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Geotechnical Aspects of Pavements Reference Manual
Chapter 6.0 Pavement Structural Design And Performance
6.1 Introduction
Previous chapters have described in qualitative terms the many geotechnical factors influencing pavement design and performance, the wide range of geotechnical properties required as input to the design procedures, and the various methods for determining the values of these geotechnical inputs. Now it is time to evaluate quantitatively the importance of these factors and properties. These are the primary objectives of the present chapter:
- To illustrate via examples how the geotechnical properties described in Chapter 5 are incorporated in the pavement design calculations; and
- To highlight the effects of the geotechnical factors and inputs on pavement design and performance.
These objectives will be met through a series of design scenarios. First, a set of reference or baseline flexible and rigid pavement designs are developed for a hypothetical and simple project scenario. Then, the effects of various deviations from the baseline conditions will be investigated and quantified. These include:
- Soft/weak subgrade conditions
- Subgrade stabilization
- Low quality base/subbase material
- Drainage and water conditions
- Shallow bedrock conditions
The design scenarios are intentionally highly idealized and simplified. Their point is to emphasize in quantitative terms how changes in geotechnical inputs affect the overall pavement design and performance. In a sense, these design scenarios are examples of the types of sensitivity studies one should perform during design to evaluate the importance of the various design inputs, especially with reference to the quality of the information used in their estimation.
All of the design scenarios described in this chapter are for new construction (or reconstruction). This is not to minimize the importance of rehabilitation design; as described in Chapter 1, most pavement design today is in fact for rehabilitation and not new construction. However, most structural rehabilitation designs focus on restoration of the surface layer, either through asphalt concrete overlays, concrete pavement restoration, or a combination of the two. For these types of scenarios, the geotechnical inputs are essentially the same as for new construction design - e.g., a subgrade is a subgrade whether it is beneath a new or an existing pavement. The principal things that change are the methods by which the geotechnical inputs are determined - e.g., MR backcalculated from FWD tests instead of measured in the laboratory. The ways that these geotechnical inputs are used in the design calculations and the effects that they have on the design pavement structure are similar for new construction, rehabilitation, and reconstruction designs.
Both the current 1993 AASHTO Design Guide and the forthcoming new design guide from NCHRP Project 1-37A are applied to the scenarios in this chapter. Summaries of each of these design procedures are provided in Appendices C and D, respectively. Calculations for the 1993 AASHTO Guide designs are based on simple spreadsheet evaluation of the flexible and rigid pavement design equations. The calculations for the mechanistic-empirical designs were performed using Final Report Release version 0.700 (4/7/2004) of the NCHRP 1-37A software. This is the final version of the software as submitted to NCHRP at the conclusion of Project 1-37A.
It is important to keep in mind the significant differences between the two design procedures. The 1993 AASHTO Design Guide is an empirical methodology in which typically the design traffic, environmental conditions, and maximum serviceability (performance) loss are specified, and the corresponding required pavement structure-typically described just in terms of layer thicknesses-is determined. The NCHRP 1-37A procedure is a mechanistic-empirical methodology in which the design traffic, environmental conditions, and pavement structure are specified, and the corresponding pavement performance vs. time is predicted. In the NCHRP 1-37A procedure, several trial designs generally need to be evaluated in an iterative fashion in order to find the one (or more than one) that meets the design performance requirements.
Typically, there are multiple pavement designs that can provide the required performance for any scenario. This is true for both the 1993 AASHTO and NCHRP 1-37A design methodologies. The final selection of the "best" design should be based upon life-cycle costs, constructability, and other issues. Crude economic evaluations can be made in terms of initial construction costs, although even this is difficult because of large region-to-region variations in unit costs.
One final note regarding units of measure in this chapter: FHWA policy is to report values in SI units in all reports, with the corresponding U.S. Customary equivalent in parentheses. This is not done here. All values for the design examples in this chapter are reported in U.S. Customary units. There are two important practical reasons that dictate this choice. First, the structural layer coefficients, empirical correlations, and other data in the 1993 AASHTO Design Guide were developed and are presented in U.S. Customary units only. Although many of these could be converted to SI units, the consequences would be confusing. For example, expressing flexible pavement layer thicknesses in millimeters would require changing the values of the structural layer coefficients to quantities that would be unfamiliar to pavement engineers (e.g., the a1 value for asphalt concrete would change from 0.44 to 0.017 if asphalt thickness were expressed in millimeters). Second, although an SI version of the NCHRP 1-37A software is planned, at the time of this writing, only the U.S. Customary version is available. All of the outputs from the software are expressed in U.S. Customary units, and, consequently, the inputs described in this report are left in U.S. Customary units as well for consistency. The general SI-U.S. Customary conversion table included at the beginning of this reference manual can be used, if necessary, for converting units in this chapter.
6.2 Baseline Designs
The baseline designs for flexible and rigid pavements are intended to provide very simple and ordinary reference cases that can be used as the basis for subsequent exploration of the effects of various geotechnical inputs. The design scenario is based on the following assumptions:
- New construction
- Simple pavement structure
- Hot mix asphalt concrete (HMA) over crushed stone graded aggregate base (GAB) over subgrade (SG) for flexible pavements
- Jointed plain concrete pavement (JPCP) over graded aggregate base (GAB) over subgrade (SG) for rigid pavements
- Excellent, non-erodable base material (AASHTO A-1-a crushed stone)
- Nonexpansive subgrade
- Benign environmental conditions - e.g., no frost heave/thaw or expansive soils
- Good drainage
- Simple traffic conditions - e.g., no traffic growth over design period
Reference pavement designs consistent with these assumptions have been developed for a hypothetical new arterial highway outside of College Park, MD. The roadway is assumed to have two lanes in each direction and significant truck traffic consisting primarily of Class 9 5-axle tractor-trailer units. The subgrade conditions are a non-expansive silty clay subgrade (AASHTO A-7-5/USCS MH material), a deep groundwater table, and no shallow bedrock. Environmental conditions in the Mid-Atlantic region are mild, so frost heave/thaw is not a design issue, and seasonal variations of the unbound material properties are expected to be minor.
