Rolling tests are important because several types of compaction equipment used in construction works used to overlap. Test strips helps to make final determination of the most efficient compactor and compaction procedures. It also helps to avoid economic and time losses in field compaction. The test strip for rolling tests should be located adjacent to the project site.
The following information are obtained from the test strip:
1. Most effective type of compaction equipment through the determination of maximum dry density and optimum moisture content of the proposed roller.
2. Optimum depth of layer.
3. Optimum compactor speed.
4. Number of passes required.
5. Amount of ballast required.
6. Vibration frequency required.
Steps in rolling tests according to General Specifications for roads and bridges (Nigeria)
1. Prepare a formation of reasonable length (about 48 m) adequately compacted to serve as a good platform for the rolling test.
2. Spread the loose material on the prepared platform to a desired loose depth (between 200 mm and 300 mm for laterite).
3. Divide the test section into at least five (5) bays (each bay not less than 8 m) and leaving a gap of two (2) metres between bays to avoid intermixing of materials from adjacent bays.
4. Determine the moisture content of material in each bay. The speedy moisture tester may be used and sampling in various points and taking the mean result of four as the moisture content.
5. Adjust the moisture content in each bay with a view to straddling the optimum moisture content (OMC). This could be achieved by progressively increasing the passes of the water tank in different bays as shown below (Table 1).
Table 1: Number of passes per bay for OMC
6. Mix the loose soil and water to ensure thorough moisturization using either hand, pulveriser or rotovator to ensure uniform moisturization. The watered material could be left overnight before mixing.
7. Compact the loose material to refusal whose OMC and MDD is required with the available plant. For practice purposes 15 to 20 passes of 10 to 15 tonnes of three wheeled roller or 10 to 15 passes of vibratory roller will be adequate for lateritic materials. The rear wheel is the compacting wheel for three wheeled roller while the front wheel is the compacting layer for the vibratory roller.
8. Determine the insitu dry density and moisture content of each bay based on the mean of four tests concentrating the measurement as the case may be on the rear or front wheel of the respective rollers.
9. Plot graphs of dry density and moisture content and determine the OMC and MDD appropriate to the plant.
Determination of economic passes
10. Prepare another test section as shown above and bring the moisture content to the OMC determined in (9).
11. Divide the section into bays and vary the number of passes of roller on each bay as shown below (Table 2).
Table 2: Number of passes per bay for MDD
12. After rolling, determine the dry density of each bay (based on the mean of four tests)
13. Plot dry density and number of passes relationship to determine the minimum number of passes that will achieve the maximum dry density (MDD).
Table 4 provides the relationship between compaction plant and laboratory test (MDD and OMC) while Table 5 provides the typical operating speed of different compaction equipment (kph means kilometer per hour).
Table 4: Relationship between compaction plant and laboratory test results
Table 5: Typical operating speed of different compaction equipment
References
FM 5-434 (2000): Earth moving operations. Department of Army, Washington, USA.
General Specifications for Roads and Bridges (Revised 1997). Federal Republic of Nigeria.
