BTSYM | SOIL TRANSITION MODELS
x-z plane Right, figure 2:
Structure of voxel-based grid within the tunnel envelope at each ring resolution
Single cell
Resolution in z direction
Resolution in x, y direction
Multiple cells stacked in a ring
y-z plane
considered equiprobable under the assumption
of reproduction of probability distributions and variograms. Further, it offers a flexible framework, aiming to
reproduce: (1) the contact relationships between units; (2) the unit proportions; (3) spatial correlation structure; and, (4) available conditioning data (i.e. available borehole data). The PGSIM technique has found application in
modeling petroleum reservoirs and also addressing environmental science problems.
GROUND MODELING: ANACOSTIA RIVER TUNNEL An example of the probabilistic approach is its application for a research study, in 2021, on the Anacostia River Tunnel (ART) project, in Washington, DC, that is part of DC Water’s Clean Rivers initiative. The probabilistic approach was applied to estimate the soil transition location uncertainty between cohesive (G1/ G2) and cohesionless (G3/G4) soils within the tunnel envelope. For the geostatistical modeling, a 3D simulation grid
was generated that extended out 50m on either side of the ART alignment. Borehole data were discretised
(representatively sampled) into the 3D grid cells (also termed ‘voxels’). The dimensions of each cuboid cell, in this case, was 1.8m long to match the tunnel ring length, 1m vertical to match the resolution of vertical sampling in the borehole, and transversely they were assumed to extend 1.8m. It is recommended that 3D models of ground
conditions should be developed to quantify the location uncertainty of soil transitions, conditioned to the borehole data samples and reflecting them having been drilled offset to the tunnel alignment, providing spatial variability – unlike 2D models, which cannot be used to quantify uncertainty. On the ART project, the PGSIM modeling technique
was applied to the borehole data and nearly 500 equiprobable realizations of the ground conditions were generated in terms of soil units. These SUs were simulated, at each grid voxel, by considering their proportion from the borehole data, the probability of transition between SUs, and their spatial variability. The number of realizations were determined to
find confidence in the repeatability of results from PGSIM modeling – meaning the model is maintaining consistency in outputting a soil type at a voxel. SUs with the maximum probability of occurrence (ratio of
ESU Fill Right, figure 3:
Longitudinal profile of the boreholes and the
interpreted most-probable ground conditions along 2km ART alignment
-50 -40 -30 -20 -10 -0
-45 -35 -25 -15 -5
0 alignment Tunnel
Alluvium
G1
G2
G3
G4
Transition 2 Tunnel envelope Transition 1
100 200 300 400 500 600 700 800 900 1000 1100 1200 Ring #
46 | Fall 2023
Elevation (m)
Elevation (m)
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