Table 6-1 provides some typical in-place initial construction unit costs for paving materials in Maryland. These costs will be used for rough economic evaluations of the designs developed in this chapter.
Material | Reasonable Range | Typical Unit Price | Typical Unit Price |
---|---|---|---|
Hot mix asphalt concrete (12.5 mm PG 64-22) | $30-$50/ton | $36/ton | $14,250/lane-mi-in |
Portland cement concrete (PCC) without steel | $110-$180/cy | $144/cy | $28,200/lane-mi-in |
Graded aggregate base | $24-$60/cy | $42/cy | $8,200/lane-mi-in |
6.2.1 1993 AASHTO Design
Flexible Pavement
The baseline flexible pavement design is a three-layer system consisting of an asphalt concrete (AC) surface layer over a nonstabilized graded aggregate base (GAB) layer over subgrade (SG). The input parameters for the baseline design using the 1993 AASHTO flexible procedure for new pavements are summarized in Table 6-2. Refer to Chapter 5 for detailed explanations of all input parameters and the methods available for their determination.
Input Parameter | Design Value | Notes |
---|---|---|
Initial service life | 15 years | 1 |
Traffic (W18) | 6.1x106 ESALs | 2 |
Reliability | 90% | 3 |
Reliability factor (ZR) | -1.282 | |
Overall standard error (So) | 0.45 | 1 |
Allowable serviceability deterioration (ΔPSI) | 1.7 | 4 |
Subgrade resilient modulus (MR) | 7,500 psi | 5 |
Granular base type | AASHTO A-1-a | 1 |
Granular base layer coefficient (a2) | 0.18 | 6 |
Granular base drainage coefficient (m2) | 1.0 | 7 |
Asphalt concrete layer coefficient (a1) | 0.44 | 1 |
Notes:
- Typical value for flexible pavement design.
- Consistent with more detailed traffic input in the NCHRP 1-37A design (Section 6.2.2).
- Typical value for a principal arterial (AASHTO, 1993).
- Typical value for flexible pavements. No serviceability reduction for swelling or frost heave.
- Consistent with the Level 3 default input for this soil class in the NCHRP 1-37A design after adjustment for seasonal effects (Section 6.2.2).
- Corresponds to an MR value of 40,000 psi, which is consistent with the Level 3 default input for this soil class in the NCHRP 1-37A design (Section 6.2.2).
- Representative of good drainage and moderate (5-25%) saturation conditions; matches value typically used by the Maryland State Highway Administration for design.
The methodology by which the input parameters in Table 6-2 are used to determine the final structural design in the 1993 AASHTO Guide is described in Appendix C. The calculations are sufficiently straightforward that they can be easily performed using a spreadsheet. The key output from the 1993 AASHTO design methodology is the required pavement structure, which is determined as follows:
- Required overall structural number SN = 4.61
- Required structural number for asphalt concrete surface layer SN1 = 2.35
- Required minimum thickness of asphalt D1 = SN1 / a1 = 5.3 inches1
- Remaining structural number required for granular base layer SN2 = SN - D1a1 = 2.28
- Required thickness of granular base inches D2 = SN2 / m2a2 = 12.7 inches1
Since the ratio of the layer coefficients (a1 / a2 = 0.44 / 0.18 = 2.44) is greater than the ratio of the associated in-place unit costs per lane-mile-inch of thickness in Table 6-1 ($14,250 / $8,200 = 1.74), there is an economic benefit from substituting granular base thickness with additional asphalt in this design - i.e., replacing 2.4 inches of granular base with an additional 1.0 inch of asphalt concrete is both structurally feasible (at least in terms of the 1993 AASHTO Guide) and economically beneficial (at least in terms of initial construction costs) since it would result in a savings of about $5400 per lane mile at the same SN value. However, in order to avoid complicating comparisons between the various design scenarios later in this chapter, the baseline flexible pavement structure will be kept at 5.3 inches of AC over 12.7 inches of GAB (or 5.5 inches of AC over 13 inches of GAB after rounding).
Although the 15-year initial service life specified for this scenario is typical for flexible pavements and equal to the values used in the design examples in the 1993 AASHTO Guide, current trends are toward longer life or "perpetual" pavement designs. The required pavement section for a 30-year initial service life based on the 1993 AASHTO Guide is 6.0 inches of AC over 13.7 inches of GAB. The "premium" for an additional 15 years of pavement life is thus only about three-quarters of an inch of asphalt and one inch of crushed stone base.
Rigid Pavement
The baseline rigid pavement design is a three layer JPCP system consisting of a Portland cement concrete (PCC) slab over a nonstabilized graded aggregate base (GAB) layer over subgrade (SG). The input parameters for the baseline design using the 1993 AASHTO rigid procedure for new pavements are summarized in Table 6-3. Refer to Chapter 5 for detailed explanations of all input parameters and the methods available for their determination. The rigid pavement design inputs are consistent with those used for the baseline flexible pavement design.
Input Parameter | Design Value | Notes |
---|---|---|
Initial service life | 25 years | 1 |
Traffic (W18) | 16.4x106 ESALs | 2 |
Reliability | 90% | 3 |
Reliability factor (ZR) | -1.282 | |
Overall standard error (So) | 0.35 | 1 |
Allowable serviceability deterioration (ΔPSI) | 1.9 | 4 |
Terminal serviceability level (pt) | 2.5 | 1 |
Subgrade resilient modulus (MR) | 7,500 psi | 5 |
Granular subbase type | AASHTO A-1-a | 1 |
Granular subbase resilient modulus (ESB) | 40,000 psi | 6 |
Drainage coefficient (Cd) | 1.0 | 7 |
Loss of Support (LS) | 2.0 | 8 |
PCC modulus of rupture (Sc′) | 690 psi | 1 |
PCC modulus of elasticity (Ec) | 4.4x106 psi | 1 |
Joint load transfer coefficient (J) | 2.8 | 9 |
Notes:
- Typical value for rigid pavement design.
- Consistent with more detailed traffic input in the NCHRP 1-37A design (Section 6.2.2).
- Typical value for a principal arterial (AASHTO, 1993).
- Typical value for rigid pavements. No serviceability reduction for swelling or frost heave.
