Soil Bioengineering for Infrastructure Development in Cambodia

A Study on Vetiver Grass and Liquid Soil Catalysts for Road Projects

• Publisher: Asian Development Bank
• Year: 2022
• ISBN:
  9789292693930
• Topic: Tecnología e ingeniería
• Subtopic: Agronomía

This report proposes a research program to determine the suitability of Vetiver grass for slope protection and the potential of liquid soil catalysts for improving soil-bearing capacity in Cambodia. It presents the advantages of soil bioengineering for strengthening soil or slope structures and erosion control, which are important considerations in road and other infrastructure projects. The report proposes six trials to be conducted on National Road 23 (NR 23) in Kandal Province and on Provincial Road 312 (PR 312) in Prey Veng Province. The proposal for a comprehensive research program also emphasizes the need to develop knowledge products and increase awareness on soil bioengineering for infrastructure development.

VGeotechnical Characteristics

Geotechnical characteristics of the road embankment soils and surrounding borrow pit are summarized in this section based on the reports of the Ministry of Public Works and Transport (2018c).

Geotechnical Characteristics of National Road 23

The location of the road alignment is lacustrine sediment rich. The site is covered with diluvia from the mountain area of paleozoic sedimentary units, mostly carboniferous-permian. The presence of alluvial sediments from highland and mountain areas has also been reported. The site has different layers such as brown, gray, light gray, and reddish silts, sands, clay, and organic soil.

Three types of tests were conducted on the site: the dynamic cone penetrometer (DCP), in situ CBR, and the CBR laboratory test. The DCP test results on-site showed the CBR value ranges from 3.02 m to 25.56 m for the depth of the top surface and down to 0.90 m depth for road sections. The CBR values referring to soil sampled from road sections indicated that the CBR value varies between 2.91 and 31.87 under natural conditions.

Moreover, the soil samples retrieved from the dug pits were subjected to routine laboratory tests. Test results are summarized in Table 4 for pit soil and Table 5 for boreholes. The test results show that soaked CBR value ranges from 0.00 to 2.60 for road sections. The pit soils are classified as A-4, A-6, A7-5, and A7-6 according to the classification of the American Association of State Highway and Transportation Officials (AASHTO). The liquid limit varies between 22% and 59%, while the plasticity index varies between 5% and 28%. %Finer #200 Sieve (0.075 mm) of the soil samples ranges between 50% and 99%.

Table 4: Summary of Geotechnical Characteristics of 40 Test Pit Soils along National Road 23

Sl. No.	Parameter	Value
1	Sample\location	Embankment
2	Soil Classification (AASHTO)	A-4, A-6, A7-5, A7-6
3	Maximum Dry Density (gm/cc)	1.64–1.8
4	Optimum Moisture Content (%)	6.9–15.0
5	California Bearing Ratio (%)	0.95–8.00
6	Liquid Limit (%)	22–59
7	Plasticity Index (%)	5–28
8	%Finer # 200 Sieve (0.075 mm)	50–99
9	In situ CBR (unsoaked)	2.91–31.87
10	In situ CBR (soaked)	0.00–2.59

AASHTO = American Association of State Highway and Transportation Officials, CBR = California Bearing Ratio, gm/cc = gram per cubic centimeter.

Note: According to the AASHTO Soil Classification system, A-4, A-6, A7-5, and A7-6 fall under mostly silty and clayey soil.

Source: See Appendix I: NR 23—Geological and Material Investigation in Government of Cambodia, Ministry of Public Works and Transport. 2018. Field Survey and Laboratory Report. Vol. 4. Phnom Penh (TA 8784-CAM).

Table 5: Summary of Geotechnical Characteristics of Soils of 10 Boreholes beside National Road 23

Sl. No.	Parameter	Value
1	Soil Classification (USCS)	CL, CH, SC
2	Water Content (%)	18.7–35.7
3	Specific Gravity	2.69–2.73
4	Bulk Density (kN/m^3)	17.23–20.50
5	Dry Density (kN/m^3)	12.70–17.28
6	Void Ratio	0.58–0.97
7	Liquid Limit (%)	34–67
8	Plastic Limit (%)	19–31
9	Plasticity Index (%)	12–36
10	Cohesion (kN/m^2)	7–160
11	Angle of Internal Friction (°)	20–34
12	Undrained Cohesion (kN/m^2)	10–160
13	SPT N-value	1–24

kN/m² = kilonewton per square meter, kN/m³ = kilonewton per cubic meter, SPT = standard penetration test, USCS = Unified Soil Classification System.

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