Smart Soil Nails

Grouted soil nails are commonly used for stabilizing steep soil slopes, tunnel linings, and many such other applications utilizing their ability to counter deformation-induced tensile stresses. However, the soil–nail system is susceptible to pullout failure due to grout deterioration and cracking, especially against extreme loading and environmental exposure. Lack of accessibility is a challenge for monitoring the grout in situ. We propose a smart nail with real-time remote monitoring capabilities. The device employs the nail as a waveguide and inspects the grout surrounding it by transmitting an ultrasonic wave packet. It has the potential to offer valuable insights into the structural integrity of the soil nails.

Fig. 1(a) shows the smart nail equipped with an ultrasonic pulse generator integrated with an Internet of Things (IoT)-enabled wireless network. Several such smart nails have been fabricated by attaching piezoelectric patches (0.5-mm thickness) at the ends of stainless-steel round bars. The bars were confined in different sand–cement grouts as shown in Fig. 1(b).

The patches act as transmitter and receiver. The pristine condition of the grout is simulated as a sand–cement mixture with a 12.5% weight ratio of cement (C12.5). Deterioration of the grout is simulated by gradually reducing the cement ratio to zero representing a complete detachment of the grout from the nail along its entire length (C0 signifying nothing surrounding the nail). A 150 V peak-to-peak sinusoidal wave was generated at the transmitter patch with the pulse generator and the attenuated signals were recorded at the receiver patch.

Fig. 1(c) shows a significant difference in the received waveforms between the nails in pristine grout C12.5 and deteriorated grout C0 for the same input waveform. The signal amplitude jumped from 0.82 to 2.78 V due to the deterioration in the grout. The signal amplitude jump is a clear indicator of the degraded stiffness of the grout surrounding the smart nail. The energy norm for a specific time window is calculated using Eq. (1)where $H$ = Hilbert transform of the received time signal $y(t)$ between the intervals $t_a$ and $t_b$. In the present case, they are 25 and 60 μs, which represent the first wave packet.

The energy norms of the deteriorated nail ($E_d$), pristine nail ($E_p$), and fully deteriorated one ($E_0$) have been calculated. $E_0$ is the characteristic of the unconfined steel bar. Upon full deterioration of the grout, the signal coincides with that of the unconfined nail. The deterioration index ($D$) is calculated as in Eq. (2)

For field application, $E_p$ is to be measured on the nail following the initial set of the grout, whereas $E_0$ can be measured from the unconfined bar prior to its installation in the slope. For monitoring the nails, $E_p$ is to be evaluated at different periods using Eq. (1). Fig. 1(d) illustrates the deterioration indexes for the corresponding actual deterioration. An excellent correlation between the actual deterioration and the nondestructive index is clearly evidenced.