B. Determination of As-Built Condition (Original Grade) B. Determination of As-Built Condition (Original Grade)

B. Determination of As-Built Condition (Original Grade)

The engineer is often responsible for determining the As Constructed and Subsequently Improved Condition" (ACSIC).

Since by definition a repair is the restoration to the "As Constructed and Subsequently Improved Condition" (ACSIC), the engineer is often placed in the position of determining that condition. Repair activity beyond the ACSIC can affect adjacent natural resources, such as public waters, navigable waters, and wetlands. Consequently, the regulatory authorities that administer these resources (LGU, DNR and USACOE) have the responsibility to monitor drainage system repairs.

There is no single best method for establishing the ACSIC. Original construction plans are seldom complete and frequently reference benchmarks which in most situations have been destroyed or removed. "As-built" construction plans rarely exist on older systems, and the actual grades constructed may not match those shown on the construction plans. Therefore, establishment of the original grade line is very challenging and becomes a matter of professional judgment. The drainage authority is ultimately responsible for making this determination, after consultation with their engineer.

However, if the repair may affect a public water, Minn. Stat. § 103E.701, Subd. 2, stipulates that before a repair is ordered the commissioner of DNR must be notified about the proposed repair and its associated repair depth. The DNR may request additional information to support their review. If the commissioner disagrees with the repair depth as provided to the drainage authority, a 3-member panel, consisting of the engineer, a representative of the DNR, and a local soil and water conservation district technician will make a joint recommendation to the drainage authority.

There are several methods of utilizing field data to determine the grades of the ACSIC. These methods (in order of expected reliability) include:

1. Test pits;
   
2. Soil borings;
   
3. Existing culvert comparison; and
   
4. Natural ground/cut sheets
   

1. Test Pits

Test pits are one of the most reliable ways to observe the depth and cross-section of construction.

One of the most reliable ways to observe the depth and cross-section of the original construction (or as subsequently improved) of an open channel system is through the use of test pits. A test pit is an excavation perpendicular and through the open channel bottom and sideslopes, typically completed using a backhoe or excavator. Ideally, the test pit should be excavated to a depth extending at least 1 foot below the bottom of the historic channel bottom (and into parent base soils). The excavation should reveal soil horizons which include native organic soils (e.g. topsoil), native mineral (inorganic) soils, spoil materials, and accumulated sediment. In most cases, the historic channel bottom can be observed at the interface between the accumulated sediment and native mineral soils. This interface can then be surveyed to determine the ACSIC grade at the location of the test pit.

Because of the expense of contracting excavation equipment for test pits, it is typically impractical to complete a sufficient number of test pits to describe the as-constructed channel profile solely based on the test pit observations. Instead, the preferred methodology is to utilize the test pit data to correlate historic design plans to a common sea level datum (e.g. NAVD 88). A minimum of four test pits is recommended to develop this datum correlation, although more may be required if the data is inconclusive or if multiple historic plan sets were used to construct the system.

There are several limitations to test pit determinations of the ACSIC. First, the open channel section must be situated in native mineral soils to yield a conclusive result. Where the open channel crosses deep organic soils (e.g. peat), it is difficult to distinguish the interface between the sediment (usually organic) and the organic native soils at the historic channel bottom. Likewise, test pit excavations where the open channel bottom is partially submerged can be challenging or even infeasible, as the water in the excavation may not allow for the interface of the sediment and the native soils to be viewed. Finally, the use of test pits is impractical for portions of the public drainage system that were deepened through undocumented modifications, since this methodology relies on the use of historic plans to develop the ACSIC profile.

2. Soil Borings

Well executed soil borings are also a good way to identify the historic open channel bottom as it was originally constructed or subsequently improved.

Like test pits, soil borings can be utilized to identify the interface between the organic soils in the channel bottom and the native mineral soils, which is evidence of the historic open channel bottom as it was originally constructed or subsequently improved. Similar to the test pit methodology, the surveyed soil borings can be used to develop a correlation between the historic design plans and a common sea level datum. However, since soils borings are relatively inexpensive, they can also be utilized to define the ACSIC open channel profile without a design plan, by completing the soil borings at intervals along the entire length of the public drainage system open channel and creating best-fit profile lines through the surveyed elevations.