- Consistent with the Level 3 default input for this soil class in the NCHRP 1-37A design after adjustment for seasonal effects (Section 6.2.2).
- Consistent with the Level 3 default input for this soil class in the NCHRP 1-37A design (Section 6.2.2).
- Representative of good drainage and moderate (5-25%) saturation conditions.
- Within AASHTO-recommended range for unbound granular materials.
- Typical value for JPCP with tied PCC shoulders and dowelled joints.
The methodology by which the input parameters in Table 6-3 are used to determine the final structural design in the 1993 AASHTO Guide is described in Appendix C. The calculations are sufficiently straightforward that they can be easily performed using a spreadsheet. The key output from the 1993 AASHTO design methodology is the required pavement structure. This is determined from the input parameters in Table 6-3 and the following additional intermediate steps:
- Assume a 6-inch design thickness for the granular subbase. This value is typical for rigid pavements on reasonably competent subgrades.
- Determine the composite modulus of subgrade reaction k∞ representing the combined stiffness of the subgrade and the subgrade layer. For a 6-inch subbase thickness and unbound moduli as given in Table 6-3, k∞ = 423 pci.
- Correct k∞ for loss of support to determine the design modulus of subgrade reaction keff. (NOTE: No shallow bedrock correction is required for the assumed subgrade conditions.) For k∞ = 423 pci and LS = 2, keff = 38 pci.
- Determine the required slab thickness D = 10.4 inches from the rigid pavement design equation.2
The final design selection based upon the design inputs in Table 6-3 is therefore a 10.4 inch PCC slab over 6 inches of GAB (or 10.5 inches of PCC over 6 inches of GAB after rounding).
6.2.2 NCHRP 1-37A Design
Flexible Pavement
Consistent with the 1993 AASHTO design, the baseline flexible pavement structure for the NCHRP 1-37A design methodology is a three-layer new construction consisting of an asphalt concrete (AC) surface layer over a nonstabilized graded aggregate base (GAB) layer over subgrade (SG). However, the input parameters required for the NCHRP 1-37A methodology are considerably more extensive than those for the 1993 AASHTO Design Guide. The NCHRP 1-37A design inputs are summarized in Table 6-4. Refer to Chapter 5 for detailed explanations of all input parameters and to Appendix D for a summary of the NCHRP 1-37A design procedure. All of the inputs correspond to Level 3 quality, and the default values provided within the NCHRP 1-37A software are used wherever appropriate.
The NCHRP 1-37A procedure requires the evaluation of several trial pavement sections in order to find the design that best meets the performance requirements. The baseline pavement structure from the 1993 AASHTO design procedure can be conveniently taken as the initial trial section. The predicted rutting performance for a trial section corresponding to the 1993 AASHTO design of 5.3 inches of AC over 12.7 inches of GAB is shown in Figure 6-1; total rutting (after adjustment for reliability) at the end of the 15-year initial service life is 0.646 inches. Fatigue and thermal cracking are negligible for this design scenario, and rutting is the controlling distress type. The design limit for predicted total rutting is an explicit input in the NCHRP 1-37A procedure that would, in general, be set by individual agency policy. For the examples in this chapter, however, the design limit for total rutting is taken as the predicted rutting for the 1993 AASHTO design section in order to make the 1993 AASHTO and NCHRP 1-37A designs equivalent for the baseline conditions. The design limit for total rutting (after adjustment for reliability and rounding) is thus 0.65 inches, which is slightly less than the 0.75 inch default value in the NCHRP 1-37A software.
Figure 6-1. Predicted rutting performance for NCHRP 1-37A baseline flexible pavement design.
Input Parameter | Design Value | Notes |
---|---|---|
General Information | ||
Design life | 15 years | 1 |
Base/subbase construction month | September | 2 |
Pavement construction month | September | 2 |
Traffic open month | October | 2 |
Site/Project Identification | ||
Functional class | Principal Arterials - Others | |
Analysis Parameters | ||
Initial IRI | 63 in./mi | 2 |
Terminal IRI | 172 in./mi | 2 |
Alligator cracking limit | 25% | 2 |
Total rutting limit | 0.65 in | See text |
Reliability | 90% | |
Traffic | ||
Initial two-way AADTT | 2000 | |
Number of lanes in design direction | 2 | |
Percent of trucks in design direction | 50% | 2 |
