LID Standards Manual and Grading/LID and...Boring Percolation Test Procedure ... surrounded by a bentonite cap, and placed at any depth in the borehole. It requires the application

ADMINISTRATIVE MANUAL GS200.1COUNTY OF LOS ANGELES 06/30/14

DEPARTMENT OF PUBLIC WORKSGEOTECHNICAL AND MATERIALS ENGINEERING DIVISION

GUIDELINES FOR DESIGN, INVESTIGATION, AND REPORTINGLOW IMPACT DEVELOPMENT STORMWATER INFILTRATION

Urbanization impacts the water resources of Los Angeles County by decreasing theamount of stormwater that infiltrates into the subsurface, and by increasing the potentialfor conveyance of pollutants into watersheds and the flood control system. Low ImpactDevelopment (LID) stormwater infiltration is a strategy that is used to mitigate some ofthese hydrological impacts. The goal of LID stormwater infiltration is to reduce runofffrom the site using stormwater quality control measures that retain runoff. The objectiveof these guidelines is to facilitate stormwater infiltration in areas of Los Angeles Countywhere the ground conditions are suitable.

Compliance with the Los Angeles County LID Ordinance (Title 12, Section 12.84)is required before the issuance of a building or grading permit. The Department ofPublic Works prepared an updated LID Standards Manual in February, 2014 to compileprevious documents, update standards, and assist applicants with the developmentprocess. The LID Standards Manual is available online at:

http://dpw.lacounty.gov/ldd/lib/fp/Hydrology/Low%20Impact%20Development%20Standards%20Manual.pdf

The geotechnical guidelines presented herein have been incorporated into theLID Standards Manual in "Section 4: Site Assessment and Design Considerations"and on the Fact Sheets in Appendix E. They provide technical guidance and specificrequirements for geotechnical investigations that evaluate ground conditions forproposed stormwater infiltration sites. All proposed stormwater quality control measureBest Management Practices (BMPs) with an infiltration component requirea geotechnical report. These LID stormwater quality control measures include but arenot limited to:

BioretentionInfiltration TrenchPermeable Pavement

Infiltration BasinDry Well

Geotechnical reports prepared for LID stormwater quality control measure infiltrationBMPs must address the Site Requirements discussed in these guidelines. Data andanalyses must be provided to substantiate the recommended infiltration rates andgroundwater elevations. Geotechnical issues that must be addressed include pollutantand sewage mobilization, slope stability, static and seismic settlement, surcharge onadjacent structures, expansive soil and rock, potential impacts to offsite property,and any other geotechnical hazards.

P:\Gmepub\Development Review\Policy Memos\GS200.1.docx Page 1 of 1406/30/14

SITE REQUIREMENTS FOR STORMWATER INFILTRATION

1. Subsurface materials shall have a corrected infiltration rate equal to or greaterthan 0.3 inches per hour (in/hr). Procedures for performing in-situ infiltrationtests and application of correction factors are described later in these guidelines.

2. The invert of stormwater infiltration shall be at least 10 feet above thegroundwater elevation. Procedures for determining the groundwater elevationare described later in these guidelines.

3. Stormwater infiltration is not allowed in areas that pose a risk of causing pollutantmobilization. Areas with known groundwater contamination include sites listedon the State Water Resources Control Board's "GeoTracker" website.

4. Stormwater infiltration is not allowed in areas that pose a risk of causing sewageeffluent mobilization from septic pits, seepage lines, or other sewage disposal.

5. Stormwater infiltration BMPs shall not be placed on steep slopes and shall notcreate the condition or potential for slope instability.

6. Stormwater infiltration shall not increase the potential for static or seismicsettlement of structures on or adjacent to the site. Potential geotechnicalhazards that shall be addressed include collapsible soils and liquefaction.

7. Stormwater infiltration shall not place an increased surcharge on structures orfoundations on or adjacent to the site. The pore-water pressure shall not beincreased on soil retaining structures on or adjacent to the site.

8. The invert of stormwater infiltration shall be set back at least 15 feet, and outsidea 1:1 plane drawn up from the bottom of adjacent foundations.

9. Stormwater infiltration shall not be located near utility lines where the introductionof stormwater could cause damage to utilities or settlement of trench backfill.

10. Stormwater infiltration is not allowed within 100 feet of any groundwaterproduction wells used for drinking water.


GEOTECHNICAL INVESTIGATION

A site-specific geotechnical investigation performed for proposed stormwater infiltrationquality control measures shall include subsurface exploration, laboratory testing,soil type classification, groundwater investigation, and in-situ infiltration testing.The investigation must be conducted by or under direct supervision of a State ofCalifornia licensed engineering geologist, geotechnical engineer, or civil engineerexperienced in the field of soil mechanics.

Subsurface Exploration

Subsurface exploration shall be performed to characterize the subsurface soil or rockthrough which water will infiltrate. Explorations shall be performed to a depth of at least10 feet below the proposed invert of infiltration. Explorations should be performed ateach proposed infiltration BMP location. For continuous infiltration improvements,enough exploration shall be performed to sufficiently characterize the soil or rock.

Laboratory Testing

Laboratory testing shall be performed to characterize the subsurface soil orrock through which water will infiltrate and confirm visual classifications made inthe field. Tests shall be performed on samples collected at and below the proposedinvert of stormwater infiltration. Sieve analysis, hydrometer, plasticity index, density,and moisture content tests are the best indicators of infiltration potential. Classificationsmust be made according to the two systems discussed below. A discussion shouldbe provided on how the soil porosity and moisture content will affect the proposedstormwater quality control measure BMP.

Soil Type Classification

Soil types are one of the best indicators to determine whether or not a proposed site willbe suitable for infiltration. Classifications of subsurface soils at and below the proposedinvert of infiltration shall be made in accordance with the following systems:

1. Unified Soil Classification System (USCS). The USCS is defined by the AmericanSociety for Testing and Materials (ASTM) International Standard D2487.

2. Hydrologic Soil Group (HSG). The HSG specifically classifies soils withregard to infiltration potential. The United States Department of AgricultureNatural Resources Conservation Service, National Engineering Handbook,Chapter 7 Hydrologic Soil Groups, is available online at:

http://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17757.wba


Coefficient of Permeability

For practitioner applications, the coefficient of permeability is a soil index property that isunderstood to be closely related to the infiltration potential of soils. The figure belowpresents typical coefficients of permeability for different soil type classifications. It isprovided as a general reference. As shown, the minimum corrected infiltration raterequirement is 0.3 in/hr.

Permeability and Drainage Characteristics of Soils from Terzaghi and Peck

Groundwater Investigation

For sites where the historic high groundwater is greater than 10 feet below the proposedinvert of stormwater infiltration, the historic high groundwater elevation may be used.Historic high groundwater elevations may be obtained from the Seismic HazardEvaluation Open-File Reports prepared by the California Geological Survey at thefollowing link: http://www.consrv.ca.gov/cgs/shzp/pages/index.aspx.

For sites where the historic high groundwater is within 10 feet of the proposed invert ofinfiltration, but existing well data in the vicinity of the proposed site shows an elevationgreater than 10 feet below the invert of infiltration, existing well data may be used.Monitoring wells operated by the Department of Public Works Water Resources Divisionmay be accessed online at the following link: http://dpw.lacounty.gov/wrd/wellinfo/.

For sites where the historic high groundwater and existing well data are within 10 feetof the proposed invert of infiltration, a site-specific groundwater investigation mustbe performed to justify using a deeper groundwater elevation. At least two borings mustbe drilled to depths at least 10 feet greater than the proposed invert of infiltration.The borings must be monitored for a period of at least 24 hours.

0.3 inches per hour


IN-SITU INFILTRATION TESTING

Infiltration tests must be performed to determine a corrected infiltration rate for design ofthe proposed stormwater infiltration quality control measures. An infiltration test shall beperformed at each location and elevation where a stormwater infiltration BMP isproposed. Due to site variability and potential uncertainty in the testing procedures,it is recommended that multiple tests be performed for each BMP and a representativecorrected infiltration rate be selected for design.

Six acceptable testing procedures and the corresponding correction factors that must beapplied for design are discussed below. In general, the double-ring infiltrometer andwell permeameter tests are preferred because their procedures are standardized andwell-documented. All of the procedures have significant soaking and data collectionperiods in an attempt to model the behavior of the stormwater quality control measureduring a design storm event.

Double-Ring Infiltrometer Test

A double-ring infiltrometer consists of two concentric metal rings. The rings are driveninto the ground to preclude leakage, and then filled with water. Water in the outer ringkeeps the flow in the inner ring vertical and the drop in water level in the inner ring isused to establish the vertical infiltration rate. This testing procedure is useful forLID features that are proposed close to the ground surface, or can be performed atdepth in a trench excavation. Procedures and example data forms for double-ringinfiltrometer testing are provided in ASTM D3385. See photo below for example testsetup. Field log template with example are attached on Plates 1-A, 1-B, and 1-C.

Double Ring Infiltrometer (ASTM D3385) Test Setup


Well Permeameter Test

The well permeameter procedure consists of introducing water into the subsurfacethrough a slotted PVC pipe inserted into a borehole. This testing procedure is useful forLID features that are proposed at depth, since slotted sections of PVC pipe can beplaced at any depth in the borehole. Careful attention must be paid to isolate the depthof the test section with an impermeable cap above and below it. The annulus betweenthe slotted PVC and native materials in the test section depths must be backfilled withwell-draining sand. The borehole below the desired test section depths, and theannulus between solid PVC and native materials above the desired test section,must be backfilled with bentonite or other low-permeability material. The borehole itselfcannot create a path of less resistance for the water than the in-situ materials that arebeing tested.

Details for this test can be found in the Procedure for Performing Field PermeabilityTesting by the Well Permeameter Method (USBR 7300-89) attached in Appendix A.See photo below for example test setup. Field log template with example are attachedon Plates 2-A, 2-B, and 2-C.

Well Permeameter (USBR 7300-89) Test Setup


Boring Percolation Test Procedure

This procedure is similar to the USBR 7300-89 Well Permeameter Testing Procedureand is useful for LID features that are proposed at depth, since the depth of testing canbe isolated with slotted sections of PVC pipe, surrounded by a bentonite cap,and placed at any depth in the borehole. It requires the application of a reduction factorto account for non-vertical flow. A figure is attached on Plate 3-A. Field log templatewith example are attached on Plates 3-C and 3-D.

1. Using a hollow-stem auger, advance the boring at least 12 inches below theelevation of proposed invert of infiltration. Rotate the auger until all cuttings areremoved. Care shall be taken to ensure smearing of clayey soils does not occuralong augered surface as this will dramatically reduce the final calculatedinfiltration rate. Record the boring diameter and depth to be tested.

2. Install through the auger, a 2- to 4-inch-diameter perforated PVC casing with asolid end cap. Perforations shall be 0.02 inch slot or larger. Pour filter packdown inside of auger while withdrawing the auger such that the PVC casing issurrounded by the filter pack. The filter pack and perforated casing must have alarger hydraulic conductivity than the soil or rock that is to be tested.

3. For boreholes drilled below the proposed invert of infiltration that are beingconverted to boring percolation tests, careful attention must be paid to isolate thedepth of the test section with an impermeable cap above and below it.The annulus between the slotted PVC and native materials in the test sectionmust be backfilled with well-draining sand. The borehole below the desired testsection, and the annulus between solid PVC and native materials above thedesired test section, must be backfilled with bentonite or similar low-permeabilitymaterial. The borehole itself shall not create a path of less resistance for thewater than the in-situ materials being tested.

4. Presoak the hole immediately prior to the percolation testing. Presoaking the testhole shall maintain a water level above the percolation testing level and at least12 inches above the bottom of the boring. If the water seeps completely awaywithin 30 minutes after filling the boring two consecutive times, and thesubsurface exploration has yielded permeable soils beneath the proposed invertof infiltration, presoaking can be considered complete and the testing canproceed. If the water does not completely drain within 30 minutes, presoak thehole for at least 4 hours before conducting the infiltration test. A sounder orpiezometer may be used to determine the water level. Record all water levels tothe nearest -inch increment.


5. After presoaking, determine the time interval that will be used to measure thewater drop readings for the percolation test. Fill the hole to a minimum depth of12 inches above the top of the bentonite plug. Observe the drop in the waterduring the next 30 minutes and compare with the condition that applies below.This will determine the standard time interval for this test location:

a. If no water remains in the hole, the time interval between readings shall be10 minutes.

b. If water remains in the hole, the time interval between readings shall be30 minutes.

6. Once the time interval for the test has been determined, add water to the casingto the depth of soil to be tested. The water depth must be less than or equal tothe water level used to presoak the hole and a minimum depth of 12 inchesabove the bentonite plug. For each successive percolation test reading,the starting water level must be at this initial water depth.

7. Conduct the percolation test by taking readings of the water drop from the initialwater depth. Record the time and the drop in water level during the standardtime interval determined in Step 5. Fill the boring back to the initial water depth.

8. Repeat the percolation test readings a minimum of eight times or until astabilized rate of drop is obtained, whichever occurs first. A stabilized rate iswhen the highest and lowest readings are within 10 percent of each other fromthree consecutive readings.

9. The average drop of the stabilized rate over the last three consecutive readingsis the preadjusted percolation rate at the test location, expressed in inches perhour. The preadjusted percolation rate must be reduced to account for thedischarge of water from both the sides and bottom of the boring (i.e., non-verticalflow). Use the following formula to determine the infiltration rate:

Reduction Factor (Rf) = 1d2

R1

DIA

df

With:d1 = Initial Water Depth (in.)

DIA = Diameter of the boring (in.)


Excavation Percolation Test Procedure

Similar to the double-ring infiltrometer, this testing procedure is useful for LID featuresthat are proposed to be constructed close to the ground surface, or can be performed atdepth in a trench excavation. It requires the application of a reduction factor to accountfor nonvertical flow. A figure is attached on Plate 3-B. Field log template with exampleare attached on Plates 3-C and 3-D.

1. Excavate a 1 cubic foot hole (1 foot deep x 1 foot wide x 1 foot long) at theelevation of the proposed invert of infiltration. Insert a wire-cage to support thewalls. The actual excavation depth may be deeper than 12 inches; however,during the test the water shall be limited to 12 inches in depth.

2. Presoak the hole by filling it with water immediately prior to the percolationtesting. If the water seeps completely away within 30 minutes after filling theexcavation two consecutive times, and the subsurface exploration and hasyielded permeable soils beneath the proposed invert of infiltration, presoakingcan be considered complete and the testing can proceed. If the water does notcompletely drain within 30 minutes, presoak the excavation maintaining12 inches of water for at least 4 hours before conducting the infiltration testing.Record all water levels to the nearest -inch increment.

3. After presoaking, determine the time interval for recording the water dropbetween readings. Fill the excavation 12 inches above the bottom. Observe thedrop in the water during the next 30 minutes and compare with the condition thatapplies below. This will determine the standard time interval for this test location.

a. If no water remains in the hole, the time interval between readings shall be10 minutes.

b. If water remains in the hole, the time interval between readings shall be30 minutes.

4. Once the time interval for the test has been determined, add water to 12 inchesabove the bottom of the excavation. For each successive percolation testreading, the starting water level must be at this initial water depth.

5. Conduct the percolation test by taking readings of the water drop from the initialwater depth. Record the time and record the drop in water level during the timeinterval determined in Step 3. Fill the excavation back to the initial water depth.

6. Repeat the percolation test readings a minimum of eight times or until astabilized rate of drop is obtained, whichever occurs first. A stabilized rate iswhen the highest and lowest readings are within 10 percent of each other forthree consecutive tests.


7. The average drop of the stabilized rate over the last three consecutive readingsis the preadjusted percolation rate at the test location, expressed in inches perhour. The preadjusted percolation rate must be reduced to account for thedischarge of water from both the sides and bottom of the boring (i.e., non-verticalflow). Use the following formula to determine the infiltration rate:

Reduction Factor (Rf) = 15.13

d2R

1 df

d1 = Initial Water Depth (in.)= Water Level Drop of Final Period or Stabilized Rate (in.)

DIA = 13.5 (Equivalent Diameter of the boring) (in.)

High Flowrate Percolation Test Procedures

If the water is draining faster than an infiltration rate of 14 inches per hour during any ofthe previous testing procedures, a modified test must be performed to record theinfiltration rate. This test is conducted in the following manner:

1. Determine the surface area (sides and bottom) through which the water isinfiltrating.

2. Flood that area in a suitable manner where the rate of water discharging into thetest pit can be measured.

3. Calculate the infiltration rate by dividing the rate of discharge (i.e., cubic inchesper hour) by the infiltration surface area (i.e., square inches).

Policy for New Percolation Basin Testing, Design and Maintenance

The County implemented the Policy for New Percolation Basin Testing, Design andMaintenance on October 10, 2007 for private development projects. The policy wasimplemented due to an increase in development and a lack of drainage features incertain areas of Los Angeles County. The hydrologic criteria and water quality portionsof the policy have been superseded by the 2014 LID Standards Manual; however, thetesting procedure is still applicable for infiltration basins proposed as part of largeprivate development projects. The testing procedure is outlined in Attachment 1 of thedocument attached to this policy as Appendix B.


CORRECTION FACTORS

Measured infiltration rates must be reduced with correction factors to determine designvalues that will represent long-term performance of the proposed infiltration BMPs.Test-specific correction factors are applied to account for the direction of flow during thetest and calculations. The correction factor for site variability, number of testsperformed, and thoroughness of subsurface investigation should be selected bycomparing the size and scope of subsurface exploration to similar projects.The correction factor for siltation, plugging, and maintenance should be selected basedon the specified levels of pre-treatment and maintenance for the proposed BMPs.For example, stormwater infiltration BMPs that are proposed with pretreatmentcomponents and regular maintenance programs, a correction factor of 1 may beappropriate; for BMPs that are proposed to infiltrate untreated flow with unspecifiedmaintenance programs, a high level of siltation and plugging is to be expected and acorrection factor of 3 is likely more appropriate.

The following table provides guidance for the range of values used for each factor.The geotechnical consultant shall determine site-specific correction factors and providesubstantiating data and analyses to justify the selection. All correction factors will besubject to review and approval by the County.

Correction Factors Applied to Measured Infiltration Rates

Double-ring infiltrometer

Well permeameter

Boring percolation

Excavation percolation

High flow-rate percolation

Policy for new percolation basins

CFt = 1

= 1

See test procedures = Rf

See test procedures = Rf

= 2

= 2

Site variability, number of tests, andthoroughness of subsurface investigation

CFv = 1 to 3

Long-term siltation, plugging andmaintenance

CFs = 1 to 3

Total Correction Factor, CF = CFt x CFv x CFs

Design Infiltration Rate = Measured Percolation Rate/CF


REPORTING

The geotechnical report shall provide an evaluation of the specific stormwater qualitycontrol measures that are proposed, and their suitability for use at the specified projectlocation based on the subsurface conditions. The report shall address any potentialgeotechnical hazards. The report shall contain a description of the subsurfaceconditions with logs of subsurface exploration, results of laboratory testing,soil classifications, depth to groundwater, and in-situ infiltration test results. There shallbe a discussion on the infiltration test procedure that was performed including field datasheets, test results, and correction factors. The compilation of data must provide areasonable understanding of the subsurface conditions and the ability to infiltrate at theproposed location and depth. The report must be signed and stamped by a State ofCalifornia licensed engineering geologist, geotechnical engineer, or civil engineerexperienced in the field of soil mechanics.

At a minimum, the following must be discussed in all infiltration reports submitted forCounty regulatory compliance:

Existing and Proposed Site ConditionsPotential Geotechnical HazardsLogs of Subsurface ExplorationGeotechnical Map with Subsurface Exploration LocationsResults of Laboratory TestingSoil Classifications (USCS and HSG)Groundwater ElevationMeasured Infiltration RateCorrection Factors and Design Infiltration RateProposed Stormwater Quality Control Measure Locations and Invert Depths

The report shall specify the recommended invert depth of the proposed stormwaterquality control measure. The invert depths shall be noted on the geotechnical map foreach location of proposed LID feature. Infiltration tests must be conducted when thefinal grades of the subject site have been established. Guidance should be provided tothe developer such that no on-site grading or construction will disturb soils at or belowthis specified invert depth of stormwater infiltration. If operation and maintenance of theproposed LID feature is critical to maintaining the design infiltration rate,the geotechnical consultant shall discuss the best practices to maintain the structureand provide suggestions for design use and life. All recommendations from thegeotechnical consultant must be incorporated into the design or shown as notes on theplans.


RESOURCE DOCUMENTS

1. American Standard Test Method (ASTM) Standard, Designation D 3385,Standard Test Method for Infiltration Rate of Soils in Field Using Double-RingInfiltrometer (latest edition). http://www.astm.org/Standards/D3385.htm

2. California Department of Conservation, Seismic Hazard Zone Reports,Division of Mines and Geology, Los Angeles County, 1998.http://www.consrv.ca.gov/cgs/shzp/pages/index.aspx

3. California Regional Water Quality Control Board Los Angeles Region,Waste Discharge Requirements for Municipal Separate Storm Sewer System(MS4), NPDES Permit No. CAS004001, Order No. R4-2012-0175,November 8, 2012.http://www.waterboards.ca.gov/rwqcb4/water_issues/programs/stormwater/municipal/

4. County of Los Angeles, Code of Ordinances, Title 12, Chapter 12.84, Low ImpactDevelopment Standards.https://library.municode.com/html/16274/level2/Tit12EnPr_Ch12.84loimdest.html

5. County of Los Angeles, Department of Public Health, A professional Guide toRequirements and Procedures for Onsite Wastewater Treatment Systems(OWTS), 2013. http://www.publichealth.lacounty.gov/eh/EP/lu/lu_owts.htm

6. County of Los Angeles, Department of Public Works, Low Impact DevelopmentStandards Manual, February 2014.http://dpw.lacounty.gov/ldd/lib/fp/Hydrology/Low Impact Development StandardsManual.pdf

7. State of California, Department of Transportation, Division of EngineeringServices, Soil and Rock Logging, Classification, Presentation Manual, 2010.http://www.dot.ca.gov/hq/esc/geotech/sr_logging_manual/srl_manual.html

8. Terzaghi, K., Peck, Ralph B., and Mesri, G., Soil Mechanics in EngineeringPractice, Third Edition, 1996.

9. United States Department of the Interior, Bureau of Reclamation(USBR), Procedure for Performing Field Permeability Testing bythe Well Permeameter Method, USBR 7300-89.http://www.usbr.gov/pmts/wquality_land/DrainMan.pdf

10. United States Department of Agriculture, Chapter 7: Hydrologic Soil Groups,Natural Resources Conservation Service National Engineering Handbook,http://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17757.wba


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Appendix A

United States Bureau of Reclamation

Test Method 7300 89

UNITED STATES DEPARTMENT OF THE INTERIORBUREAU OF RECLAMATION

P R O C E D U R E F O RUSBR 7300-89

PERFORMING FIELD PERMEABILITY TESTINGBY THE WELL PERMEAMETER METHOD

INTRODUCTION

This procedure is under the jurisdiction of the Geotechnical Services Branch, code D-3760, Research and Laboratory Services

Division, Denver Office, Denver, Colorado. The procedure is issued under the fixed designation USBR 7300. The number immediately

following the designation indicates the year of acceptance or the year of last revision.

1. Scope

1.1 This designation is used to determine the coefficient

of permeability of semipervious and pervious soils. Thetypes of soil for which the test is applicable range from

mixtures of sand, sil t, and clay with coefficients of

permeability greater than 1 X cm/s to relatively cleansands or sandy gravels with coefficients of permeability

less than 1 X cm/s. There is lack of experience with

the test in soils with coefficients of permeability outsidethese limits. The effects of capillarity on permeability test

results were not taken into account during developmentof the theoretical background.

l.-This test is similar to the “Shallow Well Pump-in

Test for Hydraulic Conductivity” in the Drainage ManualHowever, some of the float valves allow greater waterflow from

the water reservoir than the carburetor valve of the DrainageM anual t e s t .

2. Auxiliary Tests

2.1 Soil sampling by USBR 7010 and classificationof

soil from different strata by USBR 5005 are required to

identify soil stratification and location of any water table.

3. Applicable Documents

3.1 USBR Procedures:

USBR 3900 Standard Definitions of Terms and Symbols

Relating to Soil MechanicsUSBR 5005 Determining Unified Soil Classification

(Visual Method)

USBR 7010 Performing Disturbed Soil Sampling Using

Auger Boring Method

3.2 ASTM Standard:

E 1 ASTM Thermometers

4. Summary of Method

4.1 The method consists of measuring the rate at which

water flows out of an uncased well under a constant gravity

N um b e r i n br a ck e ts r ef e r s t o th e r ef er e n ce .

head. The coefficient of permeability of the soil is calculated

using (1) the relatively constant flow rate which is reachedafter a period of time, (2) the water temperature, (3) the

constant height of water in the well, and (4) the radius

of the well.

5. Significance and Use

5.1 The method is used to determine the average

coefficient of permeability for soil in its natural condition,

primarily along proposed canal alignments or at reservoirsites. The permeability results are used in appropriate

equations for calculating approximate seepage rates to aid

in decisions on lining requirements. Although the test isusually performed in auger holes, it can also be used in

test pits.

6. Terminology

6.1 Definitions are in accordance with USBR 3900.

7. Interferences

7.1 Proper use of the test requires soil characteristics

which allow excavation of an uncased well of reasonablyuniform dimensions with the soil sufficiently undisturbed

to allow unrestricted outward flow of water from the hole.

7.2 Test results are adversely affected by using uncleanwater for the permeant.

7.3 When relatively impervious or highly pervious soil

layers are present around the well, this should be consideredwhen evaluating test results.

7.4 For tests during cold weather, a shelter with heatshould be used to maintain ground and water temperatures

above freezing.

8. Apparatus

8.1 General Apparatus:8.1.1 Augers.-Hand augers suitable for excavating

permeability test holes. Power-driven augers may be used

if it is determined that disturbance of soil around the wellis no more than for a hand auger.

1227

USBR7300

t a p p e d f o rf - 2 0 t h r e a d s p e r

I

Threadsp e r

Cover

b r a z e dto d ru m - - -

/

i m pa c ttransparent plastic

s u c h ce l lu lose

2 0 0 - Lequivalent,l o

brass block

set screw

No Brass,

plated safety

waste-nut to drum

Threadsper inch

- 2 - f ’ a n dwashers

I ASSEMBLY

SCALE OF INCHES

Plastic disk

-Coupling

b e

with tape

-Schedule 40PVC pipe

REOUIRED

28 Gaugeiron tubing

b e n o n co r r os i v ep l as t i c

or redwoodfdipped inp r e s e r v a t i v e o f

Figure Drawing of well test apparatus (101-D-38)

USBR 7300

8.1.2 Thermometer.-0 to 50 0.5 divisions,

conforming to the requirements of ASTM E 1.8.1.3 Hammer, surveyors’ stakes, and string for

depth measurements in the well.8 . 2 Equipment Unique to This Procedure (see figs. 1

and 2).8.2.1 Water Reservoir.-A clean, covered, watertight

reservoir of sufficient capacity which can be convenientlyrefilled at intervals to provide a continuous supply of waterduring the test. A drum with a volume gaugetube of cellulose acetate butyrate has been found to besuitable for normal usage. Wooden blocking is requiredto raise the reservoir above the ground level.

8.2.2 K&e.-A float valve with operating arm (seefig. 3 for valve size).

8.2.3 Float.-A wooden, plastic, or metal float withbrass stem.

8.2.4 Float Guide.-A guide of galvanized iron, PVC

(polyvinyl chloride) or other materials to allow the floatto move vertically.

8.2.5 Counterwe&hts.-Brass counterweights for armof float valve.

Figure 2. Typical well test set-up.

Figure Maximum permeability coefficients measurable with typical

float valves commonly used on stock-watering tanks.

NOTE may be other appropriate valve-float equip-ment available for maintaining a constant water level in thetest well.

8.2.6 Water Truck.-A water tank truck or tank trailerof sufficient capacity to provide a continuous supply ofclean water for the number of test reservoirs insimultaneous use.

9. Reagents and Materials

9.1 Density San&Clean, dry, pervious, coarse sand (orfine gravel) calibrated for density and with a coefficientof permeability at least 1 X cm/s greater than thatof the soil to be tested is to be used for backfilling thetest well. A washed sand graded between the U.S.A.Standard series No. 4 to No. 8 sizes (4.75 to 2.36 mm)or grave1 graded between the to No. 4 (9.5 to 4.75mm) sizes is recommended. The purpose of the perviousbackfill is to (1) distribute water evenly in the well, (2)

support the wall of the well and prevent sloughing duringsaturation of the soil, and (3) provide a means of indirectlydetermining the average radius of the well. The radiusof the well is required for permeability calculations and,as explained later, a standard sand calibrated for mass per

unit volume (density) can serve this purpose.9 . 2 Water.-The water for this test is to be clean. Small

amounts of suspended soil or other foreign material inthe water may become deposited in the soil around thewell and may greatly reduce the flow, causing erroneousresults. When there is sediment in the water, arrangementshould be made to remove the particles by settling orfiltration. In some instances, a chemical reaction can take

place between water of a particular quality and the soilbeing tested, which may cause an increase or decrease insoil permeabili ty. Therefore, water similar in quality(exclusive of suspended sediment) to that expected topermeate the soil during project operation should be used

for the permeability test.

10. Precautions

10.1 SafetyPrecautions.- Norma1 precautions taken forany fieldwork.

10.2 Technical10.2.1 In windy areas, protection from blowing soil

may be needed to prevent interference to the operationof the valve-float mechanism and to prevent infiltrationof soil into the top of the well.

10.2.2 Test equipment must be protected fromdisturbance by animals, moving equipment, children, or

other sources.

11. Calibration

11.1 Water Reservoir (fig. the volumeof the water reservoir and mark the gauge tube inconvenient increments for volume readings. For a 200-Lreservoir, mark the volume gauge tube at 5-L intervalswith the largest volume reading near the top of the tube

1227

so volume readings will decrease downward and permit

volume determination by subtracting figures.

a volume tube of cellulose acetate butyrate (which

is recommended because it is durable for use under field

conditions), ink with an acetate base makes a permanent mark

on the tube. India ink can be used for marking if the surface

of the plastic is first roughened with emery cloth or steel wool;

the tube then should be coated with clear lacquer to preserve

the ink marks.

11.2 Density San&Calibrate the sand by finding the

density obtained by pouring the sand into a pipe or cylinderwith dimensions approximately those of the test well. The

pouring height above the top of the pipe should beapproximately the same as that for the well. The calibrated

density of sand is calculated from the mass of sand used

to fill the pipe and the volume of pipe occupied by thesand; i.e., density equals mass per volume.

12. Conditioning

12.1 Special conditioning requirements are not needed

for this procedure.

13. Procedure

13.1 Soil Logs.-Prior to performing field permeability

tests for a seepage investigation, exploratory borings shouldbe made at appropriate intervals and logs of the boringsshould be prepared to show a representative soil profile.

Soil classifications of the different strata encountered shouldbe recorded. The form shown in figure 4 can be used forthis purpose.

The minimum depth of borings below a proposed canal

invert or reservoir bottom should be to the ground-watertable, to an impervious soil layer, or to a depth about twice

the design water depth, whichever is reached first (seefig. 8). The location of soil layers that appear to be

impervious and the depth to a water table, if reached, willaffect permeability and seepage calculations. For depthsbelow a canal invert or reservoir bottom greater than twice

the water depth, the presence of a water table or soil layersof significantly different permeability than that of overlyingsoil will not influence permeability test results.

13.2 Size of Test We&For a low water table condition

(see condition I, fig. the depth of the well may beof any desired dimension provided the ratio of water height

in the well to well radius is greater than 1. To fulfill

theoretical considerations in development of the equationsfor high water table conditions (conditions II and III, fig.the ratio of water height h in the well to well radius should

be greater than 10. A practical well diameter is usually150 mm. Normally, in a canal seepage investigation, thewater surface elevation in the well and the well bottomshould correspond to the elevations of the proposed canal

water surface and canal bottom, respectively. Test resultswould then provide an average permeability for the soilsin the canal prism. For pervious soils, well size is limited

USBR 7300

by the capacity of the equipment to maintain a continuous

supply of water at the desired constant head level. Ifnecessary, more than one reservoir can be interconnected

to increase water capacity. Figure 3 shows the maximumcoefficients of permeability that can be measured in wellsof various diameters using float valves of different sizes.This is of assistance in selecting the valve size to be used,although a valve of approximately size is often

used for general purposes.13.3 Soil Permeability in Test Pits.-The well

meameter test method also can be adapted for use in testpits in a low water table condition if the ratio of water

depth to pit radius is greater than 1, and sand or gravelbackfill is used to prevent soil in the sides of the pit from

sloughing. In this case, calibration of backfill is not necessarysince dimensions of a test pit of regular shape can be foundby averaging linear measurements. If a rectangular pit is

used, the effective cylindrical radius for use in permeabilitycalculations can be determined from the pit dimensions(see fig. 5).

13.4 Excavation of the Test for permea-

bility tests should be prepared carefully to cause as littledisturbance to surrounding soil as possible. Where moisture

content of the soil is high, the wall of the hole can becomesmeared and outward flow of water restricted. In this case,the well should be excavated using two hand augers, one

having a diameter at least 25 mm smaller than the other.

First, auger a pilot hole with the smaller auger and followthis with the larger auger. This causes less disturbanceto the wall of the well than if a single auger is used. Ifit is still apparent that the wall of the well is smeared,

the walls should be scraped or scratched with improvisedtools to remove the smeared surface. Remove any loosesoil from the bottom of the well.

13.5 of (figs. 1 and 4).-Depth

measurements in the well should be measured (andrecorded) from a common base line. A convenient methodis to measure from a horizontal string line stretched

between two stakes driven firmly into the ground onopposite sides of the well (fig. 1). When the bottom ofthe well extends below ground-water level, insert a casing

during excavation to prevent the wall from caving. Carefullypull the casing as the well is backfilled with sand throughthe casing.

4.-For a very high ground-water condition, aout” test for saturated soils is often more satisfactory than the

well permeameter test or other “‘pump in” types of tests.

13.6 Backfilling the Test Well.-Pour calibrated sand

into the well in the same manner as during calibrationof the sand for density. The top of the sand should beabout 150 mm below the water level to be maintained.

After completion of pouring, determine the remainingmass of sand and subtract from the original mass to findthe mass of sand in the well. Measure and record the depthto the top of the sand and calculate the height of sand

in the well. From the density of the calibrated sand andthe mass and height of sand in the well, calculate the

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WELL PERMEAMETER METHOD(SOIL CLASSIFICATIONS AND WELL DIMENSIONS)

22 Example Example

Station

122.6 I 7 . 9 m I m

0 0 .45

0 45 I 77

I 77 3 8 7

SILTY CLAY approx. 85% fines with medium plasticity,

slow dilatancv, medium drv strenath, medium

opprox. 15% fine sand; maximum size, fine sand, moist,

dark gray; easy to auqer; some roots present; no

reaction with HCI (CL-ML).

SILT approx. 95% fines with low plasticity,

dilalancy, low dry low toughness;

approx. 5% fine sand; maximum size, fine sand; wet,

brown; easy to auqer; no reaction with HCI (ML-CL).

S ILT Y approx . 60% f ine to coarse , hard ,

angular sand; approx. 20% non-plastic fines; approx.

20% predominantly fine, hard, anqular lo subanqular

qravel; maximum size, 30mm; moist, brown; moderately

hard to auger; sliqht reaction to HCI

3.75 q

0.213 q m q

1.222q m q

0 . 3 7 5 q_ _ _

0 . 8 4 7 q_ _ --

0 . 2 8 0 q

0.942 q m q

400 q

3 4 . 0 2 q q

2 . 8 6 q

3 1 . 1 6 q q

0.0223 q m3

0 . 0 9 2 q__

Figure 4. Well method (soil classifications and well dimensions) example

1 2 3 1

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b

Figure Effective cylindrical radius of rectangular test pits.

(fig. 44 of ref. 2)

equivalent radius of the well (fig. 4). Development of the

equation for determining the radius is:

=

where:V. volume of sand

density of sandheight of sand

mass of sandequivalent radius of well

1 3 . 7 Test Equipment Set Up.-Place the float guide,with the float inside, on top of the sand in the well. Holdthe float guide in place vertically and pour sand aroundit. When a test is to be conducted with the water level

more than an arm’s length below the ground surface, lower

the float guide by the chain and drop sand around theguide to hold it in place during the test. The rubber slip

washer on the float stem is to prevent particles of sandfrom becoming lodged between the float stem and the

float guide. The mass of sand around the guide need notbe known because it is not used in computations for well

radius. Set up the water reservoir and valve-floatarrangement with the flexible tube from the float valve

to well and the chain attached to the float stem as shownon figures 1 and 2. The reservoir should be set on a firmplatform or cribbing at a convenient height.

13.8 Performing the Test:13.8.1 Open the valve on the reservoir and gradually

fill the well with water.13.8.2 After the water enters the float casing,

readjust the counterbalance on the operating arm of the

valve and the chain length as necessary to maintain thedesired water level in the well.

13.8.3 After the water level in the well has stabilized,

begin reading the volume gauge on the reservoir and recordthe gauge readings at convenient time intervals using the

form as shown on figure 6. The well must be keptcontinuously full of water until the test is completed. Ingeneral, dry soil at the start of the test absorbs water at

a comparatively high rate. However, as the moisturecontent of the soil increases around the well, the rategenerally decreases and usually stabilizes. It is this constant

rate after stabilization that is used to compute permeability.13.8.4 As records of water discharge from the

reservoir and time are made, plot a curve of accumulative

flow versus time as shown on figure 7.

14. Test Duration

14.1 Minimum duration for the test is the theoretical

time required to discharge the minimim volume of waterinto the soil to form a saturated envelope of hemispherical

shape with a radius B (see fig. 1).

The minimum volume of water is determined by theequation:

where:minimum volume

specific yield of the soil

h height of water in wellr well radius

NOTE quantity in brackets is the theoreticaldetermination for radius (fig. 1).

For soils in which this test would most likely be used,the specific yield varies from about 0.1 for fine-grainedsoils to 0.35 for coarse-grained soils. When the specificyield of the soil is unknown, the value of 0.35 should be

used to give a conservative value for minimum volume

and to ensure that the test duration is sufficient. Thus,

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I

Figure 6. Well permeameter method (time and volume measurements) example.

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Figure 7. Time-discharge curve for well permeameter test low

water table example.

with a known or assumed specific yield for the soil and

with the dimensions of the well, the minimum volumecan be computed and the test discontinued when the

minimum volume has been discharged through the well.In pervious soils, it may appear that the volume-time curvehas reached a uniform slope after several hours when points

are plotted over short time intervals. However, in orderto avoid discontinuing a test prematurely, it must becontinued for at least 6 hours from the starting time sothe slope can be determined over a period of 2 to hours.The first straight portion of the curve should be used for

determining the rate of discharge (fig. 7). The test mustbe conducted continuously without allowing the reservoir

to run dry until the test has been completed.14.2 Maximum Time.-If the test is continued for a

long period, a water mound may build up around the welland render the test results inaccurate. The maximum timefor test duration is the time necessary to discharge throughthe test well the maximum volume of water as determined

using equation substituting 15.0 for 2.09 and in thiscase, using an assumed minimum value (when the true

value is unknown) of 0.1 for specific yield.

V 2.05

15. Calculations

15.1 Computing Coefficient oftions or (6) are provided for calculating coefficientof permeabil ity, for the well permeameter test. The

presence or absence of a water table or impervious soillayer within a distance of less than three times that ofthe water depth in the well (measured from the water

surface) will enable the water table to be classified ascondition I, II, or III, as illustrated on figure 8.

Low Table.-When the distance from

the water surface in the test well to the ground-water table,or to an impervious soil layer which is considered for test

purposes to be equivalent to a water table, is greater thanthree times the depth of water in the well, a low water

table condition exists as illustrated by condition I (fig. 8).

For determination of the coefficient of permeability undersuch a condition, equation (4) given in subparagraph 15.2should be used.

15.1.2 High Water Table.-When the distance fromthe water surface in the test well to the ground-water table,

or to an impervious layer, is less than three times the

depth of water in the well, a high water table conditionexists as illustrated by condition II or III. Condition II shows

a high water table with the water table below the wellbottom, and for this condition equation (5) should be used.Condition III shows a high water table with the watertable above the well bottom. For this condition, equation

(6) should be used.15.2 Equations:

Condition I:

k

r

Condition II:

=

Condition III:

I

where:

coefficient of permeability at 20h height of water in the wellr radius of welly discharge rate of water from the well for

state condition (determined experimentally,

see example, fig. 7)viscosity of water at temp. T (see fig. 9)

viscosity of water at 20unsaturated distance between the water surface

in the well and the water table

15.3 The preferred metric unit for coefficient ofpermeability is cm/s (centimeters per second). The valueof X centimeters per second is approximately the

same as the inch-pound unit of foot per year.

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surface in well

= Height of water int e s t w e l l ( f t . )

h

C O N D I T I O N CONDITION

Figure 8. depth of water in test well and distance to table in well test.

Report

1 6 . 1 The report is to consist of the following completedand checked forms:

“Well Permeameter Method (Soil Classifications andWell Dimensions)” (fig. 4).

“Well Permeameter Method (Time and VolumeMeasurements)” (fig. 6).Time-Discharge Curve (example on fig. 7).Calculation of coefficient of permeability from equations

(6).1 6 . 2 All calculations are to show a checkmark and all

plotting must be checked.

References

Drainage Manual, 1st ed., Bureau of Reclamation, U.S.Government Printing Office, Washington, D.C., 1984.

Zanger, Carl Z., Theory and Problems of WaterPercolation, Engineering Monograph No. 8, (app. B “Flowfrom a Test Hole Located Above Groundwater Level,”

development by R. E. Glover) Bureau of Reclamation,Denver, Colorado, April 1953.

Ribbens, R. W. “Exact Solution for Flow From a Test

Hole Located Above the Water Table,” (unpublishedtechnical memorandum), Bureau of Reclamation, Denver,

Colorado, 1981.

1 2 3 5

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0

a

a

I

I .,I

T E M P E R A T U R E I N D E G R E E S C E N T I G R A D E , T

1236

Appendix B

Procedure for New Percolation Basin Testing