Strategies to Protect Buildings Close to Deep Excavation

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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
   • Reduce the unsupported length of the retaining wall
   • Decrease the influence of creep
   • Take the advantage of corner affect
2. Building protection by increasing stiffness of the retaining-strut system
3. 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 Wall

When 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 stage

The 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 Creep

Creep 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 Effect

When 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

2. Building Protection by Increasing Stiffness of the Retaining-Strut System

The increase of retaining-strut system would help in the decrease of excavation wall deformation and subsequently protect neighboring buildings. Techniques used to increase retaining-strut system stiffness include declining vertical and horizontal span of struts, increase retaining wall thickness, and increase strut stiffness. Reduction of vertical span of struts is proven to be the most effective stiffness improvement technique because it reduces deformation considerably. This technique would increase the rigidity of the system substantially and decline deformation effectively. It is demonstrated that, the increase of retaining wall thickness would not contribute that much in reducing deformation. Finally, increasing strut stiffness could reduce deformation. However, if it already has large stiffness, then increase of strut stiffness will not be an option. So, reducing horizontal or vertical span can be considered.

3. Building Protection by Utilizing Auxiliary Methods

The purpose of auxiliary technique application is to either decrease settlement or wall deformation. Any auxiliary techniques that lead to settlement or deformation reduction are acceptable to be applied. However, these approached need to be assessed prior to their application to find out their effects. This is because some of these methods may not offer desired affect and in fact the condition might be worsening in addition to the cost. Different auxiliary methods include ground improvement, Counterfort walls, Cross walls, Micro piles, and Underpinning.