Shallow soils and shallow depth to bedrock

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Figure 1:

Occurrence of shallow bedrock in Northeast Minnesota. Shallow bedrock is defined as bedrock occurring within six feet of the land surface. The most extensive areas of shallow bedrock occur adjacent to Lake Superior and adjacent to the Canadian border. Source: Great Lakes Association.

Bedrock and shallow soils are found in many portions of the state, but are a particular problem in the northeastern region of the state. Sites with shallow bedrock are defined for the purpose of this paper as bedrock within six feet or less of the ground surface. The stormwater management implications of shallow bedrock affect infiltration, ponding depths, and the use of underground practices. These sites present a host of challenges to the design engineer. However, these challenges can be managed and designed. Similar to karst, there are general guidelines to consider when designing stormwater management practices in these areas, as presented below. Special caution for steep slopes and hidden bedrock fractures is urged.

General Stormwater Management Guidelines for Areas with Shallow Bedrock and Soils

• Developers should conduct thorough geotechnical investigations in areas with defined shallow bedrock and soils when contact with the bedrock or lack of adequate soil depth could cause a stormwater-related problem.
• A site geotechnical analysis similar to karst is recommended.
• Where infiltration is used, BMP depths will be limited. In fact, infiltration may be altogether infeasible at the site if a minimum three foot separation between the bottom of the practice and bedrock cannot be achieved.
• Design specifications for allowable ponding depths (e.g., live storage) in filters, swales, and bioretention should be considered to up to 12 inches (typical allowable depths range from six to nine inches). This will help reduce the required surface area of these facilities.
• Underground practices such as filters will be possible but very expensive if blasting required.
• Potential Stormwater Hotspot (PSH) infiltration may not be desirable due to potential for connections with bedrock fracture zones (see later section for a detailed discussion of PSHs).
• Stormwater wetlands will have greater potential than ponds for larger storage facilities due to limitation on ponding depths. However, this means larger surface area to drainage area ratios will be required.
• Engineered soil compost amendments may be required where soils are less than three feet deep to be eligible for certain stormwater credits (see Chapter 11 for credits discussion, specifically Impervious Cover Disconnection and Rooftop Disconnection).

Table 1 provides an overview of shallow bedrock and soil related design considerations for different structural practice groups.

BMP use in settings with shallow soils and shallow depth to bedrock Table 1

Structural BMP use in settings with shallow soils and shallow depth to bedrock.

BMP	Shallow soil and shallow depth to bedrock considerations
Bioretention	Should be constructed with an underdrain if minimum separation distance of three feet is not present between practice bottom and bedrock.
Media filter	* Recommended practice in areas of shallow bedrock and soil. * Can be located in bedrock, but will be expensive due to blasting.
Vegetative filter	* Recommended practice in areas of shallow bedrock and soil. * Dry swales with engineered soil media will need and underdrain if minimum separation distance of three feet is not present between practice bottom and bedrock.
Infiltration trench or basin	* Will be limited due to minimum separation requirement. Surface area to depth ratios of practices may need to be larger. Arch pipe and other perforated storage "vault" practices can help increase treatment volumes within limited spaces. * If used, should have supporting geotechnical investigations and calculations. * Use with PSHs should be carefully considered. Pre-treatment should be extensive to limit risk of groundwater contamination if groundwater is close to the land surface. * Local review authority should be consulted for approval.
Stormwater ponds	* Will have depth limitation to consider, making surface areas larger for a given storage volume. * Shallower depths may be undesirable from an aesthetic standpoint, particularly if wide fluctuations in water level are expected. * Bedrock should act like a liner and help to maintain a permanent pool, unless fracture zone is present.
Constructed wetlands	* Applied more easily than ponds, but will also require larger surface area to drainage area ratios. * Bedrock should act like a liner and help to maintain a permanent pool, unless fracture zone is present.

Investigation for Shallow Bedrock Areas

Geotechnical investigations are recommended for all proposed stormwater facilities located in regions with shallow bedrock and soils. The recommended approach is similar to those for karst areas. The purpose of the investigation is to identify subsurface conditions which could pose an environmental concern or a construction hazard to a proposed stormwater management practice. The guidelines outlined below should not be interpreted as all-inclusive. The design of any subsurface investigation should reflect the size and complexity of the proposed project.

Subsurface Material

The investigation should determine the nature and thickness of subsurface materials, including depth to bedrock and to the water table. Subsurface data may be acquired by backhoe excavation and/or soil boring. These field data should be supplemented by geophysical investigation techniques deemed appropriate by a qualified professional, which will show the location of bedrock formations under the surface. The data listed below should be acquired under the direct supervision of a qualified geologist, geotechnical engineer, or soil scientist who is experienced in conducting such studies. Pertinent site information shall be collected which should include the following:

• Bedrock characteristics (type, geologic contacts, faults, geologic structure, rock surface configuration).
• Soil characteristics (type, thickness, mapped unit).
• Bedrock outcrop areas.

Location of Borings

Borings should be located in order to provide representative area coverage of the proposed BMP facilities. The location of borings should be:

• In each geologic unit present, as mapped by the Minnesota (MGS) and U.S. Geological Surveys (USGS) and local county records;
• Next to bedrock outcrop areas (e.g., within ten feet);
• Near the edges and center of the proposed practice and spaced at equal distances from one another; and
• Near any areas identified as anomalies from any existing geophysical studies.

Number of Borings

The numbers of recommended borings are:

• Infiltration trenches, bioretention, and filters - a minimum of two per practice.
• Ponds/wetlands - a minimum of three per practice, or three per acre, whichever is greater.
• Additional borings - to define lateral extent of limiting horizons, or site specific conditions, where applicable.

Depth of Borings

Borings should be extended to a minimum depth of five feet below the lowest proposed grade within the practice unless auger/backhoe refusal is encountered.

Identification of Material

All material penetrated by the boring should be identified, as follows:

• Description, logging, and sampling for the entire depth of the boring.
• Any stains, odors, or other indications of environmental degradation.
• A minimum laboratory analysis of two soil samples, representative of the material penetrated including potential limiting horizons, with the results compared to the field descriptions.
• Identified characteristics should include, as a minimum: color; mineral composition; grain size, shape, and sorting; and saturation.
• Any indications of water saturation should be carefully logged, to include both perched and ground water table levels, and descriptions of soils that are mottled or gleyed should be provided.
• Water levels in all borings should be taken at the time of completion and again 24 hours after completion. The boring should remain fully open to total depth of these measurements.
• When conducting a standard penetration test (SPT), estimation of soil engineering characteristics, including “N” or estimated unconfined compressive strength.

Evaluation

At least one subsurface cross section through the proposed practice should be provided, showing confining layers, depth to bedrock, and water table (if encountered). It should extend through a central portion of the proposed practice, using the actual or projected boring data. A sketch map or formal construction plan indicating the location and dimension of the proposed practice and line of cross section should be included for reference, or as a base map for presentation of subsurface data.