5.3.1 Peck's method
Peck（1969a）was the first，based on field observations，to propose a method to predict excavation⁃induced ground surface settlement.He mainly employed the monitoring results of case histories in Chicago and Oslo and established the relation curves between the ground surface settlement（δv）and the distance from the wall（d）for different types of soil，as shown in Figure 5.8.The method classifies soil into three types according to the characteristics of soil：
Figure 5.8 Peck's method（1969a）for estimating ground surface settlement
TypeⅠ：Sand and soft to stiff clay，average workmanship.
TypeⅡ：Very soft to soft clay.
1.Limited depth of clay below the excavation bottom.
2.Significant depth of clay below the excavation bottom but Nb＜Ncb.
TypeⅢ：Very soft to soft clay to a significant depth below the excavation bottom and Nb≥Ncb.
where Nb，the stability number of soil，is defined asγHe／su，whereγis the unit weight of the soil，He is the excavation depth，su is the undrained shear strength of soil，and Ncb is the critical stability number against basal heave.
Since Peck's method took the monitoring results of case histories before 1969，most of which employed steel sheet piles or soldier piles with laggings as the retaining wall，quite different and more advanced design and construction methods（e.g.the diaphragm wall method which offers higher stiffness）have been employed in excavation projects in recent years，and the relation curves proposed by Peck are not necessarily applicable to all excavations.Basically，the curves derived from Peck's method are envelopes.Because Peck's method is the first to derive an empirical formula to predict the ground surface settlement induced by excavation and is simple for application，it is still used by some engineers.