and down motions can also be reduced as compared with that of the Cheshire auger. 4. Coring bucket It is used to raise a solid core of rock. 5. Chisel - It is used break through boulder or rock. (Fig. 2.11) 8/26

9 4.1.2 Grab boring A grab consists of a clamshell bucket which is operated by means of wire ropes and suspended from a mobile crane. The grab is dropped on to the soil in an open position and is then closed. It is then raised to the surface and emptied spoil at the side of the borehole. Virtually the rope can be of infinite length. The depth of a borehole can be over a hundred meters by grab boring. Most boreholes are circular in cross section but can be rectangular by using a rectangular grab (Fig. 2.12). When boulders are encountered, they can be broken by free falling a chisel on to it. The rock fragments are then removed by the grab. Chisel and grab are often used alternatively to break through obstructions. 9/26

10 4.1.3 Down the hole (DTH) drilling A down the hole drill (DTH) is a large size hammer drill. It is equipped with a button bit that can drill holes into rock with diameters up to 750 mm and to depths of several hundred meters. A conventional hammer drill becomes less efficient as the length of borehole increases because of the loss of energy in drill stem. To overcome these difficulties the down the hole drill was developed. Its rotary motor remains above ground level while the pneumatic hammer is followed down the hole. The drill stem rotates in slow motion while the drill bit strikes rapidly. The drill debris is blown out by compressed air. (Fig. 2.13) Reverse circulation drilling (RCD) A reverse circulation drilling rig can drill at a fast rate in a wide range of ground conditions including rocks. In Hong Kong, it is commonly use to drill the bed rock sockets for bored piles. When boring in rock, rock roller bits are used. The Reverse circulation drill sits on the temporary steel casing while operating. It consists of a double wall drilling pipe and operates on the principle of air lifting. Compressed air is injected through the annular space of the drilling pipe and discharge near the base. The rising column of air and water at the centre of the drilling pipe lifts the soil or rock fragments which has been loosened by the drilling bit. (Air lifting is also commonly used to clean the bottom of a bored pile before concreting.) (Fig. 2.14) 10/26

11 4.1.6 Under-reaming The base of a bored pile can be enlarged to three times the shaft diameter to increase the bearing capacity of the pile. The method is known as under-reaming which is done by a belling bucket. The belling bucket is lowered to the bottom a borehole by a kelly bar with the cutter in retracted position. It is then rotated by the kelly bar and the cutter is jacked out to form the bell. (Fig. 2.15, 2.16) 11/26

12 4.2 Methods of supporting boreholes Self-supported (unsupported) For small diameter bored piles and in stable soils, the borehole may remain unsupported without collapsing. However, a borehole without support is not recommended in Hong Kong Supported with temporary casing (Fig. 2.17) In unstable or water-bearing ground, the bored pile shaft should be supported by a steel casing to prevent collapse. The casing can be driven into the ground by a casing oscillator (or a vibration hammer). The oscillator consists of a collar clamp and a pair of hydraulic jacks. The collar clamp grips the casing. The two hydraulic jacks extend and retract repeatly in opposite phrases. This oscillation motion helps to cut the soil by the casing shoe and sink the casing into to ground. The steel casing is usually thick and expensive and will be retrieved after the borehole is concreted. The casing oscillators (or vibrator hammer) the can be used to withdraw the casing. In a long pile the casing may be stuck by the concrete and the extraction becomes difficult. Moreover, temporary steel casings which are in contact with concrete shall be withdrawn before the initial set of the concrete has taken place. Otherwise, it shall be left in place. To overcome this problem, it is advisable to extract the casing in stages, i.e., the casing is extracted as concreting progresses upwards. However, it is important that a sufficient concrete pressure head shall be maintained within the steel casing to ensure that the pressure from external soil and ground water would not cause necking of the pile. 12/26

