CBR Tabulated Calculations - British Standard

SUMMARY

This article will describe the calculations used for CBR Test per BS standard.

SOLUTION

LOAD [L]

Recorded during test or manually entered by user.

PENETRATION [P]

Recorded during test or manually entered by user.

CORRECTED LOAD [CL]

CL = L- initial load + top cap weight.

CORRECTED PENETRATION [CP]

CP = P - initial displacement

CBR PERCENTAGE [CBR]

PENETRATION INDEXES [PI] = 2 mm, 2.5 mm, 4 mm, 5 mm, 6 mm, and 8 mm.

CBR Values are calculated at closest CP to PI.

DIVISION VALUES [DV] per BS =

11.5 @ 2 mm, 13.2 @ 13.2 mm, 17.6 @ 4 mm, 20 @ 5 mm, 22 @ 6 mm, and 26.3 @ 8 mm.

DV values are in units kN.

CBR @ PI = CL [kN] / DV * 100

CORRECTED CBR PERCENTAGE [CCBR]

Same as CBR but adjusted by correction line drawn.

From the correction line we get the correction lines Slope [S], Y-Int [B], and Corr [C].

Next, we need to determine the displacement offset of the line.  This is found with the line equation when y is zero.

CORRECTION LINE PENETRATION CORRECTION VALUE [C]

C = -B / S.  This is derived from y = mx +b and we are solving for x.

The closest matching P value for the PI is adjusted with C yielding the Adjusted Penetration value.

ADJUSTED PENETRATION [AP]

AP = CP + C

Finally, we determine where the AP value intersects with the curve.  A vertical line is used to determine intersection.  This will yield the ADJUSTED LOAD [AL] value.  

CCBR = AL / DV