Percent of trucks in design lane | 95% | 2 |
Operational speed | 55 mph | 2 |
Monthly adjustment | 1.0 throughout | 2 |
Vehicle class distribution | Level 3 defaults | Table 6-5 |
Hourly distribution | Level 3 default | Table 6-6 |
Traffic growth factor | 0% | |
Axle load distribution factors | Level 3 defaults | Table 6-7 |
Mean wheel location from edge | 18 in | 2 |
Traffic wander standard deviation | 10 in | 2 |
Design lane width | 12 ft | 2 |
Number of axles per truck | Level 3 defaults | Table 6-8 |
Average axle outside width | 8.5 ft | 2 |
Dual tire spacing | 12 in | 2 |
Tire pressure | 120 psi | 2 |
Tandem axle spacing | 51.6 in | 2 |
Tridem axle spacing | 49.2 in | 2 |
Quad axle spacing | 49.2 | 2 |
Climate | ||
Latitude | 38.98° | |
Longitude | -76.94° | |
Elevation | 48 ft | |
Depth of water table | 20 ft | |
College Park, MD climate data | Generated | 3 |
Thermal Cracking | ||
Average AC tensile strength at 14°F | 366.5 psi | 4 |
Creep test duration | 100 sec | 4 |
Creep compliance | Level 3 defaults | 4 |
Mixture VMA | 14.1% | 5 |
Aggregate coefficient of thermal contraction | 5x10-6/°F | 4 |
Drainage and Surface Properties | ||
Surface shortwave absorptivity | 0.85 | 2 |
Infiltration | n/a | 2 |
Drainage path length | n/a | 2 |
Pavement cross slope | n/a | 2 |
AC Surface Layer | ||
Cumulative % retained on 3/4 inch sieve | 4 | 5 |
Cumulative % retained on 3/8 inch sieve | 39 | 5 |
Cumulative % retained on #4 sieve | 59 | 5 |
% passing #200 sieve | 3 | 5 |
Asphalt binder grade | PG 64-22 | 5 |
Reference temperature | 70°F | 2 |
Effective binder content | 10.1% | 5 |
Air voids | 4.0% | 5 |
Total unit weight | 151 pcf | 5 |
Poisson's ratio | 0.35 | 2 |
Thermal conductivity | 0.67 BTU/hr-ft-°F | 2 |
Heat capacity | 0.23 BUT/lb-°F | 2 |
Granular Base Layer | ||
Unbound material type | AASHTO A-1-a | |
Analysis type | ICM Inputs | |
Poisson's ratio | 0.35 | 2 |
Coefficient of lateral pressure K0 | 0.5 | 2 |
Modulus | 40,000 psi | 2,6 |
Plasticity index | 1% | |
% passing #200 sieve | 3 | |
% passing #4 sieve | 20 | |
D60 | 8 mm | |
Compaction state | Compacted | 2 |
Maximum dry unit weight | 122.2 pcf | 2 |
Specific gravity of solids | 2.66 | 2 |
Saturated hydraulic conductivity | 263 ft/hr | 2 |
Optimum gravimetric water content | 11.1% | 2 |
Calculated degree of saturation | 82% | 2 |
SWCC parameter af | 11.1 psi | 2 |
SWCC parameter bf | 1.83 | 2 |
SWCC parameter cf | 0.51 | 2 |
SWCC parameter hr | 361 psi | 2 |
Compacted Subgrade (top 6 inches) | ||
Unbound material type | AASHTO A-7-5 | |
Analysis type | ICM Inputs | |
Poisson's ratio | 0.35 | 2 |
Coefficient of lateral pressure K0 | 0.5 | 2 |
Modulus | 12,000 psi | 2,6 |
Plasticity index | 30% | 2 |
% passing #200 sieve | 85 | 2 |
% passing #4 sieve | 99 | 2 |
D60 | 0.01 mm | 2 |
Compaction state | Compacted | |
Maximum dry unit weight | 97.1 pcf | 2 |
Specific gravity of solids | 2.75 | 2 |
Saturated hydraulic conductivity | 3.25x10-5 ft/hr | 2 |
Optimum gravimetric water content | 24.8% | 2 |
Calculated degree of saturation | 88.9% | 2 |
SWCC parameter af | 301 psi | 2 |
SWCC parameter bf | 0.995 | 2 |
SWCC parameter cf | 0.732 | 2 |
SWCC parameter hr | 1.57x104 psi | 2 |
Natural Subgrade (beneath top 6 inches) | ||
Unbound material type | AASHTO A-7-5 | |
Compaction state | Uncompacted | |
Maximum dry unit weight | 87.4 pcf | 2 |
(other properties same as for compacted subgrade) | ||
Distress Potential | ||
Block cracking | None | 2 |
Sealed longitudinal cracks outside wheel path | None | 2 |
Notes:
- Typical initial service life for flexible pavement design.
- Level 3 default/calculated/derived value from NCHRP 1-37A software.
- Based on interpolated climate histories at IAD, DCA, and BWI airports.
- Level 3 default/calculated/derived values from NCHRP 1-37A software for baseline AC mixture properties. Thermal cracking is not expected for the baseline design. However, these values are included here because they will be used in subsequent design scenarios.
- Based on a Maryland State Highway Administration 19.0mm Superpave mix design.
- Default input value at optimum moisture and density conditions before adjustment for seasonal effects (adjustment performed internally within the NCHRP 1-37A software).
Class 4 | 1.3% |
Class 5 | 8.5% |
Class 6 | 2.8% |
Class 7 | 0.3% |
Class 8 | 7.6% |
Class 9 | 74.0% |
Class 10 | 1.2% |
Class 11 | 3.4% |
Class 12 | 0.6% |
Class 13 | 0.3% |
By period beginning: | |||
Midnight | 2.3% | Noon | 5.9% |
1:00 am | 2.3% | 1:00 pm | 5.9% |
2:00 am | 2.3% | 2:00 pm | 5.9% |
3:00 am | 2.3% | 3:00 pm | 5.9% |
4:00 am | 2.3% | 4:00 pm | 4.6% |
5:00 am | 2.3% | 5:00 pm | 4.6% |