13 4.2.3 Permanent casing Sometimes a permanent casing will be lowered into a temporary cased borehole. The permanent casing is made of light gauge corrugated steel sheet. It is much cheaper than the thick temporary casing. The permanent casing prevents the expensive temporary casing being stuck by the fresh concrete. It also reduces the risk of necking of the pile after extraction of the temporary casing and prevents the fresh concrete being contaminated by soil and ground water. Corrugated steel casings are often used in long piles and in difficult grounds such as reclaimed land Supported with bentonite slurry (Fig. 2.18) Bentonite is a kind of clay. When mixed with the correct amount of water bentonite slurry produces thixotropic properties, it gives a liquid behaviour when agitated and a gel structure when undisturbed. During boring, the borehole is filled with bentonite slurry. The boring action stirs the slurry so it remains liquid state. But the slurry penetrates slightly into the subsoil and forms a soft gel or so called 'filter cake' at the interface of the excavation sides. Hydrostatic pressure caused by the slurry thrusting on the 'filter cake' that prevents the borehole from collapse. This method is not suitable for soil with high permeability. 4.3 Examples of Replacement Piles Bored pile Bored pile is perhaps the most popular type of pile in Hong Kong currently. The maximum diameter can be over 3 m and the pile length can be over 100 m. It can be designed to carry loads up to kn. 13/26

14 There are various techniques for boring pile shafts and supporting them. The choices of the methods and their combinations depend on the ground condition and the pile design. Some typical examples are as follows: Barrette (Fig. 2.19) (Source: General Specification for Civil Engineering Works) By definition, a barrette is exactly the same as a bored pile supported with bentonite slurry. However, a barrette is often referred as a pile in rectangular cross section. Its formation is the same as that of a bored pile except that four concrete guide walls are used instead of the collar casing. The guide walls are about 150 mm thick, arranged as a topless and bottomless box with internal dimension the same as the barrette section Socketted steel H-piles (Fig. 2.20) A socketted steel H-pile is a replacement pile with an H-pile in the core. It is formed by pre-drilling a borehole which is temporary supported with a steel casing. The borehole shall be of adequate size to allow a minimum grout cover of 40 mm to the H-pile. The borehole shall be sunk to sound rock level and the bottom of the borehole shall then be cleaned by air lifting. A H-pile is inserted into the borehole and the annular space is subsequently filled with cement grout. The temporary casing can then be withdrawn. 14/26

15 4.3.4 Minipile (Micropile) Minipiles are defined as piles having a diameter of less than 300 mm. Generally they range in shaft diameter from 50 to 250 mm, with working loads in the range of 50 to 500 kn. There are many ways of forming minipiles. A typical method currently used in Hong Kong is (Fig. 2.21): 1. Drill a borehole of about 150 mm diameter with a rotary drilling rig and line the borehole with a steel casing. 2. Grout the borehole with cement slurry from the bottom of the hole. 3. Insert the pile reinforcement into the casing. Normally it consists of 2-4 numbers of T40 or T50 bars. (The steel casing is left in place to enhance corrosion protection.) The principal use of minipiles is for underpinning work or in steep slope where large piling machine cannot access Installation procedure of Bored Piles i) Bored pile supported with temporary and permanent casing Procedure 1. Pitch a temporary steel casing in the correct position and drive it into the ground with a casing oscillator. 2. Bore within the casing with a hammer grab. Boring shall not go beyond the bottom of the casing to prevent collapse of the borehole. 3. If rocks or boulders are encountered, use a chisel to break them and then use the grab to remove the fragments. 4. Extend the steel casing and drive it into the ground if necessary. 5. Repeat steps 2 to 4 until the founding rock is reached. 15/26

16 6. Place a reverse circulation drill on top of the steel casing. Form the socket of desired depth into the bed rock with a roller cutter. The debris is removed by air lifting. 7. Remove the RCD. (Lower a permanent casing into the borehole.) 8. Lower the reinforcement cage into the borehole. 9. Check the rate of ingress of ground water. If it does not exceed 0.3 L/s, place high slump concrete into the shaft immediately after it is pumped dry. Otherwise, fill the shaft with tremie concrete. 10. Extract the temporary steel casing as the concreting is progress upward. When the concrete level reaches the desired lever, the piling process is completed. ii) Bored pile supported with bentonite slurry Procedure 1. Pitch a steel collar casing in the correct position and drive it into the ground with a vibration hammer. 2. Fill bentonite slurry within the casing and excavate the borehole with a hammer grab. 3. Maintain the slurry level to about 500 mm below the top of the casing during the whole piling process. 4. If rocks or boulders are encountered, use a chisel and the grab alternatively to break the rocks and remove the fragments. 5. Repeat steps 2 to 4 until the founding rock is reached. 6. Use the chisel and the grab alternatively to form a socket in the rock. 7. Lower the reinforcement cage into the borehole through the slurry. 8. Insert an air lift pipe to the bottom of the borehole. Clean the bottom of the borehole with air lifting. Slurry loaded with soil particles shall be pumped to a settling tank, stained and recycled. 9. Fill the pile shaft with tremie concrete. Displaced slurry shall be retrieved. 16/26

