Introduction
Landslides and geotechnical data
Effective friction angle
Geotechnical correlations and classification systems
Study aims
Materials and methods
Results and discussion
Geotechnical correlations
Classification of soils from Saint Lucia
Classification of soils based on formation and weathering
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Class A: Tropical residual soils are the classical ‘tropical residual soils’. They are reddish-brown, clayey soils with deep weathering profiles, and are particularly associated with the latosolic soils and andesitic polysols of the 1966 Regional Research Laboratory soil survey. Colluvium derived from these soils is also included as Grade VI material, based on field observations. Two subsets are defined, where ‘A1’ (n = 17) describes the upper layer of weathering Grades V and VI (soil), and ‘A’ (n = 23) is the underlying layer of weathered Grade IV material in which some of the relict rock structure and cementation is still present (which, in turn is underlain by Grades III, II and the Grade I parent material). Subset ‘A1’ contains only data which are known to pertain to the topsoil layer based on knowledge of the original samples. Subset ‘A’ contains all other ‘A’ data, excluding ‘A1’.
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Class B: Agglomerate soils derived from poorly sorted pyroclastic deposits. This soil is recognisable by the presence of boulders and gravels with a weakly cemented (and sometimes clayey) weathered soil matrix. Two subsets are defined, where ‘B1’ describes the most weathered layer in which the soil matrix is fully weathered and few large particles remain (large particles are manually removed from samples before direct shear testing), and ‘B’ the underlying less weathered layer. Because the soil matrix of Soil ‘B’ tested in the DSA is thought to be similar to that of Soil ‘A’, Soil ‘A’ is included in the statistical analysis of angle of friction: Subset ‘B1’ contains all data for Soil ‘A’, ‘A1’ and ‘B’ (n = 47). Subset ‘B’ contains all data for Soil ‘A’ (excluding A1) and Soil ‘B’ (n = 30).
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Class C: Volcanic ash soils derived from pumice or tuff. This class contains only data points described recognised as a distinctive grey ash (or tuff) soil, sometimes highly cemented, but otherwise friable (n = 14). It is typically associated with the southern geological series (Fig. 1). Wright et al. 1984 give further commentary on the soils of Saint Lucia and is part of the so called ‘Belfond Pumice’.
Classification of soils based on soil index parameters
Statistical Models
Goodness of fit tests
Exponential | Normal | Lognormal | Weibull |
n
| |
---|---|---|---|---|---|
ϕ'
peak
| 13.37 | 0.93 | 1.03 |
0.61
| 85 |
c'
| 5.43 | 1.78 | 1.26 |
0.47
| 86 |
PI
| 7.06 | 1.00 | 0.68 |
0.49
| 61 |
w
nat
| 9.59 | 0.99 |
0.68
| 0.79 | 58 |
Exponential | Normal | Lognormal | Weibull | Generalised Extreme Value |
n
| |
---|---|---|---|---|---|---|
ϕ'
peak
| 714 | 639 | 643 |
635
| 636 | 85 |
c'
| 715 | 712 | 697 |
690
| 694 | 86 |
PI
| 519 | 497 | 484 | 485 |
483
| 61 |
w
nat
| 529 | 470 | 470 |
467
| 470 | 58 |
Distribution | Saint Lucia soil type (number of data points) | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
A1 (n = 17) | A (n = 23) | B1 (n = 47) | B (n = 30) | C (n = 14) | U (n = 24) | |||||||
ϕ'
peak
|
c'
|
ϕ'
peak
|
c'
|
ϕ'
peak
|
c'
|
ϕ'
peak
|
c'
|
ϕ'
peak
|
c'
|
ϕ'
peak
|
c'
| |
Normal |
0.35
| 0.69 | 0.28 | 0.80 |
0.31
| 1.45 |
0.25
| 0.73 | 0.65 |
0.21
| 1.15 | 0.64 |
Lognormal | 1.33 |
0.49
| 0.56 | 0.64 | 1.72 | 0.55 | 0.74 | 0.78 |
0.42
| 0.94 |
0.34
| 0.42 |
Weibull | 0.36 | 0.50 |
0.27
|
0.42
| 0.44 |
0.41
| 0.25 |
0.38
| 0.60 | 0.60 | 0.77 |
0.28
|
Exponential | 4.16 | 1.96 | 3.03 | 1.99 | 8.04 | 2.29 | 4.54 | 2.42 | 2.65 | 1.28 | 3.14 | 2.46 |