6:00 am | 5.0% | 6:00 pm | 4.6% |
7:00 am | 5.0% | 7:00 pm | 4.6% |
8:00 am | 5.0% | 8:00 pm | 3.1% |
9:00 am | 5.0% | 9:00 pm | 3.1% |
10:00 am | 5.9% | 10:00 pm | 3.1% |
11:00 am | 5.9% | 11:00 pm | 3.1% |
Axle Weight (lbs) | Truck Class | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | |
3000 | 1.80 | 10.05 | 2.47 | 2.14 | 11.65 | 1.74 | 3.64 | 3.55 | 6.68 | 8.88 |
4000 | 0.96 | 13.21 | 1.78 | 0.55 | 5.37 | 1.37 | 1.24 | 2.91 | 2.29 | 2.67 |
5000 | 2.91 | 16.42 | 3.45 | 2.42 | 7.84 | 2.84 | 2.36 | 5.19 | 4.87 | 3.81 |
6000 | 3.99 | 10.61 | 3.95 | 2.70 | 6.99 | 3.53 | 3.38 | 5.27 | 5.86 | 5.23 |
7000 | 6.80 | 9.22 | 6.70 | 3.21 | 7.99 | 4.93 | 5.18 | 6.32 | 5.97 | 6.03 |
8000 | 11.47 | 8.27 | 8.45 | 5.81 | 9.63 | 8.43 | 8.35 | 6.98 | 8.86 | 8.10 |
9000 | 11.30 | 7.12 | 11.85 | 5.26 | 9.93 | 13.67 | 13.85 | 8.08 | 9.58 | 8.35 |
10000 | 10.97 | 5.85 | 13.57 | 7.39 | 8.51 | 17.68 | 17.35 | 9.68 | 9.94 | 10.69 |
11000 | 9.88 | 4.53 | 12.13 | 6.85 | 6.47 | 16.71 | 16.21 | 8.55 | 8.59 | 10.69 |
12000 | 8.54 | 3.46 | 9.48 | 7.42 | 5.19 | 11.57 | 10.27 | 7.29 | 7.11 | 11.11 |
13000 | 7.33 | 2.56 | 6.83 | 8.99 | 3.99 | 6.09 | 6.52 | 7.16 | 5.87 | 7.32 |
14000 | 5.55 | 1.92 | 5.05 | 8.15 | 3.38 | 3.52 | 3.94 | 5.65 | 6.61 | 3.78 |
15000 | 4.23 | 1.54 | 3.74 | 7.77 | 2.73 | 1.91 | 2.33 | 4.77 | 4.55 | 3.10 |
16000 | 3.11 | 1.19 | 2.66 | 6.84 | 2.19 | 1.55 | 1.57 | 4.35 | 3.63 | 2.58 |
17000 | 2.54 | 0.90 | 1.92 | 5.67 | 1.83 | 1.10 | 1.07 | 3.56 | 2.56 | 1.52 |
18000 | 1.98 | 0.68 | 1.43 | 4.63 | 1.53 | 0.88 | 0.71 | 3.02 | 2.00 | 1.32 |
19000 | 1.53 | 0.52 | 1.07 | 3.50 | 1.16 | 0.73 | 0.53 | 2.06 | 1.54 | 1.00 |
20000 | 1.19 | 0.40 | 0.82 | 2.64 | 0.97 | 0.53 | 0.32 | 1.63 | 0.98 | 0.83 |
21000 | 1.16 | 0.31 | 0.64 | 1.90 | 0.61 | 0.38 | 0.29 | 1.27 | 0.71 | 0.64 |
22000 | 0.66 | 0.31 | 0.49 | 1.31 | 0.55 | 0.25 | 0.19 | 0.76 | 0.51 | 0.38 |
23000 | 0.56 | 0.18 | 0.38 | 0.97 | 0.36 | 0.17 | 0.15 | 0.59 | 0.29 | 0.52 |
24000 | 0.37 | 0.14 | 0.26 | 0.67 | 0.26 | 0.13 | 0.17 | 0.41 | 0.27 | 0.22 |
25000 | 0.31 | 0.15 | 0.24 | 0.43 | 0.19 | 0.08 | 0.09 | 0.25 | 0.19 | 0.13 |
26000 | 0.18 | 0.12 | 0.13 | 1.18 | 0.16 | 0.06 | 0.05 | 0.14 | 0.15 | 0.26 |
27000 | 0.18 | 0.08 | 0.13 | 0.26 | 0.11 | 0.04 | 0.03 | 0.21 | 0.12 | 0.28 |
28000 | 0.14 | 0.05 | 0.08 | 0.17 | 0.08 | 0.03 | 0.02 | 0.07 | 0.08 | 0.12 |
29000 | 0.08 | 0.05 | 0.08 | 0.17 | 0.05 | 0.02 | 0.03 | 0.09 | 0.09 | 0.13 |
30000 | 0.05 | 0.02 | 0.05 | 0.08 | 0.04 | 0.01 | 0.02 | 0.06 | 0.02 | 0.05 |
31000 | 0.04 | 0.02 | 0.03 | 0.72 | 0.04 | 0.01 | 0.03 | 0.03 | 0.03 | 0.05 |
32000 | 0.04 | 0.02 | 0.03 | 0.06 | 0.12 | 0.01 | 0.01 | 0.04 | 0.01 | 0.08 |
33000 | 0.04 | 0.02 | 0.03 | 0.03 | 0.01 | 0.01 | 0.02 | 0.01 | 0.01 | 0.06 |
34000 | 0.03 | 0.02 | 0.02 | 0.03 | 0.02 | 0.01 | 0.01 | 0.00 | 0.01 | 0.02 |
35000 | 0.02 | 0.02 | 0.01 | 0.02 | 0.02 | 0.00 | 0.01 | 0.00 | 0.00 | 0.01 |
36000 | 0.02 | 0.02 | 0.01 | 0.02 | 0.01 | 0.01 | 0.00 | 0.00 | 0.00 | 0.01 |
37000 | 0.01 | 0.01 | 0.01 | 0.01 | 0.01 | 0.00 | 0.01 | 0.00 | 0.01 | 0.01 |
38000 | 0.01 | 0.01 | 0.01 | 0.01 | 0.00 | 0.00 | 0.00 | 0.02 | 0.01 | 0.01 |
39000 | 0.01 | 0.00 | 0.01 | 0.01 | 0.01 | 0.00 | 0.01 | 0.01 | 0.00 | 0.01 |
40000 | 0.01 | 0.00 | 0.01 | 0.01 | 0.00 | 0.00 | 0.04 | 0.02 | 0.00 | 0.00 |
41000 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
Total % | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 |
Vehicle Class | Single Axle | Tandem Axle | Tridem Axle | Quad Axle |
---|---|---|---|---|
Class 4 | 1.62 | 0.39 | 0.00 | 0.00 |
Class 5 | 2.00 | 0.00 | 0.00 | 0.00 |
Class 6 | 1.02 | 0.99 | 0.00 | 0.00 |
Class 7 | 1.00 | 0.26 | 0.83 | 0.00 |
Class 8 | 2.38 | 0.67 | 0.00 | 0.00 |
Class 9 | 1.13 | 1.93 | 0.00 | 0.00 |
Class 10 | 1.19 | 1.09 | 0.89 | 0.00 |
Class 11 | 4.29 | 0.26 | 0.06 | 0.00 |
Class 12 | 3.52 | 1.14 | 0.06 | 0.00 |
Class 13 | 2.15 | 2.13 | 0.35 | 0.00 |
Rigid Pavement
Consistent with the 1993 AASHTO design, the baseline rigid pavement structure for the NCHRP 1-37A design methodology is a three-layer JPCP construction consisting of a Portland cement concrete (PCC) slab over a nonstabilized graded aggregate base (GAB) over subgrade (SG). However, the input parameters required for the NCHRP 1-37A methodology are considerably more extensive than for the 1993 AASHTO Design Guide. The NCHRP 1-37A design inputs are summarized in Table 6-9. Refer to Chapter 5 for detailed explanations of all input parameters and to Appendix D for a summary of the NCHRP 1-37A design methodology. All of the inputs correspond to Level 3 quality, and the default values provided within the NCHRP 1-37A software are used wherever appropriate.
