## 1 Introduction  
Information and communication technology (ICT) are actively used in construction [1; 2], and the characterization of geotechnical data and the facilitation of site-specific data are considered important [3]. In addition to enabling us to respond in real-time [4], the measurement equipment on a construction machine also has the potential to improve the reliability of the structures [5; 6]. In the field of piling, the press-in piling method, which is one of the pile penetration methods, can obtain contiguous piling data. This method can not only potentially be used for safety confirmation, but also for the prediction of ground information in real-time [7], as shown in Figure 1. However, while the piling data are collected during piling, it is not yet being fully utilized. As an active learning method for dealing with construction data, the observational method (OM) was first introduced by [PERSON] [8], and was later supported by many other researchers [9; 10; 11; 12], with its use being accepted by guidelines [13; 14; 15]. Because the Bayesian model can deal with probabilistic problems, and OM and \"Bayesian updating\" are easy to use together, these methods can account for uncertainties and update the posterior probability based on observations [16; 17; 18; 19; 20]. The OM has advantages in geotechnical structures, which have a large degree of uncertainty compared to artificial materials such as concrete and steel. However, it is said that there are several conditions for its implementation [21; 22]: (1) the structure must have considerable uncertainty; (2) the response time and cost for monitoring must be appropriate; (3) the field condition must be observable with some reliability; and (4) the building must consist of multiple stages and be reinforceable after the observations. Condition (1) has been met because geotechnical structures have some large uncertainties [23]. Conditions (2) and (3) can be expected to be met due to improvements in ICT. Thus, only the last condition remains unsolved for conducting the OM. In excavation/embankment, for example, where the OM is actively used, measurements in the early stages of building are used to predict the displacement/settlement and are also applied in the next step (e.g., the cuts and fill stages and the prediction of consolidation durations [9; 25]). Conversely, few cases have been reported on pile foundations or embedded cantilever retaining walls (i.e., single excavations), in which it is difficult to use construction data [26]. In this paper, such a condition is referred to as \"single stage.\" However, is condition (4) always necessary? The following uses of construction data come to mind: (a) for updating the design for the next step using the construction data (i.e., ordinary OM); (b-1) for assessing the performance of the structure and completing the construction (i.e., construction confirmation); (b-2) for designing cheaper structures in advance, anticipating that the performance of the structure can be verified using the construction data; and (c) for reviewing the maintenance or buying insurance based on the construction data [27]. As projects with rotary press-in piling are often not so large (mainly 20-40 piles [28]), method (a) is not practical for pile foundations to take time away from conducting load tests in order to design, order, and prepare the piles. In method (b-1), the cutting of pre-fabricated piles when the pile stops at a high level is expensive and time-consuming, and the pile length mainly depends on the depth of the bearing layer; therefore, no significant changes can be expected. Method (c) has been reported by [29]; however, to the best of our knowledge, it has not been realized. Thus, it is difficult to apply these methods in both small and in single-stage projects, with the exception of method (b-2).