Characteristics of Ground Movements Induced by Excavations

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It is crucial for engineer to have adequate information regarding possible ground movements and identify its characteristics to consider proper measures against such undesired movement. Characteristics of ground movements include shape and type of ground surface settlement, influence zones of settlement, locations of maximum settlement, magnitude of maximum settlement and relationship between ground surface settlements and soil movements are discussed in the following sections.

Fig.1: Ground Settlement due to Excavations

Characteristics of Ground Movements Induced by Excavations

1. Shape and Type of Ground Surface Settlement

Generally, there two major types or shapes of ground surface settlement due to foundation excavation namely concave shape and spandrel shape. The magnitude and shape of retaining wall deflection are the cause of these two types of ground surface settlement. If the initial stage of excavation induces greater retaining wall deflection or the retaining wall deflect similar to a cantilever beam, then the settlement will be spandrel type and the maximum settlement will be close to the excavation area, as it is illustrated in Figure 2.

Fig.2: Spandrel shape ground surface settlement induced due to cantilever shape deflection of retaining wall

However, concave type settlement will occur provided that the wall has a deep inward movement as shown in Figure 3 and the largest settlement magnitude will be positioned at a distance from the excavation.

Fig.3: Concave shape ground surface settlement caused by deep inward movement of retaining wall

Moreover, it is demonstrated that spandrel settlement type is frequently occurred in sandy soil and stiff clay since the deflection of retaining wall is smaller and it is like cantilever beam deflection. However, spandrel shape settlement is mostly occurred in soft clay soil since the retaining wall would suffer from considerable deep inward deflection. These statements are valid for normal conditions. Furthermore, formula has been developed to anticipate the shape of settlement based on the retaining wall deflection. Parameter used to predict ground surface settlement are area of cantilever component deflection (A_c=max (A_c1, A_c2)), and area of total deflection minus area of cantilever component deflection (A_s). Lastly, if the A_s is large than 1.6A_c, then concave shape ground surface settlement will be highly likely to cure whereas spandrel type will occur provided that A_s is smaller than 1.6A_c. Figure 4 illustrate the definition of different parameters used for the prediction of shape of ground surface settlement.

Fig.4: Parameter (As, Ac1, and Ac2) are illustrated

2. Influence Zones of Settlement

Generally, the influence zone of settlement is claimed to be two to three times the depth of the excavation. It is affected by number of factors for instance width of excavation, depth of excavation, and types of soil. Table 1 provides influence zone of settlement based on the type of soil. Hsieh and Ou proposed a conception that the influence zone of settlement curve is composed of primary influence zone and secondary settlement zone as illustrated in Figure 5. As it may be observed from the figure, the curve steeper in primary influence zone which means buildings will be affected greatly whereas the curve is gentler in the secondary influence zone and hence the affect on the structure will be smaller. It is possible that the influence zone of settlement exceeds primary and secondary settlement zone, but it is so small that its influence on the structure can be neglected. The evaluation of influence zone of settlement range is considerably significant to evaluate the extent of damage that adjacent structures might suffer due to deep excavations.

Fig.5: Distribution of influence zone settlement curve

Table-1: Influence zone of settlement based on the type of soil

Types of soil	Induced Influence zone of settlement
Sandy soil	Two times excavation depth
Stiff to very stiff clay	Three times excavation depth
Soft or medium soft clay	Two times the depth of excavation

3. Location of Maximum Settlement

In previous sections, spandrel shape and concave shape settlement which are two major type of settlement induced by excavation were discussed. When the former type occurs, the maximum settlement will be close to the retaining wall. However, the retaining wall is reported to occur at a distance of 1.5 times the height of retaining wall in the case concave settlement. Nonetheless, it is practically measured that maximum settlement distance from the retaining wall, in the case of concave settlement shape, is equal to the primary influence zone divided by three. The primary influence zone is explained in the influence zone of settlement in the above section.

4. Magnitude of Maximum Settlement

Several attempts have been made to evaluate the magnitude of maximum settlement for example clough and O'Routke set a relationship between maximum settlement and excavation depth in different types of soil to estimate. This method does not provide accurate outcome because the excavation depth is not the only factor that affect the settlement. Moreover, (Mana and Clough, and Ou et. el.) established a relationship to compute the maximum settlement magnitude. They have derived a relationship between maximum retaining wall deformation and maximum ground settlement because factors that influence maximum retaining wall deformation are the same as those affecting magnitude of maximum settlement induced by excavation. Figure 6 show the relationship between maximum retaining wall settlement and magnitude of the maximum settlement for different types of soil with upper limit for sandy soil and lower limit for clay soil. So, the maximum deformation of retaining wall caused by excavation can be evaluated using finite element method or beam on elastic foundation method. Finally, the maximum deformation of retaining wall will be used to compute maximum settlement magnitude with the help of the figure.

Fig.6: Relationship between maximum ground surface settlement and lateral deformation of retaining wall