17 10. Stop concreting with the concrete reaches 1 m about the cut-off level. Remove the collar casing and the piling is completed. 17/26

18 4.4 Advantages and Disadvantages of Replacement Piles Advantages of replacement piles: 1. Soil or rock removed in boring can be inspected for comparison with site investigation data. 2. Can be installed in very large diameters. 3. Can be installed in very long length. 4. End enlargements up to three diameters are possible 5. Can be installed without appreciable noise or vibration. 6. Can be installed in conditions of very low headroom. 7. No risk of ground heave Disadvantage replacement piles: 1. Drilling a number of piles in group can cause loss of ground and settlement of adjacent structures. 2. Squeezing or necking may occur in soft ground that reduces the bearing capacity of the pile. 4.5 Placing Concrete in Piles Concreting by trunking In boreholes where the rate of ingress of water does not exceed 0.3 L/s, the piles shall be dried immediately before concrete is placed. Then the concrete can be placed using a readily workable mix (slump 100 mm, which is self-compacting but does not segregate) through a trunking. 18/26

19 4.5.2 Concreting under water (tremie method) (Fig. 2.22) If the excavations for piles are supported by bentonite slurry or if the rate of ingress of water exceeds 0.3 L/s, concrete shall be placed by a tremie. A tremie is a steel tube suspended in the water by a crane, with a hopper fixed to the top end to receive the concrete. The tube shall be watertight, smooth-bored, diameters of mm. The tremie concrete shall be of high workability (minimum 150 mm slump). Procedure: Erect the tremie pipe (with hopper) vertically and resting on the bottom of the pile shaft. Place a traveling plug (formed with foamed plastic in plastic bag) into the tremie pipe. Feed high slump concrete into the hopper to push the traveling plug downward. When the plug reaches the bottom, slightly raise the tremie pipe to let the plug and the concrete to flow out. Continuously feed fresh concrete into the tremie pipe. Pump away displaced water/bentonite slurry. Raise and remove sections of the tremie pipe progressively to maintain the concrete flow rate, but keeping at least 3 m embedment of the tremie pipe in the concrete. Continue concreting until the concrete lever reaches 750 mm above the cut-off level. Remove the tremie pipe. The concreting process is completed. (The surplus concrete shall be trimmed away hardening.) 19/26

20 5 Quality Control of Piles Construction Technology B (CON4313) As the design and construction of buildings become more complex, it is necessary to exercise closer supervision during construction to ensure quality of the building works. The foundation of a building is a fundamental structural element which supports the whole building. Moreover as the foundation, when constructed, is buried in the ground, its built quality is not readily visible. There is a need to enhance the supervision of foundation works during construction and testing of the completed works in order to ensure that the quality of the foundation works is up to standard. There are various methods to control the quality of piling works: 5.1. Pre-drilling for piles founded on rock For piles founded on rock, sufficient pre-drilling should be carried out before the installation works, such that the quality of the founding rock can be identified and the appropriate founding levels can be determined. The pre-drilling should be sunk to at least 5m below the tentative founding rock levels of the piles. Pre-drilling should be carried out for each of the large-diameter bored piles, barrettes and the like. For minipiles, socketted steel H-piles and similar small diameter-bored piles, founding on rock, the number of pre-drill boreholes required should be such that the pile tip of every such pile should be within 5 metres from a pre-drill hole, or at a larger distance from it as decided by the Engineer. 5.2 Proof drilling For Large diameter bored piles 20/26

