Earth Datum

NHDOT Earthworks Level Datum Strings

The Level Datum string represents the grade break from which the earthworks strings are generated.

There are two methods for generating Level Datum strings for earthwork computations. The method one chooses to use ultimately depends on the proposed shoulder cross slope. Most road designs will require using both methods. Larger projects, specifically those containing reversing curves, will likely require constructing a segmented earthworks datum string as constraints are likely to evolve from beginning to end. Only one side of a roadway can be done at a time.

When to use Method 1, Horizontal Offset...

For any portion of a roadway whose superelevation does not exceed 5%. This includes standard straight roadway portions and curves with small superelevations.
For any portion of a roadway outside of a curve (highside).

When to use Method 2, Extension of Cross Slope...

For any portion of a roadway inside of a curve (lowside), specially when the roadway superelevation exceeds 5%.

Method 1, Horizontal Offset...

This option can be found from the MX menu bar,

Design => Design a String => Horizontal Offset...

From the Details tab, specify the Design Model, the Reference String Name, and the Subsidiary String Name.

Check Create a New String. Specify the Model to Store New String (usually the design model), and the New String Name, in this case select Shoulders (Earthworks Datum)(EH).

From the Horizontal Offset tab, select Constant for the horizontal offset. Specify the Constant Offset value of 0.3 meters (1.0 feet). Use the left or right arrow to control the offset direction. The diagram at the bottom right of the panel shows the current offset direction.

From the Elevations tab, select Constant Cross Slope. Specify a Cross Slope value of 0.05 (5%). Select the down arrow.

From the Start and End tab, Specify the limits of your application.

Method 2, Extension of Cross Slope...

This option can be found from the MX menu bar,

Design => Design a String => Extension of Cross Slope...

From the Details tab, specify the Design Model, and the Reference String Name. Subsidiary String 1 Name should be the inner most string defining the slope to be extended (usually a CE?? string). Subsidiary String 2 Name should be the outer most string defining the slope to be extended (usually a ES?? string).

Check Create a New String. Specify the New String Name, in this case select Shoulders (Earthworks Datum)(EH).

From the Extension tab, select Apply Horizontal Offset and Normal Offset. Specify a Horizontal Offset value of 0.3 meters (1.0 feet). Use the left or right arrow to control the offset direction. Specify a Normal Offset value of 0.

From the Start and End tab, Specify the limits of your application.

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