Geotechnical_Recommendations_-_Final.pdf

that the river will continue to aggressively impact this bank and any wall that is constructed will be subject to hydrodynamic forces and potential flanking due to erosion beyond the limits of the wall. The magnitude of the hydrodynamic forces is unknown as a detailed hydraulic model has not been developed for the current river configuration.

The most robust wall solution to account for the uncertainties associated with the river flow is a wall on deep foundations such as a secant wall, soldier pile wall, or sheet pile wall. However, these walls have been deemed cost prohibitive due to expensive construction methods and materials required and/or the possible presence of sedimentary bedrock layers within the construction profile.

An alternative solution that may provide more stability than a gabion wall is to construct an MSE wall. The advantages associated with an MSE wall are that the select borrow wall fill consisting of predominantly granular soil will drain rapidly if it becomes saturated, MSE walls can tolerate large deflections before failure, and the reinforcement will provide additional stability in the event of another large flood. The disadvantage is that the wall is more susceptible to erosion and hydrodynamic forces than a secant wall. To potentially provide additional protection against the unknown magnitude of hydrodynamic forces, the intact lower gabion baskets should be left in place in front of the new wall. For the portion of the wall where the lower baskets have been damaged, the rounded rock from the remainder of the wall may be used and placed in a similar configuration but should be partially grouted to provide additional stability.

2.0 Geotechnical Conditions

A technical memorandum was prepared by CFLHD in December 2011 presenting geotechnical data and recommendations for the previous gabion wall. Three borings were drilled in the vicinity of the proposed wall. The boring logs and the geotechnical parameters presented in the memo were reviewed to develop a design soil profile.

Based on visual observations, the new wall will be constructed in an area where the soils are likely to be most similar to those encountered in boring B-6. The soils in this boring consisted of loose to medium dense silty sand underlain by sedimentary bedrock consisting of sandstone, siltstone, or claystone at a depth of approximately 14 feet. The sand was generally fine grained with uncorrected Standard Penetration Test (SPT) blowcounts ranging from 6 to 17 blows per foot. Soil properties presented in the memo for this soil type are a soil friction angle of 32 degrees and a total unit weight of 120 pounds per cubic foot.

3.0 Wall Design Recommendations

Preliminary external design including global stability, bearing resistance, sliding, and eccentricity was performed to evaluate the proposed wall geometry. The final MSE wall bearing pressures and internal stability will be designed and submitted by the contractor. Design walls in general accordance with AASHTO “LRFD Bridge Design Specifications” (7th edition), and FHWA Publication NHI-10-024 and NHI-10-025, Volumes 1 and 2, respectively, entitled “Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes” dated November 2009. Acceptance criteria used for preliminary design are shown in Table 1.

TABLE 1 - Minimum Acceptance Criteria for MSE Walls

Stability Mode Minimum Acceptance Criteria

Global Stability *FS = 1.25

Bearing Resistance CDR = 1.0

Sliding CDR = 1.0

Limiting Eccentricity emax = L/4 Notes: FS = Factor of Safety, CDR = Capacity to Demand Ratio, Eccentricity (emax) for soil foundations *Considered Acceptable for this Project

The capacity to demand ratios (CDR) for bearing resistance and sliding are used to quantify the ratio of the factored resistance to the factored load. Resistance factors used in the preliminary external design for bearing and sliding are 0.65 and 1.0, respectively. Load factors for bearing, sliding, and the eccentricity check are shown in Tables 4-1 and 4-2 of FHWA (2009).

Material properties were estimated for wall design based on observations during the field investigation, results of laboratory testing, and known empirical strength characteristics of like materials. The selected design values for soil unit weight (γ), internal friction angle (), and cohesion intercept (c) are shown in Table 2.

TABLE 2 - Estimate Material Properties for MSE Design

Material Unit

Weight γ (pcf)

Cohesion c (psf)

Friction Angle (deg)

Reinforce Material:

Select Borrow (704.07)

135 0 34

Retained Material:

Unclassified Borrow (704.06)

130 0 32

Foundation Material:

Silty SAND (SM)

120 0 32

The reinforced and retained portions of the wall system are backfilled with select borrow and backfill material in accordance with the requirements in Section 704 of the FP-14. Based on laboratory testing results, on-site, existing fill and native soils will generally not meet the requirements for select borrow. In addition, local rock found on-site within the vicinity of the wall will generally not meet the requirements of wall facing fill.

The proposed MSE walls were preliminarily evaluated for feasibility based on the existing site conditions and available subsurface information using the program Mechanically Stabilized Earth Walls (MSEW 3.0) developed by ADAMA Engineering. The design methodology used by MSEW is consistent with current AASHTO and FHWA guidelines for assessment of external stability of MSE walls. Global stability analysis was performed using Slide, Version 7.0, a two dimensional, limit equilibrium computer program from RocScience.

Cross sections were evaluated at both the minimum (15 feet) and maximum (22 feet) wall heights. A backfill slope of 4H:1V and traffic surcharge of 250 psf was modeled in the analyses.

A minimum wall embedment of 6 feet is recommended with the material in front of the wall consisting of the existing gabion baskets with angular rock and/or the existing gabion baskets with rounded rock that has been grouted. Groundwater was not detected in the borings but will be present during high flow events, although the select borrow is expected to drain rapidly. In the event that the wall drains slowly during a flood event, a stability analyses including a groundwater table within the reinforced backfill was performed to simulate a rapid drawdown condition.

Based on the results of the analyses, the proposed MSE wall system will meet the required acceptance criteria provided the wall system is constructed with a minimum reinforcement length equal to 85 percent of the wall height (0.85H) or 8 feet, whichever is greater. Summary results of the most critical cross section cases from Slide, Version 7.0 and estimated static factored bearing resistances assuming a Resistance Factor () of 0.65 (AASHTO, Table 11.5.6-1) and an acceptable maximum settlement of 1 inch.

TABLE 3 - Summary of Stability Analyses

Station Description

Estimated Factored Bearing

Resistance

Global Stability

Required F.S. Calculated F.S.

1+00 Height = 16 ft *5,760 psf 1.25 1.60

2+45 Height = 26 ft *8,870 psf 1.25 1.54

* Assumes minimum reinforcement length of 85 percent of wall height.