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1.13 Additional problems

1.13.1

For the design of a cut slope (grey dashed line), 3 boreholes were drilled. The geological description of the encountered formations, their USCS classification and the SPT test results are provided below. Use the canvas to create a geotechnical model of the area to be introduced in the slope stability calculations, and propose the necessary parameters to be employed in the corresponding stability analyses.

Cross-section of a slope with high point to the left, low point to the right. Three boreholes show from left to right - Borehole 1 with elevation 29.9m, Borehole 2 with elevation 23.0m, and Borehole 3 with elevation 17.2m. A grey dashed line shows the proposed straight cut into the slope.
Additional Problem 1.13.1. Problem description and input parameters.

 

Visual representations of the test results from each borehole (BH1 to BH3) on the sample slope, showing different soil layers with depth, as well as the variation of N-values with depth measured in each one of the boreholes
Additional Problem 1.13.1. Problem description and input parameters.

Answer:

Layer I: Dense clayey sand (SC) φ′≈34°, γ=18 to 20 kN/m3

Layer II: Very dense clayey gravels, poorely graded (GC-GP) φ′≈40° to 43°, γ=18 to 20 kN/m3

Layer III: Fault zone: infinite strength for slope stability analyses

Layer IV: Slightly to moderately weathered limestone (Lm): infinite strength for slope stability analyses

1.13.2

For the design of the deep foundation of a tall building, two (2) sampling boreholes were drilled and two (2) CPT soundings were performed. Results of the geotechnical investigation revealed a (more-or-less) consistent geotechnical profile: a medium-to-stiff low plasticity (PI ≈ 20%) clay formation prevails, interrupted by a loose-to-medium silty-clayey sand layer. The bedrock, a sandstone formation is encountered at depths 12.0 to 16.0 m.

Use the in situ SPT and CPT test results, combined with the provided empirical relations/charts, to determine the characteristic undrained shear strength (Su) of the clay layer and the effective friction angle of the sand layer (φ′). Assume the bulk unit weight of the clay/sand layers to be constant γ = 17 kN/m3.

The graphs on the left of the figure presents the different geotechnical units identified during drilling of borehole BH1, and the variation of N60 values with depth measured from that borehole. The graphs on the right show the variation of cone resistance and friction ratio values measured in CPT1, and the soil behaviour type chart, where each pair of measurements is assigned of a different soil behaviour type zone.
Additional Problem 1.13.2. Problem description and input parameters.
The graphs on the left of the figure presents the different geotechnical units identified during drilling of borehole BH2, and the variation of N60 values with depth measured from that borehole. The graphs on the right show the variation of cone resistance and friction ratio values measured in CPT2, and the soil behaviour type chart, where each pair of measurements is assigned of a different soil behaviour type zone.
Additional Problem 1.13.2. Problem description and input parameters.
Example 1.13.2. CPT1 and CPT2 test results
CPT1 CPT2
Depth (m) qt/pa FR (%) Depth (m) qt/pa FR (%)
1 3.5 2.5 1 3 2.5
2 4 1.3 2 5 0.8
3 6 0.8 3 6.5 1
4 5.5 1 4 9 1.5
5 20 1 5 7 0.6
6 11 0.6 6 4 4
7 20 0.6 7 5.5 4.5
8 17 0.5 8 6 2
9 19 0.5 9 8 6
10 5 3 10 9 3
11 3 4.5 11 7.5 5
12 11 2.5
13 10.5 6
14 12 1.4
15 13 2

Answer:

Clay layer: Su = 20 kPa for depth z<4m and Su = 20+2(z-6) kPa for depth z>6m

Sand layer: φ′ = 30o

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Fundamentals of foundation engineering and their applications Copyright © 2025 by University of Newcastle & G. Kouretzis is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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