CBR TEST

Dhananjoy Ray October 09, 2020 

<script data-ad-client="ca-pub-6864624463949406" async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
DETERMINATION OF CBR VALUE based on IS2720/Part-XVI

A. GENERAL- The CBR test is an empirical test giving an indication of the shear strength of soil. The CBR test is essentially a laboratory test but in some instances the test is carried out on the soil in-situ. CBR values may vary from less than 1% on soft clays to more than 100% on dense crushed rock samples. CBR test is generally done on compacted soil of around 97% compaction.

IS: 2720, Part-XVI covers the laboratory method for determination of California Bearing Ratio (CBR). This method was used by the California State Highway Department in USA for evaluating the stability of subgrade and other material layers for flexible pavements.

It is the ratio expressed in percentage of ‘Force per unit area required to penetrate a test soil mass, to a specified depth, of a circular plunger of 50 mm diameter and  the force that required for the same penetration in a standard material i.e crushed stone. Piston moves  at the rate of 1.25 mm/min while penetrating.

Load vs penetration value of  soil sample is determined in the laboratory and load is recorded at a certain interval of penetration, generally at 0, 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 7.5, 10 and 12.5 mm. In this regard a typical  graph-A is shown Para F. The load value corresponding to  penetration of 2.5 and 5 mm is considered standard and used for the calculation of CBR value.

B. PURPOSE OF THE TEST:

1. CBR is an important test as far as road construction is concerned. Road crust thickness depends on CBR value of subgrade material. Higher CBR vaule of sub grade implies lesser thickness of Road layers which is highly preferable in terms of cost effectiveness of construction of road. Whole road layers are designed based on the CBR value of the subgrade soil which may be the top soil, a natural soil prepared to receive the stresses from the top layers. It shall be well compacted and achieved the desired dry density at OMC (optimum moisture content). CBR test measures the strength of unbound material.

A typical cross section of the road is shown in fig.2



C. APPARATUS- 


. Moulds with Base Plate, Stay Rod and Wing Nut - Moulds in fig C-1, Shall be used for collecting undisturbed sample, preparation of remoulding sample and for CBR test as required shall comform to IS : 
9669 - 1980.
Fig-C-1

2. Cutting Collar- shown in fig-C2. It shall conform to 4.2 of IS : 9669 - 1980. while collecting undisturbed soil sample cutting collar is fitted with the CBR mould.

Fig-C2



3. Spacer Disc - It shall conform to 4.4 of IS : 9669 -1980. 
The spacer disk is used in CBR test as a false bottom in a soil mould during the compaction process to get the desired height of soil sample.

4. Metal Rammer - As specified in IS : 9198 - 1979. used for pushing the core cutter into the soil by ramming.

5. Expansion Measuring Apparatus –

6. The adjustable stem with perforated plates and tripod shall conform to 4.4 of IS : 9669 – 1980. The apparatus is used to record the expansion during soaking of soil sample. 

7. Weights - This shall conform to 4.4 of IS : 9669 – 1980.These are surchage load placed on the sample during CBR test.

8.Loading Machine

9. Penetration Plunger –50mm dia  as per para 4.4 of IS : 9669 – 1980.

10. Dial Gauges –are used to record penetration and load.

11. Sieves – 19 mm IS Sieve

D. SELECTION OF SAMPLE

The test may be performed on: a) undisturbed specimen or on  b) remoulded specimen.

a) Undisturbed Specimens -Para 4.2 of IS 2027 part-XVI shall be followed to get undisturbed sample. The brief  for the collection is mentioned that  the steel collar with sharp cutting edge  of 150 mm internal diameter shall be fitted at the bottom of the CBR mould and extension collar at the top.Then the system is to placed on a leveled surface from where sample is to be collected, then pushed the mould as gently as possible into the soil. After a certain push the soil around the mould shall be dug out and the system is again pushed gently until the mould and a part of extension collar is filled or as per height of sample required.  Then the mould  shall be removed by under digging. The cutting collar and extension collar is removed slowly  The top and bottom surfaces are then trimmed flat so as to give the required length of specimen ready for testing. 

b) Remoulded Specimens - In the laboratory CBR test, a sample of soil is prepared in a cylindrical steel mould.

The dry density for preparing  remoulding sample, shall be either the field density or the value of the maximum dry density. Sample shall be prepared by the light or heavy compaction method as per IS : 2720 ( Part 7)- 1980  and IS : 2720 (Part 8) - 1983 or any other density at which the bearing ratio is desired.  

The material used in the remoulded specimen shall pass a 19-mm IS Sieve. Allowance for larger material shall be made by replacing it by an equal amount of material which passes a 19-mm.IS Sieve but is retained on 4.75-mm IS Sieve. 

E.PROCEDURE OF TEST

1. The specimen in the mould is soaked for 96 hours to check the free swelling characteristics of soil, measured duly fitted a dial gauge during soaking. Swelling and water absorption is measured during this period. Soaking of sample is necessary as it is likely that soil will accumulate moisture after the construction of embankment.

The mould with soaked sample is put on the perforated disc, tightened with  Stay Rod and Wing Nut. fig-1.

