SM 1110-02 Surfacing Manual INTRODUCTION Soil cement is a mixture of Portland Cement, a mineral soil (usually a graded sand) and water that is placed and compacted in one lift. It is an alternate to granular or asphalt concrete base courses. SOIL CEMENT BASE THICKNESS DETERMINATION The thickness of soil cement base course required to limit the strain imposed on the subgrade by the design traffic loading can be determined by: 1. Using the Chevron 5 Layer Program as described in SM 1110-03, Other Design Methods. 2. Using a graphic solution that provides the relationship between base course thickness and stiffness, and asphalt course thickness and stiffness. On a log-log plot, there is a straight-line relationship between base course thickness and base course stiffness for a given subgrade support and traffic level. To determine this straight-line relationship, one requires the thickness and resilient modulus of the granular base along with the thickness and stiffness of the AC base. Once the thickness and stiffness values for the two materials are determined, the values can be plotted on a log-log plot and joined with a straight line. This line can then be used to obtain the thickness of any other base type provided its stiffness is known. 3. Using SM 1110-01, select a Full Depth Asphalt Concrete and a granular base structure with an asphalt concrete wearing course for the traffic and subgrade conditions of the highway section. The thickness of AC on the granular structure is subtracted from the full depth thickness to get the thickness of asphalt concrete base course. With the thickness of granular course and the AC base, use the following steps to calculate the equivalent soil cement base thickness. a) Determine the modular ratio (K 2 ) from Figure 1, for the thickness of unbound granular course considered. b) Calculate the Resilient Modulus (M R ) of the granular course as follows: 2013-08-15 1 of 5
M R = 10,000 * K 2 * CBR of the subgrade c) Calculate the stiffness modulus of the asphalt concrete base course (Smix) for the asphalt cement and aggregate gradation to be used in the asphalt concrete (if such data is available at the time of the surfacing design). NOTE: For design purposes, the stiffness of asphalt concrete can be assumed to be 6.2 x 10 6 kpa. This is based on typical Saskatchewan asphalts and conditions. 4. Plot the co-ordinates of the granular base thickness versus the Resilient Modulus of the granular base course (see Figure 2). 5. Plot the co-ordinates of the asphalt concrete base course thickness versus the stiffness of the asphalt concrete. 6. Join the two sets of co-ordinates by a straight line. 7. The stiffness of the soil cement base course can be assumed as follows: Table 1. course. Material vs. Stiffness of soil cement base Material Silty sand mix (100% smaller than the 400 µm sieve) Sandy gravel mix Stiffness 2.8 x 10 6 kpa 3.4 x 10 6 kpa 8. Enter the thickness-strength plot (constructed in steps 4, 5 and 6 above) at the appropriate soil cement stiffness. Determine the required soil cement thickness at the point where the straight-line plot and the line representing the soil cement stiffness intersect. NOTE: The minimum thickness of soil cement that should be used is 140 mm. 2 of 5 2013-08-15
EXAMPLE OF SOIL CEMENT THICKNESS DETERMINATION Assuming a design subgrade CBR of 8.0 and a design traffic loading (N 5 ) = 1.6 x 10 5, full depth asphalt concrete and paved granular base structures can be generated (see SM 1110-01). The paved granular structure selected has a 60 mm asphalt concrete mat; 60 mm is then deemed to be the thickness of the asphalt concrete surface course for all structures considered in the comparison, as follows: Table 2. Thickness of the Asphalt Concrete Surface. Alternate Asph. Conc. Surf. Course Asph. Conc. Base Course Granular Base Course Total Thickness 1 60 110 -- 170 2 60 -- 270 330 1. K 2 = 2.6, from Figure 1 and M R = 10,000 x 2.6 x 8 = 2.1 x 10 5 kpa. 2. S mix = 6.2 x 10 6 kpa. 3. Plot the granular thickness vs granular M R, (270 mm and 2.1 x 10 5 kpa), and plot the asphalt concrete base thickness vs S mix (110 mm and 6.2 x 10 6 kpa) on a log-log scale. Join the two sets of co-ordinates by a straight line. 4. Assuming a graded, sandy gravel soil cement mix, the soil cement stiffness is 3.4 x 10 6 kpa. Enter the thickness strength log-log plot and pick off the equivalent soil cement base thickness (130 mm, in this case). See Figure 2. NOTE: Since the minimum soil cement thickness is 140 mm, 140 mm would be used for design purposes instead of the 130 mm thickness. 2013-08-15 3 of 5
Figure 1. Modular Ratio vs. Granular Layer Thickness. 4 of 5 2013-08-15
Figure 2. Base Course Thickness vs. Stiffness 2013-08-15 5 of 5