Chapter 6. TRIGRS Model for Guwahati City
6.6. TRIGRS – SEEP/W comparative analysis
Figure 6.6 Depth (m) of basal boundary map
The alluvial plains of this region remain saturated all year round, owing to the ground water table (GWT) being controlled by the mighty Brahmaputra River flowing through the city. However, detailed data on the ground water table in the hills of the study area does not exist. Observing the wells dug for obtaining drinking water and from the few in-situ borehole investigation data, it can be assumed that the basal rock strata remain saturated at all times.
Thus, in the TRIGRS analysis, the initial ground water table is considered same as the depth of the basal rock. Finite depth basal boundary is considered and the ground water table is allowed to rise due to the rainwater infiltration.
considered 2.57 m, as that given by Equation 6.28. The soil strata is considered composed of two layers. The top soil layer is assigned with saturated hydraulic conductivity, ks = 1.0×10-6 m/s while the bottom soil layer is assigned with ks = 1.0×10-5 m/s, the basal boundary is considered impermeable. The initial ground water table is considered to be at the interface of the soil and basal boundary. The soil layers of the slope were discretized with a finite element mesh of combined 4-noded quadrilateral elements with a fineness of 0.1 m and the sizes of the mesh gradually increased in the basal strata to reduce the total number of elements and decrease the computational time requirement. Surface elements of 0.1 m thickness were applied to define the ground surface of the slope. Figure 4.4 shows the SWCC and the van Genuchten (1980) parameters used to define the SWCC in the SEEP/W analysis. The unsaturated hydraulic conductivity curve is derived with the van Genuchten (1980), UHCC model coded in SEEP/W. The SEEP/W model was developed to represent the closest generalized approximation of the in-situ condition.
Figure 6.7 (a) Slope model as developed in SEEP/W (b) Cut-slope at Fatasil hill showing close resemblance to developed model
In the SEEP/W model, the SWCC is defined using the van Genuchten (1980) four parameter model, while the Richards’ (1931) flow equation is solved using the finite element method to evaluate the pore pressure distribution within the slope. The soil parameters obtained through laboratory and in-situ testing were used as input to define the soil characteristics. The model comprises two soil layers with different permeability giving a much closer approximation to the in-situ condition. TRIGRS uses the Gardner’s (1958) exponential hydraulic parameter model to define the SWCC and the unsaturated hydraulic conductivity, while applying analytically derived one dimensional infiltration model to
estimate the transient pore pressure changes. Modelling of layered soil profile is therefore, not possible in TRIGRS. The pore pressure distribution obtained from SEEP/W is likely to be a much closer approximation of the in-situ condition and different from that obtained with TRIGRS. To define the values of TRIGRS input parameters an analysis consisting of a finite element numerical model and TRIGRS simulation were conducted.
Unsaturated infiltration condition and finite depth boundary condition was considered and the ground water level was allowed to rise with infiltration of rainwater in TRIGRS simulation. Pore pressure profiles obtained for the above considered slope angle and depth of basal boundary from the TRIGRS model were compared with that obtained from SEEP/W model, for identical intensity of rainfall, as shown in Fig. 6.8. From the in-situ observation of landslides occurrences reported in the morning of 26th June 2012 at various locations in the Guwahati city, the landslides were inferred to occur along the soil-basal rock interface, with depth of slip surface ranging from 2.0 m to 2.5 m. The reported landslides were accompanied by profuse interfacial seepage indicating saturation of basal rock and landslide soil mass (Goswami, 2013). The numerical modeling conducted in the present study efficiently illustrated that the rise of water table led to the saturation and loss of strength of the overlying soil, leading to landslides that were conforming to those actually occurred in the field. Hence, the rise of water table is adjudged as the predominant condition leading to the widespread landslides in the region. It is worth mentioning that the rise of water table is not a field observation, as there were no subsoil instrumentations available yet in the landslide-affected areas to recognize the rise of water table. The TRIGRS parameters are so attuned such that the temporal rise in water table simulated by TRIGRS is in closest possible similitude to that obtained from SEEP/W, and that the corresponding location of slope instability, along with its time of occurrence, matched with that of in-situ conditions. The values of the parameters themselves should be consistent with that of experimentally observed values. Thus, for the present study, instead of focusing on the pore pressure profile itself, emphasis is given on the location and time of triggering of slope failure due to rising water table. Table 6.1 and Table 6.2 gives the values of the hydraulic soil parameters and soil shear strength parameters respectively, considered for conducting the rainfall induced landslide hazard analysis of the study area.
(a) (b)
(c) (d)
Figure 6.8 Pore pressure profiles and Ground Water Table for different rainfall intensities: (a) 75 mm/day for 72 hours duration; (b) 100 mm/day for 60 hours duration; (c) 125 mm/day for 48 hours duration; and (d) 125 mm/day for 60 hours duration
6.6.1. Sub–Surface Flow Parameters
Table 6.1 gives the hydraulic parameters used in the TRIGRS analysis. Previous studies report the soil diffusivity (Do) values ranging approximately 5–500 times that of the
2010; Viet et al., 2018). The general trend being with increase in saturated permeability (ksat) the ratio of Do to ksat also increases. For this study, the soil diffusivity (Do) is assumed to be 10 times the saturated permeability (ksat) (Baum et al., 2010). Figure 6.9 shows the soil water characteristics curve applied in the analysis and the corresponding unsaturated hydraulic conductivity curve. As can be observed from the figure the unsaturated hydraulic conductivity is highly sensitive to the soil moisture and can vary by an order for change in volumetric water content of only 0.1. The soil water characteristic has a significant effect on the hydraulic conductivity and soil shear strength.
Table 6.1 Hydraulic parameters used in TRIGRS analysis
ks (m/s) Do (m/s) θs θr α
2.5×10-6 2.5×10-5 0.45 0.05 0.8
Figure 6.9 Soil water Characteristics Curve (SWCC) and corresponding Unsaturated Hydraulic Conductivity Curve (UHCC) as used in the TRIGRS analysis
6.6.2. Shear Strength Parameters
Table 6.2 shows the Mohr-Coulomb shear strength parameters and the unit weight used as input for the TRIGRS analysis. The shear strength parameters were so adjusted such that the corresponding location of slope instability, along with its time of occurrence, due to the rise in water table, matched with that of in-situ conditions. The values of the parameters, consistent with that of experimentally observed values, were only considered for the analysis.
Figure 6.10 shows the factor of safety (FoS) profile with depth, output from TRIGRS corresponding to the pore pressure changes as shown in Figure 6.8 (c) and (d).
Table 6.2 Soil shear strength parameters used in TRIGRS analysis c′ (kPa) φ′ (⁰) γs (kN/m3)
10 27⁰ 18.5
Figure 6.10 Factor of safety profile for rainfall intensity of 125 mm/day