GM12-14_Naukati_Road_Reconstruction_Design_Modifications_Final10June2014..pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

Date: June 10, 2014 In Reply Refer To: HFL-17

From: Diann Morehouse, Geotechnical Engineer

To: Neal Christensen, Project Manager

Copies: Brian Collins Malcolm Ulrich

Subject: GEOTECHNIAL MEMORANDUM GM12-14 Geotechnical Subexcavation Modifications Naukati Road Reconstruction

AK USFS 2060(1)

INTRODUCTION

This memorandum summarizes the modification to the subexcavation recommendations which were provided in the geotechnical report Naukati Road Reconstruction, Final Geotechnical Investigation and Design Recommendations Report, GR10-13 dated January 2014.

Due to the high estimated costs associated with converting the roadway to a paved driving surface the scope was changed to a gravel driving surface. The purpose of this memorandum is to provide documentation of the re-evaluation of embankment construction methods over muskeg and soft soil deposits.

The original roadway design was a 22-foot paved roadway with 2-foot gravel shoulders for a total of 26-foot top width. The pavement structure design consisted of asphalt concrete, 4 inches thick, overlying 8 inches of base material.

At the 70% plan review the subexcavation quantity for this project was estimated at 48,000 cubic yards of material and the engineers estimate total cost was approximately $11.7 million or $4.5 million per mile. Subexcavation and replacement construction method accounts for about $3.5 million of the project cost.

In February 2014, a decision was made to re-examine the project design and provide cost reduction design alternatives. At that time the project alignment and grade was set and right-

Geotechnical Subexcavation Modifications Page 2 of 3

AK USFS 2060(1)

of-way acquisition had begun so no substantial changes to the alignment and grade could be made.

COST REDUCTION DESIGN ALTERNATIVES

Settlement that occurs when embankments are constructed across muskeg and soft soil deposits must be mitigated prior to constructing asphalt concrete pavement. The GR10-13 report provides a discussion of four embankment construction methods that were considered for the proposed roadway design: Surcharge, Lightweight Fill, Partial Depth Subexcavation and Replacement, and Full Depth Subexcavation and Replacement. The geotechnical recommendation in GR10-13 was to utilize the full depth subexcavation and replacement method that has been successfully used on previously on numerous WFLHD projects on Prince of Wales Island.

The following options were discussed by the WFLHD cross-functional-team:

• Rock Fill Bridge. This option will greatly reduce project costs. In areas where the perceived resulting risk of substantial post construction settlement is high, the risk must be partially mitigated by implementing other design changes.

• Partial Depth Subexcavation. This option will reduce project costs. Partial depth subexcavation will reduce risk of substantial post-construction settlement in areas perceived to be high risk.

• Elimination of Asphaltic Concrete Surfacing. This option will greatly reduce project costs and the risk of substantial pavement damage will be high if full-depth subexcavation is not implemented. Paving can be delayed until the embankments until at time that settlement will be within tolerable levels. Settlement monitoring during and after construction can be used to determine when the asphaltic concrete surfacing can be placed.

• Surcharge. Surcharging option will have a high cost; however, if asphalt concrete surfacing is not placed as part of the project it would not be necessary.

• Lightweight Fill. This option was not pursued due to the high cost of lightweight materials and proximity of groundwater table to ground surface.

RECOMMENDATIONS

WFLHD decided to pursue the following where embankments are planned to be constructed in muskeg and soft soils locations:

• Eliminate the asphalt concrete surface course from the project and construct a gravel surfaced road, maintaining the foot print for paving in the future,

• Eliminate subexcavation of muskeg and soft soils located between the toe of the existing embankment and the toe of the proposed embankment, Geotechnical Subexcavation Modifications Page 3 of 3

AK USFS 2060(1)

• Require only clearing, but no grubbing, of the area between the toe of the existing embankment and the toe of the proposed embankment,

• Utilize layers of geogrid reinforcement in the pavement section to partially mitigate the development of surface distress related to settlement,

• Reduce embankment heights over muskeg and soft soils where possible,

• Use settlement plates, staged embankment construction, and shear keys in areas of high embankments over soft deposits,

• Monitor embankment settlement during construction and post-construction, and

• Utilize a separate project to install chip seal surfacing at least 2 years following initial construction. Use settlement data to determine when the surfacing can be placed.

Asphaltic pavement may be placed at a future date at least 5 years after initial construction. Placing the chip seal could identify potential problem areas that could be mitigated before placing the asphalt concrete surfacing.

Embankment construction details that illustrate the recommended embankment construction methods, as discussed above, are provided in Attachment A.

This approach will reduce the project cost by approximately $3.9 million. The total project cost based on this approach is estimated at $7.8 million or roughly $3.0 million per mile.

However, this approach will require more work from DOT maintenance in the future but the lifecycle costs would likely be significantly reduced.