The NCHRP 1-37A procedure requires the evaluation of several trial pavement sections in order to find the design that best meets the performance requirements. The baseline pavement structure from the 1993 AASHTO design procedure can be conveniently taken as the initial trial section. The predicted faulting performance for a trial section corresponding to the 1993 AASHTO Design section of 10.4 inches of PCC over 6.0 inches of GAB is shown in Figure 6-2; total faulting (after adjustment for reliability) at the end of the 25-year initial service life is 0.117 inches. Transverse fatigue cracking is negligible for this design scenario, and faulting is the controlling distress type. The design limit for predicted faulting, which is an explicit input in the NCHRP 1-37A procedure, would in general be set by individual agency policy. For the examples in this chapter, however, the design faulting limit is taken as the predicted faulting for the 1993 AASHTO design section. This is done in order to make the 1993 AASHTO and NCHRP 1-37A designs equivalent for the baseline conditions. The design limit for faulting (after adjustment for reliability and rounding) is thus 0.12 inches, which coincidentally equals the default value in the NCHRP 1-37A software. Note that initial service life for the baseline rigid pavement is 25.5 years after rounding of the faulting limit.
Figure 6-2. Predicted faulting performance for NCHRP 1-37A baseline rigid pavement design.
Input Parameter | Design Value | Notes |
---|---|---|
General Information | ||
Initial service life | 25 years | 1 |
Pavement construction month | September | 2 |
Traffic open month | October | 2 |
Site/Project Identification | ||
Functional class | Principal Arterials - Others | |
Analysis Parameters | ||
Initial IRI | 63 in./mi | 2 |
Terminal IRI | 172 in./mi | 2 |
Transverse cracking (% slabs cracked) | 15% | 2 |
Mean joint faulting | 0.12 in | See text |
Reliability | 90% | |
Traffic | ||
Initial two-way AADTT | 2000 | |
Number of lanes in design direction | 2 | |
Percent of trucks in design direction | 50% | 2 |
Percent of trucks in design lane | 95% | 2 |
Operational speed | 55 mph | 2 |
Monthly adjustment | 1.0 throughout | 2 |
Vehicle class distribution | Level 3 defaults | Table 6-5 |
Hourly distribution | Level 3 defaults | Table 6-6 |
Traffic growth factor | 0% | |
Axle load distribution factors | Level 3 defaults | Table 6-7 |
Mean wheel location from edge | 18 in | 2 |
Traffic wander standard deviation | 10 in | 2 |
Design lane width | 12 ft | 2 |
Number of axles per truck | Level 3 defaults | Table 6-8 |
Average axle outside width | 8.5 ft | 2 |
Dual tire spacing | 12 in | 2 |
Tire pressure | 120 psi | 2 |
Tandem axle spacing | 51.6 in | 2 |
Tridem axle spacing | 49.2 in | 2 |
Quad axle spacing | 49.2 | 2 |
Wheelbase spacing | Level 3 defaults | Table 6-10 |
Climate | ||
Latitude | 38.98° | |
Longitude | -76.94° | |
Elevation | 48 ft | |
Depth of water table | 20 ft | |
College Park, MD climate data | Generated | 3 |
Design Features | ||
Permanent curl/warp effective temperature difference | -10°F | 2 |
Joint spacing | 15 ft | 2 |
Dowel bar diameter | 1 in | 2 |
Dowel bar spacing | 12 in | 2 |
Edge support | Widened slab | |
Slab width | 14 ft | |
Bond at PCC-base interface | Unbonded | |
Base erodibility index | 4 (Fairly Erodable) | |
Drainage and Surface Properties | ||
Surface shortwave absorptivity | 0.85 | 2 |
Infiltration | Minor (10%) | 2 |
Drainage path length | 12 ft | 2 |
Pavement cross slope | 2% | 2 |
PCC Surface Layer | ||
Unit weight | 150 pcf | 2 |
Poisson's ratio | 0.2 | 2 |
Coefficient of thermal expansion | 5.5x10-6/°F | 2 |
Thermal conductivity | 1.25 BTU/hr-ft-°F | 2 |
Heat capacity | 0.28 BTU/lb-°F | 2 |
Cement type Type | 1 | 2 |
Cement content | 600 lb/yd3 | 2 |
Water/cement ratio | 0.42 | 2 |
Aggregate type | Limestone | 2 |
PCC zero-stress temperature | 120 °F | 4 |
Ultimate shrinkage at 40% relative humidity | 632 με | 4 |
Reversible shrinkage | 50% | 2 |
Time to develop 50% of ultimate shrinkage | 35 days | 2 |
28-day PCC modulus of rupture | 690 psi | 2 |
28-day PCC elastic modulus | 4.4x106 psi | 4 |
Granular Base Layer | ||
Unbound material type | AASHTO A-1-a | |
Analysis type | ICM Inputs | |
Poisson's ratio | 0.35 | 2 |
Coefficient of lateral pressure K0 | 0.5 | 2 |
Modulus | 40,000 psi | 2,5 |
Plasticity index | 1% | |
% passing #200 sieve | 3 | |
% passing #4 sieve | 20 | |
D60 | 8 mm | |
Compaction state | Compacted | 2 |
Maximum dry unit weight | 122.2 pcf | 2 |
Specific gravity of solids | 2.66 | 2 |
Saturated hydraulic conductivity | 263 ft/hr | 2 |
Optimum gravimetric water content | 11.1% | 2 |
Calculated degree of saturation | 82% | 2 |
SWCC parameter af | 11.1 psi | 2 |
SWCC parameter bf | 1.83 | 2 |
SWCC parameter cf | 0.51 | 2 |
SWCC parameter hr | 361 psi | 2 |
Compacted Subgrade (top 6 inches) | ||
Unbound material type | AASHTO A-7-5 | |
Analysis type | ICM Inputs | |
Poisson's ratio | 0.35 | 2 |
Coefficient of lateral pressure K0 | 0.5 | 2 |
Modulus | 12,000 psi | 2,5 |
Plasticity index | 30% | 2 |
% passing #200 sieve | 85 | 2 |
% passing #4 sieve | 99 | 2 |
D60 | 0.01 mm | 2 |
Compaction state | Compacted | |
Maximum dry unit weight | 97.1 pcf | 2 |
Specific gravity of solids | 2.75 | 2 |
Saturated hydraulic conductivity | 3.25x10-5 ft/hr | 2 |
Optimum gravimetric water content | 24.8% | 2 |
Calculated degree of saturation | 88.9% | 2 |
SWCC parameter af | 301 psi | 2 |
SWCC parameter bf | 0.995 | 2 |
SWCC parameter cf | 0.732 | 2 |
SWCC parameter hr | 1.57x104 psi | 2 |
Natural Subgrade (beneath top 6 inches) | ||
Unbound material type | AASHTO A-7-5 | |
Compaction state | Uncompacted | |
Maximum dry unit weight | 87.4 pcf | 2 |
(Other properties same as for compacted subgrade) |
Notes:
- Typical initial service life for rigid pavement design.
