DHARM
700 GEOTECHNICAL ENGINEERING
Table 17.1 Suggested gradings for mechanically stabilised base
and surface courses (adapted from HMSO, 1952)
Per cent passing Base or surface course
I.S. Sieve size Base course Surface course Max. size
Max. size Max. size
80 mm 40 mm 20 mm 20 mm 10 mm 5 mm
80 mm 100 — — — — —
40 mm 80–100 100 — — — —
20 mm 60–80 80–100 100 100 — —
10 mm 45–65 55–80 80–100 80–100 100 —
5 mm 30–50 40–60 50–75 60–85 80–100 100
2.36 mm — 30–50 35–60 45–70 50–80 80–100
1.18 mm — — — 35–60 40–65 50–80
600 micron 10–30 15–30 15–35 — — 30–60
300 micron — — — 20–40 20–40 20–45
75 micron 5–15 5–15 5–15 10–25 10–25 10–25
Note: 1. Not less than 10% should be retained between each pair of successive sieves specified,
excepting the largest pair.
- Material passing I.S. Sieve No. 36 shall have the following properties:
For base courses: For surface courses:
Liquid limit |> 25% Liquid limit |> 35%
Plasticity Index |> 6% Plasticity Index : between 4 and 9.
Gravel is used for base courses of pavements and for filter courses. The presence of fines
to an extent more than the optimum might make the gravel unsatisfactory.The limit for the
fines may be 3 to 7%, depending upon its intended use. The upper limit is for filter purposes.
An obvious treatment, for a gravel with larger amount of fines than desired, is to wash
out excess fines. This may sound very easy, but it is not that simple in practice. When supplies
of dirty gravel along with a satisfactory source of water are available locally, the procedure of
removal of fines from gravel by washing is employed.
Mehra’s method of stabilisation
The procedure advocated by S.R. Mehra (IRC, 1976) for mechanical stabilisation has been
widely used in the construction of base and surface courses for low-cost roads. The method
involves the use of just three sieves for the mechanical analysis (instead of ten as advocated by
ASTM and HMSO) and only the plasticity index. The three sieves are equivalents of I.S. Sieves
of sizes 1.18 mm, 300–μ and 75–μ.
Mehra recommends a compacted thickness of 76 mm for the base course with a mini-
mum of 50% sand (fraction between 300–μ and 75–μ), and a plasticity index of 5 to 7. The
