Erosion Risk Index Notes

Erosion Risk Index: Correlation of ROM-scale and Mackintosh Probe at Sungai Langat Tributary

Abstract

The study aims to develop an empirical correlation between the number of blows per unit penetration (M-value) from the Mackintosh Probe test and the ROM-scale number from soil particle size distribution analysis.
Both tests were conducted at three different points in one of the Sg. Langat tributary riverbanks near Universiti Kebangsaan Malaysia, Bangi.
The soil at each location was analyzed at three different layers, from 0 m to 1.5 m, with intervals of 0.3 m and 0.5 m for the Mackintosh Probe test and ROM-scale, respectively.
The result indicates that the ROM-scale number is inversely proportional to the M-value with a coefficient of determination of 0.5885.

1 Introduction

Soils are made up of minerals, organic matter, living organisms, gas, and water.
Soil is a natural medium for planting crops and acts as the foundation for construction.
Soils have a heterogeneous composition and residual soils in Malaysia are made up of different compositions of soil particles, namely sand, silt, and clay. More than three-quarters of the land in Peninsular Malaysia is covered by residual soils.
Soil heterogeneity influences the process of soil erosion.
The determination of soil erosion risk is crucial to ensure that preventive measurements can be taken appropriately before any catastrophic events happen that may lead to the loss of life and change the surrounding ecosystem.
Empirical models such as Universal Soil Loss Equation (USLE) and its derivatives, namely Modified Universal Soil Loss Equation (MUSLE) and Revised Universal Soil Loss Equation (RUSLE), are used to estimate soil loss.
Developing a soil erosion risk index can be an alternative to estimate the future occurrence of soil erosion through the development of a correlation between existing and conventional methods.
ROM-scale was developed in 2001 by Malaysian researchers (Roslan and Mazidah) to categorize the degree of soil erodibility using the percentage of sand, silt, and clay of the soil sample.
ROM-scale focuses on categorizing the soil erodibility (K-factor) using the particle size distribution.
The ROM-scale can be evaluated using the mathematical expression as in Equation 1:
$EI = \frac{\% \text{ sand } + \% \text{ silt }}{2 \times \% \text{ clay}}$
Table 1 shows the range of ROM-scale and the degree of soil erodibility:
ROM-scale Soil erodibility category
<1.5 Low
1.5 - 4.0 Moderate
4.0 - 8.0 High
8.0 - 12.0 Very high
>12.0 Critical
Local researchers have used ROM-scale because it was developed specifically for soil series in Malaysia that can cover different conditions of SI location.
Soil stiffness also contributes to the possibilities of soil erosion events aside from the soil erodibility factor.
Mackintosh Probe (MP) test is a conventional site investigation (SI) method that is widely used to find soil strength. The soil strength is directly reflected by counting the number of blows for every 0.3 m penetration into the soil.
Limitations of the Mackintosh Probe (MP) test:
- Cannot penetrate hard layers.
- Not suitable for hard clay or soils containing gravel or cobbles.
- The test needs to stop when the depth reaches 15 m or the number of blows reaches 400.
In 1994, Sabtan and Shehata established the correlation between the consistency of cohesive soil based on the M-value from MP test. They reported that the M-value could be regarded as the soil stiffness as its consistency can be related to the unconfined compressive strength and undrained shear strength as shown in Table 2.
Table 2. Consistency of residual cohesive soil based on Mackintosh M-value [13]
Mackintosh M-value/ 0.3 m penetration Consistency
< 15 Very soft
15 - 33 Soft
33 - 72 Medium
72 - 147 Stiff
147 - 322 Very stiff
> 322 Hard
Previous literature papers have discussed the correlation of Mackintosh Probe with Spectral Analysis of Surface Waves (SASW) to find the bearing capacity, Standard Penetration Test (SPT) together with resistivity test for soft soil assessment and Vane Shear Test to find undrained shear strength.
The description of the correlation between Mackintosh Probe test and ROM-scale for soil erosion risk index is limited in the literature. Thus, the developed correlation from this case study will allow future work to assess soil erosion potential using a simpler method.

ROM-scale	Soil erodibility category
<1.5	Low
1.5 - 4.0	Moderate
4.0 - 8.0	High
8.0 - 12.0	Very high
>12.0	Critical

Mackintosh M-value/ 0.3 m penetration	Consistency
< 15	Very soft
15 - 33	Soft
33 - 72	Medium
72 - 147	Stiff
147 - 322	Very stiff
> 322	Hard

2 Methodology

2.1 Site investigation (SI) – Mackintosh Probe (MP) test and collection of soil sample for ROM-scale evaluation

The location to conduct this case study is at one of Sg. Langat tributary riverbank nearby Universiti Kebangsaan Malaysia, Bangi. There are three different points had been chosen, namely, Point 1, 2 and 3.
Both MP test and collection of soil samples were conducted at every location starting from topsoil until 1.5 m deep with interval of 0.3 m and 0.5 m, respectively.

