Reset of Secondary Compression Clock for Peat by Cyclic Straining

Peat soils are widely recognized as being highly compressible. The virgin compression index, $C_c$, is often 5 to 20 times larger than for soft clay, and the secondary compression index, $C_{\alpha }$, is in the range of (0.05 to 0.07) $C_c$, which is also higher than for typical clays (Mesri and Ajlouni 2007). Secondary compression is often the dominant source of volume change in peat over time. The compressibility of peat has typically been investigated due to total stress increase imposed by static loads. This paper discusses time-dependent volumetric strains due to cyclic shear straining.

A multistage cyclic simple shear laboratory testing program using the University of California, Los Angeles (UCLA) digitally controlled direct simple shear device (Duku et al. 2007), modified for constant height testing, was used to investigate postcyclic volume change of relatively undisturbed specimens of fibrous peat from Sherman Island in the Sacramento–San Joaquin Delta. During each stage, 15 strain-controlled cycles (1 Hz) were imposed on peat specimens while maintaining constant height by varying the vertical stress, after which specimens were reconsolidated to the initial vertical stress (${\sigma }_{vc}^{\prime }$). Postcyclic reconsolidation was monitored for approximately 20 min following each loading stage. Odometer tests indicate primary consolidation was complete after 1 min, leaving the remaining time to record the rate of secondary compression.

Test results for a peat specimen with 55% organic content, ${\sigma }_{vc}^{\prime }=12\mathrm{kPa}$, overconsolidation ratio (OCR) = 4.9, $C_c=2.21$, and $C_r=0.14$ are shown in Fig. 1. Fig. 1(a) shows that excess pore pressure ratios ($r_u$) are nonzero when ${\gamma }_{cyc}>3\%$, with a residual $r_u=0.2$ for ${\gamma }_{cyc}=10\%$. As shown in Fig. 1(b), although these $r_u$ values are modest, postcyclic volumetric strains are significant when ${\gamma }_{cyc}>1\%$ because of the peat’s high compressibility. Fig. 1(c) shows that the secondary compression rate, which is related to the difference between ${\epsilon }_v$ at 1 and 20 min, increases with ${\gamma }_{cyc}$. This suggests that cyclic straining can increase the secondary compression rate, which has not been previously recognized. Similar increases in secondary compression rate are routinely observed when total stress is increased in laboratory odometer tests, which can be viewed as resetting the clock to zero at the time the load is applied. The data in Fig. 1 indicate that cyclic straining can at least partially reset the secondary compression clock without total stress increase.