- Level 3 default/calculated/derived value from NCHRP 1-37A software.
- Based on interpolated climate histories at IAD, DCA, and BWI airports.
- Level 3 default/calculated/derived values from NCHRP 1-37A software for baseline PCC mixture properties.
- Default input value at optimum moisture and density conditions before adjustment for seasonal effects (adjustment performed internally within the NCHRP 1-37A software).
Short | Medium | Long | |
---|---|---|---|
Average Axle Spacing (ft) | 12 | 15 | 18 |
Percent of trucks | 33% | 33% | 34% |
6.2.3 Summary
Baseline flexible and rigid pavement designs were developed using the 1993 AASHTO Design Guide and the input parameters in Table 6-2 and Table 6-3. The final designs are 5.3 inches of AC over 12.7 inches of GAB (5.5"/13" after rounding) and 10.4 inches of PCC over 6 inches of GAB (10.5"/6" after rounding), respectively. Initial construction costs for these designs, based on the unit cost data in Table 6-1, are $180,000 and $342,000 per line-mile, respectively ($185,000 and $345,000 per lane-mile using rounded layer thicknesses).
These baseline designs were then analyzed using the NCHRP 1-37A procedures and the input parameters in Table 6-4 and Table 6-9 to determine the corresponding distress levels at the end of initial service life. Permanent deformations are the controlling distress type for the flexible pavement scenario; the predicted total rutting (after adjustment for reliability) for the baseline flexible pavement section is 0.65 inches, as compared to the 0.75-inch default value in the NCHRP 1-37A software. Joint faulting is the controlling distress type for the rigid pavement scenario; the predicted joint faulting (after adjustment for reliability) for the baseline rigid pavement design is 0.12 inches, identical to the default value in the NCHRP 1-37A software. Note that the baseline design scenarios are not greatly different from the pavement conditions at the AASHO Road Test (except perhaps for climate), and therefore the 1993 AASHTO designs should be in general agreement with those from the more sophisticated NCHRP 1-37A methodology. Discrepancies between the design procedures should become more pronounced as conditions increasingly deviate from the AASHO Road Test conditions.
6.3 Soft Subgrade
The design scenario for this case is identical to the baseline conditions in Section 6.2, except for a much softer and weaker subgrade. The subgrade is now postulated to be a very soft high plasticity clay (AASHTO A-7-6, USCS CH) with MR = 6000 psi at optimum moisture and density before adjustment for seasonal effects. Note that this MR value is even lower than the NCHRP 1-37A default values for an A-7-6/CH material in order to accentuate the effects of low subgrade stiffness. The groundwater depth is left unchanged from the baseline scenario in order to focus on the subgrade stiffness effect. Intuitively, more substantial pavement sections are expected for this scenario as compared to the baseline conditions to achieve the same level of pavement performance. This can be achieved by increasing the thicknesses of the AC/PCC/granular base layers, increasing the quality of the AC/PCC/granular base materials, stabilizing the granular base layer, treating the soft subgrade soil, or some combination of these design modifications. In order to keep the comparisons among scenarios simple, only increases in AC or PCC thickness will be considered here.
6.3.1 1993 AASHTO Design
Flexible Pavement
The only modification to the 1993 AASHTO flexible pavement baseline inputs (Table 6-2) required to simulate the soft subgrade condition is a reduction of the seasonally-adjusted subgrade resilient modulus MR from 7,500 to 3,800 psi3. The W18 traffic capacity for baseline flexible pavement section under the soft subgrade conditions is only 1.25x106 ESALs, corresponding to an 80% decrease in initial service life.
The required pavement structure for the soft subgrade condition is determined from the 1993 AASHTO design procedure, as follows (assuming changes only in the AC layer thickness):
- Required overall structural number SN = 5.76 (compared to 4.61 for the baseline conditions)
- Structural number provided by granular base (same thickness D2 as in baseline design) SN2 = m2a2D2 = (1.0)(0.18)(12.7) = 2.28
- Required asphalt structural number SN1 = SN - SN2 = 5.76 - 2.28 = 3.48
- Required asphalt layer thickness D1 = SN1 / a1 = 7.9 inches
The design for the soft subgrade condition is thus 7.9 inches of AC over 12.7 inches of GAB (before rounding). This represents a 50% increase in AC thickness as compared to the 5.3 inches in the baseline design, which, in turn, translates to a 20% initial construction cost increase of about $37K per lane-mile (using the typical unit cost data in Table 6-1).
Rigid Pavement
The only modification to the 1993 AASHTO rigid pavement baseline inputs (Table 6-3) required to simulate the soft subgrade condition is a reduction of the seasonally-adjusted subgrade resilient modulus MR from 7,500 to 3,800 psi. The W18 traffic capacity for the baseline rigid pavement section under the soft subgrade conditions is reduced to 15.5x106 ESALs, corresponding to a 6% decrease in initial service life.
