Roadway widening over existing walls and embankments, conflicts with settlement-sensitive utilities, and accelerated schedule delivery have increased demands for alternative lightweight fill materials. Engineers and contractors are increasingly considering Lightweight Cellular Concrete (LCC) backfills for abutments, embankments, and Mechanically Stabilized Earth (MSE) retaining walls; however, the absence of a consistent design methodology has led to a wide range of design approaches with no consensus standard. The most common class of LCC used in previous highway projects does not strictly behave like a soil or like concrete and must be investigated as a new material for engineering applications. Controversy exists within the industry regarding whether LCC should be modeled as a frictional or a cementitious (cohesive) material. In addition, earth pressures for retaining wall design and potential failure mechanisms of LCC are poorly understood for retaining wall applications, including uncertainty in LCC interaction with internal wall reinforcement in MSE wall applications.

The overall objective of this study is to measure engineering design parameters and failure mechanisms for unreinforced and reinforced LCC backfills based on large-scale laboratory tests.

Funded tasks for this study include the following: 1. Perform literature review and survey to determine methods currently used in design of MSE walls with LCC backfill, and review performance of these walls since construction (where possible). 2. Conduct Unconfined Compressive Strength (UCS), triaxial shear, direct shear, unit weight, and other laboratory tests to define basic material properties of LCC backfill (Caltrans Class II) that is used during the course of each of the five large-scale laboratory tests. 3. Perform a large-scale test on unreinforced LCC using a reinforced concrete, cantilever retaining wall on the open side of an existing BYU test box. Measure pressures on wall, wall deformations, and eventual failure planes during fill placement, curing, and after application of a surcharge load at the top of the cured fill surface. (This test will be performed after reviewing results of a similar test previously performed on a separate UDOT research project.) 4. Within the BYU test box, perform the following four large-scale tests using MSE wall panels with various arrangements of LCC fill reinforced with inextensible ribbed strip reinforcements (or other reinforcement as indicated): > Reinforced LCC Test 1 – MSE wall with LCC backfill, > Reinforced LCC Test 2 – MSE wall with LCC backfill against soil slope, > Reinforced LCC Test 3 – MSE wall test with lower strength LCC backfill, > Reinforced LCC Test 4 – Pull-out tests on MSE wall, and > Reinforced LCC Test 5 – MSE wall test with welded-wire reinforcement In these MSE reinforced LCC backfill tests, measure pressures on wall panels, wall deformations, force in reinforcements, and internal failure planes during fill placement and after application of a surcharge load at the cured fill surface. The pull-out tests of reinforcements will be performed at a variety of vertical effective stress levels with and without surcharge. These pull-out tests will include some welded-wire reinforcements in addition to the originally planned ribbed-strip reinforcements. 5. Compare results with design methods. Define earth pressure coefficients, wall displacement, and failure surface geometry for the unreinforced LCC backfill test and the reinforced MSE wall LCC backfill tests. Define reinforcement pull-out resistance as a function of vertical stress and LCC strength. Compare measured earth pressure, tensile force, and pull-out resistance with available design methods. 6. Prepare two Final Reports that describe the test setup, test results, and provides comparisons with existing design procedures for (a) the unreinforced LCC test and (b) the reinforced LCC tests. The reports will also provide recommendations for design procedures based on test results and analyses of data relative to existing procedures. 7. Disseminate study results in periodic TAC update meetings and in other venues as funding allows.

The Principal Investigators for this study are Dr. Kyle Rollins of Brigham Young University and Ryan Maw, a principal engineer at Gerhart-Cole, Inc. Dr. Rollins has extensive experience with large scale testing of piles, MSE walls and bridge abutments. Gerhart-Cole has designed several MSE walls with LCC backfill and conducted triaxial shear tests on LCC. The project began in May 2020 and is anticipated to take two years to complete. The minimum partner commitment expected is $30,000, in 2019 or 2020, or split between both years. The 100% SPR approval has been received.