GRAVEL ROAD DESIGN


The primary design requirements for aggregate surfaced roads include:

o Predicted future traffic for the design period (see Article 2)

o The lengths of the seasons (see Article 6.6.1e)

o Seasonal resilient moduli of the roadbed soil (see Article 6.6.1f)

o Elastic modulus, EBS(psi), of aggregate base layer (from HVEEM or other testing. MR value)

o Elastic modulus, EBS(psi), of aggregate subbase layer (from HVEEM or other testing. MR value)

o Design serviceability loss, ΔPSI (Article 6.6.1c)

o Allowable rutting, RD(inches), in surface layer (Article 6.6.1a), and

o Aggregate loss, GL(inches), of surface layer (Article 6.6.1b)

These design requirements are used in conjunction with the computational chart in Table 2 in Appendix 2 and the design nomographs for serviceability (Figure 18, Appendix 1) and rutting (Figure 19, Appendix 1) The following steps outline the procedure:

Step 1: Select four levels of aggregate base thickness, DBS, which should bound the probable solution. Prepare four separate tables, one for each trial thickness, identical to Table 2. On each of the four tables enter the trial base thickness, DBS; design serviceability loss, ? PSI; and the allowable rutting, RD in the appropriate boxes.

Step 2: Enter the appropriate seasonal resilient (elastic) moduli of the roadbed (MR) and the aggregate base material, EBS, in columns 2 and 3, respectively, of Table 2. The base modulus values may be proportional to the resilient modulus of the roadbed soil during a given season. However, a constant value of 30,000 psi was used in the example which follows since a portion of the aggregate base material will be converted into an equivalent thickness of subbase material (which will provide some shield against the environmental moisture effects).

Step 3: Enter the seasonal 18-kip ESAL traffic in column 4 of Table 2. Assuming that truck traffic is distributed evenly throughout the year, the lengths of the seasons should be used to proportion the total projected 18-kip ESAL traffic to each season. If the road is load-zoned (restricted) during certain critical periods, the total traffic may be distributed only among those seasons when truck traffic is allowed. Total traffic of 36,500 18-kip ESAL applications (the minimum 5 EDLA and a 20 year design period) and a seasonal pattern corresponding to U.S. Climatic Region VI was used in the example.

Step 4: Within each of the four tables estimate the allowable 18-kip ESAL traffic for each of the four seasons using the serviceability-based nomograph (Figure 18) and enter the result in column 5. If the resilient modulus of the roadbed roil (during the frozen season) is such that the allowable traffic exceeds the upper limit of the nomograph, assume a practical value of 500,000 18-kip ESAL.

Step 5: Within each of the four tables estimate the allowable 18-kip ESAL traffic for each of the four seasons using the rutting-based nomograph (Figure 19) and enter the result in column 7. Again, if the resilient modulus of the roadbed soil is such that the allowable traffic exceeds the upper limit of the nomograph, assume a practical value of 500,000 18-kip ESAL.

Step 6: Compute the seasonal damage values in each of the four tables for the serviceability criteria by dividing the projected seasonal traffic (column 4) by the allowable traffic in that season (column 5). Enter these seasonal damage values in column 6 of Table 2 corresponding to serviceability criteria. Next, follow these same instructions for rutting criteria, i.e., divide column 4 by column 7 and enter in column 8.

Step 7: Compute the total damage for both the serviceability and rutting criteria by adding the seasonal damages. When this is accomplished for all four tables, a graph of total damage versus base layer thickness should be prepared. The average base layer thickness, DBS, required is determined by interpolating in this graph for a total damage equal to 1.0. Figure A3-5 provides an example in which the design is controlled by the serviceability criteria.

Step 8: The base layer thickness determined in the last step should be used for design if the effects of aggregate loss are negligible. If, however, aggregate loss is significant, the design thickness is determined using the following equation:

DBS = DBS + (0.5 x GL)

where GL = total estimated aggregate (gravel) loss (in inches) over the performance period.

Step 9: The final step of the design chart procedure for aggregate surfaced roads is to convert a portion of the aggregate base layer thickness to an equivalent thickness of subbase material. This is accomplished with the aid of Figure 20. Select the final base thickness desired, DBSf (6 inches is used in the example). Draw a line to the estimated modulus of the subbase material, EBS. Go across and through the scale corresponding to the reduction in base thickness, DBSi -- DBSf. Then for the known modulus of the base material, EBS, determine the required subbase thickness, DSB.

As an example to illustrate the described procedure and the requirements of Article 6, assume the following:

o HVEEM R value of 20 for the roadbed soil.

o The minimum required EDLA of 5, over a 20 year design period for a total traffic of 36,500 18-kip ESAL.

Assume 6, 8, 10, and 12 inches of base thickness for preparation of the four tables. Per Article 6.6.1, the design serviceability loss is 3, and the allowable rutting is 2.

Proportion the total projected 18-kip ESAL traffic into the seasonal traffic values for column 4 according to the lengths of season specified in 6.6.1e.

The results of proceeding according to steps 4, 5, and 6 above are shown in the example tables, Tables A3-1 through A3-4.

Figure A3-5 shows the graph of total damage versus base layer thickness for this example. The serviceability criteria require a larger thickness of base than the rutting criteria. Use the higher value (11.6 inches) for design.

Gravel loss is specified for design purposes in 6.6.1b as 2 inches, therefore the required thickness, DBS, is:

DBS = DBS + (0.5 x GL) = 11.6 + (0.5 x 2) = 12.6 inches.

Use Figure 20 (reproduced showing the example as Figure A3-4) to determine the amount of subbase material required to reduce the base thickness by 6 inches.

