Designing Strong Walls on Weak Soils

RETAINING WALLS Designing strong walls on weak soils

Civil engineers have options to remedy foundation soil problems and meet project cost and schedule requirements.

By Fadi Faraj, P.E.; Michael H. Garrison, P.E.; and Brendan FitzPatrick, P.E.

he demand for new roadway stabilized earth (MSE) walls. While foundation soil problems include mas- construction or expansion both systems are commonly used, many sive overexcavation and replacement, of existing infrastructure for public and private owners have adopted deep foundations, or staged construc- both public and private own- MSE wall solutions, which represent a tion. Each of these options provides Ters continues to grow. These projects more economical and faster wall con- distinct advantages and disadvantages commonly involve grade-separation struction approach than cast-in-place and is selected based on the project- construction and projects are often cantilevered retaining walls. MSE walls specific needs. restricted by tight schedules, limited can also be designed to tolerate more For instance, overexcavation is most funding, public opposition, and right- settlement. commonly used when the depth of of-way limitations, among other things. However, weak or compressible weak or compressible soils is relatively Although project challenges may vary, foundation soils present significant shallow (less than 10 feet). Removal of one universal question remains for design and construction challenges. shallow, unsuitable soils and replace- engineers, contractors, and owners on Wall heights commonly range from 10 ment with compacted, engineered every project: How do you design and to 40 feet and apply pressures ranging fill is often an inexpensive approach build the project to meet the owner’s from 4,000 to 7,000 pounds per square to provide improved foundation soils cost and schedule requirements? foot (psf) near the wall face, depending in these conditions. Overexcavation Infrastructure construction involv- on the specific wall design. The increas- and replacement may become less ing grade separations in urban environ- ing size of these walls poses geotechni- cost-effective, however, when poor ments is often challenged by tight or cal challenges, including inadequate soils extend to deeper depths, dewa- difficult access and limited right-of- factors of safety for global stability and tering is required because of shallow way restrictions for construction. As bearing capacity, as well as excessive groundwater, temporary shoring is an alternate to sloped embankments, total and differential settlement. required to stabilize an excavation next which typically require large work Traditional solutions for remedying to an existing roadway, or the presence areas and property acquisition, grade- separation solutions typically involve construction of retaining walls using Construction of cantilevered or mechanically stabilized earth retaining walls to create grade separa- either conventional cast-in-place con- tions presents significant design and construction challenges when faced with weak or compressible crete cantilevered walls or mechanically foundation soils.

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Rammed Aggregate Pier installation involves drilling a 30-inch-diameter hole; placing thin lifts of aggregate within the cavity; and vertically ramming the aggregate using a high-energy, patented of contaminated soil results in high beveled impact tamper. costs of disposal. Overexcavation and replacement is significantly affected by inclement weather, which could present pressure is applied, settlement occurs able to support high loads through very schedule challenges as well. and the weak foundation soils become soft soils — delivering superior perfor- When compressible or weak soils stronger, thereby permitting higher mance — but are an expensive solution. extend to depths of 30 or 40 feet embankment construction. This process The costs incurred include not only the or greater, options for supporting is repeated in multiple stages until the deep foundations (driven steel or con- embankments or wall construction may embankment reaches the final design crete piles, augercast-in-place piles, or include staged construction or deep height. This approach is well suited caissons) but also a load-transfer plat- foundations. Staging involves embank- when there is significant time in the form constructed using either multiple ment construction to specific heights, construction schedule. This approach layers of structural geogrid or a concrete temporarily stopping construction and is not often a viable solution when the mat to transfer embankment pressures monitoring the embankment until embankment construction is part of the to the deep foundation elements. settlement is complete, followed by critical path for the project. The balance of cost, schedule, perfor- continuation of construction to greater Deep foundations are used in similar mance, and ease of construction has led heights. The purpose is to build the situations with deep, compressible soils design teams to an alternative approach embankment to specific heights where to support and transfer the embank- for embankment and wall construction the existing soils will provide suitable ment pressures to more competent called an Intermediate Foundation support. As the new embankment bearing layers. Deep foundations are solution. This approach uses Rammed

