DETERMINATION OF CALIFORNIA BEARING RATIO AIM: California bearing ratio (CBR) test is a penetration test developed for the evaluation of sub-grade strengths for roads and pavements. The results obtained by these tests are used in conjunction with empirical curves based on experience for the design of flexible pavements. APPARATUS: 1) CBR mould 150 mm dia. x 175 mm high with collar and base plate.
2) Space disc 3) Hammer 4) Expansion measuring apparatus 5) Adjustable stem with perforated plates 6) Tripod 7) Load frame 8) Penetration plunger. 9) Dial gauge 10) Filter paper 11) Proving ring 10 kg capacity.
PROCEDURE: 1) The dry density for remoulded specimens shall be either field density or
the maximum dry density .Take about 10 kg of soil sample. Add water corresponding to optimum moisture content and mix thoroughly. 2) Place the spacer disc over the base plate in the mould and put filter paper on the top of the spacer disc. 3) Compact the soil-water mixture into the mould in this layer, each being given 55 blows of the 2.5 kg hammer. 4) Remove the extension collar and trim the compacted soil even with the top of the mould by means of a straight edge. Remove the perforated box plates and spacer disc and take weight of the mould and the compacted soil specimen. 5) Place a filter paper on the perforated base plate, invert the mould and place it on the base plate and compact it. 6) Take soil sample in aluminium container for moisture content determination.
7) Place the mould on the lower plate of the load frame. Put surcharge weights, sufficient to produce an intensity of loading equal to the weight of the base material and permanent on the specimen. Seat the plunger in the angular hole under a load of 4 kg. To establish the full contact. Set the stress and strain gauges to zero. The initial load applied to the plunger shall be considered as the zero load when determining the load penetration relation. Apply load on the penetration plunger so that the penetration is approximately 1.25 mm per minute and take readings. After completion of the test, take soil sample for water content determination.
OBSERVATIONS: Empty Mould Weight of SPT apparatus= 2.190kg
) c 1.66 c / g ( . 1.64 t w t i n1.62 U y r D 1.6 1.58 1.56 0
5
10
15
20
25
30
Moisture Content (%)
Optimum Moisture Content=21.425% Maximum dry unit weight=1.68 g/cc
Penetration (mm)
Dial (load)
Load (kN)
Load (kg)
0.5
1.85
46.25
4.715
1
2.5
62.5
6.371
1.5
3
75
7.645
2
3.4
85
8.665
2.5
3.8
95
9.684
3
4.2
105
10.703
3.5
4.6
115
11.723
4
4.8
120
12.232
4.5
5
125
12.742
5
5.2
130
13.252
6
5.6
140
14.271
7
6
150
15.291
8
6.4
160
16.31
9
6.8
170
17.329
10
7.2
180
18.349
12.5
8
200
20.387
25.00
20.00
) 15.00 g k ( d a o L
10.00
5.00
0.00 0
5
10
15
Penetration (mm)
RESULTS: CBR (at 2.5 mm) = Test load/1370 * 100
= 968.4/1370 * 100 = 70.68 % CBR (at 5 mm) = Test load/2055 * 100 = 1325.2/2055 * 100 = 64.48 % 70.68% shall be taken as the CBR.
SIGNIFICANCE: Field in place CBR tests are used for evaluation and design of flexible pavement components such as base and sub-base course and sub-grade and for other applications (such as unsurfaced roads) for which CBR is the desired strength parameter. Soils and flexible pavement components at the same location may exhibit significantly different load deflection relationship. No method presently exists to evaluate the precision of a group of non-repetitive plate load tests on soils and flexible pavement due to the variability of these materials.
