2-2013 Sandy River Lane PS and Vulnerability Mit. app D.pdf

Memorandum

15575 SW Sequoia Pkwy, Suite 100 l Portland, OR 97224 l 503.968.8787

To: Ron Weigel From: George Saunders

Company: Century West Engineering

Corporation

Date: September 17, 2013

Address: 5331 SW Macadam Avenue, Suite 207

Portland, OR 97239 cc: n/a

GDI Project: ClackCo-75-01

RE: Sandy River Lane Force Main and the Sandy River Lane Pump Station – Vulnerability

Mitigation

Geotechnical Engineering Considerations

INTRODUCTION

We are pleased to present this memorandum providing our preliminary observations and recommendation regarding vulnerability mitigation for the Sandy River Force Main and the Sandy

River Lane Pump Station. As outlined in Clackamas County Service District No. 1’s Request for

Proposals, erosion of the Sandy River bank has jeopardized the integrity of the Sandy River Force

Main and Sandy River Lane Pump Station.

SCOPE

Our scope has included the following elements:

Reviewed the existing geotechnical and geologic information.

Attended a June 19, 2013 site meeting and reconnaissance of the two sites.

Worked with the design team to identify and evaluate geotechnical aspects of the alternatives.

BACKGROUND INFORMATION

GENERAL

Our evaluation has been based on the site-specific information discussed below for the two subject areas of this project.

SANDY RIVER FORCE MAIN

The existing geotechnical information for the area includes well logs and an October 10, 2007 memorandum referencing sonic borings completed on both banks of the Sandy River. Copies of the salient components of these documents are included in Attachment A.

The well logs are associated with sites on the south side of the river (Attachments A-1 through A-5) and completed from higher elevations on the river terrace relative to the river crossing. Materials described are consistent with the re-worked alluvium and lahar (volcanic debris flow) deposits.

An October 10, 2007 memorandum by Clackamas County Water Environment Services (Attachment

A-6) summarizes sonic borings completed by Boart Longyear in advance of the 2007 horizontal directional drilling (HDD) to relocate the force main. Well logs from the Oregon Water Resource

Department (OWRD) associated with the borings are included in Attachment A (Attachment A-7 through A-10). The approximate locations of the borings are included in the well logs. The logs are highly generalized and do not provide detailed descriptions of the materials encountered. At our request, Boart Longyear reviewed their records but could not locate copies of detailed well logs. The memorandum indicates that an approximately 10-foot-thick layer of silty clay was encountered at between 26 and 36 feet and 35 and 45 feet on the south and north side of the river, respectively. A drawing showing the alignment of the 2007 HDD is included in Attachment A (Attachment A-11);

however, it appears that the 10-foot-thick layer of silty clay is labeled sand on the drawing.

SANDY RIVER LANE PUMP STATION

The existing geotechnical information of the area is limited to a January 19, 2001 geotechnical report by Squire Associates. The report included an 11-foot-deep test pit (TP-1) at the location of the pump station (see Attachment B). As encountered at the Sandy River Force Main site, the primary geological unit described is alluvial sediments consisting of sand and gravel with variable amounts of cobbles and boulders. Based on site observations of the eroded cut-bank on the Sandy River adjacent to the pump station, a berm of semi-cemented gravel, sand, cobbles, and boulders (ancient mud flow or lahar) exists between the Sandy River and the pump station. The adjacent parcel downstream of the pump station is currently armored with riprap. The above-referenced erosion of the riverbank in this area is potentially jeopardizing the riprap.

DISCUSSION

GENERAL

Additional information associated with the project includes the following:

Inter‐Fluve’s September 12, 2013 technical memorandum

Staheli Trenchless Consultants’ (STC) September 12, 2013 technical memorandum

Inter‐Fluve evaluated the historical meandering of the Sandy River and presented their results in their technical memorandum. Several options were discussed to mitigate the vulnerability of the two areas. STC presents a discussion regarding different trenchless options for realignment of the sewer. The geotechnical considerations associated with these options are presented below.

SANDY RIVER FORCE MAIN

Bank stabilization using either riprap or large wood jam revetments were presented in the Inter-Fluve memorandum. Both options require realignment of the sewer, with the new alignment entry point on the south side of the river between 130 and 290 feet south of the existing riverbank. STC presented a discussion on the trenchless option for the new alignment and concluded that HDD methods are better suited for the project. Three alignments options are presented on their Figure 3, which is included in Attachment A (A-12). We understand that Clackamas County would like to limit considerations to the 800- and 900-foot alignments (referenced as Option 4 and 4A).

The proposed alignment of the new HHD targets the same layer of material identified in 2007 HDD alignment. The information on the extent of the clay/sand layer is limited to the drawing showing the 2007 HDD alignment (Attachment A-11). Moreover, the OWRD logs do not provide detailed logs describing the amount of cobbles and boulders associated with the overburden material above the clay/sand layer nor accurate locations of the borings. We recommend that additional sonic borings be completed on both sides of the river further explore favorable layers for HDD and to determine the characteristics of the overburden material. The boring on the south side of the river should be completed along the access road to the Timerline Pump Station. The boring on the north side of the river should be completed along E Sandy River Lane as close as possible to the proposed exit point of the proposed HDD alignment. A detailed log of the subsurface conditions should be produced to assist the contractors in preparing bids.

