Basement

3 basement 3 Basement 1 Quick Revision 1. 1 Problems arising from basement construction a. jab method. b. Surface and ground piddle control c. Lateral stability of basement excavation. d. Stability of adjoining building. 1. 2 flat coat water control dewatering a. Open sinkhole Pumping b. Wellpoint System. c. Shallow-Bored Well System d. Deep-Bored Well System 1. 3 Basement Excavation a. Open excavation b. Sheet pile cofferdam back up with raking struts Supported with struts and walings Supported with ground anchors Ground support and ground water control for deep excavationCut off argueings c. Thin fastened diaphrage d. Mixed in place skirt e. Ground treatment f. Circular cofferdam g. Diaphrage groyne 1/11 3 basement 2. Cut-off seawallings 2. 1 Thin grouted diaphragm A series of touching widely distributed beams or columns argon driven into the ground to the required depth. A grout dispenseion pipe is fixed to the web the section and this is connected to a conclave p ump at the ground. As the sections are withdrawn the void created is filled with cement grout to form the thin membrane. This is a non-structural wall and services solely as a cut-off wall. It is uitable sub grime of silt, sand and gravel. 2. 2 Thin grouted diaphragm (Source R. Chudley) Mixed-in-place wall A hollow churn drill is drilled into the ground. Intrusion grout is pumped through the hollow kelly bar during the drilling operation. When the required depth is r separatelyed, the auger rotation is reversed and withdrawn while grout shaft continues. The grout is thus mixed with the existing soil to form a mixed-in-place pile. The above process is repeated with the tons being cast against each other until the all told wall is formed. Mixed-in-place pile (Source Jufri & Wellmen) /11 3 basement 2. 3 Ground treatment The major disadvantage of pumping is causing settlement of surrounding ground. In urban areas, settlement would serious affect the stability of adjacent properties. In this situation, grouting method can be employ to control ground water. The basic method is to inject the soil or rock with fluids which, on setting, seal or reduce the permeability of the secular. There are various types of grouts. The choice of grouts, the pattern and spacing of the injection pipes will depend upon the particle sizes of the soil or the size of fissures in the rock.Cement Grouting (Source R. Chudley) 2. 3. 1 Cement grouting Cement grout is suitable for injecting into coarse fabrics which have a high permeability. This method not only can form an impermeable barrier to ground water but also solidify loose soils which prevents from collapsing during excavation. It is chronic to commence grouting with a batch of thin grout and then(prenominal) to increase the viscosity of the grout as the process continues, by reducing the water-cement ratio. The grout utilize may be composed of neat cement grout, or a mixture of cement and 3/11 3 basement sand in the ratio of 1 4.The adding of sand reduces the cost of material but it also reduces the workability. On the other hand, pulverized fuel ash (PFA) may be employ to replace part of the cement (up to 50%) to reduce the cost of the material. The spherical particle shape of PFA improves the flow quality of the grout without reducing its strength. 2. 3. 2 Bentonite grouting Bentonite grouting is used in ground conditions where the particles of the soil are too small for cement grouting. While bentonite grout adds little strength to the soil, it has a high resistance to water flow and thitherfore produces an excellent barrier.Bentonite is produced from montmorillionite clay, which has thixotropic properties when it coagulates it forms a gel, and if mixed with certain additives, such as Portland cement or soluble silicates, the barrier formed will be permanent. 2. 3. 3 Chemical grouting There are two main processes of chemical grouting one-shot and two shot. In the two-shot process, pipes are driv en into the ground at closely 600 mm centres, and the first chemical, normally sodium silicate, is injected. This is followed immediately by the injection of the second chemical, calcium chloride.The reaction between the two chemicals is immediate, resulting in a tough, water-insoluble silica-gel. The process gives considerable strength to the soil and greatly reduces its permeability. Two-shot process for chemical grouting 4/11 3 basement The two-shot process has been largely superseded by the one-shot process, which consists of assortment together prior to injection two chemicals whose gel cadence can be sufficiently delayed to allow full penetration of the soil earlier gel occurs. The extent of the delay can be accurately controlled by varying the proportions of the two chemicals.The extra time available for placing this grout allows wider spacing of the boreholes. 4. 2. 6 Circular cofferdam Steel sheet pile cofferdams may be arranged in circular shape in plan. Circular cof ferdams may be supported with ring walings. The walings are subject to tangential thrust and act as a continuous arch. Thus transverse struts are not required. openhanded diameter circular cofferdams may be formed of concrete diaphragm wall and reinforced with ring concrete beam. The advantage of this method is that it provides a soak up excavation un-hampered by struts. However, the method is restricted to circular shape.Circular cofferdam support with ring walings Basement construction of IFC 5/11 3 basement 5 Diaphragm seawalls Diaphragm walling describes the construction of continuous concrete walls into the ground. The process, sometimes called the slurry infringe method, involves the excavation of a narrow trench to the required depth which supported with bentonite slurry. The trench is then filled with concrete to form the wall. Construction of diaphragm wall 5. 1 Advantages of using diaphragm walls for basement construction 1. The process of constructing the wall is re latively quiet and has little vibration. . The wall can be constructed to a great depth. 