2. Surcharge weight is placed at the top of the specimen in the mould and the assembly is placed under the plunger of the loading frame.

3. Load applied and values corresponding, to the standard penetration of 0, 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 7.5, 10, 12.5 were recorded.

4. Load vs Penetration graph with load values along the Y-Axis is plotted. A graph with arbitrary values shown in graph-A below

5. Load at 2.5mm penetration is measured=P2.5

6. CBR at 2.5mm penetration= (P2.5 / Pstd)*100. Similarly CBR at 5mm penetration = (P5 / Pstd)*100.

F. STANDARD VALUES OF LOAD Vs PENETRATION IS TABULATED: 

Penetration of Plunger(mm)

Standard Load(Kg), (Pstd)

2.5

1370

5

2055

7.5

2630

10

3180

12

3600

 A standard Penetration vs Load curve is shown in the graph-B 

The normal curve is with convexity upwards and the loads corresponding to 2.5mm and 5.0mm penetration are noted from the graph to find CBR. Sometimes at the initial, the curve may be seen with upward concavity as shown in graph-A, curve-3. This may be due to


1. the bottom surface of the plunger or top surface of the specimen may not be truly horizontal.

2. The plunger surface not being in full contact with the top of the specimen.

3. The soil is soft at the top layer. In all these cases specimen  did take no load or little, while dial gauge shows reading fully.

A typical graph of penetration vs load is shown in the graph-A with arbitrary values and another Graph-B showing the standard curve.


 In graph A,  03 types of curves which may be obtained during CBR test of soil specimen are shown. The normal curve shall be with convexity upwards like curve-2.

CURVE-1: It is a wrong curve. There may have defects in the instrument, in calibration, missing records or wrong operation. Generally such curve does not come in CBR test. In such cases, test shall be repeated.

CURVE-2: The shape of the curve as that of normal curve with convexity upward, is the most desirable one and no correction is required.

CURVE-3: The shape of the curve have some deviation w.r.t normal curve at the initial stage i.e concavity upward and then the curve changes direction. For such curve if found, test need not be repeated, however a correction is required, thereafter values shall be taken from the corrected curve as shown in the curve-3 of the graph-A. A tangent is drawn from the point where the curve changes its direction to meet the X-axis. The meeting point away from the origin by 0.8mm, is the new 0 penetration, accordingly position of 2.5mm and 5.0mm will also  be shifted by same amount.

GRAPH-B: It is the standard curve which gives the load values at 2.5mm, 5mm, 7.5mm, 10mm and 12.5 mm penetration also tabulated in para F in the top. At the initial stage nearly up to 5 mm the curve is increasing upward at a greater rate than the rest portion and soil specimen resists maximum load within 5 mm penetration which is main criteria to take the load at 2.5mm and 5.0mm penetration.

G- CALCULATIONS

From the curve-3 in graph-A, corresponding to 2.5mm penetration(shifted), load=395 Kg and to 5mm penetration load= 590Kg, all values are tabulated below.

Penetration, mm

 

Load of Soil Specimen, Kg

Standard load, Kg

CBR Value %

(Col-2/Col-3)*100

Col-1

Col-2

Col-3

Col-4

2.5

395

1370

28.83

5.0

590

2050

28.78

REMARKS: Normally CBR value at 2.5mm is higher than at 5mm. If CBR value at 2.5mm is higher than at 5mm then the value corresponding to 2.5mm is recorded as the CBR value. Here, CBR value =28.83.  In case CBR value at 5mm is higher, test shall be repeated. If the repeat test shows the same result, then the value at 5mm will be considered as CBR value of the soil sample. Average CBR value shall be determined of three test specimen following the same procedure. Generally CBR is checked for the subgrade soil of 97 % compaction.

Foundation Strength of Rigid Pavement:  It is estimated on K value, the modulus of subgrade reaction in terms of Kg/cm2/cm which is pressure required per unit deflection obtained by plate bearing test. 

Strength of subgrade soil is largely affected due to water content, therefore strength reduces considerably during monsoon. Design strength of foundation soil shall be considered minimum at highest/worst moisture content. In any case if test of subgrade strength is to carryout in dry season, necessary allowance for loss of strength at worst moisture content has to be taken into account. 

The expected reduction in strength of foundation soil can be obtained from laboratory CBR value of compacted field soil sample comparing with Table-3 of IRC-58-1988. Sample of compacted subgrade  at optimum moisture content is taken from the field  and CBR test is carried out  in laboratory before and after saturation. After saturation  CBR value  shall be least which is compared with Table-3 of IRC-58 to obtain K value. Value depicted in Table-3 as below: 

CBR Value % :               2       3       4       5       7       10      20      50      100

K-Value(Kg/cm2/Cm):  2.08   2.77  3.46  4.16   4.84   5.54   6.92  13.85   22.16

If laboratory soaked CBR value of the sample is 10%, designed strength of subgrade shall be 5.54 Kg/cm2/cm. This K-vale can be utilised as one design parameter in the design procedure of Pavement.

However K-value can also be obtained directly by Plate Bearing Test in the field which shall be carried out at the end of monsoon.

 Also See the Important Link.

Water content- dry density-relation

 



Post a Comment

0 Comments