36 CE NEWS April 2008 www.cenews.com RETAINING WALLS

Retaining wall heights commonly range from 10 to 40 feet and apply pressures ranging from 4,000 to 7,000 pounds per square foot near the wall face, depending on the specific wall design.

of Loop 363 to create a new highway interchange along Interstate 35, as well as widening nearby portions of Loop 363 to accommodate the traffic demand created from the new interchange. In one location, an existing embankment was used to facilitate a grade separation over an existing railroad crossing. The plan called for widening the existing two-lane roadway to accommodate a total of four lanes. In another location, proposed interchange construction required new grade- separation construction. Because of the presence of a nearby telecommunica- tion substation, acquiring additional right-of-way to facilitate widening of the existing embankments was not a Aggregate Pier (RAP) systems to reduces the compressibility of the sur- viable solution. After a preliminary reinforce poor soils to intermediate rounding soil and promotes positive analysis was conducted to compare depths, typically ranging from 10 to 40 coupling of the RAP element and the construction of an extended bridge feet (see Figure 1). As described in the soil to create a composite, reinforced with an embankment/retaining wall, recently published Highway Innova- soil zone. the project team concluded that a taller tive Technology Evaluation Center Additionally, when constructed using MSE wall would be the most economi- (HITEC) evaluation report, RAP open-graded stone, the RAP elements cally feasible solution. elements use highly densified aggre- act as vertical drains to promote radial Led by transportation engineers at gate piers to improve the composite drainage and accelerate settlement PBS&J working for the Texas Depart- engineering characteristics of poor or within the reinforced zone. Overall, the ment of Transportation, the project unsuitable soils to support high applied system provides the benefits of increased team developed plans for walls as high pressures. Installation involves drilling shear resistance for stability and bear- as 38 feet at a railroad overpass and as a 30-inch-diameter hole; placing thin ing capacity improvement coupled with high as 22 feet at the new I-35 inter- lifts of aggregate within the cavity; and reduction in settlement magnitude and change. Wall construction was expected vertically ramming the aggregate using duration by improving the strength and to apply pressures greater than 4,250 a high-energy, patented beveled impact stiffness of soft or compressible soils at psf at the 22-foot-tall wall and 7,500 tamper. intermediate depths. psf at the 38-foot-tall wall. During construction, the high-fre- Geotechnical engineers at HVJ quency energy delivered by the modi- Foundation challenge Associates, Inc., investigated existing fied hydraulic hammer, combined with Engineers designing the Loop 363 soil conditions at the wall locations the beveled shape of the tamper, not South Interchange project in Temple, and evaluated performance of the walls. only densifies the aggregate vertically Texas, were confronted by design Soil conditions for the project consisted to create a stiff aggregate pier with challenges for a series of new grade- of newly placed clay fill, in some areas internal friction angles on the order of separation walls — inadequate factors extending to depths of about 8 feet, 50 degrees, but also forces aggregate of safety for global stability and bearing underlain by very soft to stiff clay. The laterally into the sidewall of the hole, capacity, as well as excessive total and clay ranged from low to high plasticity, resulting in lateral stress increase in sur- differential settlement. The project with moisture contents ranging from rounding soil. The lateral stress increase involved reconstruction of portions 15 percent to 38 percent. The clay was

38 CE NEWS April 2008 www.cenews.com Weak or compressible foundation soils present significant design and construction challenges.

underlain by bedrock at depths as shal- analyses (slope stability programs), Bearing pressures were calculated low as 13 feet in some locations and HVJ Associates concluded that the at the retaining walls and, using con- more than 30 feet in other locations. shear strength (resistance) along the ventional Terzaghi bearing capacity HVJ Associates identified early in critical slip surface extending behind approaches, engineers determined that the design that construction of the tall the reinforced portion of the wall and the factors of safety for bearing would walls would result in significant increase through the weak clay was insufficient fall below the required minimum factor in the shear stress (demand) on the for supporting the walls. Factors of of safety of 2.0 for wall heights greater underlying weak clay foundation soils. safety for stability may be determined than about 16 feet. While settlement In addition, the high applied pressures as the ratio of the shear strength within in the areas with relatively shallow rock at the wall face resulted in unacceptably the contributing soil layers along the was less of a concern, the variability of low factors of safety for bearing capac- slip surface to the applied shear stress. the clay stiffness and the deeper depth ity. While settlement control was less The calculated factor of safety for the to rock coupled with the high design of a concern in areas with shallow rock, long-term case was less than 1.25 for pressures resulted in estimated post- the high wall pressures applied in areas walls taller than 15 feet, and approxi- construction settlement of more than of deeper rock were expected to result mately 1.0 for walls taller than 20 feet, 5 inches. An alternative solution was in unacceptable long-term settlement. indicating a strong likelihood of global required to limit the post-construction