SANDY RIVER LANE PUMP STATION

Without stabilization, Inter‐Fluve concludes that the riverbank may continue to erode until the pump station and associated sewer lines are breached. As discussed in the Inter‐Fluve memorandum, the close proximity of the Sandy River to the pump station precludes the use of large wood jam revetments. A riprap solution is presented; however, the riprap would need to extend from the existing riprap downstream of the pump station, upriver sufficient to prevent the river from outflanking the riprap, and tie into the riverbank mud flow/lahar berm.

A geotechnical option considered during this evaluation was installing secant piles around the pump station. Secant piles are drilled shafts constructed in such a way that the shafts overlap each other to form a reinforced wall that can cantilever relatively high embankments. As with the riprap approach, the secant piles would need to be aligned to prevent the river from outflanking the wall. In addition, specialized equipment would be required to work within the close proximity of the pump station and the exiting river berm. Lastly, the construction process would likely damage the trees and river berm and accelerate the erosion of the riverbank. Accordingly, while potentially feasible, we understand that secant piles will not be used for the project.

We understand that the most favorable option being considered is to move the pump station to a vacant parcel across E Sandy River Lane. If this approach is selected, we recommend that an exploration be completed in the close vicinity of the proposed pump station. As show in the logs in

Attachment B, excavation refusal was encountered because of caving conditions at depths of 11 to

14 feet. We anticipate that similar caving conditions will be encountered at the adjacent parcel, and if deeper explorations are required for the design, then a sonic boring should be completed compared to a test pit exploration.

GPS:kt

Attachments

One copy submitted (via email only)

Document ID: ClackCo-75-01-091713-geom.docx

© 2013 GeoDesign, Inc. All rights reserved.

ATTACHMENT A

Page A-1

ATTACHMENT A

SANDY RIVER FORCE MAIN INFORMATION

Background information associated with the Sandy River Force Main referenced in this memorandum is presented in this attachment.

A-1 through A-5: Well logs

A-6: October 10, 2007 memorandum by Clackamas County Water Environment

Services

A-7 through A-10: Well logs of sonic borings referenced in memorandum by Clackamas County

Water Environment Services

A-11: Drawing showing the alignment of the 2007 HDD

A-12: Figure 3 from STC’s September 12, 2013 technical memorandum gsaunders Text Box Attachment A-1

Attachment A-2

Attachment A-3

Attachment A-4

Attachment A-5

Michael S. Kuenzi, P.E.

Director

M E M O R A N D U M

TO: File

FROM: Dewayne Kliewer

DATE: October 10, 2007

SUBJECT: Boart Longyear Sonic Drill Results

Boart Longyear completed a Sonic Drill exploration on both banks of the Sandy River adjacent to the existing forcemain and waterline crossing points.

Boring results indicate a 10-foot thick lens of silty clay below the rivers bottom.

On the southerly side it is at a depth between 26 and 36 feet.

On the northerly side it is at a depth between 35 and 45 feet.

The ground elevation on the northerly side is about 5 feet higher than the south side.

The material from the surface down to this sand layer is sand, cobbles and boulders.

Should be a good "envelope" for getting our replacement forcemain and Timberline Water Districts waterline through.

Attachment A-6

CLAC 64206

Attachment A-7

CLAC 64206

Attachment A-8

CLAC 64207

Attachment A-9

CLAC 64207

Attachment A-10

NOTES:

Dots represent elevation shots and soundings taken on 10-25-2012.

Dashed lines in river, where readings could not be taken.

gsaunders

Attachment A-11

1” = 10’ Ver 1” = 100’ Hor

0+00 1+00 2+00 3+00 4+00 5+00 6+00 7+00 8+00 9+00 10+00

HDD Alignment Option 4A (900’)

Pipe Ramming Option 3A (400’)

Pipe Ram Launch/Exit Shafts

MIN 36-IN CASING W/ PRODUCT PIPE

HDD Alignment Option 4 (800’)

Near-Surface Conductor Casing

Note:

Vertical datum is assumed.

Effort has been made to overlay this section with elevations taken from 2013 topographic survey in support of this technical memorandum.

All HDD Options depicted with 500’ vertical and horizontal bend radii.

Existing Gravity Main To Remain in Place

New Gravity Main

Existing Force Main Connects to MH TR-2

Estimated Sand Layer based upon sonic drilling by Boart Longyear, Oct. 2007

Existing Gravity Main (To Be Abandoned with Option 4 and 4A)

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HDD Alignment Option 3 (600’) gsaunders

Attachment A-12

ATTACHMENT B

Page B-1

ATTACHMENT B

SANDY RIVER LANE PUMP STATION INFORMATION

Background information associated with the Sandy River Lane Pump Station referenced in this memorandum is presented in this attachment and includes excerpts from the January 19, 2001 geotechnical report by Squire Associates.