200B-2: The S2 Event

Subsurface Investigation 

Note that parts of the overall subsurface investigation are necessary for, and directly used for, S2 purposes as discussed in this section.  However, a large part of the overall subsurface investigation is also used later for S3, S4, and/or other purposes as listed below.  The overall purpose of the subsurface investigation for a roadway project is to provide sufficient soils related information for the following:

• Overall geotechnical design for the final roadway location, both horizontal and vertical.

• Suitability of materials obtained from cuts, borrow, or channel changes for use in embankments.

• Final geotechnical design of the roadway section, foreslopes, and backslopes, including possible geotechnical remediations, etc.

• Identification of required subgrade treatment and determination of type of subgrade treatment.

• Identification of select soil locations and quantities available.

• Identification of areas where geotechnical features such as subsurface drains are needed, and associated final design of such features.

• Final geotechnical design of embankments in the vicinity of structures (including around bridges, when used in conjunction with the bridge borings), based on adequate determination of strength and consolidation characteristics of the subsoils from tests on soil samples obtained with the borings.

Include the entire length of the project under consideration in the scope of the soils investigation.  The field exploration should be performed as outlined in Section 200C-1.  All information and all aspects of drilling as included in Section 200C-1 must be fully understood, and must be appropriately and fully carried forward into the subsurface investigation so that all requirements of the investigation are fulfilled.  As such, part of the overall subsurface investigation program (drilling) may, when necessary and under certain circumstances, be delayed until after the drilling that is required for S2 purposes has been completed.

Design of Stability Features

The plan/profile sheets and the cross sections along the alignment must be reviewed along with the soil boring logs and laboratory test data from the Soils Survey to identify all potential cut/fill slope stability issues which would require additional ROW.  Once a critical area has been identified, a detailed stability analysis of the entire area must be performed using geotechnical software (such as GEO5, Slope/W, etc.) and/or other procedures to determine if the cut/fill slope is stable and whether a stability feature will be required to improve stability.  Types of slope stability analyses include rotational slope failure, translational failure, irregular surfaces of sliding, and infinite slope failure.  Special attention should be given to areas of the project where:

• Fill heights are > 10 feet.

• Backslope heights are > 25 feet.

• Soft soil is present.

Possible stability features (remediations) that can be used for increasing the stability of embankment slopes include, but are not limited to:

• Stability berms (such as 10H:1V bench slope, 3H:1V toe slope, minimum 3 feet in height).

• Backslope benches (usually standard design).

• Undercutting/replacement of soft subgrade soils, and/or ground improvements such as rammed aggregate piers, and/or staged construction or similar techniques.

• Subsurface drains and/or similar drainage features.

Stability analyses should be performed as outlined in Section 200F-1 for both undrained (end-of-construction) and drained (long term) cases.  Soil and groundwater properties, as well as assumptions used in the analyses, should be documented in the stability file for the project.  Slopes should generally be designed to have:

• Factor of Safety > 1.3 (undrained case).

• Factor of Safety > 1.5 (long term case).

After completion of the slope stability analyses, the necessary stability-related geotechnical remediations that require additional ROW must be depicted/defined/delineated on plan/profile sheets and/or cross sections so that the Design Section, Survey/Photogrammetry, ROW Office, District Office, Office of Location and Environment, and others can clearly understand the additional stability related ROW requirements that are necessary for S2 purposes.

Final Borrow Site Selection 

The S1 Event is reviewed and the final S2 borrow sites are based on some or all of the following considerations:

• Minimizing the number of borrows and land use impact.
• Total borrow need, including amounts needed for shrinkage and cushion.
• Distribution of the total borrow need (where it is needed on/along the project, which requires a mass diagram or equivalent for the Soils Design Section to best understand).
• Project breaks.
• Hauling distance (generally < 2 miles in any direction).
• Hauling barriers (rivers, etc.).
• A borrow site’s potential to produce select for subgrade treatment (this is not always accomplished).
• Use of landlocked parcels, irregular shaped parcels, or total acquisition parcels.
• Proximity to farmsteads, homesteads, lakes, parks, and other sensitive features.
• Avoidance of environmental/cultural sites and issues (wetlands, historical sites, etc.).
• Desired or needed borrow design (pond or drainable).
• Potential for integration of final design details with wetland mitigation sites.
• Restrictions such as the Loess Hills region, Century Farms, pipelines, etc.

