Calculation Discussion Rapid Drawdown Stability Analysis

Problem Definition

The Rapid Drawdown Case is often a critical condition for stability of submerged slopes, whereby in-situ pore water pressures first equilibrate to hydrostatic levels under steady state seepage conditions, then upon sudden drawdown of the water adjacent to the slope, the stabilizing force of the water against the slope is lost and pore water pressures in low permeability materials within the slope are not able to dissipate. This phenomenon increases the shear stresses for equilibrium.

This analysis considers a rapid drawdown on the lakeward side of the lake berm from the Maximum Storage Pool elevation to elevation 388, and a rapid drawdown of prolonged floodwaters on the riverward side of the lake berm to the base flow elevation in the river channel. For the Urban/Natural Lake, the riverward side rapid drawdown is from elevation 403 to elevation 382. For the West Dallas Lake, the riverward side rapid drawdown is from elevation 405 to elevation 387. This analysis is considered to be appropriate in its consideration of the possibility of a sudden drawdown of water in the Urban/Natural or West Dallas Lakes, and is considered to be very conservative for the riverward side of the lake berm due to the unlikely possibility that the low-permeability material of the embankment will become fully saturated during a relatively brief flood event.

Analysis Geometry

The embankment geometry analyzed is an assumed maximum section with a minimum allowable crest width (20 feet for the Urban/Natural lake and 44 feet for the West Dallas Lake). In most cases, the embankment is expected to be wider (and perhaps much wider) than the section analyzed. The geometry of erosion control and slope retention concepts reflects those shown in the drawing set for the corresponding embankment options (A, B, C, or D). The subsurface profile shown conservatively considers the entire subsurface to be the upper clay unit as defined by Fugro in the Geotechnical Data Report (Fugro, 2009). This neglects the presence of the flaggy zone, basal sands, and eagle ford shale/Austin chalk formations, all of which have been found to exhibit higher strength than the upper clay unit. Interior slopes for the lakes are 3H:1V for all proposed lakes, a recompacted clay liner 18 thick is applied for uniform seepage control.

Option A

Option A involves river-side slopes 4H:1V or flatter with vegetative cover for erosion protection. This option is not applicable to the Urban/Natural lake berm, but is expected to be used frequently on the West Dallas Lake.

Option B

Option B involves erosion protection of river-side slopes 4H:1V or flatter, protected by articulated concrete block or soil cement with a gravel filter. This option is expected to be used frequently for the Urban/Natural.

Option C

Option C involves oversteepened slopes steeper than 3H:1V where tiered retaining walls are needed to conform to the desired geometry. These retaining walls will need to be tied back with grouted soil anchors (described below). This option is expected to be used for both the Urban/Natural lakes and the West Dallas Lake.

Option D

Option D involves river-side slopes 3H:1V or flatter that have an architectural treatment with stone slabs or boulders. These stone slabs provide erosion protection for the slope and improve stability. This option is expected to be used only on the Urban/Natural Lakes.

Soil Strength

Effective and Total stress strengths were used to develop strength parameters for use in accordance with the Duncan and Wright (1990) three-stage analysis of Rapid Drawdown for the Upper Clay Unit and for the Compacted Clay Fill/Liner soils. Strength parameters for these materials were interpreted from the results of Isotropically Consolidated Undrained triaxial strength tests with pore pressure measurements (ICU-bar tests). These ICU-bar tests were performed on relatively undisturbed Shelby Tube samples to characterize the Upper Clay Unit, and performed on remolded samples compacted to 98% of ASTM D 698 at the optimum water content for the Compacted Clay Fill/Liner soils.

Drained soil strengths are used in this analysis to represent the strength of materials which are expected to freely drain during drawdown scenarios. These materials include the riprap, topsoil, and the Articulating Concrete Block/soil cement/gravel filter materials.

Alluvial Overbank Deposits

It is assumed that the alluvial overbank deposits (as described in the Geotechnical Data Report by Fugro, 2009) will generally be removed beneath the embankment prism and replaced with compacted clay fill. The extents of this fill are indicated on the analysis cross-sections, and are expected to represent a conservative estimate. Where unsuitable soils are encountered, it is assumed that they will be overexcavated and replaced with compacted clay fill.

Upper Clay Unit

Both drained (effective stress) and undrained (total stress) shear strengths for the upper clay unit (embankment foundation material) were estimated using the results of ICU-bar tests performed on relatively undisturbed Shelby tube samples. These results were used to develop a bilinear strength envelope for analysis as recommended by the Duncan and

Wright (1990) three-stage slope stability procedure. Additional discussion regarding the development of the bilinear envelope and incorporation of the three-stage procedure into stability analysis discussions is presented in Chapter 4 of the Geotechnical Report.