TABLE 2a -- EXAMPLE ASSUMING 6 INCHES BASE COURSE

TRIAL BASE THICKNESS, DBS (INCHES)____6____
SERVICEABILITY CRITERIA
RUTTING CRITERIA
PSI = ______3_____
RD (INCHES) ____2_____
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
SEASON (ROADBED MOISTURE CONDITION)
ROADBED RESILIENT MODULUS MR (psi)
BASE ELASTIC MODULUS EBS (psi)
PROJECTED 18 -- KIP ESAL TRAFFIC W18
ALLOWABLE 18 -- KIP ESAL TRAFFIC (W18)PSI
SEASONAL DAMAGE W18/(W18)PSI
ALLOWABLE 18 -- KIP ESAL TRAFFIC (W18)RUT
SEASONAL DAMAGE W18/(W18)RUT
WINTER (FROZEN)
20,000
30,000
9,125
32,000
0.29
350,000
0.03
SPRING/THAW (SATURATED)
1,500
30,000
4,563
2,200
2.07
3,500
1.30
SPRING/FALL (WET)
3,300
30,000
9,125
5,000
1.83
4,500
2.03
SUMMER (DRY)
4,900
30,000
13,687
7,000
1.96
7,500
1.82

TOTAL

TOTAL

TOTAL

TRAFFIC =
36,500
DAMAGE =
6.15
DAMAGE =
5.18


TABLE 2b -- EXAMPLE ASSUMING 8 INCHES BASE COURSE

TRIAL BASE THICKNESS, DBS (INCHES)___8___
SERVICEABILITY CRITERIA
RUTTING CRITERIA
PSI = ______3_______
RD (INCHES) ____2_____
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
SEASON (ROADBED MOISTURE CONDITION)
ROADBED RESILIENT MODULUS MR (psi)
BASE ELASTIC MODULUS EBS (psi)
PROJECTED 18 -- KIP ESAL TRAFFIC W18
ALLOWABLE 18 -- KIP ESAL TRAFFIC (W18)PSI
SEASONAL DAMAGE W18/(W18)PSI
ALLOWABLE 18 -- KIP ESAL TRAFFIC (W18)RUT
SEASONAL DAMAGE W18/(W18)RUT
WINTER (FROZEN)
20,000
30,000
9,125
70,000
0.13
400,000
0.02
SPRING/THAW (SATURATED)
1,500
30,000
4,563
4,200
1.09
7,000
0.65
SPRING/FALL (WET)
3,300
30,000
9,125
12,000
0.76
11,000
0.83
SUMMER (DRY)
4,900
30,000
13,687
13,500
1.01
16,000
0.86

TOTAL

TOTAL

TOTAL

TRAFFIC =
36,500
DAMAGE =
2.99
DAMAGE =
2.36


TABLE 2c -- EXAMPLE ASSUMING 10 INCHES BASE COURSE

TRIAL BASE THICKNESS, DBS (INCHES)____10____
SERVICEABILITY CRITERIA
RUTTING CRITERIA
PSI = ______3______
RD (INCHES) ____2______
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
SEASON (ROADBED MOISTURE CONDITION)
ROADBED RESILIENT MODULUS MR (psi)
BASE ELASTIC MODULUS EBS (psi)
PROJECTED 18 -- KIP ESAL TRAFFIC W18
ALLOWABLE 18 -- KIP ESAL TRAFFIC (W18)PSI
SEASONAL DAMAGE W18/(W18)PSI
ALLOWABLE 18 -- KIP ESAL TRAFFIC (W18)RUT
SEASONAL DAMAGE W18/(W18)RUT
WINTER (FROZEN)
20,000
30,000
9,125
120,000
0.08
400,000
0.02
SPRING/THAW (SATURATED)
1,500
30,000
4,563
8,000
0.57
11,000
0.41
SPRING/FALL (WET)
3,300
30,000
9,125
20,000
0.46
21,000
0.43
SUMMER (DRY)
4,900
30,000
13,687
28,000
0.49
28,000
0.49

TOTAL

TOTAL

TOTAL

TRAFFIC =
36,500
DAMAGE =
1.60
DAMAGE =
1.35



TABLE 2d -- EXAMPLE ASSUMING 12 INCHES BASE COURSE

TRIAL BASE THICKNESS, DBS (INCHES)____12____
SERVICEABILITY CRITERIA
RUTTING CRITERIA
PSI = _____3______
RD (INCHES) _____2______
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
SEASON (ROADBED MOISTURE CONDITION)
ROADBED RESILIENT MODULUS MR (psi)
BASE ELASTIC MODULUS EBS (psi)
PROJECTED 18 -- KIP ESAL TRAFFIC W18
ALLOWABLE 18 -- KIP ESAL TRAFFIC (W18)PSI
SEASONAL DAMAGE W18/(W18)PSI
ALLOWABLE 18 -- KIP ESAL TRAFFIC (W18)RUT
SEASONAL DAMAGE W18/(W18)RUT
WINTER (FROZEN)
20,000
30,000
9,125
200,000
0.05
400,000
0.02
SPRING/THAW (SATURATED)
1,500
30,000
4,563
18,000
0.25
22,000
0.21
SPRING/FALL (WET)
3,300
30,000
9,125
30,000
0.30
31,000
0.29
SUMMER (DRY)
4,900
30,000
13,687
40,000
0.34
45,000
0.30

TOTAL

TOTAL

TOTAL

TRAFFIC =
36,500
DAMAGE =
0.82
DAMAGE =
1.35