Soil borings are best utilized where the open channel is well-defined and is situated in native mineral soils. As with test pits, soil borings are often inconclusive in locations where the open channel crosses through deep organic soils. Soil borings can also be inaccurate if they are not located close to the historic excavated channel bottom. Over time (particularly when the public drainage system has not been maintained) the open channel bottom can shift laterally as sedimentation and erosion occurs. This lateral movement of the channel bottom can typically be observed through a test pit, but a soil boring placed in the current channel bottom may actually be located in the historic channel sideslope and infer a channel bottom elevation higher than it actually was constructed. For this reason, it is important when using this methodology to plot the soil borings in profile along the open channel and use statistics and engineering judgment to remove “outliers” where necessary. Providing good documentation of these decisions is critical.

3. Existing Culvert Comparison

When test pits and soil borings are infeasible, the ACSIC channel profile can be determined by surveying existing culverts along the public drainage system and use the elevations to either 1) correlate the historic plans to a common sea level datum, or 2) independently define grades along the ACSIC profile. This methodology assumes that the culverts were installed at the as-constructed channel bottom. While this generally has been the preferred placement in a standard roadway crossing design, in practice, culverts often were placed at the elevation of the current open channel bottom. This channel bottom elevation may be substantially higher than the ACSIC due to sedimentation, particularly when the culvert is installed without an engineered design (e.g. private driveway and field crossings). Culverts also could have been placed after excavation to provide bedding foundation, in which case the culvert could be lower than ACSIC. When using existing culverts to establish the ACSIC, preference should be given to “major” (collector and arterial) roadway crossings and documentation provided as to how the culvert represents an accurate, reliable, or defendable reference point.

4. Natural Ground/Cut Sheets

Historic cut sheets from Anoka County Ditch 31 in Rice Creek Watershed District.

One other methodology that has been utilized to determine the ACSIC is to survey the natural ground elevation adjacent to the open channel, and subtract the excavation depth from cut sheets in the original engineer’s report to calculate the design channel bottom elevation. This methodology assumes that 1) the natural ground elevation can be accurately identified and surveyed in the field and is using the same datum as the original survey; and 2) the natural ground elevation has not changed since the construction of the ditch. Because of spoil piles, vegetation, and undulating topography, it can be difficult to ascertain what portion of the adjacent topography (if any) would be representative of natural ground elevation at the location of the open channel. This problem is compounded when the adjacent grades have changed due to land use practices or subsistence of organic soils. Because of these factors, this methodology can be highly inaccurate and should only be used to verify the ACSIC grades determined through other methodologies.

5. Drainage Records Modernization

Because the determination of the ACSIC is critical to defining the depth and cross-section for repairs and maintenance of the public drainage system, and since these determinations rely on the historic record of the drainage system from its inception to its most recent repairs, it is imperative that the drainage authority develop an efficient and sustainable system to manage these historic records. Modern record-keeping practice includes scanning historic and current drainage system files into an electronic format (typically .pdf) and organizing these files via an electronic database. Many drainage authorities have also created GIS layers, geodatabases, and applications to map the public drainage systems they manage and provide georeferenced attribute data (including as-designed or as-constructed dimensions and grades) along the drainage system alignments. These electronic databases greatly increase efficiency and accuracy in managing repairs and maintenance to the public drainage systems and streamline the processes for regulatory review. The State of Minnesota has created (August of 2016) a geodatabase template for any drainage authority to use to organize and store their drainage system hydrographic and management data. Click here to find more information regarding drainage records modernization.

For an example of a public drainage system portal, see the RCWD Drainage System Information Portal.

Note: In support of these efforts, the engineer, when carrying out surveys, is strongly encouraged to geo-reference and report all benchmarks and centerline alignments with a common coordinate system and datum (e.g. county coordinates, state plane, & NAD83 UTM).