How to obtain optimum moisture content in field compaction of soils in road construction

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Most soils used in road construction works are borrowed materials from borrow pit. Most times, these are lifted and brought to site and spread and sprinkled with water, then compacted. Without considering much whether the soil was soaked with the required quantity of water or weather there was uniform soaking of the soil, the relative density test is often the only tool used to access the condition of the compacted soil. The ability to achieve the required maximum dry density of the soil in the field compaction is dependent on the ability to moisten the soil uniformly at the optimum moisture content (OMC).

If the soil is too dry, friction between particles tends to resist close packing. If the soil is too wet, the water between particles prevents close packing. Typically, OMC varies from 12 to 25 % for fine-grained soils and 7 – 12 % for well-graded granular soils. It is advisable to work within ±2% of the optimum since it is often difficult to achieve the optimum. As rule of thumb, the optimum moisture content of the soil can be checked in the field by squeezing a sample in the hand:

1. If the material is too dry, it will not stick together and must be thoroughly mixed with water before compacting it.
2. if water runs out of the material, it is too wet and should not be compacted but left to dry out until the moisture content has reduced.
3. if the material is wet enough to stick (you can form a ball), it has the optimum moisture content and it is suitable for compaction.

If option 1 is the case which is often what is encountered on site, the optimum moisture content can be achieved by considering the following factors:

1. The amount of water required
2. The rate of water application
3. The method of application
4. The effect of weather

Generally, water can be added to the soil at borrow pit or in place (at the construction site). When processing granular materials, it is more advisable to add water in place because granular materials easily loses water through evaporation. In Nigeria, the common practice is to add water at the construction site. Therefore, this post is necessary to guide to ensure there is neither over-watering nor under-watering of site. After adding water, thoroughly and uniformly mix it with the soil.

Amount of water required

It is important to determine the amount of water required in order to achieve soil water content within the acceptable moisture range. The formula stated below is used per stretch (30 m length of 150 mm thick layer of soil).

Amount of water required (m³) = ρ_ddesired x ((w_desired – w_borrow)/100) x (v_sc/1000)

Where ρ_ddesired = desired dry density of the soil (kgm^-3) which is gotten from laboratory tests.

W_desired = desired moisture content (%) which is gotten from laboratory tests.

W_borrow = moisture content (%) of the soil at borrow pit.

V_sc = compacted volume of the soil (m³) which is gotten by the product of length, width and compacted depth of the place to be compacted.

1000 = the weight (kg) of cubic meter of water.

Example

Prepare to place water in 150 mm (compacted) soil. The desired dry density of the embankment is 1920 kgm^-3. The desired OMC is 12 %. The moisture content of the borrow pit is 5%. The roadway width = 12m. Compute the amount of water to be added per 30 m stretch of the road.

Solution

ρ_ddesired = 1920 kgm^-3

w_desired = 12 %

w_borrow = 5 %

v_sc = 12 x 30 x 0.15 = 54 m³

Volume of water required (m³) = 1920 x ((12-5)/100) x 54 = 7257.6/1000 = 7.26 m³.

As noted earlier, it is good practice to adjust the moisture content to OMC ±2% but this depends on the environmental conditions (temperature and wind) and the soil type. A negative value from the computation above indicates that removal of water from the borrow material is necessary before compacting the material on the fill.

Rate of water application

Having determined the water required, it is also necessary to determine the rate of application. The rate of application is measured in m³/m²

Rate of application, R = ρ_ddesired x (w_{added or removed}/100) x compacted thickness x 1/1000

Using the example above, R = (1920 x 0.07 x 0.15)/1000 = 0.02 m³/m²

Method of application

It is also important to achieve the proper application rate and the uniform distribution of water. This necessitate the consideration of the method of application. There are two proper methods:

1. Water distributor: water distributors are designed to distribute the correct amount of water evenly over the fill. It distributes water by pressure or gravity feed. The water application rate can be maintained by controlling the forward speed of the vehicle and the water distribution pressure. The site engineer should provide the operator with the application rate and should ensure that the Speedometer of the vehicle is working. With the water application rate, the operator can determine the appropriate spray bar length, pumping pressure and vehicle speed to achieve the required application rate.

2. Ponding: if time is available add water by ponding the area until achieving the desired depth of penetration. This method is time consuming as many days are required to achieve uniform moisture distribution.

Effects of weather

Cold, rainy, cloudy or calm weather cause soil to retain water but hot, dry, sunny or windy weather cause water loss by evaporation. Depending on the ambient weather condition, the percentage of the OMC should be added or subtracted from the OMC to achieve the required result. Acceptable range could be ±2- 6 %.

In the situation where there is excess water in the soil due to rain or other causes, the following measures can be adopted to reduce the excess water.

1. Simple soil aeration.
2. Use of stabilization agents like lime or fly ash.
3. Use of subsurface drainage in case of high water table.
4. Scarifying the soil prior to compaction. It can be done using the scarifier in grader or stabilizer mixer (pulveriser).

Compaction of the soil

It is also very necessary that suitable compaction is applied to the soil to achieve the maximum dry density at the optimum moisture content. Niphadkar (2016) opined that there are two general specifications for earthwork compaction. These are: Method specifications and End-product specifications.

Method Specifications (MS)

According to him, method specifications, requires the determination of the type and weight of the compaction equipment, the number of passes, and the maximum lift thickness by the design Engineer. The method does not require any quality assurance (QA) test as the engineer must be certain that the specifications should be able to achieve the desired result. To achieve this, he must have prior knowledge of the fill material. In a situation where he is not familiar with the fill material, rolling tests (according to Nigerian General Specifications for Roads and Bridges) or similar tests should be performed to determine the number of economic passes.

Advantages of MS

It is more economical

It helps to avoid over-compaction and under-compaction

Disadvantages of MS

It is time consuming

It requires expertise

End Product Specifications (EPS)

This is the method common in Nigeria and in most road construction projects. In the method, the engineer specifies a relative compaction which the Contractor must achieve. Relative compaction (%) is the ratio of field dry density to laboratory dry density. This methods find the application of sand cone method of insitu density test and similar methods for the determination of field dry density/relative density.

Advantages of EPS

It saves time

Disadvantages of EPS

There is the risk of over-compaction and under-compaction of soil

It is not economical

Reference

FM 5-434 (2000): Earth moving operations. Department of Army, Washington.

Niphadkar, N. (2016) Relationship Between Number of Passes of Compactor and Compaction Characteristics of Soil. International Research Journal of Engineering and Technology (IRJET) Volume: 03 Issue: 05. PP 728 – 732.

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