The next cap supported on a bridge is set and temporarily supported on a single post € Side spiles are driven for their full lengths € A heavy horizontal beam is pushed forward to support the forward cap € The breast board is extended to grade level by adding new sheets *UNDERWATER TUNNEL Immersed-tube method The immersed tube tunnel technique uses hollow box sectioned tunnel elements that have been prefabricated in reinforced concrete. These are floated out into the harbor and placed into a trench that was pre-dredged in the harbor bed. When in position, the elements are joined together to form a tunnel. The trench is then refilled and the harbor bed returned to its original level. *PROBLEM FACED DURING TUNNELING Moisture, water and chemical damage to concrete Even concrete of high quality is a porous material – Excess water evaporation during hardening will leave millions of pores and capillaries in concrete – The zones between cement and aggregates are prone to cracking during hardening due to drying shrinkage, temperature stress and outside forces Porosity of concrete – Allows moisture, water and chemicals to move freely throughout the concrete Increases absorption of deleterious chemicals Cracks may develop as tunnel structures are constantly moving and developing strains due to earth loads, stress redistribution and tectonic seismic influences. If not waterproofed, cracks can allow water to pass into the structure and possibly damage utilities, interior finishes and even the structure itself. Moisture, water and chemical intrusion – Results in corrosion of the concrete due to chemicals dissolved in water –
Results in concrete neutralization (carbonization) Results in alkali-aggregate reaction Freeze/thaw cycles can lead to concrete cracking and damage Reduces structural property *TUNNEL SHAFT SINKING Excavation from the surface of an opening in the earth. Shafts, which are generally vertical, are usually distinguished from tunnels, which are horizontal. Little difficulty is experienced in shaft sinking through solid rock, which contains little water. When loose, water-bearing strata have to be contended with, careful shoring and sealing of the shaft lining become necessary, and pumping facilities are needed. Shafts are usually circular or rectangular and are generally lined with wood, masonry, concrete, steel, or cast iron. Shafts sunk in loose water-bearing soils, where there is great external pressure on the shaft sides, are nearly always circular; rectangular shafts with wood lining are often used in mining work, as the shafts are frequently of a temporary nature. Shaft sinking through rock is generally accomplished by blasting. When the loose surface material has been removed, holes are drilled, and the charges are placed and are fired by electricity. The broken rock is removed and the process is repeated. In an ordinary rectangular shaft the lining consists of timbers 8 or 12 in. (20 or 30 cm) square placed horizontally around the shaft. Shafts of a more permanent nature are generally circular in form and lined with cast iron or with concrete masonry 1 to 2 ft (30–61 cm) thick, built in sections as the work advances. When excessive quantities of water are met with, cast-iron tubbing is sometimes used. This consists of heavy cast-iron rings made in segments, with flanges for connecting, and bolted together in place. Cement grout is forced into the space between the outside of the tubbing and the surrounding earth to form a seal. Shaft sinking by the freezing process in very watery soil is accomplished by sinking pipes in the area to be excavated and circulating brine at low temperature in them until the earth is frozen and hard so that it can be excavated in the same manner as rock. In the grouting method, liquid cement is forced into the water-bearing earth under very high pressure. On mixing with the water, the cement solidifies the adjacent area, and it is removed by drilling and blasting as with rock. *BEDDING OF CONDUIT Pipe bedding comprises pipe underlay, compacted pipe side support, and pipe overlay to 300mm above the conduit. Bedding material shall be a material conforming to AS 2758.1 for Fine Aggregate. Do not commence the placement of bedding in trenches until the trench bottom has been inspected and approved and the bedding type confirmed by the Superintendent.
Place bedding material in uniform layers for the full width of the trench in accordance with the Drawings. *PROBLEMS ENCOUNTERED IN UNDERWATER CONSTRUCTION The most common concrete defects found in drilled shafts and their causes are summarized below. 1. Voids or lenses in concrete and excessive wash-out of cement due to loss of tremie pipe seal during tremie concrete placement; 2. Voids behind the steel cage due to loss of concrete workability during placement, or insufficient clearance between reinforcing steel. 3. Significant washout of concrete and trapping of water in concrete due to unbalanced concrete surface or placing concrete directly through water 4. Necking of the shaft due to cave-in of the soft zone of the ground (e.g., thick clay seams in rock); 5. Entrapment of debris in concrete due to insufficient bottom cleaning prior to concreting. *UNDERWATER DRILLING AND BLASTING The drilling of blast holes for underwater blasting is carried out with drilling rigs mounted on pontoon or barge. Initially, the required position of drilling is finalized using various positioning systems operated from the barge. The drilling barge is brought into the predetermined location to drill a line of holes and is held in position by anchoring. The drilling towers are positioned over the specified drill hole location and drilling commences. The most commonly used drilling method for underwater blasting is called Over Burden Drilling (OD). In this method of drilling, a casing pipe is driven separately into the rock through the overburden for a distance sufficient to provide a seal to prevent small stones, sand, or silt from filling the drill hole. After the casing pipe is fixed, the inner drill rods are inserted through the casing pipe and the shot hole is drilled to the required depth. Upon reaching the required depth, drill rod is retrieved and the hole is ready for charging with explosives. In underwater blasting usually number of holes are drilled in a line and after completion of drilling and loading of all the holes in the line, the barge is shifted to the next position for drilling and charging of another row of holes and the operation is repeated until required number of holes has been completed for a particular blast. For drilling of blast holes at MbPT site, the entire blasting area was divided into 21 different blocks and each block was subdivided with rectangular grid of 1.5 m × 2.8 m. At each grid point, 150 mm diameter holes with 1.5 m burden and 2.8 m spacing were drilled to the required depth. Truck mounted hydraulic down-the-hole (DTH) drilling machine was used for drilling of blast holes.
*TYPES OF FORMWORK (SHUTTERING) Formwork is an ancillary construction, used as a mould for a structure. Into this mould, fresh concrete is placed only to harden subsequently. The construction of formwork takes time and involves expenditure up to 20 to 25% of the cost of the structure or even more. A good formwork should satisfy the following requirements: 1. It should be strong enough to withstand all types of dead and live loads. 2. It should be rigidly constructed and efficiently propped and braced both horizontally and vertically, so as to retain its shape. 3. The joints in the formwork should be tight against leakage of cement grout. 4. Construction of formwork should permit removal of various parts in desired sequences without damage to the concrete. 5. The material of the formwork should be cheap, easily available and should be suitable for reuse. 6. The formwork should be set accurately to the desired line and levels should have plane surface. 7. It should be as light as possible. 8. The material of the formwork should not warp or get distorted when exposed to the elements. 9. It should rest on firm base. Timber Formwork: Timber for formwork should satisfy the following requirement: It should be 1. well-seasoned 2. light in weight 3. easily workable with nails without splitting 4. free from loose knots