DCP to CBR 01

THE USE OF DYNAMIC CONE PENETROMETER (DCP) TO DETERMINE SOME USEFUL RELATIONSHIPS FOR SANDY AND CLAYEY SOILS Mohammadi, S.D. and Nikoudel, M.R. Tarbiat Modares University, Tehran, Iran

Khamehchiyan, M. Tarbiat Modares University, Tehran, Iran

ABSTRACT: This paper explains a laboratory investigation carried out to develop some useful relationships for the use of Dynamic Cone Penetrometer (DCP) for road subgrade evaluation and building foundation. Series of laboratory tests were performed by varying the moisture content and the dry density. Bad graded sands (SP) and Clayey Silt (CL-ML) were used for the study. Experimental procedure, statistical analysis of the results and the developed equations are presented in this paper. Keywords: Dynamic Cone Penetrometer (DCP), California Bearing Ratio (CBR) test, Plate Load Test (PLT), Moisture content, Dry density

with the CBR value [3-4]. Different correlations were suggested between the DCP-PR in (mm/blow) and CBR values. Kelyn [3] conducted DCP tests on 2,000 samples of pavement materials in standard moulds directly following CBR determination. Based on his results the following correlation was recommended:

1. INTRODUCTION Dynamic Cone Penetrometer (DCP) is an instrument, which can be used to evaluate California Bearing Ratio (CBR) value and Elastic Modulus (E) of soils. It is an insitu test, simple to use and inexpensive. Scala [1], originally developed the DCP in Australia. After that various researchers developed both the testing instrument and the testing procedure. The relationships were developed between DCP and CBR, shear strength, soil index properties etc. [2]. There are various types of DCPs available in the world. They are operated on the same principle. A DCP consists of an 8 kg weight dropping through a height of 575 mm and a 600cone having a base diameter of 20 mm as shown in Figure 1. The penetration of the cone is measured using a calibrated scale. It is possible to measure up to 800 mm depth without an extension rod and up to 1200 mm depth when fitted with an extension rod. It needs three operators, one to hold the instrument, one to raise and drop the weight and the other to record the penetration.

Log CBR = 2.62 – 1.27 log PR

(1)

Based on a field study, Smith and Pratt [5] suggested the following correlation: Log CBR = 2.56 – 1.15 log PR

2. EXISTING CORRELATION BETWEEN DCP AND CBR To assess the structural properties of the pavement subgrade, the DCP values are usually correlated

Fig. 1.Dynamic Cone Penetrometer (DCP) [6]

1

(2)

Livneh and Ishia [7] conducted a correlation between the DCP-PR and the in-situ CBR values using a wide range of undisturbed and compacted fine-grained soil samples, with and without saturation. Compacted granular soils were tested in flexible moulds with variable controlled lateral pressures. The equation 3 was obtained between CBR and DCP-PR:

3. EXISTING CORRELATION BETWEEN DCP AND PLT

Log CBR = 2.2 – 0.71 (log PR) 1.5

Log (EPLT) = (-0.88405) Log (PR) +2.90625 (7)

Based on a regression analysis, Konard and Lachance [13] suggested a relationship between the PR of a large DCP with a 51 mm diameter cone and the elastic modulus of unbound aggregates and natural granular soils back-calculated from plate load tests (EPLT), and it is as equation 7:

(3)

Where EPLT is expressed in MPa.

Harrison also suggested equations 4 and 5 for different soils [8]:

As in the case for other stress-strain tests, different elasticity moduli can be obtained from the PLT. Soil elasticity moduli can be defined as: (1) the initial tangent modulus; (2) the tangent modulus at a given stress level; (3) reloading and unloading modulus and; (4) the secant modulus at a given stress level. In this study, the initial tangent modulus was determined for all plate load tests. To determine the initial modulus (EPLT(i)), a line was drawn tangent to the initial segment of the stressstrain curve; then an arbitrary point was chosen on this line and the stress and deflection corresponding to this point was used to determine the initial modulus. Figure 2 describes the deflection and stress used for determining EPLT(i) from δ 1 and p. German Code for the design of flexible pavement structures specifies performing in-situ plate-bearing tests on constructed pavement layers [13]. For the second cycle of the regular plate-bearing test, the German code defines a reloading stiffness modulus called EPLT(R2) using the following equation [14]:

Log CBR = 2.56 – 1.16 log PR for clayey-like soil of PR > 10 (mm/blow) (4) Log CBR = 2.70 – 1.12 log PR for granular soil of PR