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Designing Strong Walls on Weak Soils

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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. www.cenews.com April 2008 CE NEWS 35 RETAINING WALLS 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 embank- ment was used to facilitate a grade separation over an existing railroad crossing. The plan called for widen- ing 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
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