Repeat Problem 19-14 with the subgrade Mr values for each month from January through December being 18,000, 18,000, 12,000, 8000, 8000, 9000, 9000, 9000, 9500, 9500, 12,000, and 20,000 lb/in2, respectively. The pavement structure will be exposed to moisture levels approaching saturation for 20% of the time, and it will take about 4 weeks for drainage of water from the pavement. Use untreated sandy gravel with Mr of 15 × 103 lbs/in2 for subbase and untreated granular material with Mr of 28 × 103 lbs/in2 for the base course.

Question

What will be an ideal response?

Answer 1

First determine the relative damage each month.

Using Figure 19.6 or the following equation and table:



Average uf = ?uf / n = 0.704/12 = 0.0587

Roadbed soil, Mr = 10800 lb/in2

From Problem 19-14, ?PSI = 1.7

Using Figure 19.10, SN = 3.4

Determine the structural layer coefficient for each layer:

From Figure 19.5, EAC = 4.5 × 105 (asphalt concrete, from Problem 20-14)

a1 = 0.44

From Figure 19.4, Mr = 28 × 103 (untreated gravel base)

a2 = 0.13

From Figure 19.3, Mr = 15 × 103 (sandy gravel subbase)

a3 = 0.11

Find layer thicknesses, starting with surface course:

SN of surface course is dependent on base course Mr, therefore enter Figure 19.10

with Mr = 28 × 103 SN = 2.5

D1*= 2.5/0.44 = 5.68 in. Use 6 in.

SN1*= a1D1* = 0.44(6) = 2.64

Repeating this procedure for the base course:

Mr= 1.5 × 104 (Subbase)

From Figure 19.10, SN = 3.2

D2* = (3.2 – 2.64) / [(0.13)(0.7)] = 6.15 in. Use 6.5 in.

SN2* = (a2)(m2)(D2*) + SN1* = (0.13)(0.7)(6.5) + 2.64 = 3.23

D3* = (3.6 – 3.23) / [(0.11)(0.7)] = 7.4 in. Use 7.5 in.

Check design:

SN = 0.44(6) + (0.13)(0.7)(6.5) + (0.11)(0.7)(7.5) = 3.81 (which is greater

than SN of 3.4), so the design is satisfactory

The pavement will consist of 6 in. of asphalt concrete surface, 6.5 in. of untreated

gravel base, and 7.5 in. of sandy gravel subbase.