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67 ence. In summary, long-term strength behavior, with cylin- Background Information ders subjected to similar time-temperature histories, can serve as a better indicator of field behavior and excavatabil- In September 1996 NRMCA cast two CLSM mixtures into ity, but even this approach would not recognize that the spe- the two trenches evaluated in this field study. The two CLSM cific materials and proportions (e.g., presence or lack of sand) mixtures were cast despite the heavy rain from a hurricane. can profoundly impact excavatability. For convenience, the two trenches are referred to herein as As reported in Chapter 3, the splitting tensile strength of Northeast (NE) and Northwest (NW). The CLSM mixture in CLSM might be a better indicator of excavatability than com- NE consisted of 29.6 kg/m3 portland cement; 1406 kg/m3 pressive strength, because the actual excavation of CLSM mim- high-carbon, Class F fly ash; and 292 kg/m3 water (similar in ics a tensile failure in the material. In this field trial, the tensile nature to the Paste mixture from the UTAustin study). The strength values correlated quite well with DCP indexes and CLSM mixture in NW was composed of 28.5 kg/m3 portland ease of manual or mechanical excavation. Although tensile cement, 180.9 kg/m3 Class F fly ash, 1409 kg/m3 concrete sand, strength may be a better index of excavatability, the inherent and 270 kg/m3 water. variability in tensile results is higher than that for compression testing, and therefore, precautions should be taken to lessen Testing Program observed variations. The use of a removability modulus, as proposed by Hamil- A range of tests was performed on the two trenches; in ad- ton County (Ohio), successfully predicted the excavatability dition, limited testing (compression and splitting tensile) was of the six CLSM mixtures. As described in Chapter 3, this performed on cylinders that remained in the fog room from approach takes into account the 28-day laboratory-cured the original mixtures. The following tests were performed strength of a given mixture and its in-situ density to calcu- (results are described later in this section): late a removability modulus. Values of RE greater than 1.0 are assumed to be non-excavatable. The RE data shown in Excavatability (square-head shovel, round-head shovel, pick, Table 4.6 was based on 28-day laboratory-cured strength val- backhoe) ues, as per the Hamilton County approach. In addition, the Needle penetrometer (field version of ASTM C 403) field-cured 28-day values were also used to calculate RE, Soil penetrometer (hand or pocket) which slightly increased the RE values for the non-excavat- Torvane shear tester able mixtures and had a negligible effect on the excavatable Kelly ball (after ASTM D 6024) mixtures. GeoGauge Dynamic cone penetrometer Compressive strength Excavation Study at NRMCA 75 150 mm and 150 300 mm cylinders (capped with (Silver Spring, Maryland) sulfur) Introduction 100 200 mm cylinders (capped with polyurethane pads) Splitting tensile strength (150 300 mm cylinders) A major concern historically with using CLSM in backfill applications is related to ease of excavation, for instance, when CLSM is used in utility applications. During the course Results and Discussion of this project, major efforts were undertaken to investigate Excavatability this issue by evaluating CLSM that was cast either in the lab- oratory or field and then later excavated by various methods. The two CLSM trenches evaluated in this study were buried However, because of the finite duration of the project, exca- approximately 0.6 m below grade, with a layer of soil above the vatability was assessed within a matter of months (or a year trenches. A backhoe was first used to remove the soil and to or so in some cases) after CLSM placement. This limitation expose the CLSM. Groundwater was found on the exposed NE was addressed in a unique way in a field test performed at the trench, and a lower elevation was formed to drain the water. NRMCA facility in Maryland: two CLSM trenches were ex- Both CLSM trenches were visibly in good condition, with no cavated that had been placed about 6 years earlier as part of a signs of freeze-thaw damage or other forms of distress. How- separate CLSM research effort. Because the trenches were cast ever, the CLSM in the NW trench had segregated, especially in with the intention of tracking long-term CLSM properties, the upper 80 to 100 mm. quite a bit of information and data were available, including Table 4.7 summarizes the results of various evaluations earlier attempts at excavation. This section describes the either directly or indirectly related to excavatability. The NE excavation study and relates this experience to various engi- trench was quite easy to excavate manually, but the NW trench neering properties. was very difficult, if not impossible, to remove manually, which

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68 Table 4.7. Evaluation of excavatability of test trenches at NRMCA. Method NE Trench NW Trench Square-head shovel Easy Nearly impossible: only shallow dents were made on the surface Round-head shovel Easy Very difficult: small pieces were removed Pick Easy Difficult: pick could penetrate into the mixture Kelly ball (ASTM D 6024) Average diameter 95 mm Average diameter 87 mm; C.O.V. 4.0% Torvane shear tester Average 3.9 kg/cm2; Average 3.2 kg/cm2; C.O.V. 9.9% C.O.V. 18.1% Needle penetrometer (field 5.7 MPa Out of range version of ASTM C 403) Soil penetrometer 4.0 MPa Out of range Stiffness (using GeoGauge) Average 10.3 MN/m, Average 47.4 MN/m, C.O.V. 18.3% C.O.V. 1.8% DCP 4.5 mm per blow 1.3 mm per blow RE 0.23 1.04 Backhoe Easy Easy was somewhat surprising because manual excavation of this trench was possible 4 years earlier (2 years after placement). Both trenches were easily excavated using a backhoe (after the completion of the other tests). The Kelly ball test and Torvane shear test (which measures the shear resistance of soil as the device twists) were not able to differentiate the manual excavatability of the two trenches. This inability may be because these two tests involve near- surface measurements of CLSM, and the surfaces of these trenches were somewhat disturbed during the removal of the top soil that covered the trenches. The needle penetrometer (field version of ASTM C 403) and soil or pocket penetrometer were both used on these trenches, but as expected, the NW trench was impenetrable because of its higher strength. These devices are better for measurements of earlier CLSM properties and impact on constructability. The GeoGauge (Model H-4140), which did not perform very reliably in the laboratory trials described in Chapter 3, was able to discern the difference in excavatability between the two trenches, with the measured stiffness of the NW trench found to be almost 5 times as high as the NE trench. Figure 4.2. The DCP being used in the NW trench. Figure 4.2 shows the DCP being used in the NW trench. The DCP is often used in pavement construction to evaluate the CBR compaction or density of subgrade, subbase, and base ma- 1 10 100 terials. One advantage of this method is it allows for evaluation 0 0 of CLSM penetrability as a function of depth of placement. The 100 5 DCP index is defined as the penetration per blow and it has DEPTH, mm DEPTH, in. 200 been correlated empirically with CBR values. Based on infor- 10 mation provided by the DCP manufacturer, the CBR values 300 15 along the depth of NE and NW materials were calculated and 400 are shown in Figures 4.3 and 4.4, respectively. CBR values of 20 500 100 (which NW surpassed) correspond to a well-compacted stone backfill, which presumably would be difficult to excavate, 25 600 1 10 100 as was NW. One interesting observation from the NW trench was the significant difference in the DCP values (and calculated Figure 4.3. The CBR profile of NE trench.