The reduction in foundation stiffness in this scenario has a direct effect on the design modulus of subgrade reaction keff, which decreases from its original value of 38 pci for baseline conditions to a value of 27 pci for the soft subgrade case. However, the required slab thickness is relatively insensitive to this reduction in foundation stiffness, increasing only 0.1 inches for a final design of 10.5 inches of PCC over 6 inches of GAB for the soft subgrade condition. Note that after rounding to the nearest half-inch, this design is identical to the rigid pavement design for the baseline conditions.
A common constructability concern under these soft in-situ soil conditions is the requirement for a stable working platform. The 6-inch granular subbase is unlikely to provide adequate stability. Consequently, a realistic final design would require either a thicker granular subbase, a separate granular working platform (not included in the structural design calculations), and/or subgrade improvement (see Chapter 7) for constructability.
6.3.2 NCHRP 1-37A Design
Flexible Pavement
Changing the subgrade soil type to A-7-6 changes many of the other Level 3 default inputs for the subgrade in the NCHRP 1-37A design methodology. The altered input parameters for the soft subgrade condition are summarized in Table 6-11. Figure 6-3 summarizes the predicted rutting vs. time for the baseline flexible pavement section (5.3" AC over 12.7" granular base); the time to the 0.65 inch total rutting design limit is only 93 months (7.75 years), corresponding to a 48% decrease in initial service life due to the soft subgrade conditions.
The trial designs (assuming only increases in AC thickness) and their corresponding predicted performance at end of the initial service life are listed in Table 6-12. Rutting is again the critical distress mode controlling the design in all cases; the design limit of 0.65 inches for total rutting is based on the performance of the baseline pavement section, as described previously in Section 6.2.2. Interpolating among the results in Table 6-12, the final flexible pavement design section for the soft subgrade conditions consists of 7.9 inches of AC over 12.7 inches of GAB. This design section is identical to that obtained from the 1993 AASHTO Design Guide for this scenario.
Input Parameter | Design Value | Notes |
---|---|---|
Compacted Subgrade (top 6 inches) | ||
Unbound material type | AASHTO A-7-6 | |
Analysis type | ICM Inputs | |
Poisson's ratio | 0.35 | 1 |
Coefficient of lateral pressure K0 | 0.5 | 1 |
Modulus | 6,000 psi | 2,3 |
Plasticity index | 40% | 1 |
% passing #200 sieve | 90 | 1 |
% passing #4 sieve | 99 | 1 |
D60 | 0.01 mm | 1 |
Compaction state | Compacted | |
Maximum dry unit weight | 91.3 | 1 |
Specific gravity of solids | 2.77 | 1 |
Saturated hydraulic conductivity | 3.25x10-5 ft/hr | 1 |
Optimum gravimetric water content | 28.8% | 1 |
Calculated degree of saturation | 89.4% | 1 |
SWCC parameter af | 750 psi | 1 |
SWCC parameter bf | 0.911 | 1 |
SWCC parameter cf | 0.772 | 1 |
SWCC parameter hr | 4.75x104 psi | 1 |
Natural Subgrade (beneath top 6 inches) | ||
Unbound material type | AASHTO A-7-6 | |
Compaction state | Uncompacted | |
Maximum dry unit weight | 82.2 pcf | 2 |
(Other properties same as for compacted subgrade) |
Notes:
- Level 3 default/calculated/derived value from NCHRP 1-37A software.
- Set artificially low to simulate a soft subgrade condition.
- Input value at optimum moisture and density conditions before adjustment for seasonal effects (adjustment performed internally within the NCHRP 1-37A software).
Figure 6-3. Predicted rutting performance for soft subgrade scenario.
AC Thickness (in.) | Base Thickness (in.) | Total Rutting (in.) |
---|---|---|
5.3 | 12.7 | 0.765 |
6.0 | 12.7 | 0.730 |
8.0 | 12.7 | 0.643 |
10.0 | 12.7 | 0.572 |
Design Limit: | 0.65 |
Rigid Pavement
Changing the subgrade soil type to A-7-6 changes many of the other Level 3 default inputs for the subgrade in the NCHRP 1-37A design methodology. The altered input parameters for the rigid pavement soft subgrade condition are the same as those summarized earlier in Table 6-11 for the corresponding flexible design condition. Figure 6-4 summarizes the predicted faulting vs. time for the baseline rigid pavement section (10.4" PCC over 6.0" granular base); the time to the 0.12 inch faulting design limit is only 22.2 years, corresponding to a 13% decrease in initial service life due to the soft subgrade conditions.
The trial designs (assuming only increases in PCC slab thickness) and their corresponding predicted performance at end of design life are listed in Table 6-13. Faulting is again the critical distress mode controlling the design in all cases; the design limit of 0.12 inches for faulting is based on the performance of the baseline pavement section, as described previously in Section 6.2.2. Interpolating among the results in Table 6-13, the final rigid pavement design section for the soft subgrade conditions consists of a 10.9 inch PCC slab over 6.0 inches of GAB. This slab thickness is 0.5 inch (5%) greater than that obtained from the 1993 AASHTO Design Guide for this scenario; this corresponds to an initial construction cost increase of $14K (8%) per lane-mile.
Again, the subgrade soil is so soft and weak in this scenario that some additional design features may be required to provide a stable working platform during construction. The 6 inch granular subbase is unlikely by itself to provide an adequate working platform.
Figure 6-4. Predicted faulting performance for soft subgrade scenario.
PCC Thickness (in.) | Base Thickness (in.) | Faulting (in.) |
---|---|---|
10.4 | 6 | 0.131 |
10.7 | 6 | 0.125 |
11.0 | 6 | 0.118 |
Design Limit: | 0.12 |
Notes
- The 1993 AASHTO Design Guide recommends rounding the asphalt layer thickness to the nearest half inch and unbound layer thicknesses to the nearest inch. However, all layer thicknesses are rounded to the nearest 0.1 inch in this chapter to make the comparisons between the various scenarios more meaningful. Return to Text
- The 1993 AASHTO Design Guide recommends rounding the slab thickness to the nearest inch (nearest half inch if controlled grade slip form pavers are used). However, all slab thicknesses are rounded to the nearest 0.1 inch in this chapter to make the comparisons between the various scenarios more meaningful. Return to Text
- Based on the results from the NCHRP 1-37A analyses for these conditions. Return to Text
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