Knowing the soils in the area of a proposed infiltration best management practice (BMP) helps determine the suitability and design of the BMP. Soils determine how quickly stormwater will infiltrate, affect plant growth, and affect the fate and transport of pollutants. Section 16.10 of the Construction Stormwater general permit states “Permittees must provide at least one soil boring, test pit or infiltrometer test in the location of the infiltration practice for determining infiltration rates”. The Minnesota Stormwater Manual recommends that soil borings or pits be dug to verify soil types and infiltration capacity characteristics and to determine the depth to groundwater and bedrock.
This article presents a discussion of soil sampling, soil borings, and interpreting soil information.
This video provides an example of the impact of even thin layers of clay on infiltration.
The recommended number of borings is shown in the folowing table.
Recommended number of soil borings, pits or permeameter tests for bioretention design. Designers select one of these methods.
Link to this table
|Surface area of stormwater control measure (BMP)(ft2)||Borings||Pits||Permeameter tests|
|1000 to 5000||2||2||10|
|5000 to 10000||3||3||15|
1an additional soil boring or pit should be completed for each additional 2,500 ft2 above 12,500 ft2
2an additional five permeameter tests should be completed for each additional 5,000 ft2 above 15,000 ft2
Prior to ordering a soil investigation, it is highly recommended that historical records are evaluated for the site. What other use(s) has the site had? Are historical soil borings available for the site? Could site soils be contaminated? A list of possible site soil constraints and additional information can be found here. This page provides a summary of useful information sources for investigating the history of a site.
The more variable the site history, the greater variety of soil types you may expect to find. For example, a rural 1 acre site that has always been farmed will probably have more consistent soils from one location to the next compared to an urban 1 acre site which has had many different uses, including importation of fill materials. An urban site with many previous uses may require a larger amount of soil investigation to properly identify the soil profile.
There are several types of soil sample collection methods which can be used to determine soil characteristics. Sample collection methods are listed in the following table from simplest/quickest/cheapest to complex/slower/expensive. The table describes advantages and limitations and provides references and photographs for each of these methods.
Of these six types of soil sample collection methods, soil borings are highly recommended. Soil borings provide the greatest amount of data. Knowing existing conditions as a result of proper soil investigation allows the design team to develop an accurate design and cost estimate prior to bidding or construction. Finding unexpected soil conditions during construction can result in time delays and costs associated with modifying the design.
Summary of drilling methods to collect soil samples for infiltration basins. Click on an image to enlarge.
Link to this table
|Drilling Method||Description||Sample Collection Possible||Depth||Advantages||Limitations||Photo reference/credit||Example Photo|
|Soil Probe||Soil probes are a basic form of soil sampling, used to collect shallow or surface samples by pushing a T-shaped probe into the ground. The probe cuts a core sample smaller than the diameter of the probe body. The sample can also be collected within a liner inside the probe.||Small diameter soil surface profile||≤ 3 feet||Inexpensive Quick Easy to use||May not be able to collect enough information to determine if infiltration is possible due to depth and limited soil information collected. Unable to perform blow counts.1||Iowa State University - University Extension|
