Building structures close to the excavation for deep foundations may suffer settlement and subsequent cracking and even failure. Therefore, it is necessary to practice utmost cares while deep excavation is carried out to reduce its undesired effect on the surrounding buildings.
Measures considered to protect buildings near deep foundation are discussed.
Fig.1: Structures Close to Deep Excavation
- Strategies to Protect Buildings Close to Deep Excavation
- 1. Building Protection Using the Characteristics of Excavation Induced Deformation
- 2. Building Protection by Increasing Stiffness of the Retaining-Strut System
- 3. Building Protection by Utilizing Auxiliary Methods
Strategies to Protect Buildings Close to Deep Excavation
- Building protection using the characteristics of excavation induced deformation
- Reduce the unsupported length of the retaining wall
- Decrease the influence of creep
- Take the advantage of corner affect
- Building protection by increasing stiffness of the retaining-strut system
- Building protection by utilizing auxiliary methods
1. Building Protection Using the Characteristics of Excavation Induced Deformation
1.1 Reduce the Unsupported Length of the Retaining WallWhen retaining wall used as a safety measure for deep excavations, struts are provided at a designated level to support the retaining wall. It is observed that, the unsupported height as shown in Figure 2 of the retaining wall affects its deformation due to earth pressure. As the unsupported retaining wall length is decreased, the amount of deformation will reduce as well and vice versa.
Fig.2: Deep excavation with retaining wall and strut level, unsupported length specified in figure left for each excavation stageThe unsupported height of retaining wall can be decreased by placing struts close to the excavation level. The closer the struts to the ground level the lesser the unsupported length as explained in Figure 3. Finally, it is recommended to consider 0.5m distance between strut and ground level at each excavation stage.
Fig.3: Deformation in figure A is less than that of figure B since supported length smaller in figure. The only difference between figure A and B is the distance of strut from ground level which 0.5m in figure A and 1.5m in figure B
1.2 Decrease the Influence of CreepCreep is the increase of deformation with time under constant stress. it occurs in clay soil. So, to decrease the affect of creep on the excavation, it is advised to install struts as soon as the excavation stage is completed. Usually, strut installation could take time and creep effect may increase and worsen the condition of excavation. Therefore, it is recommended to lay 100mm concrete on the excavation surface to contain the effect of creep till struts are installed.
1.3 Take the Advantage of Corner EffectWhen diaphragm walls are used to contain earth pressure in excavation, deformation and settlement will commonly lesser at corners and short direction of the excavation area compare with long direction. So, if buildings are located at corners or along short direction of the excavation area, then it is recommended to employ diaphragm walls to take advantage of corner affects.
Fig.4: Taking the advantages of corner effect to decrease effect of foundation on adjacent buildings