2.2 Laboratory testing – Sieve analysis and hydrometer test

A portion of soil samples was collected from the SI locations (separated based on the location and soil depth) had been subjected to a 24-hour drying process before conducting laboratory testing for particle size distribution analysis.
The dried soil samples then were submitted for sieve analysis. The fines particles of the soil samples that pass through the 75μm sieve opening were collected and used for the second laboratory test, the hydrometer test.
Sodium hexametaphosphate $(Na<em>6(PO</em>3)<em>6)$ and sodium carbonate $(NaCO</em>3)$ were used as a dispersing agent during the hydrometer test.
All laboratory testing was conducted according to the British Standard (BS 1377) system.
Based on British Standard, the particle size (Table 3) of the soil collected from the SI location was determined, and the corresponding soil type percentage was calculated. Later, the values obtained from the calculation were used to evaluate the ROM-scale and its corresponding degree of erodibility.
Table 3. Particle size distribution according to British Standard (BS).
Type of Soil Particle Size (mm)
Clay < 0.002
Silt
Fine 0.006 – 0.002
Medium 0.02 – 0.006
Coarse 0.06 – 0.02
Sand
Fine 0.2 – 0.06
Medium 0.6 – 0.2
Coarse 2.0 – 0.6
Gravel > 2.0

Type of Soil	Particle Size (mm)
Clay	< 0.002
Silt
Fine	0.006 – 0.002
Medium	0.02 – 0.006
Coarse	0.06 – 0.02
Sand
Fine	0.2 – 0.06
Medium	0.6 – 0.2
Coarse	2.0 – 0.6
Gravel	> 2.0

3 Result and discussion

3.1 Mackintosh Probe test

The Mackintosh Probe test results were tabulated as in Table 4, and the graph of M-value versus soil depth was illustrated in Fig. 4.
Table 4. M-values from Mackintosh Probe test at three different locations.
Depth (m) No of blow/0.3m penetration (M-value)
Location 1 Location 2
0.0 – 0.3 56 18
0.3 – 0.6 50 23
0.6 – 0.9 48 17
0.9 – 1.2 46 21
1.2 – 1.5 15 35
According to Sabtan and Shehata (1994), the M-value for every 0.3 m of penetration obtained at different locations can be directly used to categorize the consistency of the soil at each layer based on Table 2 [13].
Based on Fig. 4, the trends show that for the first layer (topsoil to 0.3 m) of the MP test, Location 1 has stiffer soil compared to Point 2 and 3.
The soil consistency for Location 1 after that drastically dropped from medium to a soft region when the cone penetrometer traveled from 1.2 m to 1.5 m, which gives the smallest M-value (15 blows) for this case study.
In contrast, the trend for Location 2 and 3 show that they have soft soil from the topsoil until 1.2 m deep. After the depth of 1.2 m, Location 2 and 3 became stiffer as the M-value exceeded the limit for soft soil, which is 33 blows for every 0.3 m penetration.
In general, the soil at this SI location falls between soft to medium consistency.

Depth (m)	No of blow/0.3m penetration (M-value)
	Location 1	Location 2
0.0 – 0.3	56	18
0.3 – 0.6	50	23
0.6 – 0.9	48	17
0.9 – 1.2	46	21
1.2 – 1.5	15	35

3.2 Soil sample – Particle size distribution analysis and ROM-scale

In total, there are nine soil samples collected from three different locations at the interval of 0.5 m deep, as shown in Fig. 5. The physical appearance of the samples does reflect its composition, especially for sandy soil. It can be seen from the picture, Location 3 (1.0 m - 1.5 m) seems like it contains the most sand particles compared to the rest of the soil samples.
Through the sieve analysis and hydrometer test, the percentage of sand, silt, and clay can be determined from the particle size distribution curve (Fig. 6-8) and later was compared to the given particle size according to British Standard (BS) as tabulated in Table 3. The experimental results and the computed ROM-scale values are tabulated in Table 5. The soil compositions from this case study were grouped and categorized into three different levels, while ROM-scale were classified based on Table 1.
The soil samples from Location 1 have a relatively high percentage of clay and a low percentage of sand, resulting in a low ROM-scale at this location. Low ROM-scale means that the soil at this location is less susceptible to erosion since it has a low erodibility degree.
According to the results from MP test in Table 4, the M-values for this location are relatively higher compared to Location 2 and 3, starting from topsoil until 1.2 m deep. Higher M-values indicates that the soil in Location 1 within the range of 0.0 m-1.2 m is stiffer and hard to erode, as suggested by ROM-scale.
The soil at Location 2 contained a low clay percentage for each layer but with a combination of a moderate and high percentage of sand. ROM-scale shows that the soil at this location has a moderate risk of soil erosion and has relatively low M-values. Low M-values show that the soil at Location 2 is softer than soil at Location 1 and easier to get eroded.

