Soil-Cement Stabilization: Determination of Optimum Stabilizer Content and Application

Reading Time: 6 minutes

Soil-cement stabilization (SCS) is one of the commonest system of stabilizing soils which may possibly loose pre-eminence in the developed countries of the world due to cost and unsustainability, if a viable equivalent alternative is found. However, in developing countries like Nigeria, it is the major stabilization agent for flexible pavement in spite of large carbon foot print associated with it. Soil-cement involves the mixing of pulverized soil with cement and hydrating the mix with water to obtain a solid strong mass that can bear heavier loads when compacted and hardened than ordinary soil. Soil-cement are often categorized into four, depending on the purpose of stabilization, amount of cement used, mixing condition and platform etc. These are Cement Modified Soil (CMS) with cement dosage range of 3 – 5%, Cement Stabilised soil (CSS) with cement dosage range of ˃ 5%, Cement Treated Base (CTB) with cement recommended dosage range of 3 – 10% and Full Depth Reclamation (FDR) with cement dosage range of 5 – 8%. Table 1 shows the summary of category of soil-cement stabilization, cement content and areas of application.

Table 1; Category of SCS, cement content and areas of application

The processes leading to the use of soil-cement in construction work can be categorized into four (4) viz:

1. Discover why soil conditions should be improved: this could be due to the amount of load coming on the soil
2. Discover the type of soil present: this involves a geotechnical engineer, and laboratory tests would be done to classify the soil. The tests to be done include: particle size analysis, compaction tests, Atterberg limits tests etc.
3. Carry out the mix design process: this is often a desk work which can be done by highway engineer based on some laboratory test results.
4. Implement the construction methods

Effects of SCS on soils

1. Alters the soil cohesion.
2. Change capillary and pore structures of the soil.
3. Change the chemical composition of the soil.

Advantages of SCS

1. It improves soil bearing capacity.
2. It is often cheaper than the traditional method of remove and replace bad soil.
3. More cost effective than the use of granular base.
4. Soil changes are permanent, thereby it provides strength to the soil for long time.
5. Future maintenance options are usually cheap.
6. It reduces soil’s liability to moisture.

Going forward in this post, I would show how to carry out the mix design process according to Nigerian General Specifications for Roads and Bridges (1977) and how to implement the design in the construction methods.

A. The Mix Design Process:

The following tests for the selection of soil to be stabilised shall be carried out on soils sampled at suitable interval and depths.

1. Atterberg limit tests
2. Particle size analysis
3. Compaction test
4. Laboratory CBR test
5. Unconfined compression test

Procedure for Estimation of Cement Content

The most important test for the determination of cement content is the laboratory CBR test.  The specimen shall be cured with a wax cover for 6 days and soaked for 24 hours before testing, after allowing the specimen to drain for 15 minutes. Table 2 shows the design CBR values for soil cement mix-in-place and plant mix.

Table 2; Design CBR values for soil cement mix-in-place and plant mix

The following processes should be scrupulously adopted:

1. Based on the index properties tested, classify the soil according to US public road administration system
2. The estimated range of cement content is determined using AASHO soil group (Table 3)
3. Perform BS compaction on the material to establish MDD and OMC using the middle cement content for example (4.5%) cement A-2 as shown in Table 3. The CBR and UCS specimens shall be moulded to this density and moisture content.
4. Establish the relationship between cement content vs CBR and UCS (unconfined compressive strength) for soaked and unsoaked samples.
5. Mould 3 CBR samples and 6 UCS at each cement content, with a minimum of 3 cement content for example A-2 soil (3%, 4.5% and 6%).
6. Wax and cure all samples for 6 days (except 3 UCS samples at each cement content which should be cured for 7 days and tested without soaking)
7. Test all CBR samples and the 3 UCS samples at each cement content after 24 hours soaking by complete immersion in water and allowed to drain for 15 minutes
8. Plot graphs of cement contents vs CBR (soaked) and cement contents vs UCS (soaked and unsoaked).

Establish the required cement content at 180% CBR for site mix or 160% for plant mix and the corresponding UCS values soaked and unsoaked for quality control in the field.

Table 3; modified cement content requirements of AASHO soil groups

Note: Heavy clays soils ranging from A-5 to A-7 should be avoided where possible

B. Implementation of Construction Methods

Before spreading cement, the soil should be well pulverized often pre-wetting soil to aid pulverization. Cement shall be spread uniformly over the pulverized soil. Cement may be supplied in bags or bulk. If bags of cement are used, the bags shall be placed at calculated intervals along the verge, the bags opened and the cement spread evenly on the pulverized soil by shovels and rakes (See Figures 1 – 3).

If cement is supplied in bulk, it shall be spread by an approved mechanical cement spreader and each batch in the spreading equipment shall be weighed so that the average rate of spread can be determined by the Engineer. The rate of spread per linear meter shall not vary more than 10% from the approved rate.

Example

Assuming a given road, 276 m length x 9.5 m width is required to be stabilised with 3% cement as determined from the procedure outlined above, if the materials are spread by bags, show how to estimate the quantity of bags required and how to spread them (Note that 3% is calculated on the mass of dry soil when estimating the quantity of bags required) .

Volume of road to receive stabilizer = 276 x 9.5 x 0.2 = 524.4 m³            (0.2 m = thickness of subgrade)

Dry density of soil = 1970 kg/m³

Mass of soil = density of soil x volume of soil = 524.4 x 1970 = 1033068 kg.

Cement content = 3% of mass of soil = 3% of 1033068 = 30992.04 kg of cement required for the entire road.

Using 50 kg bags, number required = 30992.04/50 = 620 bags.

Determination of area to receive one bag of cement on ground (methods vary according to Engineer’s discretion)

The width of the road is first divided into 6 equal parts (9.5/6) = 1.58 x 276 m across section

620/6 bags of cement would be required per 1.58 x 276 m strip. This would be 103.33 bags.

Strip to receive one bag along the length of 276 m = 276/103.33 = 2.671 m. Thus, one bag of cement would be required per 2.671 x 1.58 m of the entire road section. These sections are marked on the roads as shown above with suitable equipment, the cement are place on each segment and spread by rakes (Figure 1 – 3). After even spread of the cement on the soil, pulverizer moves and mixes the cement thoroughly with the soil (Figure 4).

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