3. The wall provides temporary ground supports during excavation hence expensive steelwork is eliminated or minimized. 4. The wall provides a good water cut-off no dewatering is required and hence has little effects on adjacent structure. 5. The wall serves both as external wall for the basement and the foundation for the superstructure. 6. For deep basement, diaphragm walling is more economical than using steel sheeting cofferdam. 6/11 3 basement 5. 2 method of construction of concrete diaphragm wall 1. A edge trench, 1 to 1. m deep and with a width equal to the wall thickness positive(p) 300 mm is excavated. 2. The trench is lined on both sides with 150 mm thick in-situ concrete. (The linings provide a guide for the grab and comfort the top of the excavation from collapse. ) 3. The wall is divided into to panels. The width of each panel is some 4. 5 to 7 m. The sequence of construction f or the panels is in an alternatively way. That is, an mediocre panel is excavated aft(prenominal) panels on its both sides have been cast. 4. The trench is then filled with bentonite slurry from a large storage tank before excavation.Excavation is done by a grab. During excavation, it is important to maintain the slurry level at about 0. 5 m below the top of the trench. (The function of the bentonite is to prevent the ingress of water and soil into the trench. ) Excavation is continued until the founding is reached. 5. Stop end pipes of diameter equal the wall thickness are placed at both ends of the panel. A keep cage is lowered through the bentonite and panel is cast with tremie concrete. The displaced bentonite is recovered, strained to remove the soil particles and then stored for future use. 6.When the concrete has set, the stop end pipes are removed. Semi-circular sockets are formed which provide key for the adjacent panels. 7. Further panels of the diaphragm wall are then formed in a similar manner until the wall is completed. 5. 3 Precast Concrete Diaphragm Walls Diaphragm walls can also be formed with precast concrete panels. The construction sequence is 1. A perimeter trench, lined on both sides with concrete walls, is prepared. 7/11 3 basement 2. The trench is filled with bentonite slurry and excavated with a grab. During excavation, the slurry level is maintained at about 0. m below the top of the trench. Excavation is continued until the founding is reached. 3. Cement grout is introduced to the bottom of the trench with a spreader. The displaced bentonite is recovered, strained to remove the soil particles and then stored for future use. 4. Precast posts or panels are inserted into the trench to form the wall 5. When the grout has set, it seals the joints of the precast panels effectively, and the diaphragm wall is formed. Precast Concrete Diaphragm Wall Precast Concrete Diaphragm Wall (Source R. Holmes) (Source R. Holmes) 8/11 3 basement 6 S oldier pile wallSince the wall of a basement must(prenominal) be watertight, if a soldier pile wall is used as a basement wall, the scads of the wall will be constructed to overlap with each other. This type of wall is also called secant pile wall. The overlapping is about 120 to 200 mm. It is cut by a hydraulic actuated slip fitted with a cutting ring at the lower end. The cutting must be done within one or two days after the concrete has been cast to prevent it become to hard. Secant pile wall Alternatively, the piles of the wall are cast contiguous. Boreholes are drill at the joints of the piles which cut the edges of the piles. The boreholes are then filled ith bentonite-cement grout to seal the joints. Secant pile wall (Source R. Chudley) 9/11 3 basement 7 top-down Basement Construction It is to construct the basement in the downward direction to the final levels of the basement. The sequence of work is as follows1. Construct the permanent basement wall using diaphragm wall or soldier piles wall. 2. Construct bored piles with temporary steel casings as the foundation of the basement. The cut-off level of the piles is at the final basement level. 3. Place steel columns inside the steel casings of the bore piles from the foundation level up to the ground level. . Backfill the shafts with granular material to hold the steel columns. 5. Construct the ground al-Qaida slab supported on the steel columns and the diaphragm walls. (This slab provides a working platform for superstructure construction and acts as the squint support for the basement excavation. ) Access opening in this slab shall be provided for further excavation. 6. Excavate beneath the ground floor slab down to next basement floor soffit. 7. Cut the steel casing, remove the gravel and expose the steel column. 8. Cast the second basement slab. The slab is connected to the columns with shear studs and connected the diaphragm wall with dowels) 9. seize the process for the successive basement floors until down to the final basement slab. The top-down method provides the following main advantages1. Superstructure can be constructed at the same time as the basement construction takes place and therefore the overall construction time is reduced. 2. Temporary and permanent works are combined and therefore there is no need for extensive steel temporary works. 3. Stiff rigid walls allow an almost watertight construction and basement construction to great depth.These ladened walls provide at the same time a limitation to ground movement. 10/11 3 basement Steel casing infilled with granular material Steel column on top of bored pile Reference Control of groundwater for temporary works (1986), S. H. Somerville, CIRCA Construction Technology (1987), R. Chudley, Longman Building Construction vade mecum (1992), R. Chudley, Newnes Introduction to Civil Engineering Construction (1985), R. Holmes, College of Estate Management Civil Engineering Construction IV (1991), S. A. R. Jufri & R. J. Wellmen, Hong Kong Polytechnic 11/11