Underground Singapore 2011 2.2 Monitoring of strut loads An intensive programme of strut monitoring was implemented during the excavation works. In line with the APD method outlined by Peck (1969), the maximum value of the strut load was extracted for the entire monitoring period for each strut at the various excavation sites. More than 1500 strain gauges and load cell readings were extracted and these are presented in Figure 2.
Figure 2. Maximum strut loads monitored for each strut at the various station locations
3 ANALYSIS OF STRUT MONITORING DATA 3.1 Analysis of strut forces using apparent pressure method For each strut location, the maximum strut loads were back-analysed into the definition of Peck’s apparent pressure. This is done by dividing the strut loads (F) into the tributary area derived from interstrut spacings (A), and then further normalising the average pressures by dividing this with H. The normalized apparent pressure (F / A / H) is then plotted against the location of the strut with respect to the final excavation depth (i.e. z / H). The back-analysed apparent pressures from actual strut monitoring are then compared against the original APDs that would have been estimated using Peck’s recommendation. The results are plotted in Figure 3 and Figure 4. In order to compare against Peck’s APDs, it is necessary to estimate the stability number for the excavation conditions. Table 2 calculates the stability number corresponding to each cut-and-cover excavation. The undrained shear strength was estimated based on the average value corresponding to the strutted excavation depth. At some locations, the excavation was done in rock and without struts at the lower levels. In such cases, the maximum excavation depth and the average undrained shear strength were estimated based on the soils above the unsupported excavations in rock. When the stability number is more than 4, the ground would comprise of soft to medium stiff clays and the APD in Figure 1b would be used to estimate the strut loads. As seen in Table 2, the excavation at Pasir Panjang is an example of such an excavation, whose high stability number reflects the thick Kallang Formation encountered during excavation. When the stability number is less than 4, the ground comprises of stiffer clays and the APD in Figure 1c would be used to estimate the strut loads. It is also to be noted that the lower the stability number, the stiffer would be the ground. For example for the excavations at Telok Blangah and at Marymount, although the lower half of the excavation is done in rock
292