Pavement Concepts

The vertical dimension of a void is not important. Yet, prediction of the length of voids under a slab is very difficult. Factors such as subgrade movements and settlements make predictions even more difficult. Whenever existing pavements are being overlayed, these movements can be assumed to be almost zero. However, where clayey areas or collapsible sands are being traversed by new pavements, enlarged voids can be expected. The extent of this enlargement is taken into consideration by introducing the input variable called Voids Extra. A guide to the magnitude of the practical values to be used as input is shown in the right-hand column of the table below.

The following may be used as a guideline for void length under a pavement. The suggested values take curling and pumping into account.

No	Subbase type and subgrade types	Erosion factor	Voids extra
1	Cemented crushed stone subbase	0 - 2
2	Highly erodible gravel subbase (more than 15 % passing the 0.075 mm sieve, PI > 6, liquid limit > 25	6 - 8
3	Low eroding good draining gravel (less than 20 % passing the 0.6 mm sieve, less than 8 % passing the 0.075 mm sieve)	2 - 4
4	Newly laid continuously graded hot mix asphalt with low stripping potential (60/70 pen bitumen)	0 - 1
5	Old and stiff hot mix asphalt	1 - 3
6	Highly active clay subgrades with <1.5 m low activity fills		1
7	Untreated collapsing sand subgrades with low fills		0.4
8	Medium activity clay subgrades and clayey fills.		0.1