|Hand held or machine auger||The auger is held vertically and is driven into the ground and rotated by the handle while applying leverage. At every 30 cm of depth penetrated, the auger is taken out and the samples of the soils are collected separately for examination. This method works best in loose materials ranging from clay to sand to gravel.||Bag/jar samples of material||≤ 50 feet||Inexpensive, Easy to use. Large selection of auger types||Only suitable for unconsolidated deposits (loose materials) Slow compared to other methods Equipment can be heavy. Unable to perform blow counts1.||University of New Hampshire - Institute for the Study of Earth, Oceans, and Space|
|Excavation Test Pit or Trench||A pit or trench is dug manually or with an excavator to depth desired in order to visually observe subsurface soil conditions.||Grab samples or bulk samples of material||≤ 20 feet||Quick, Easy to do, Shallow exploratory drilling method||Must be onsite during excavation to coordinate locations and depth as well as observe subsurface soils and behavior, taking photos and soil samples Shallow form of subsurface drilling Must be cautious of collapsable soils when digging Requires back fill and compaction. Unable to perform blow counts1.||United States Department of Agriculture - Natural Resources Conservation Service New Hampshire|
|Direct Push Technology (DPT)||Geoprobe or Strataprobe is pnuematic hammer driven with continuous sampling into plastic tubes. Blow counts1 are not provided through this method. Single tube and dual tube systems are available.||Bag/jar samples of material||usually up to 60 feet but can go to over 100 feet||Minimal to no cuttings Quick and efficient Small, relatively mobile rig Continuous sampling||Unable to perform blow counts1||Minnesota Pollution Control Agency|
|Rotosonic||Rotosonic drilling is a method where continuous samples are taken. It is good for penetrating through most formations, but due to the ground disturbance, is not suitable for collecting undisturbed samples. Samples are brought out of the subsuface in clear plastic sleeves.||Bag samples of material||>100 feet||Continuous sampling Quick and efficient Produces little to no waste||Mobilization cost can be expensive. Unable to perform blow counts1.||United States Geological Survey - Eastern Mineral and Environmental Resources Science Center|
|Standard Penetration Test (SPT) using Hollow Stem Auger (HSA) drilling method||Augers act as casing for borehole (prevent caving) Center of augers is hollow for sampling||Bag/jar sample Split-spoon (or other disturbed samples) Thin-wall samples (or other undisturbed samples)||>100 feet||Augers do not need to be removed to sample Can be used above and below water table. Can collect blow counts1.||Heaving sands may clog the hollow stem auger. Can be slow when drilling in soils with cobbles and boulders.||U.S. Department of Transportation - Federal Highway Administration|
There are two categories of soil samples, disturbed and undisturbed.
Having a geotechnical engineer/technician (engineer) present during drilling investigations is highly recommended as they specialize in field reconnaissance, coordinating and directing the drillers, logging boreholes, and collecting soil samples. Having an engineer onsite during drilling investigations will cost more initially, but if subsurface information is inaccurate or inadequate, a greater number of assumptions need to be incorporated into the design, which increases the potential for negative impacts later in the design life and/or costly surprises during construction.
If an engineer cannot be onsite during drilling investigations, good communication with the drillers prior to performing soil borings will help ensure the most valuable information is obtained. It is important the drillers understand the purpose of the investigation. Explain that drilling is being done to find soils indicative of restrictive layers; i.e. soils that inhibit the flow of water. Listed by most restrictive to least restrictive, these soils are: clay, silt, sand, and gravel. Provide drillers with the following guidance:
Soil samples help provide a general subsurface profile and can be used to measure engineering properties, including those properties indicating the presence of a restrictive soil layer. When a soil boring is performed to determine if a site is suitable for infiltration, the following information should be requested from a soil boring company or geotechnical engineer (see attached Table 2 for greater detail):
Summary of information to include on a boring log. To enlarge an image, click on the image.