Disadvantages:

• Settlement of the road could become severe in a few locations and general perception could be that the project did not properly address the issue, and

• Will require more maintenance and construction from DOT in the future.

Advantages:

• Significant lower cost,

• Project will have a better chance at obtaining construction funding, and

• Reduce the high probability of change orders during construction to cover additional excavation.

These modified recommendations will result in substantial project construction savings but the resulting disadvantages and implications outlined above must be recognized and accepted.

If you would like to discuss this memorandum further, please feel free to contact Diann Morehouse at (360) 619-7609.

Attachments: Embankment Construction Details – Sheet C.3 and C.4.

Variable

Geogrid, biaxial

LC

Select borrow

Borrow, rock

Profile grade m a x

Variable Slope stake

Earthwork geotextile, Type IV-A

Toe of existing embankment

DETAILS

EMBANKMENT CONSTRUCTION

Variable

LC

Profile grade

Variable Slope stake

Earthwork geotextile, Type IV-A

Toe of existing embankment

NOTE:

FOOTNOTE:

Subgrade

See Section 204.

Construct slopes in the Staking Report.

See Mainline Typical Section for proposed surfacing structure.

3.

2.

1.

excavation or borrow, rock

Suitable roadway

LC

Biaxial geogrid spaced 6" apart

Subgrade

S e le c t b o r r o w

1' c o u r s e

S u r fa c in g

Geotextile Type IV-A

Rock Borrow

1'

Profile grade

Original ground

Original ground

Subgrade

2 3

1'

CONSTRUCTION

NORMAL EMBANKMENT

C.3

STATE PROJECT

NUMBER

SHEET

AK

USFS 2060(1)

P

M a y

U

S S u r ft

D c m y fi le s p w p r o d u c ti o n d a s c c .d g n

C h e c k e d b y

D e s ig n e d b y sections and to original ground in cut sections.

Extend geogrid to within 1 foot of embankment slope in fill directed by C.O.

In areas where subgrade is less than 22-inches, place geogrid as road, place geogrid at existing road elevation.

In areas where subgrade is less than 24-inches above existing

EMBANKMENT CONSTRUCTION

SINGLE LAYER REINFORCEMENT

SOFT GROUND

DETAIL

TRI-LAYER GEOGRID REINFORCEMENT

SOFT GROUND

DRAFT

Diann.Morehouse Rectangle

Slope stake

Profile grade

Existing ground

24" max.

20'

NOTE:

FOOTNOTE:

Var iab le

W

Embankment width

0.5W

CONSTRUCTION DETAILS

EMBANKMENT

Va ria bl e

3. See Section 204.

2. Construct slopes as shown in the Staking Report.

1. See mainline typical section for proposed surfacing structure.

See sheet C.1 for locations and quantities.

sections and to original ground in cut sections.

Extend geogrid to within 1 foot of embankment slope in fill directed by the CO.

In areas where subgrade is less than 22", place geogrid as place geogrid at existing road elevation.

In areas where subgrade is less than 24" above existing road, 21+75 to 23+50

-3.19% 1:3

1:1.5

1:3

1:

1.

E x c a v a ti o n L im it s

E x c a v a ti o n L im it s

3.20%

LC

E x c a v a ti o n L im it s

1:2

1:

1.

1:3 6.00% -6.00% 1:3

1:1.5

E x c a v a ti o n L im it s

113+50 to 115+75

LC

Settlement plate

Existing ground

Slope stake Toe of existing embankment

Borrow, rock

Select borrow Subgrade

Profile grade

Settlement plate assembly

Geogrid, biaxial

NO SCALE

24" max.

10'

Earthwork geotextile, type IV-A

C.4

STATE PROJECT

NUMBER

SHEET

AK

USFS 2060(1)

a s c d .d g n

U

S ft

D

J O

H N

S O

N

C h e c k e d b y

C

K N

U T

H

D e s ig n e d b y

WITH SETTLEMENT PLATE

EMBANKMENT CONSTRUCTION

KEYED IN SINGLE REINFORCEMENT

WITH SETTLEMENT PLATE

STAGED EMBANKMENT CONSTRUCTION

SOFT GROUND

WITH SETTLEMENT PLATE

REINFORCEMENT EMBANKMENT CONSTRUCTION

KEYED IN TRI-LAYER

1'

Stage thickness

LC

DRAFT

Existing ground

Profile grade

Subgrade

Slope stake

Toe of existing embankment

Embankment geotextile, type IV-A

SETTLEMENT PLATE LOCATION

114+25, 30' RT

114+75, 30' RT

Geogrid, biaxial

Toe of existing embankment

Earthwork geotextile, type IV-A

1'

Subgrade

Borrow, rock

Select borrow

Geogrid, biaxial, 6" spacing

Settlement plate

SETTLEMENT PLATE LOCATION

22+75

22' RT

Diann.Morehouse Rectangle