Professor Dr. Md. Jahangir Alam BScEngg – Structure MScEngg – Geotechnical PhD – Earthquake Engineering Post Doc – Soil-Structure Interaction Compare between precast and bored RCC pile Precast pile Slump value 50-100 mm Retarder and plasticizer is not needed in general Compaction by vibration is necessary Clear cover > 50 mm Durability should be considered Bore pile Slump value 150 – 200 mm Retarder and plasticizer should be used Compaction is prohibited Clear cover > 75 mm Durability should be considered Mix ratio of cast in situ piling concrete without plasticizer Cement = CEM-II, FA = Coarse Sand, CA = Stone chips or Shingles 1 : 1.25 : 2.00 Minimum fc’ = 28 Mpa (4000 psi) 3 Mix ratio of cast in situ piling concrete with plasticizer Cement = CEM-II, FA = Coarse Sand, CA = Stone chips or Shingles 1 : 1.5 : 2.5 Minimum fc’ = 28 Mpa (4000 psi) 4 Wet method of construction Or Slurry method of construction Excavation of bore hole can be done by 1. Percussion method of drilling a. Usually done in Bangladesh b. Good for dia 20” – 24” and L = 90’ 2. Rotary method of drilling a. Expensive b. Now used in big projects c. Any dia and length ok d. Economic for large dia Steps of cast-in-situ piling Boring Insertion of rebar cage Concreting through tremie pipe Boring concreting Use of casing Temporary casing Permanent casing Steel casing water soil Permanent casing examples Steel casing Thick caving soil: Loose sand or Loose nonplastic silt Dense sand or Stiff clay Temporary casing necessary when caving soil (loose sand and silt) exist at top layer To maintain the piling location To maintain the slurry above the ground level Length of casing depends of soil layers Temporary casing example Temporary steel casing Loose sand or Loose nonplastic silt or soft clay Dense sand or Stiff clay Precautions for temporary casing Cleaning after each use Must be as smooth as possible Casing with bonded concrete should not be allowed Slurry column should extend well above the level of the piezometric surface so that any fluid flow is from the excavation outward Drilling Slurry Bentonite or polymer slurry may be used Drilling fluid serves to put soil particles in suspension and will form a membrane or a filter cake at the walls of the borehole. The membrane acts to prevent caving or collapse of the borehole. Also called as drilling mud Dense Sand Loose Sand Drilling slurry is essential For piling into caving soil Caving soil types Loose sand and gravel Nonplastic Silt Important note: avoid drilling slurry if only water is enough, because Bentonite slurry is expensive Reduces skin friction of pile Slurry types Bentonite slurry Polymer slurry Properties of drilling slurry Item to be Measured Range of Results at 20 C Test Method Density during drilling to support excavation greater than 1.05 g/ml Mud density Balance Density prior to concreting less than 1.25 g/ml Mud density Balance Viscosity 30 - 90 seconds Or 5 - 25 cP Marsh Cone Method Or Fann viscometer 9.5 to 12 pH indicator paper strips or electrical pH meter pH New Slurry Intake Pump Slurry tank Sediment Solids (to be disposed daily) Drilling fluid circulation New Slurry Intake Pump Slurry tank Sediment Solids on inclined vibrating sieve Modern Drilling fluid circulation Problems of Cast in situ piling - 1 Solution: measure the depth of the excavation two or more times after drilling ceases to see that sediment is not settling out and the hole is as deep as indicated by the penetration of drilling tools. Problems of Cast in situ piling - 2 Solution: do an appropriate job of sampling and testing the slurry Solution: ensure the location of the bottom of tremie pipe is just far enough above the bottom of the excavation so that concrete can start flowing Problems of Cast in situ piling - 3 Solution: the casing should not be pulled until it is filled with concrete with good flow characteristics Pile reinforcement (cage) The cage is designed to meet two requirements The structural requirement for bending and for column action (sometimes slender column) Stability requirements of the rebar cage during its placing, during the placing of concrete and during the withdrawal of the casing Minimum longitudinal reinforcement If the pile has sufficient axial strength using only half the gross concrete area, Ag/2, the longitudinal reinforcement ratio can be reduced to 0.5 percent of the gross concrete area, Ag. Longitudinal bar in the cage Maximum longitudinal reinforcement at the top if no drag load Symmetrical arrangement of longitudinal rebar is recommended unless there is compelling reasons Minimum 5 or preferably 6 longitudinal bars are needed 16mm dia bar is the minimum size of longitudinal bar Clear spacing between bars is 3-5 times maximum size of CA If a very large amount of rebar is needed, concentric multiple cages or bundled bars may be used. Splices and sizing hoops Splices should be staggered Lapping at the same location is economical in piling Spiral Longitudinal bar Sizing hoop Sizing hoops simply provide guides for the fabrication of cage Centering Devices Two purposes: 1. Clear cover 3” 2. To flow concrete Rebars should not be used for centering devices unless they are epoxy-coated Better solution is to use concrete roller Installation of concrete rollers Pile Cap Casting Break 3 ft weak concrete at top of drilled shaft / bored pile before casting pile cap Use top, bottom and side reinforcements in pile cap even it is not needed by calculation Use thick pile cap to ensure rigid action of pile cap to distribute column load to individual piles uniformly Take care of basement construction at pile cap level Basement floor Placement of concrete in drilled shaft Basic characteristics of concrete for drilled shafts Excellent fluidity SCC (Self Compacting Concrete) Compaction under self-weight Resistance to segregation Controlled setting Measurement of Workability Slump Test 4” 12” 8” 1. 