After borrow site selection is completed, the recommended ROW acquisitions for those sites must be depicted/defined/delineated in some fashion, typically on air photos that can be related to the project’s alignment, so that the Design Section, Survey/Photogrammetry, ROW Office, District Office, Office of Location and Environment, and others can clearly understand the ROW acquisition areas for the selected S2 borrow sites.  In addition to the air photos, an electronic file for referencing the design file is advantageous and provided if feasible.

Include a general (simplified) conceptual borrow design in the S2 submittal at this time so that all individuals noted above can visualize the basic borrow concept/design, and so that the basic borrow concept/design can be carried forward into the S3 Event: Final Determination of All Geotechnical Related Items Required for Grading Plans.  This general/simplified conceptual design is typically shown on an aerial photo that can be related to the alignment.  That depiction is usually not much more than a general shape and size for a pond and drainable borrow concept.  However, some details may be provided for pond borrow concepts (such as deep versus shallow ends of the pond), and probable surface drainage directions for a drainable borrow are usually depicted by a few arrows.

S2 Event Submittals 

Draft S2 Submittal

A draft S2 submittal is usually prepared for internal peer review and/or other purposes such as initial, pre-distribution review of unusual items with other sections or offices.  The draft submittal consists of a memo following the S2 memo boiler plate and should include full and detailed information/delineation/documentation of: 1) all areas where there are stability problems that require additional ROW; 2) each final selected borrow site, with identifying boundaries, size (acreage), type of borrow, type of material available, and estimated quantities; and 3) any other pertinent information.  Also included are other items that might have been identified during the field exploration program (such as cisterns).  If stability analyses are performed, a calculation package that includes the assumptions, input parameters, and output should also be available for peer review.  Use the following boiler plates for final submittals: 1) S2 email and 2) S2 memo.

The draft S2 submittal should first be submitted to others in the Soils Design Section who can provide peer review comments, and then to the Soils Design Section Engineer.  Comments should be documented, discussed, and addressed in the final submittal.  The overall Soils Design Section review team should ultimately include:

• Soils Design Section Engineer,

• Soils Design Section Assistant Engineer,

• Soils Design Section Geologists, and

• Others as applicable.

Final S2 Submittal

The Final S2 Submittal Memo (pdf) is placed in the S2 Submittal folder of the Soils folder of the Project Directory and an email (see S2 email) is sent to the recipient list below with links or electronic files of the Final S2 Submittal.  For consultants, the Final S2 Submittal is forwarded via email or ftp.  The S2 submittal should be directed to the Design Section Engineer, Soils Design Section, and District Engineer with copies to the following:

• Office of Design Engineer, Office of Design Assistant Engineer – Development, Office of Design Assistant Engineer – Support.

• Assistant District Engineer and District Construction Engineer.

• Office Director of the Office of Location and Environment, Wetlands Section Supervisor, and Cultural/Historic Resource Team Leader.

• Office Director of the Office of Right of Way, Right of Way Design Supervisor, and Right of Way Acquisition Team Leader.

• Photogrammetry Engineer and Survey Supervisor.

• Roadside Development Supervisor.

• Office of Construction and Materials – Earthwork Field Engineer.

• Design Section Assistant Engineer and Design Section Technician.

• Assistant Soils Design Section Engineer and those others in Soils Design who worked on the S1 and/or S2.

Electronic Files

For internal Iowa DOT use, electronic files are copied to the project directory and links to the files are included in the email, see S2 email.  Consultant generated electronic files will be submitted with final submittal via email or ftp.  Electronic files should be developed according to Section 20B-71.

Electronic files for the S2 submittal should include and be named:

• County# - 2 digits, Route# 3 digits, and Paren # - 3 digits_S2memo.pdf, S2 Submittal Memo

• County# - 2 digits, Route# 3 digits, and Paren # - 3 digits_S2.kml, Google Earth

• County# - 2 digits, Route# 3 digits, and Paren # - 3 digits _S2.sol, Microstation 

• County# - 2 digits, Route# 3 digits, and Paren # - 3 digits _S2borrowXX.jpg, Aerial Photos

S2 Email

S2 Memo