Transcript Calculating SNP

Establishing Pavement
Strength
HTC
Inf rastructure Management
Specialists
HTC INFRASTRUCTURE MANAGEMENT LTD
Contents

Definitions

Purpose

Methods

Output
SNP - Background




‘Structural Number of the Pavement’
SNC, modified structural number, a measure
of a pavement’s structural capacity
Based on the AASHTO’s structural number
Pavement layers are assigned a structural
layer coefficient which represents the layer’s
contribution to pavement performance
Equations:
n
SN  0.0394 ai hi
i 1
n
SNC  0.0394 ai hi  SNSG
i 1
Background . . .

SNP, adjusted structural number, used in
HDM-4 to characterise pavement strength

Applies a weighting factor, which reduces
with increasing depth

Pavement strength for deep pavements not
over predicted, a concern with SNC
Why SNP?

A good overall measure of pavement
strength for pavement performance
predictions

Required by HDM Models

Unfortunately, not available in RAMM
database
Requirements

Must have SNP for each section
– either based on existing RAMM data, or,
– from other survey/testing data available

Must be able to import this information into
the dTIMS dt2699.dbf file for each section
Solution

Using the strength program in conjunction
with the interface program
Other
Data
RAMM
Database
Strength
Program
Interface
Program
dT2699
What does the Strength Program do?

Not a lot … except:
Allows the user to calculate a structural
number for a particular section by a variety
of methods
Currently Available Methods






FWD with Layer Thickness Method
FWD without Layer Thickness Method
CBR Layer Method
Benkleman Beam Method
ARRB Method
Typical Pavement Design Method
Falling Weight Deflectometer (FWD)
Method
SNPS  SNS  s  SNSGS
SNSGS  3.51log10 CBR  0.85log10 CBR  1.43
2
FWD with Layer Thickness Method

Used when pavement thickness data are
available
known
deflection
100 AM
150 BASE
known thicknesses
150 SUB-BASE
FWD with Layer Thickness Method

Rhode’s method:
a
SNS  a0SIPS 1HPa2
SIPS  a3 (D0  D1.5HP )

Subgrade contribution:
 Esg 

CBRS  

 41.19 
2.6
a
Esg  10a4 SISS 5 HPa6
SISS  a7 D1.5HP  D1.5HP  450 
Source: HDM-4 Study
FWD only Method

Used when pavement thickness data are not
available
known
deflection
AM
BASE
?
SUB-BASE
FWD only Method

Roberts’ formula:
SN  12.992  4.167  log10 D0  1000  0.936  log10 D900  1000

Jameson’s formula:
SN  1.69 

0.8428
0.04294

D 0  D1500
D 900
CBR Contribution:
CBRS  103.264 1.018 log 10 D900 1000 
CBR Layer Method

Based on the thickness of each layer,
strength coefficients and subgrade CBR
SNPS  SNBASUS  SNSUBAS  SNSUBGS
n
a h
SNBASUS  0.0394
is i
i1
CBR Layer Method
SBSUBGS  b 0  b1 exp b 2 z m exp b 3 z m  
3.51log
10 CBRS  0.85log10 CBRS   1.43
2

zz
 b o exp b 3 z  b1 exp b 2  b 3 z   j
SNSUBAS  0.0394 a js 





b
b

b
3
2
3

 z  z j 1
j1
m

Source: Parkman and Rolt (1997)
Benkleman Beam Method

Granular base pavements:
SNPS  3.2DEFS0.63

Cemented base pavements:
SNPS  2.2DEFs0.63
Source: Paterson (1987)
ARRB Method

An approximate method

Estimation based on traffic volume

Assumes pavement is correctly designed for
traffic loading based on design period of 20
years
ARRB Method
SNP  ISNP 1  0.01 n
ISPN  0.6951CUM_ ESAL0.0993
CUM_ ESAL  ESA _ init  365  GF


1  r   1
GF 
m
r
ntype
ESA _ init  AADT _ init 
 P  F 
i
i
i1
AADT _ init  AADT
1
1  r n1
Source: Roberts and Roper (1998)
Typical Pavement Design Method



Useful when no data is available to utilise
the previous methods
Consists of a series of typical pavement
configurations and subgrade strengths
Select the most appropriate pavement
configuration
– dependant on thickness of layers

Select subgrade strength
– low, medium or high
Example ...
Pavement
Subgrade Strength
Type
Low
Medium
High
1
1.3
1.7
2.1
2
1.8
2.2
2.6
3
2.3
2.7
3.1
4
2.8
3.2
3.6
5
3.3
3.7
4.1
Strength Program Data Sources

RAMM database

Other sources - Excel spreadsheets
Importing RAMM Data

Export required data from RAMM for
Windows as text files then save as Excel
files
– it is important that the fields are in the correct
order and that no fields are missing

Import Excel files into Strength program
through File - Data Import/Append
Importing Other Data - Excel
Spreadsheets

These must be in the format shown in the
Strength Discussion Document
– it is important that the fields are in the correct
order and that no fields are missing (will be
improved in later releases)

They are imported into the Strength program
the same way as data originating from the
RAMM database
Hierarchy of Methods

Pre-defined by the user prior to calculating
SNP values

Determines which SNP value is taken when
more than one is calculated
Output File / Interface Program

Output can be exported as an Excel file

SNP values are imported into the dt2699 file
by the Interface program to be used by the
HDM models in dTIMS