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Underpinning structures is the introduction of extra support to the foundation of a structure to extend or enlarge its bearing value. Extending or enlarging a foundation because of deeper recent construction close to the building is label as precautionary underpinning. One of the first large underpinning projects was at the great cathedral in Winchester, England, which settled for 900 years until it was underpinned in the early twentieth century by a diver who installed bags of concrete into pits dug under water through peat and build up to a gravel layer without anyone assist (see figure 1.1). In the course of early subway work, it was essential to underpin literally hundreds of buildings of a great many types and sizes and other new method were work out. The furthermost improvement was made in shoring and temporary support techniques prior to underpinning. In many buildings, mostly the large ones, it was found out that it was possible to excavate small pits under footings to install piers or piles, making it workable to reduce temporary support. The loss of support below the foundation for a short time could be accepted until the underpinning support elements were bed in. Nowadays, the quantity of underpinning work has multiply due to large, deep structures and the huge volume of subways throughout the world. In this report will discuss what is underpinning and how it works.
Before designing underpinning, the structure to be underpinned should be analyzed to determine the loads to be supported (John Wiley and Sons 2007, 31). If original plans are available, column loads may be tabulated or the assumed earth bearing value may be found so that column loads may be computed from the footing sizes. The height of building, column spacing, stories, wall thickness and window spacing, type and materials of construction, live loads assigned to each floor and room, earthquake factors where required, superimposed loads caused by part of the new structure, condition of building should all be considered (Fang 1991, 637).
In designing underpinning, the ground condition under the building requires close study. The most uncertain element of underpinning systems is the under laying soil supporting the system rather than the structural capacity of concrete or steel used. Seams of soft or compressible material, boulders, filled ground, groundwater conditions, and the location of bedrock, hardpan, or satisfactory bearing material will have an influence on the extent, depth, and type of underpinning necessary. For this reason, it is essential to have adequate, properly made borings available, preferably with samples that can be examined, consolidation tests, groundwater-level observations and grain size analysis.
Of the many methods of underpinning, the most common is concrete pit underpinning. Plain concrete pits are installed by excavating sheeted pits by hand under existing structures to the proper depth and to suitable bearing strata, and filling the pit with concrete and dry packing between the pit and foundation to transfer the load into the new concrete piers. This method is basically limited to dry ground because it is difficult to dig below groundwater level without loss of ground causing settlement of the building to be underpinned. The breadth of the pit may vary from four to five feet, although larger pits may be dug. The usual procedure is to dig an approach pit 3â€™ wide and 4â€™ long to a depth 5â€™ below the bottom of the footing in front of the footing to be underpinned. The pit is completed by deepening until it is several feet below subgrade of the new excavation, provided the bearing capacity of the ground is adequate on this level. Next, provide support to sides of excavation and to existing footing if necessary. In sand or non cohesive soils, insert joggle timbers that interlock as the depth increases. Timber bracing is also required to prevent timber joggles from slipping. All sides of excavation require support until desired depth is reached.
Concrete piers may be intermittent or they may be installed next to each other to form a continuous wall. This is determined by the load of the structure being underpinned and by the bearing value of the material under the pit. If the foundation wall is heavy masonry or reinforced concrete or has a sound reinforced grade beam, the wall will span from pier to pier. However, if it is found that the strength of the foundation footing is not strong enough to span between the intermittent piers, it is necessary to install lintels between the pits to support footing.
Continuous underpinning is installed when intermittent pits do not provide sufficient bearing support for the building. Alternate pits are installed first to provide preliminary support. When the filler pits between the alternate pits are excavated, the side pit boards of the first pits installed are removed. If either intermittent or continuous underpinning does not have adequate bearing capacity, it is often advantageous to bell out the bottom of the pits in the last three or four feet of the pit to provide extra pit area and therefore obtain additional carrying capacity.
If piling is to be driven after concrete pit underpinning is installed and a deep excavation made adjacent to the building, it is prudent to install jacking notches in the concrete underpinning in order to be able to install jacks and maintain the building if the underpinning pits settle due to consolidation of the ground caused by pile driving.
The limitation of pit underpinning is the problem of excavating a small pit through water-bearing ground, particularly through fine-grained or silty material. To overcome these problems, compressed air caissons were used but they were expensive and caused loss of ground and settled the structures underpinned. Section steel piles were developed in 1912 for underpinning buildings where there were water problems with indifferent result, as the building underpinning settled even though the piles were jacked to overloads of 50% above their design load. These problems had been overcame by Lazarus White and Edmund A. Prentis during the construction of the William Street subway in New York City (Prentis and White, 1950) It was the rebound and recompressing of the ground that caused the settlement of the buildings when the piles were not pretested.
Pre-test piles are installed by digging a carefully excavated and sheeted approach pit under the footing or wall. As the piles are jacked down, they are periodically cleaned out with special digging tools. Cleaning of the piles reduces the jacking pressure required and enables the piles to be advanced. Water jetting and pumping can be used to excavate the piles but should be used with great care to avoid softening the soil around the pit. Closed end or solid piles should not be used because they cannot be advanced if the jacking pressure too high or if obstruction prevents the pile from being jacked.
When the piles has reached the required depth and has been excavated to approximately the bottom of the pipe, concrete is poured in the pile if dry. If the pile is wet, the pipe can be sealed without a grout plug. The pretesting is accomplished by using two hydraulic jacks installed side by side with enough room between them to install a wedging beam. The pile may settle under the pretest load until the bulb of pressure is established. When the settlement stops, the load should be held for one hour without further settlement. While the load is still being applied, a section of steel I beam is cut and set on end between the two jacks. Filter plates are used with forged steel wedges inserted between the I beam and top plate. The steel wedges are driven tight with a sledge hammer. After that, the jack can then be deactivated and removed and pin up remaining openings.
Compaction grouting can be either a soil treatment system or a combination of soil treatment and load transfer. Soil in loose or soft state can be densified by injecting a very viscous sand-cement grout into soil zones. Compaction grouting uses displacement to improve ground conditions. With adequate confinement stress and slow injection rates, the low mobility grout will displace the soil into a denser arrangement, thus increasing its bearing capacity and reducing its compressibility. In cases where ground dropouts are a problem, settled structures can be pushed back to near original position as part of the grout treatment program.
Underpinning techniques present a unique blend of experience, theory, and economics. Improved soil mechanics has been helpful in analyzing underpinning problems, and there have been many improvements in techniques and method. Mechanization has not advanced as much as in other fields of construction and underpinning costs have increased along with labor costs. The selection of the best underpinning method depends on the best combination of safety and cost of installation, and it is essential that the work be performed by an organization familiar with both the design and execution of underpinning. On the other hand, it is also important to point out that underpinning works requires expertise in the design and execution levels, along with safe working practices. An experienced geotechnical consultant can offer much to the success of an underpinning system.