Prior to the construction of overlying layers of pavement, it is necessary to determine the subgrade surface modulus of the foundation. Different moduli are encountered in foundation design for pavement but each is drawn from the stiffness modulus which is the ratio of applied stress to induced strain. Our focus would be on the subgrade surface modulus which is an estimated value of ‘Stiffness Modulus’ based on subgrade CBR and used for foundation design. The Subgrade Surface Modulus used for design must be determined using the lowest value of the long term and short term CBR. See Figure 1 for illustration of different kinds of modulus.
The four Foundation Classes encountered in pavement design are usually defined by the Foundation Surface Modulus values:
Class 1 ≥ 50MPa
Class 2 ≥ 100MPa
Class 3 ≥ 200MPa
Class 4 ≥ 400MPa
How to Determine Subgrade Surface Modulus
The construction subgrade surface modulus shall be determined at intervals suitable for the type of subgrade material and its condition, with a maximum spacing of 60 m along each lane of prepared subgrade and staggered to the mid-point between adjacent lanes as shown below.
Important Application Rules
- At least 10 tests shall be carried out for each prepared foundation area.
- The measurement of the construction subgrade surface modulus shall be taken at formation level or at sub-formation level if capping is part of the foundation design.
- The test site shall be free from standing water, ice and snow.
- Where the construction subgrade surface modulus is found to be less than the design subgrade modulus, the area shall either be improved or the foundation redesigned in accordance with CD 225 (DMRB).
Main Methods
The subgrade surface modulus can be determined using one of the following devices:
(i) Dynamic Cone Penetrometer (DCP)
(ii) Falling Weight Deflectometer (FWD)
(iii) Lightweight Deflectometer (LWD)
Determination of Subgrade Surface Modulus using DCP
Precautions to Take before/during Test
- The cone shall be inspected before use.
- The cone shall be replaced if there are signs of visible damage or when its diameter is reduced by 10%.
Procedure
The DCP test is a simple and rapid method for assessing the strength and thickness of the foundation layers. The DCP has an 8 kg hammer that drops through a height of 575 mm and drives a 60° cone with maximum diameter of 20 mm vertically through unbound material. The strength of the material can be assessed by measuring the rate of penetration per drop or “blow”. See Figure 2.
The depth of penetration shall be recorded at approximately 10 mm increments, together with the number of blows to achieve this. Note that this number of blows between readings can vary depending on the strength of the layer being penetrated.
The DCP can penetrate most types of granular or lightly stabilised materials fairly easily. However, in strongly stabilised layers, very dense, high quality crushed stone and granular materials with large particles; progress is much slower or negligible.
Where there is less than 4 mm measurable penetration after 40 consecutive blows it should be assumed that the DCP cannot penetrate the material and the test terminated.
After the Test,
A mechanical extractor mechanism can be used to remove the cone and shaft from within the pavement. This reduces both manual handling and stress on the apparatus. Alternatively, the use of push fit (sacrificial) cones can also ease removal of the shaft.
Results
a. The results (blows against depth) shall be recorded on a DCP test log which shall include;
1) unique DCP reference number;
2) core hole depth;
3) zero error (the reading on the ruler when the DCP is placed on a hard, flat, level surface which can be negative where an extension shaft is used). The value is used, along with the depth of the core hole, to calculate the depth of penetration.
4) section reference and chainage;
5) traffic direction;
6) lane and offset (and datum used e.g. nearside lane edge);
7) Global Positioning System (GPS) coordinates;
8) date and time of test.
Where the test was undertaken in a core hole or test pit, the DCP log reference should link to the reference number for the core or test pit.
b. Plot the data as the cumulative number of blows (positive x-axis) against depth of penetration relative to the pavement surface (negative y-axis) which represent the depth below the surface of the pavement as shown below.
Observations from the Graph
Where there is a change in slope of the plotted data, it indicates a change of strength and/or material type. The thicknesses of different strength layers are usually determined by inspection and the average penetration rate, in mm per blow, calculated for each.
Determination of CBR
The penetration rate expressed as mm/blow may be converted to a nominal California bearing ratio (CBR) value expressed as a percentage using the following relationship developed by the Transport Research Laboratory given in Overseas Road Note 8.
Log10 (CBR) = 2.48 – 1.057 x Log10 P (mm/blow) >> CBR = 10 (2.48 – 1.057 x Log10P)
Where,
P = the penetration rate in mm per blow
Note the accuracy of this relationship reduces for CBR values below 10%
The CBR value obtained shall then be converted to surface modulus (E) using the following equation:
Subgrade Surface Modulus, E = 17.6(CBR)0.64 MPa
