Pavement Concepts > Erosion and Pumping

Surface erosion of the subbase is an important mechanism by which fines are redistributed under a concrete slab thus leading to voids below the slab. In extreme cases when fines are pumped to the surface through cracks or unsealed joints, this erosion is called pumping. Little research has been done to mechanistically predict the size of the void that is created below the slab but all researchers agree that for pumping to take place, three factors need to be present:

(i) heavy load repetitions and the deflection of the concrete layer,

(ii) saturation or near-saturation of the layer below the slab, and

(iii) material susceptible to pumping.

Erosion resistance of materials and its shear resistance against erosion can be increased by:

•increasing the average particle size of material where erosion is to be expected,

•using angular particles (crushed) instead of rounded,

•increasing the indirect tensile strength (ITS) of the layer, and

•ensuring a well compacted as opposed to a lightly compacted layer.

The table below may guide the user to a selection of the subbase type:

Subbase Type	Stiffness E (mPa)	Erosion Factor
New asphalt: No stripping	2500 - 8000	0 - 1
New asphalt: medium strip	1200 - 2500	1 - 3
New asphalt: high strip	400 - 1000	3 - 6
Old asphalt: no strip	2000 - 7000	0 - 1
Old asphalt: medium strip	800 - 1200	1 - 3
Old asphalt: high strip	250 - 600	5 - 7
Strong concrete	30000 - 50000	0
Medium strength concrete	15000 - 30000	0
Weak concrete	8000 - 15000	0 - 1
Newly cemented G1 & G2: C1	2500 - 9000	0 - 2
Old cracked C1	200 - 1000	1 - 3
Newly cemented G3: C2	2000 - 4500	0 - 2
Old cracked C2	200 - 800	1 - 3
Newly cemented G4 & G5: C3	1000 - 2500	0 - 3
Cracked cemented G4 & G5: C3	120 - 500	2 - 4
Newly cemented G6 & G7: C4	1000 - 2500	1 - 3
Old cracked & weathered C4	80 - 400	2 - 5
Best quality crushed stone: G1	300 - 1000	2 - 4
High quality crushed stone: G2	150 - 800	2 - 4
Base quality gravel: G3	100 - 700	3 - 5
High quality subbase gravel: G4	80 - 500	3 - 5
Subbase quality gravel: G5	70 - 300	4 - 6
Selected quality gravel: G6	60 - 200	5 - 7
Good quality gravel: G7	50 - 180	5 - 8
Natural gravel: G8	40 - 140	6 - 9
Sandy soil: G9	30 - 190	7 - 10
Claye soil: G10	15 - 50	10

Subgrade type	Voids extra
Highly active clay below < 1.5m fill	0.5 – 1.5
Untreated collapsing sand below < 1.5m fill	0.3 – 0.5
Medium active clay below clayey fill	0.1 – 0.3

The factor can also be calculated using the following equation (Ref.13):

Erosion factor = 15(PI+1)0.15 / GM1.54 / (ITS+1)2

where

PI plasticity index

GM grading modulus

ITS indirect tensile strength, kPa.

Note: The value of the erosion factor should exceed 10.