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Publication Number: FHWA-RD-01-113
Date: October 2002

 

Back-Calculation of Layer Parameters for LTPP Test Sections, Volume II

Figure 1. Flow chart showing the major steps and decisions used in the linear elastic back-calculation process

Step 1. Data Extraction.
Step 2. Classify Test Section.

Average deflections for specific drop height at each test point location.

Load-Response Classification.

Deflection Basin Classification.

Section Uniformity Classification.

Establish Analysis Sections from Deflection Data.

Step 3. Determine Inputs to MODCOMP.

Decide on number of subgrade layers.

Determine cross-section (layer type and thickness) -- combination of similar materials.

Determine Poisson's ratio for each layer.

Determine depth to apparent rigid layer.

Select initial Layer-Sensor Assignment.

Step 4. Trial Computations.

Select random test points for trial solutions.

Execute MODSHELL for specific test points and deflection basin data.

Question 1: Are there insensitive layers?
If Yes, assume E-value for insensitive layer and revise layer-sensor assignment. Then select initial layer-sensor assignment and redo Step 4, trial computations.
If No, go to Question 2.

Question 2: Are the other results reasonable (RMS error and E-values)?
If Yes, go to Step 5.
If No, Revise one or more inputs (combine different layers, apparent rigid layer, etc.). Then go to the step where you are asked to Determine cross-section (layer type and thickness -- combination of similar materials).

Step 5. Mass Back-Calculation of Deflection Basin Data.

Question 3: Are most of the results reasonable?
If Yes, go to Step 6.
If No, Revise one or more inputs (combine different layers, apparent rigid layer, etc.). Then go to the step where you are asked to Determine cross-section (layer type and thickness -- combination of similar materials).

Step 6. Store results in summary table.

 


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