1. A field investigation of subgrade soils involves classifying soils, determining engineering properties through in-situ and laboratory tests, and documenting findings. In-situ tests identify layers, groundwater, and obtain undisturbed samples for lab tests. 2. Laboratory tests include moisture content, grain size, compaction, CBR, and strength tests. The compaction test produces a moisture-density curve to evaluate compaction in the field. CBR testing evaluates the soil's strength for pavement design. 3. Investigation results are used to classify soils according to systems like USCS and AASHTO for understanding suitability and designing pavement structures based on subgrade strength.

1. 1.SUBGRADE SOILS
General
1.1 Soil Classification
1.2 Field Investigation
1.2.1 Depth of Investigation
1.2.2 In-place Tests
1.2.3 Groundwater table
1.2.4 Field Log
1.3 Laboratory Tests

2. 1. SUBGRADE SOILS
Integral part of the pavement structure
Desirable properties
1. Stability
2. Incompressibility
3. Resistant to property change in adverse climatic
conditions
4. Good drainage
5. Ease of compaction

3. 1.1 SOIL CLASSIFICATION
Objective
 Determining suitability for use
 Creating common understanding among
professionals
A. Unified Soil Classification System (USCS)
 Classifies granular soils on grain size
characteristics
 Classifies fine-grained soils on plasticity
properties

7.  Divides soils into three general areas :USCS.pptx
1. Coarse-grained soils (G,S)
2. Fine-grained soils (M,C)
3. Peaty soils (pt)
 Uses gradation coefficients( CU, Cg) to
describe coarse grained soils.
 Subdivides fine grained soils using the
plasticity chart, with PI given as :
PI=0.73(LL-20)

8. B. AASHTO System
 Determines the relative quality of soils for use in
embankments, sub grades, sub base and base
course.
 Uses grain size distribution and Atterberg limits
for classification
 Evaluates soils within each group using the
group index formula as:
GI=(F-35)[0.2+0.05(LL-40)] +0.01(F-15)(PI-10)

9. General Classification Granular materials (35 % or less passing No. 200) Silt-Clay materials (more than 35% passing No
200)
Group Classification A-1
A-3
A-2
A-4 A-5 A-6
A-7
A-1-a A-1-b A-2-4 A-2-5 A-2-6 A-2 -7 A-7-5 A-7-6
Sieve analysis, per
cent passing
No 10 (2 mm) 50 max - - - - - - - - -
No 40 (0.425 mm) 30 max 50 max 51min - - - - - - -
No 200 (0.075 mm)
15 max 25 max 10 max 35 max 35 max 35 max 35 max 36 min 36 min
36
min
36 min
Characteristics of
fraction passing No
40
LL
-
- 40 max 41 min 40 max 41 min 40 max 41 min
40
max
41 min
PI
6 max NP 10 max 10 max 11 min 11 min 10 max 10 max
11
min
11 min
Usual types of
significant
Stone fragments,
fine gravel, and
sand
Fine
sand Silty or clayey gravel or sand
Silty soils Clayey soils
constituent materials
General rating as
subgrade
Excellent to good Fair to poor
AASHTO CLASSIFICATION TABLE

10. 1.2 FIELD INVESTIGATION
 Objectives
1. Identification of engineering properties of sub
grade materials
2. Determination of construction material sources
3. Estimation of seepage rates
 Information collected:
 Depth, thickness and properties of each soil
layer
 Location of ground water table
 Depth to bed rock

11.  Surface investigation( R/A)
1. Geophysical methods
2. Test pits or trenches
3. Hand augers
4. Boring test holes and sampling drill rigs

12. 1.2.1 DEPTH OF INVESTIGATION
 Design depth(D)
 The depth from the finished level to the depth
that the load bearing strength has no effect on
the pavement’s performance in relation to traffic
load
D≤ 1.50m for pits and borings

13. 1.2.2 IN-PLACE TESTS(R/A)
A. SPT
B. Vane Shear test
C. Pressuremeter test

16. 1.2.3 GROUNDWATER TABLE
 Determined by excavating holes
 Identified by installing piezometer when it is not
initially determined.

17. MINIMUM MATERIALS TESTING FREQUENCY( TANZANIA
PAVEMENT DESIGN MANUAL)
Road type Min number of
indicative tests per
km
Min number of
strength tests per
km
Paved trunk
roads
4 2
Other paved
roads
2 1
Gravel roads 2 1 per 2 km

19. 1.2.4 FIELD LOG
 Must include
Sample number, depth and type
Field tests, depth and results
Depth to layer thickness
Field soil description

20. Test hole log

21. 1.3 LABORATORY TESTS
Test Sample required Soil
Disturbed/undisturbe
d
Undisturbe
d
Cohesive Granular
Moisture
content
X x X
Grain size X x X
Relative density X x X
Atterberg limits x x
Unit weight X x X
Unconfined
compression
X x X

22. A. PI test
The range of moisture content over which the
soil is in plastic state
B. Compaction/Proctor/ Test
 Gives the moisture-density relation of soils

25. PROCTOR APPARATUS ZAV CURVE

27. C. Field Density Test
RC= ρ field/MDD*100% (> 80 %)

31. D. CBR Test
 Used to evaluate the strength of sub grade soil for
pavement thickness design
 Calculated as:
CBR % =B/A *100 %
where, B = Load or pressure sustained by the specimen at
2.54 or 5.08 mm penetration, read from load-
penetration curve, of the specimen.
A = Load or pressure sustained by standard
aggregates at the corresponding penetration
level
= 6.9 MPa for 2.54 mm penetration
= 10.34 MPa for 5.08 mm penetration
 Measured after 4 days of soaking for wet or moderate
climatic zones

33.  A road section for which a pavement design is undertaken
should be subdivided into sub grade areas( max 5) where
the sub grade CBR can be reasonably expected to be
uniform, i.e. without significant variations

34. Design CBR as the 90% ile value