As mentioned previously, the upper clay unit was conservatively considered to be the only foundation material beneath the embankment, neglecting the Flaggy Zone, Basal Sands, and bedrockall of which are anticipated to have a higher undrained strength.

Compacted Clay Fill/Liner

Both drained (effective stress) and undrained (total stress) shear strengths for the compacted clay embankment and liner were estimated the results of ICU-bar tests performed on remolded specimens compacted to 98% of ASTM D 698 at the optimum water content. These results were used to develop a bilinear strength envelope for analysis as recommended by the Duncan and Wright (1990) three-stage slope stability procedure. Additional discussion regarding the development of the bilinear envelope and incorporation of the three-stage procedure into stability analysis discussions is presented in Chapter 4 of the Geotechnical Report.

Riprap

The riprap material is assumed to exhibit drained behavior during drawdown. Drained (effective stress) strengths for the riprap were estimated from experience with typical values for similar material. A specific riprap source and gradation have not been identified at this time.

Topsoil

The strength of the topsoil material placed on top of the embankment is not expected to greatly impact the stability of the embankment. This material is currently not well defined, but is assumed to exhibit drained behavior under this loading condition. The topsoil placed in planters covering the top of the embankment were conservatively assumed to have a low strength. Specific information about the type of material used for the topsoil is not available at this time. These strengths were assumed, and are considered to be conservative.

Articulated Concrete Block/Soil Cement/Gravel Filter

The Articulated Concrete Block (ACB), soil cement, and gravel filter materials are assumed to exhibit drained behavior under this loading condition. Drained (effective stress) strengths for the ACB, soil cement, and gravel filter material are conservatively considered to behave as a single cohesionless material with an angle of internal friction

() of 35 degrees.

Retaining Walls

Retaining walls, where used, are assumed to be proportioned according to the recommended lateral earth pressures given in the corresponding calculation package. Internal stability (sliding, overturning, bearing capacity, etc) of these structures has not been addressed by this calculation. Global stability is, however, addressed by these calculation results.

Soil Anchors

Grouted soil anchors are required for the proposed Option C, where oversteepened slopes must be retained due to space constraints. Grouted soil anchors are estimated to develop capacity equal to 900 pounds-force per linear foot based on the recommendations of the Post Tensioning Institute (PTI) for the materials encountered on site. The estimated length required for global stability of the Option C downstream slope is approximately 62 feet per anchor. For stability, spacing of these anchors was found to be 4 feet laterally, with one row of anchors required for each tier (NOTE: 4 tiers for Option C, typ.). The specific details of this configuration are subject to change. Details of the internal retaining wall design will be addressed during subsequent levels of design. Global stability is confirmed by these calculation results.

Stone Slabs

Stone slabs, where used for the proposed Option D, are conservatively assumed to have drained (effective stress) shear strength approximately equal to that of the riprap layer. The exact nature of the slabs is not known at this time, but their size is expected to be massive, and use of riprap strength is assumed to be conservative.

Analysis Details

The UTEXAS3 computer program (Shinoak Software, 1991) was used to evaluate the stability of the riverward side and lake side stability under rapid drawdown conditions for Options A, B, and D. This program executes the Duncan and Wright (1990) method of determining shear strength for drawdown. Spencers (1967) method was implemented within the program to perform the limit equilibrium analysis. Extensive details describing the nature of the analysis are presented in Chapter 4 of the Geotechnical Report.

The SLIDE v5.03 (Rocscience, 2007) two-dimensional limit-equilibrium computer program was used to perform stability analyses Option C of the proposed sections. SLIDE utilizes the B-bar effective stress method as a means of evaluating slope stability following rapid drawdown. SLIDE was utilized because the UTEXAS3 computer program was not able to consider the tieback anchors in the analysis. Spencers method was used within SLIDE as a means of considering total force equilibrium between slices.

The phreatic surface through the embankment was manually approximated to duplicate the results seen in previous finite element analyses. Values of hydraulic conductivity and boundary conditions used are discussed in the Seepage Analysis Memo.

Analysis Results

The results of this analysis were found to be acceptable, even with the low strengths assumed for the upper clay unit and the compacted clay.

Lake Berm: Slope stabilization

option:

Upstream or Downstream

Slope* Factor of Safety

Urban/Natural Opt. B US 1.41 Urban/Natural Opt. B DS 1.37 Urban/Natural Opt. D US 1.41 Urban/Natural Opt. D DS 1.22 Urban/Natural Opt. C US 1.29 Urban/Natural Opt. C DS 1.18 West Dallas Opt. A US 1.28 West Dallas Opt. A DS 1.32

* "Upstream (US) slope considered to be the lake side, Downstream (DS) slope considered to be the river side.

** Analysis not yet performed.