Site testing

Question 1

What is a deflectograph survey and what does it measure?

What implications do measurements have?

Answer 1

A vehicle with loaded rear wheels and a falling weight measures deflection at intervals (~4m) along the road.

The deflections are directly related to the strength of the pavement and sub-grade layers.

Question 2

How do the deflection readings generate residual life?

Answer 2

PANDeFF software uses the deflection values alongside traffic figures (msa) and the existing pavement structure to calculate how much traffic is expected to use the pavement over it’s design life, and therefore how much design life is remaining in terms of traffic (msa)

Question 3

Why can deflection readings be negative?

Answer 3

When deflection measurements within the processed data exceed the ‘critical deflection’ i.e. the total amount of traffic expected to use the road through it’s design life, the calculated residual life becomes negative, as it is beyond it’s design life.

Question 4

Why does a figure of 400 microns indicate zero years residual life?

Is this figure the same for all roads?

Answer 4

Using interpolation, it can be found that at 400 microns deflection in the processed deflectograph file, the residual life equates to zero years.

This figure is dependant on the PANDeFF software calculations and relates to the pavement structure and traffic loading.

Question 5

How does the drop weight correlate to CBR?

Answer 5

The drop weight outputs in MPa (surface subgrade modulus). The software has a non-linear conversion to CBR where 35MPa is approximately 30% CBR.

Question 6

Why do LCC have lower foundation thicknesses when compared to national standards?

Answer 6

LCC have tested sites historically and refined foundation designs based on this data. There is also confidence in the Lincolnshire limestone Type-1 often used, leading to a slight reduction in foundation thickness when compared to DMRB standards.

Question 7

What document do you use for pavement design?

What is the DMRB equivalent?

Answer 7

We use the Lincolnshire Maintenance design manual. This is in place of CD 226.

Question 8

What document do you use for foundation design?

What is the DMRB equivalent?

Answer 8

We use the Lincolnshire Maintenance design manual. This is in place of CD 225.

Question 8

How does the MDM relate to CD 226?

Answer 8

the MDM assigns NRSWA categories to roads and a pre-defined pavement thickness per NRSWA category. These ranges are equivalent to the Nomograph in CD 226 for flexible pavement design.

Question 9

How does the MDM relate to CD 225?

Answer 9

The foundation requirements are reduced due to historical testing and performance of Lincolnshire limestone foundations in the county.

Question 10

How are schemes identified and prioritised for delivery?

(Scoring, %’s, composition)

Answer 10

Highway sections scored, 60% on testing (CVI, Scanner, Deflectograph), and 40% on reactive jobs, enquiries and insurance games.

Highest scoring areas of the network are put forward in the next tranche of scheme development.

Question 11

What does PRIMA stand for?

What is PRIMA equipment and what does it do?

Answer 11

Portable road infrastructure measurement apparatus.

It is a lightweight falling weight deflectometer (LFWD) used to assess stiffness of the footway.