21 Proof drilling shall be carried out on every large-diameter bored pile to check the condition at the concrete/rock interface. A base coring tube of at least 150 mm diameter shall be left at about 1000 mm above the founding level of the pile. The core-drilling shall be carried out to 1000 mm below the concrete/rock interface For minipiles and socketted steel H-piles Proof drilling shall be carried out to verify the adequacy of the socketted length of minipiles and socketted steel H-piles The depths of the proof drill holes shall be at least 5 metres below the founding levels of the adjoining piles. 21/26

22 6 Determination of the Settlement of the Piles by Load Test 6.1 Procedure of Load Test (Fig. 2.23) a. Working piles shall be tested to not less than 1.8 times working load. b. Test loads shall be applied and removed in three stages as stated in Table c. The test loads shall be applied in increments, and removed in decrements, of 25% of the working load. Increments of load shall not be applied until the rate of settlement of the pile is less than 0.1 mm in 20 minutes. d. The full test loads for Stage I shall be applied in increments and shall then be maintained for at least 24 hours after the rate of settlement has reduced to less than 0.1 mm per hour. The test loads shall be removed in decrements and the recovery of the pile determined before loading is resumed. e. The procedure stated in (d) shall be repeated for Stage II loading. f. The procedure stated in (d) shall be repeated for Stage III loading. g. The settlement of the pile shall be measured at hourly intervals. The settlement of the pile under each increment and decrement of loading shall be measured. The exact times at which increments are applied and decrements are removed shall be recorded. h. The level of the reference beam shall be checked at regular intervals agreed by the Engineer during the test. Table 8.11 Test loading stages Stage Test load I 25% of max. test load II 50% of max. test load III 100% of max. test load 6.2 Compliance criteria of load tests on piles a. The settlement at any load shall be less than twice the settlement at 90% of that load (Brinch Hansen's criteria). 22/26

23 b. Under working load the gross pile head settlement shall not exceed 20 mm for buildings and 10 mm for other structures. 23/26

24 7. Testing of concrete core from piles Concrete cores shall be 100 mm diameter. Each concrete core from a pile shall be inspected for evidence of segregation of the constituents and for the presence of voids. Specimens selected from each core shall be tested to determine the compressive strength. The concrete core shall be considered as non-compliant if it exhibits honeycombing which means interconnected voids arising from, for example, inadequate compaction or lack of mortar. For any set of cores representing a test location, the average estimated equivalent cube strength shall be at least 85% of the specified grade strength. Each individual estimated equivalent cube strength shall be at least 75% of the specified grade strength. 24/26

25 8. Sonic Test on bored pile (Fig. 2.24) Prior to concreting, three to four mild steel tubes of not less than 50 internal diameter are fixed to reinforcement cage of the cast insitu pile. The tubes shall be watertight, parallel to the axis of the pile and extend the full depth of the pile. Testing is carried out by lowering a pair of sensor probes (a transmitter and a receiver) to the bottom of the tubes. The tubes are filled with water/jelly prior to testing to provide acoustic coupling The probes are maintained at the same elevation and simultaneously raised slowly (20cm/s). The propagation of sonic signals between the tubes is recorded. The test is repeated for each pair of tubes, i.e. three runs for a pile with three tubes and six runs for a pile with four tubes. The sonic profiles of the entire length of the pile are plotted and the defects and irregularities can be assessed. 25/26

26 Reference: 1. Pile design and construction practice, fourth edition (1994), M.J. Tomlinson, E & FN Spon. 2. Introduction to civil engineering construction, third edition (1995), Roy Holmes, The college of estate management. 3. Construction Technology, second edition (1987), R. Chudley, Longman Scientific & Technical. 4. Civil Engineering Construction IV (1991), S.A.R. Jufri & R.J. Wellman, Hong Kong Polytechnic. 5. General Specification for Civil Engineering Works (1992), Hong Kong Government. 6. R.Chudley, 1985, Construction Technology Vol, 3 &4, Longman 7. S.A.R. Jufri & R.J. Wellman, 1991, Civil Engineering Construction IV, Vol 2 & 3, Hong Kong Polytechnic 8. R. Holmes, 1983, Introduction to Civil Engineering Construction, College of Estate Management 9. M.J. Turner, 1997, Integrity testing in Piling Practice, Construction Industry Research and Information Association 26/26