Table 5. Results from particle size distribution analysis and evaluation of ROM-scale.

Location	Depth (m)	Moisture Content (%)	Sieve Analysis	Hydrometer Test	ROM-% scale
			Sand %	Silt %	Clay
1	0.0 - 0.5	13.06	57.94	5.94	36.12
	0.5 - 1.0	20.36	50.04	9.26	40.69
	1.0 - 1.5	18.46	55.91	6.12	37.96
2	0.0 - 0.5	18.32	70.24	10.41	19.35
	0.5 - 1.0	17.87	64.89	12.17	22.94
	1.0 - 1.5	13.79	70.10	9.42	20.48
3	0.0 - 0.5	17.07	52.13	6.90	40.97
	0.5 - 1.0	18.61	60.93	9.79	29.28
	1.0 - 1.5	14.03	75.14	9.89	14.97

Therefore, from the analysis of experimental results, the percentage of clay and sand is inversely related.
Clay is classified as cohesive soil, while sand is cohesionless soil.
Sand particles are prone to detachment compared to clay as the cohesion force between the clay soil particles impacts the soil detachability.
Thus, the soil at Location 1 is less vulnerable to erosion than Location 2 and 3 because the soil at Location 1 has lesser loose soil grains.

3.3 Correlation between M-value and ROM-scale

The soil series in Malaysia can be divided into three different texture layers at the interval of 0.5 m deep, namely layer A (surface soil), B (subsoil), and C (substratum)
Thus, the M-value at 0.3 m was chosen to indicate the M-value from the MP test at layer A, 0.9 m for layer B, and 1.2 m for layer C. These depths for MP are reasonable as they are in between the represented layers by ROM-scale.
This adjustment was made to develop the one-to-one relationship between M-values with ROM-scale since the MP test and soil samples collection were not done on the same scale.
Hence, Table 6 summarizes the selected M-values and the corresponding ROM-scale for each layer, while Fig. 8 shows the linear regression of a scattered plot between the two parameters.
Table 6. The M-values and corresponding ROM-scale for each layer.
Location Layer M-value ROM-scale
1 A 56 0.88
B 48 0.73
C 46 0.82
2 A 18 2.08
B 17 1.68
C 21 1.94
3 A 30 0.72
B 33 1.21
C 20 2.84
Based on the scattered plot between M-value and ROM-scale, these two parameters can be correlated as inversely proportional.
The linear regression for this correlation has a coefficient of determination $(R^2)$ of 0.5885.
Aminaton (1996) stated that if the range of $(R^2)$ is between 0.25 to 0.55, the coefficient of determination is considered moderately good.
From Fig. 9, there are two outliers in the dataset which belong to Location 3-A and 3-C.
The moisture content of the soil might also contribute to the inconsistency of these results for soils in Location 3 as the MP test does get affected by the saturation of soil.

Location	Layer	M-value	ROM-scale
1	A	56	0.88
	B	48	0.73
	C	46	0.82
2	A	18	2.08
	B	17	1.68
	C	21	1.94
3	A	30	0.72
	B	33	1.21
	C	20	2.84

5 Conclusion

The Mackintosh Probe (MP) test and the gradation of soil analysis using sieve and hydrometer tests have been carried out for soil samples collected from the Sungai Langat riverbank.
The soil erosion index using ROM-scale has been determined from the result obtained from a series of laboratory testing.
Based on the result, the M-value from the MP test is correlated with the evaluated value of ROM-scale of the soil sample.
The soil with a high sand amount has a lower M-value, whereas the soil with a clay amount has a higher M-value.
By correlating the M-value with ROM-scale at different soil locations, it was found that these two data are inversely proportional with the coefficient of determination $(R^2$ of 0.5885).
This study indicates that the assessment of soil erosion index using ROM-scale is possible to be predicted by using the M-value obtained from the simple and time-efficient Mackintosh Probe test.