Various defects occur during construction in deep excavations such as leakage through retaining wall, dewatering during excavation, retaining wall construction, pulling out used pile and over excavation. Measures used to tackle these defects are discussed in the following sections.
Construction Defects in Deep Excavation and their Remedies
1. Leakage Through Retaining WallThis type of deep excavation issues occur when retaining wall is damaged or not adequately waterproofed, and the type of project site soil is sandy soil with high level of groundwater. Figure 1 shows leakage of ground water through retaining wall of deep excavation. The excavation may collapse if the leakage through retaining wall is large, so this issue is substantially problematic and pose great risk. When the leakage is small, it is possible that fine grain soil such as silt would be moved along the water into the excavation. This process may be slow, but it might cause failure in area around the project site if extra loads such as vibration or any other loads act on this area. This is because the movement of fine grain soil would destabilize the soil and hence vibration and other similar forces would trigger the failure. The settlement of the soil surrounding the excavation area may be local or overall and its consequences could seriously disastrous. Failure due to leakage through retaining wall shall be taken into consideration provided that the soil has high permeability such as sand. So, such problems need not to be considered in clay soil which its permeability is low and there is cohesion between soil particles. Causes of leakage in different types of retaining wall and techniques used to tackle this issue are shown in table. Table-1: Causes of Retaining Wall Leakage and Its Remedies Techniques
|Type of retaining wall||Cause of groundwater leakage through retaining wall||Techniques used to tackle groundwater leakage through retaining wall|
|Soldier piles or laggings||
||Prevent the application of soldier piles and sheet piles in sandy soil with ground water level|
|Adjacent retaining wall with different rigidities||Differential deflection of adjacent walls lead to cracking due to earth pressure||Joint grouting|
|Presence of pipe under retaining wall||Insufficient construction space results in poor construction due to the presence of pipes||Grout pipe locations prior to the start of construction work|
Fig.1: Leakage through retaining wall of deep excavation, water moves from lowest row of anchor plates, and this problem solved, and the leaks stopped using prime resins polyurethanes
2. Dewatering During ExcavationDewatering is used to keep excavation dry in permeable soil such as sandy soil or to lower groundwater pressure of permeable soil beneath clay soil so as to prevent upheaval failure. Pumping should be carried out with great cautions since pumped water may carry fine grain soil and lead to settlement as illustrated in Figure 2. It should be known that pumping of ground water outside excavation area may cause settlement outside the excavation area.
Fig.2: Ground Settlement Caused by Pumping in Deep Excavation
3. Retaining Wall ConstructionThe construction of retaining wall causes settlement in the excavation under normal circumstances because of unloading which is caused by trench excavation. Sometimes, trenches would suffer from movements which produced greater settlement in the excavation area while diaphragm wall is constructed. Moreover, when piles are used as a retaining wall, boreholes drilled for pile installation purpose would lose the soil and cause movement of surrounding soil and subsequent settlement. Finally, compaction effect caused by driving sheet pile in sandy soil with high level of groundwater cause settlements but if the type of soil is clays then heave is likely to occur in soils close to the excavation area.
4. Pulling Out Used PileWhen soldier or sheet pile is used as a retaining wall in deep excavation and these piles are pulled, then voids will be left after piles are removed and these voids may not be properly filled which cause settlement. The purpose of removing piles after the construction of basement is to reuse them to reduce the cost. Nonetheless, it is recommended to left piles at their place to prevent settlement.
Fig.3: Formation of Voids due to Pulling Out of Piles and Subsequent