12PH-MesaAACEN4C08Using conventional limit 3/6/08 equilibrium 4:42 PM instability. Page 1 settlement to 1 inch or less.

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The conventional approach Figure 1: Conceptual drawing of mechani- of removal and replacement was cally stabilized earth wall Rammed initially considered by the design Aggregate Pier element layout team to address the geotechnical design challenges. Concern about required depth and lateral extent of elements installed beneath the excavations combined with nega- MSE walls. RAP spacings rang- tive impacts to the construction ing from 4.75 to 8.5 feet on-center schedule and costs led the project were incorporated beneath wall team to consider other alterna- heights of 16 feet or greater. The tives. Based on previous project spacing of the piers was reduced, experiences, engineers from HVJ corresponding to increases in Associates determined that an wall heights, to provide sufficient Intermediate Foundation solu- levels of improvement. Piers were tion using RAP elements would as long as 16 feet but did not provide the level of improvement completely penetrate the clay. In required to satisfy factors of safety some locations, piers did tag the for both bearing and global stabil- shallow rock. ity and provide sufficient improvement By incorporating the high shear in the composite stiffness to control Design and installation strength afforded by each pier, the settlement of the walls. The system Working closely with the project improved strength characteristics of would also provide a cost-effective team, a solution was developed by the composite reinforced zone provided approach to soil reinforcement while Geopier Foundation Company, Inc., increases in the factors of safety for making short work of pier installation. consisting of two to four rows of RAP bearing capacity instability and global

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40 CE NEWS April 2008 www.cenews.com instability to greater than 2.0 and 1.3, Additionally, a steel telltale rod sleeved ness and impacts to the construction respectively. Additionally, the stiff in PVC and installed within the pier schedule must then be considered when RAP elements substantially reduced allows for deflection measurements evaluating the overall effectiveness of a settlement magnitudes to meet the near the bottom of the pier. By moni- solution to reinforce poor foundation stringent post-construction settlement toring deflections at both the top and soils. Each project requires a unique requirement. bottom of the pier, the modulus test solution to address the specific design Working for general contractor provides confirmation that the stiff- challenges such as global instability, Zachry Construction Corporation ness of the pier achieves the required inadequate bearing, or excessive settle- of San Antonio, Texas, Peterson design stiffness, and that the pier is ment magnitude or duration. Contractors, Inc., of Reinbeck, Iowa, sufficiently long to dissipate stress to installed the piers. During installation, act as an intermediate foundation as field monitoring was performed to opposed to a deep foundation (pile), provide quality control for the instal- which transfers loads to a better layer. lations. In addition, field performance The modulus test results showed a total Fadi Faraj, P.E., is senior project engi- verification of the RAP system was movement of 0.69 inches at a stress of neer for HVJ Associates, Inc., in Dallas. accomplished by conducting a full- more than 22,000 psf, indicating a pier He can be contacted at [email protected]. Michael H. Garrison, P.E., is senior scale modulus test. stiffness greater than twice the assumed project manager for PBS&J in Dallas. He The modulus test is similar to a design value. can be contacted at mhgarrison@pbsj. pile load test, where stress is applied com. Brendan FitzPatrick, P.E., is to a concrete cap at the top of the pier Conclusion director of engineering and development using a 100-ton jack reacting against Solutions to address the engineering – North America for Geopier Foundation a steel beam held in place with helical challenges encountered on transporta- Company, Inc., Mooresville, N.C. He can be anchors. Deflections are taken to moni- tion projects must first overcome the contacted at [email protected]. tor the movement of the top of the pier. technical challenges. Cost-effective- ACEC Directory Ad_7.375x4.8 11/29/07 3:46 PM Page 1

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