The following chart is used to determine the road structure for roads in Malta. It has been prepared by Profs. Dr. Ing. Klaus Muller in the year 2000 and forms part of the “Directive for the Standardization of Pavements for Traffic areas”. It is based on a similar chart used by road engineers in Germany. The chart is not a direct copy of the German chart but has been adapted accordingly to match local conditions. This post shows how weak soils can be improved through the use of geocells and cement stabilisation techniques.
I have already described in a previous post how this chart is used to design the road structure of a road.
Basis of road structure design
The structural design of a road is based on two important points:
- expected traffic flow (including projected traffic increase in the foreseeable future)
- available road structure thickness. Presence of existing services may limit the depth of the allowable road structure.
- sub grade bearing capacity (i.e. the strength of the ground on which the road will be built). When a road is built on a strong subgrade, it will last longer.
What does a strong sub grade mean?
The chart states that depending on the road classification, the bearing capacity of the sub grade shall be between 100 to 120N/sq.mm (minimum). For example Pavement construction 1 requires that the bearing capacity of the sub grade is minimum 120N per sq.mm. Pavement construction 1 is the thinnest of all sections for all construction classes. Hence we must ensure that the materials we have in the limited space for the road structure are laid on a very strong subgrade.
How is the bearing capacity of the sub grade measured?
In order to determine if the sub grade of a road is adequate or not a series of plate bearing tests are carried out on the subgrade. The test consist in the loading of a steel plate and reading the amount of settlement of the subgrade. Then the load on the steel plate is removed and re-applied once again. Readings of the deformation of the sub grade is noted during these stages. The palate bearing test produces the following readings Ev1 (bearing capacity during first loading), Ev2 (bearing capacity during second loading), and from these two values one can determine the ratio of Ev2 / Ev1. This ratio should always be less than 2.2 whilst Ev2 should always be greater than 120N per sq.mm. One must note that the 120 N per sq.mm is the bare minimum. If Ev2 / Ev1 is greater than 2.2 then it means that the material is not yet fully compacted. The sub grade is not suitable for road construction yet even though Ev2 is greater than 120N per sq.mm. The sub grade would thus need to be compacted further until Ev2/Ev1 is less than 2.2 whilst Ev2 is greater than 120 N per sq.m..
What happens if the sub grade is too weak to achieve the required sub grade strength?
There may be instances where there sub grade material is not adequate to reach the 120 N per sq.mm whilst EV2/Ev1 is less than 2.2. This may occur when clay soils are encountered. Clay cannot reach Ev2 values greater than 120 and ratios of Ev2 / Ev1 less than 2.2. Nonetheless roads can still be constructed in such poor geological conditions but not before stabilising the existing terrain.
How can the existing poor sub grade be improved?
Poor weak sub grades (with Ev2 less than 45 N per sq.mm.) can be improved by stabilising it through the use of :
- cement or hydraulic lime stabilisation materials
- using geo cells.
What is cement stabilisation?
Cement stabilisation is a dry mix consisting of aggregates and cement usually laid using a paver or a grader. This is a 150mm thick layer, laid just above the subgrade as per pavement construction 5 in the chart above. The mix should achieve a compressive strength between 7 to 12 N per sq.mm. The cement stabilisation layer should be left to cure and expansion joints should be cut every 3m up to c. 50mm deep. A layer of compacted type 1 aggregate is normally laid on top of the cement stabilisation to serve as a cushion between the rigid cement stabilisation and the flexible asphalt pavement. This type 1 layer serves to prevent the formation of cracks in the asphalt layers.
Stabilising roads made from compacted earth
What are geocells?
I have only used geocells for slope stabilisation purposes to stabilise steep slopes for landscaping purposes as per photos below.
I have never used geocells for road stabilisation purposes, but the principle is the same as geocells used for stabilising slopes.
A 3d honeycombed geosynthetic fabric referred to as geocell is used to stabilise weak soils. The geocell is laid over the existing subgrade and aggregates are poured into the cells using an excavator or grader similar to how the soil is being poured into the geocells for the slope stabilisation.
If you are interested, you may wish to refer to the following books: