00-31-32-01_1-3-B-12_2014_09_02_IBM13C0014_SD_Admin_Geotech_Addendum_Stantec.pdf

Stantec Consulting Services Inc.

25864-F Business Center Drive Redlands, CA 92374 Tel: (909) 335-6116 Fax: (909) 335-6120

V:\1858\active\185803308\05_report_deliv\deliverables\abide_intl_ibwc_geotechnical_report_addendum_090214.docx

September 2, 2014

Mr. Karl Lindstrom Abide International Inc.

561 First Street West Sonoma, California 95476

RE: ADDENDUM 01 - GEOTECHNICAL INVESTIGATION REPORT

United States International Boundary and Water Commission Water / Wastewater Treatment Plant New Administration Building 2225 Dairy Mart Road San Ysidro, California 92173

Dear Mr. Lindstrom, Pursuant to your request, Stantec Consulting Services Inc. (Stantec) has prepared this addendum to the Geotechnical Investigation Report (GIR) (Stantec, May 27, 2014) for the proposed administration building at the United States International Boundary and Water Commission, Water / Wastewater Treatment Plant, located at 2225 Dairy Mart Road, in the City of San Ysidro, San Diego County, California. This addendum presents the results of onsite percolation testing and the recommended design infiltration rate in the proposed permeable parking area.

GAS POWERED AUGER DRILLING

Four shallow percolation wells (PT1 through PT4), were drilled with a manual gas-powered auger on August 20, 2014 by Stantec personnel under the direction of a Stantec engineer. The shallow soil borings for this investigation were advanced using an eight-inch outside diameter auger, to a maximum depth of approximately 2.5 feet below the ground surface (bgs), at the locations shown on Figure 1. The rationale for placement of the auger borings was to have at least two pairs of percolation test locations across the proposed permeable parking area to evaluate the infiltration rate of near surface soils.

When the desired sampling depth was achieved, the bottom of the auger boring was cleaned by slowly rotating the gas-powered auger with minimal downward pressure. Approximately six inches of 3/8 inch rock were placed in the bottom of the boring followed by a four-inch diameter perforated pipe set at a depth of approximately two feet bgs. Additional 3/8 inch gravel was backfilled around the four inch diameter pipe to a depth of approximately one foot below the ground surface. Bentonite crumbles were backfilled around the four-inch diameter pipe above the gravel to the ground surface and hydrated prior to beginning the four hour soaking period.

PERCOLATION TESTING PROCEDURE

Percolation tests were performed at depths between approximately two feet bgs in accordance with the County of San Diego, Department of Environmental Health (SDCDEH), Land and Water Quality Division, Design Manual for Onsite Wastewater Treatment Systems, dated March 22, 2010. A percolation test was conducted at each of the four locations shown on Figure 1. At each percolation test location, a minimum of 14 inches of water was maintained in the hole for four hours prior to testing. Approximately 15 hours after the presoaking period, the wells were filled with approximately six inches of water. The water drop was measured in each well at 30-minute intervals with a water level indicator. The test continued for a minimum of four hours until at least three successive water level drops did not vary by more than approximately 10% within a 30-minute period. The last measurement was recorded as the infiltration rate for that point.

INFILTRATION RATE

Following the SDCDEH formula for calculating the infiltration rate, the field test results in Appendix A indicate the infiltration rate ranged between 25 and 65 minutes per inch. Based on the soil type from onsite boring logs and materials laboratory testing, Stantec recommends a design infiltration rate of 65 minutes per inch and recommends the infiltration system be connected to the storm drain system as an additional safety measure.

CLOSURE

Our conclusions and recommendations are based on the results of the field testing, combined with an interpolation of subsurface conditions between exploratory boring locations. Our findings and recommendations were prepared in accordance with generally accepted professional engineering and engineering geological principals and practice in this area of southern California and the referenced SDCEHD standards. It is noted that conditions may change over the course of time. These results represent conditions at the time of testing. If subsurface soils are disturbed as a result of grading or other activities, additional testing may be required.

All other geotechnical aspects of design and construction for the subject development should be performed in accordance with Stantec’s Preliminary Geotechnical Investigation report

(2014).

It is our pleasure to be of service to you and we look forward to providing Abide International with future engineering services. Should you have any questions regarding the information contained in the attached report, please contact the undersigned at your convenience.

Respectfully submitted, Stantec Consulting Services Inc.

Jaret Fischer, PE Kevin Miskin, PE Senior Engineer Principal Engineer Phone: (909) 335-6116 ext. 8209 Phone: (909) 335-6116 ext. 8210 Fax: (909) 335-6120 Fax: (909) 335-6120 jaret.fischer@stantec.com kevin.miskin@stantec.com

Attachments: Figure 1 – Site Plan

Attachment A – Percolation Test Data

FIGURES

PT-1

PT-2

PT-4

PT-3

FAX:PHONE:

25864-F BUSINESS CENTER DRIVE

REDLANDS, CA 92374

(909) 335-6116 (909) 335-6120

APPROXIMATE PERCOLATION TEST LOCATIONS

PROPOSED SITE IMPROVEMENTS

0 20 40

APPROXIMATE SCALE (FEET)

N

PT-4

FOR:

CHECKED BY:DRAWN BY:JOB NUMBER: APPROVED BY: DATE:

FIGURE:

SITE PLAN

UNITED STATES INTERNATIONAL

BOUNDARY AND WATER COMMISSION

2225 DAIRY MART ROAD

SAN YSIDRO, CALIFORNIA

185803308 JF JF JF 8/30/14

25864-F BUSINESS CENTER DRIVE

REDLANDS, CALIFORNIA

PH (909) 335-6116 FAX (909) 335-6120

ATTACHMENT A

abide_intl_ibwc_geotechnical_report_addendum_090214