3 layers 2. 25 tamping per layer 3. Tamping rod dia 16mm 34 Measurement of Workability Slump Test Upto 125 mm •True Slump In case of shear slump, repeat the slump test 25-50 mm Upto 150 mm •Shear Slump Disadvantage of Slump Test • Lean Mix > Shear type or collapse at low w/c ratio 150-250 mm •Collapse •Zero Slump 35 Use of retarder in tremie concrete If the required time of pouring concrete is more than initial setting time, retarder must be used Initial setting time >= 45 minute Final setting time <= 420 minute Usually initial setting time = 2 hours Precautions for tremie concrete the concrete shall contain at least 10 per cent more cement than that required for the same mix placed in the dry. Slump = 150 mm to 200 mm Successful placement of concrete under water requires preventing flow of water across or through the concrete. The first concrete placed is physically separated from the water by using a “rabbit” or go-devil or plug in the pipe, or by having the pipe mouth capped or sealed and the pipe dewatered. Once filled with concrete, the pipe is raised slightly to allow the “rabbit” to escape or to break the end seal. Concrete will then flow out and develop a mound around the mouth of the pipe. This is termed as “establishing a seal”. Once the seal is established, fresh concrete is injected into the mass of existing concrete. Precautions during concreting In the “rabbit” plug approach, open tremie pipe should be set on the bottom, the “rabbit” plug inserted at the top and then concrete should be added to the tremie slowly to force the “rabbit” downward separating the concrete from the water. Once the tremie pipe is fully charged and the “rabbit” reaches the mouth of the tremie, the tremie pipe should be lifted a maximum of 150 mm (6 inch) off the bottom to allow the “rabbit” to escape and to start the concrete flowing. After this, a tremie pipe should not be lifted again until a sufficient mound is established around the mouth of the tremie. Precautions during concreting Tremies should be embedded in the fresh concrete a minimum of 1.0 to 1.5 m (3 to 5 ft) and maintained at that depth throughout concreting to prevent entry of water into the pipe. Rapid raising or lowering of the tremie pipe should not be allowed. All vertical movements of the tremie pipe must be done slowly and carefully to prevent “loss of seal”. Underwater concrete shall be placed continuously for the whole of a pour to its full depth approved by the Engineer, without interruption by meal breaks, change of shift, movements of placing positions, and the like. Delays in placement may allow the concrete to stiffen and resist flow once placement resumes. The volume of concrete in place should be monitored throughout the placement. Under runs are indicative of loss of tremie seal since the washed and segregated aggregates will occupy a greater volume. Over runs are indicative of loss of concrete. Tremie pipe dia > 6 times of maximum size of CA Plug of cement paste is recommended as low cost solution of initial charge Minimum c/c spacing of piles Drilling and extraction of casing during construction of a pile can cause upward directed shear stresses to develop on the perimeter of adjacent piles that were installed earlier, possibly damaging those shafts So minimum c/c spacing = 3D If closer spacing is required, sequence of shaft installation must be mentioned Load distribution in pile Initial loads are taken almost completely by skin friction at very small displacement as loading continues, some load is transferred to the base of the drilled shaft At the ultimate load, a sizeable portion of load is carried by end bearing but at significant amount of downward displacement Influence of construction on soil properties of clay Stress released during excavation. The placing of concrete will reimpose a stress in the clay surrounding the drilled shaft that can be greater than the in situ stress. The magnitude of the concrete stress is dependent on the slump of the concrete, and high-slump concrete is highly recommended. Some chemical bond occur between clay and cement, so skin friction increases. So, Shear failure does not occur at interface but a short distance from interface Influence of construction on soil properties of sand A membrane of bentonite is created at the wall of the borehole Stress released during excavation. The placing of concrete will reimpose a stress in the sand surrounding the drilled shaft that can be greater than the in situ stress. The magnitude of the concrete stress is dependent on the slump of the concrete, and high-slump concrete is highly recommended. Some chemical bond occur between sand and cement, so skin friction increases. So, Shear failure does not occur at interface but a short distance far from interface Quality Assurance Pile integrity test – length and integrity Pile load test on test piles and service piles Static load test – takes time Dynamic load test (PDA) - rapid Summary of quality control of pile Ensuring length Cross checking the soil test bore log with drilling bored pile Slump and mix ratio of concrete Outlet of tremie pipe into concrete all the time Maintaining clear cover Prevention of caving by using bentonite slurry or any other mud Cleaning the bore hole properly to ensure end bearing