Link to this table
|Include on log||Description||Example / Photo|
|Boring name or number||Important to identify and distinguish each boring with a unique name||Most borings start with B or BH and end with a number Example: B-01 or BH01|
|Project name, number, client, and city or county||Information is used identification purposes and for sample storage|
|Boring coordinates and ground surface elevation||Each hole should be located and verified with coordinates in case of discrepancies If possible, record ground surface elevation with GPS Locations should be shown on a figure or map||Include Datum (NAD29 vs NAD83, NAVD 1929 or NAVD 1988) Understand coordinate systems. For example. UTM is in meters while Lat/long is in degrees or deg/min/sec|
|Start and end date and time||Helpful for tracking how long drilling is taking per borehole A known date can help identify the project for future reference|
|Type of drill rig and boring depth||Will be used to determine the hammer efficiency if performing blow counts. Examples include:
||United States Geological Survey - Eastern Mineral and Environmental Resources Science Center|
|Drilling method||Record method used or depth where there is any change in the method being used. Different drilling methods may require different sampling techniques or procedures. Drilling methods include: Hollow Stem Auger, Direct Push Technology, Mud Rotary, Rotosonic, Air Rotary, ODEX.|
|Sampling method and sample type||
|Sample recovery||Record recovery of samples for comparison to length of drilled sample Note assumed reasoning for poor recovery (i.e. bedrock, gravel stuck in splitspoon)||For example, the split-spoon was pounded 18" but only 10" of soil was recovered in the sampler|
|Soil Density or Consistency||Different characterization of coarse and fine grained samples Based on N-value of sample|
|Color||Most colors should be earth tones with descriptors. Include all mottling or redoximorphic feature colors. Note iron staining or oxidized areas for indication of previously wet soil that has been dried out||North Carolina State Cooperative Extension Resources|
|Grain size||For granular samples only (not for silts and clays). Can be called: very fine-, fine-, medium-, coarse-, or very coarse-grained||Virginia Cooperative Extension - Virginia tech/Virginia State University|
|Particle shape||Can be angular, subangular, subrounded, or rounded. Important for understanding deposition environment and weathering of soil.|
|Sedimentary structure||Refers to bedding and laminations in the soil. Provides insight to origin of soils or potential weathering or chemical reactions experienced. Provides insight to geologic hazards (i.e. weak soil layers, collapsibility)|
|Layer boundaries and thickness||Note stratigraphy breaks on logs. Unless transition is obvious while drilling, assume it is halfway between sampling intervals.|
|Blow Counts||The number of hammer blows to pound the split-spoon 6, 12, 18, and 24 inches into the soil. The 6 to 12 and 12 to 18 inch blow counts are added and recorded as the n-value for that split-spoon. "Refusal" or termination of borehole occurs if the hammer does not advance for >50 blows/6 inches. Note the advancement (e.g. 50/2").|
|Depth to bedrock and/or refusal||"Refusal" or termination of borehole occurs if the hammer does not advance for >50 blows/6 inches. Note the advancement (e.g. 50/2"). Note refusal on logs. Note sample recovery if refusal is reached in a split spoon interval.|
|Depth of groundwater||Record during and after drilling Note perched layers of water in sand or lignite. Record cave-in depth in unstable boreholes. Note if water isn't encountered. Saturated silts will have high dilatancy.||U.S. EPA|
|Plasticity||Plasticity helps determine if a clay is lean (low plasticity) or fat (high plasticity). Is very dependent on moisture content (i.e. dry fat clay can seem like it has low plasticity)|
|Moisture||Provides indication of water table. For each sample identify if it is moist, wet, or saturated.|
|Inclusions||Used to describe the secondary components in a soil sample.
||United States Geological Survey|
|Topsoil thickness||Helpful to know how much topsoil stripping may be necessary. Note organic material if present or not.||Pat Dumas. Creative Commons BY-NC-SA (cropped).|
|Photographs||Pictures should be taken of each borehole location. Take photo of stake and all four directions. Record and photograph any crop damage. Photograph any interesting soil samples that may be of concern|
Boring logs can be presented vertically or horizontally. Examples of logs for four of the drilling methods previously presented are attached.
Note that layer boundaries are well defined on each of these logs making it easy to identify the transition between different soil types. All the information discussed in Table 2 is presented on the logs as well as laboratory testing results.
Note that each of the example logs above utilize the United Soil Classification System (USCS). While this is the most commonly used classification system on boring logs, other systems, such as the United States Department of Agriculture (USDA) classification system, may be used. An example blank form that allows for both the USCS and USDA classifications is shown in the figure to the right (Source: USDA-NRCS). Comparisons between the USCS and USDA classifications are shown in the following table.
Design infiltration rate as a function of soil texture for bioretention in Minnesota
The values shown in this table are for uncompacted soils. This table can be used as a guide to determine if a soil is compacted. For information on alleviating compacted soils, link here. If a soil is compacted, reduce the soil infiltration rate by one level (e.g. for a compacted B(SM) use the infiltration rate for a B(MH) soil). Link to this table
|Soil texture (USDA classification)||Corresponding Unified Soil Classification||Corresponding hydrologic soil Group (HSG)||Design Infiltration Rate (in/hr)|
|Gravel, sandy gravel, and silty gravels||GW – Well-graded gravel or well-graded gravel with sand
GP – Poorly graded gravel or poorly graded gravel with sand
|Sand, loamy sand, and sandy loam||GM – Silty gravel or silty gravel with sand
SW – Well-graded sand or well-graded sand with gravel SP – Poorly graded sand or poorly graded sand with gravel
|Silty sand||SM – Silty sand or silty sand with gravel||B||0.45|
|Silt loam, loam||ML – Silt
OL – Organic silt or organic silt with sand or gravel or gravelly organic silt
|Sandy clay loam||GC – Clayey gravel or clayey gravel with sand
SC – Clayey sand or clayey sand with gravel
|Clay, clay loam, silty clay loam, sandy clay, and silty clay||CL – Lean clay or lean clay with sand or gravel or gravelly lean clay
CH – Fat clay or fat clay with sand or gravel or gravelly fat clay OH – Organic clay or organic clay with sand or gravel or gravelly organic clay MH – Elastic silt or elastic silt with sand or gravel
Samples collected from the soil exploration can be tested in the laboratory for Particle Size Distribution (PSD). This test is critical to determining the most limiting soil layer with respect to infiltration. A disturbed or bulk sample is appropriate for this test. The PSD test was developed to classify a soil particle based on its size according to the particle’s ability to pass through a series of standard sieves. A sieve is a carefully manufactured mesh of wires with a specified opening size. The ASTM D2487 standard is the most commonly used for particles retained on the #200 sieve or greater than 0.003 inches (0.075 millimeters) in diameter. A hydrometer analysis can be performed on particles smaller than the #200 sieve following ASTM D422 standard, but is generally not necessary for infiltration analysis. If a large percentage of soil sample particles are passing the #200 sieve, infiltration is not recommended. PSD curves are created from the sieve and/or hydrometer results which provide the information needed to classify a soil and determine if the soil is poorly-graded (does not have a good representation of soil particle sizes) or well-graded (has a good representation of soil particle sizes). An example of a PSD curve is shown to the right.
A restrictive soil layer is one continuous layer of soil that impedes the movement of water through it. A restrictive layer will also impede air movement and root growth. Bedrock, frozen soils, dense soils, cemented soils, saturated soils, and a clay layer are examples of restrictive layers. A restrictive soil layer is capable of perching groundwater above the layer. This video provides an example of the impact of even thin layers of clay in soil.
Infiltration is limited by the least permeable or most restrictive layer in the soil profile. The Minnesota Construction Stormwater general permit prohibits infiltration in areas with less than three feet of separation distance from the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock. Other published guidance found on the California Environmental Protection Agency State Water Resources Control Board website recommends that any restrictive soil layer found within ten feet of the bottom of an infiltration BMP should be evaluated by a Professional Engineer (PE) and approved by the City Engineer.
If a restrictive layer is found within ten feet of the bottom of an infiltration BMP, the Project Engineer should evaluate whether the site is a good location for an infiltration BMP, or if another type of BMP would be more successful. Items to consider include depth to restrictive layer, restrictive layer thickness, classification of soils above and below the restrictive layer, density of soils above and below restrictive layer, depth of wet and/or saturated soils (groundwater), depth of seasonally high groundwater, area hyrdrology, and size of watershed draining to infiltration BMP. Additional considerations when considering a site for infiltration can be found here in the Minnesota Stormwater Manual:
At the end of a soil exploration program, and when the soils collected from the field have been subjected to visual observation and laboratory testing, a soil exploration report is prepared for use in planning and design, clearly summarizing results and providing soil data (including boring logs). A larger exploration program will required a larger report while a smaller program might only be summarized in a short technical memo. The items listed below should be present in both types of reports, but the level of information or detail summarized will vary.
Additional information about evaluating site soils can be found in Site Evaluation for Stormwater Infiltration by the Wisconsin Department of Natural Resources Conservation Practice Standards (2004).
Information on soil investigations for petroleum contaminated sites include the following.