5.2         Field Records

Figures 5-2 and 5-3 shows the stress monitored over the construction period of both array the south and middle array respectively. From the two figures it can be noticed that the stresses monitored by the cell that are beneath the foam at level zero recorded less than 20kPa at the end of construction. The design stress estimate from the pavement loading is 27kPa, as was mentioned earlier. This was observed in both arrays and the recorded value of less than 5kPa in the middle array is even lower than for the south. The stress cells installed below the concrete slab, i.e. S9 in the south array and M8 in the middle array, recorded about 30 and 40kPa, respectively, at the end of construction. The cells located on the top of the concrete slab, i.e. SLDS in the south array and MLDS in the middle array recorded approximately 55kPa and 40kPa respectively. For all cells, the jump in the recorded stress in the middle of May 1999 was a result of fill placement. More fill was added at the end of June of the same year.

Explanation of these trends of results could be as follow:

·         There are gaps between the foam blocks as was mentioned earlier and these gaps will make non uniform contact between the successive block layers hence creating non uniform stress distribution on the foam.

·         The pressure of the load cells will create stress concentration resulting in an increase of stresses read by the cells.

·         The lower stress cell is embedded in the sand, over the soft soil. Hence settlement is expected to occur below the pressure cell upon increasing the stresses, limiting the reading to such small values shown in the figures.

·         The 55kPa stress recorded by the pressure cell above the load distribution slab in the south array is not related to the gaps between the foam layers as the 0.15m reinforced concrete slab is stiff enough to be affected by such gaps. So the reason of such high-recorded stress value must be related to the type of fill used above the slab. Bad compaction of such fills may results in gaps around the pressure cell yielding to stress concentrations at on the pressure cell.

Figure 5‑1 Recorded Stresses in the South Array

Figure 5‑2 Recorded Stresses in the Middle Array

Figures 5-4 through 5-14 show the results of the displacements measured by the magnet plates in the south array section. The magnet plates were placed in level zero and in every other level as well as level nine. That results in five readings. All readings are measured from level zero as base reference, figure 5-4. From this figure it can be seen that the distance between the magnetic plates is approximately 1.6m except between the top two plates, which approximately equals 0.8m. The creep cannot be noticed using the scale in figure 5-4. Note that more readings were obtained during the construction period.

Figure 5-5 shows the difference between the nominal height between blocks and the same distance measured from the field records for the south array. The difference is for two successive block layers. Positive values can mean it is equal to two gaps plus the thickness of the magnetic plate. All the values are positive except for the top layer; a negative value is recorded. The gaps are not of the same values between the layers. Negative values can be explained by considering the block dimension tolerance of ±0.5 percent as was mentioned before. As the magnet is almost 1cm in thickness and the total gap in this south array equals to 121.3mm the air gap can be considered equal to 80mm.

Figure 5-6 shows the settlement in the south array for portions of the geofoam measured from the level zero. The total settlement shown is for both the immediate deformation and the creep settlement. The maximum value reached is approximately 80mm. From the equation in figure 3-23 the initial modulus of this 18kg/m³ minimum density is about 4.3 MPa, hence the intermediate strain is equal to 0.64% for a stress of 27.5kPa resulting in an immediate settlement of 47mm between levels zero and nine. The gaps are not completely closed by the summer of year 2000 and there is still non-uniform stress distribution because of the existence of the gaps.

Figure 5‑3 Elevations of the Different Levels in the South Array

Figure 5‑4 Gap Heights in the South Array

Figure 5‑5 South Array Creep for Different Portions

 The settlement observed since the end of construction reached 13mm as shown in figure 5-7. The main trend is that most of the settlement occurred in the 1^st 6 months, the settlement over the following 6 months have been negligible and are within the system precision. Figure 5-8 shows the settlement occurring in the different 2 layer segments of the embankment. The top single layer experienced insignificant settlement throughout the period of record. The other layers experienced total settlements of between 15 to 26mm by June 2000. Most of the settlement occurred during the construction period, and has leveled by the year 2000.

Figure 5-9 shows strains for different heights of the south array measured from the bottom layer. A 1% strain was reached by June 2000 for all the portions. From figure 5-10 the strains obtained after construction are less than 0.2%. Figure 5-11 shows comparable strain developed in most of the intermediate portions. The top portion hadn’t experienced any immediate strain that’s why the total strain reached was less than 0.2%. Incremental strains of generally less than 0.2% were recorded since the end of construction in all intermediate portions of the south array as shown in figure 5-12.

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Figure 5‑6 South Array Creep for Different Portions Since August 1999

Figure 5‑8 South Array Strains for Different Portions

Figure 5‑9 South Array Strains for Different Portions Since August 1999

Figure 5‑10 South Array Strains for Intermediate Portions

Figure 5‑11 South Array Strains for Intermediate Portions Since August 1999

Figure 5‑12 South Array Strain Rates for Intermediate Portions

Figure 5‑13 Intermediate Portions South Array Strain Rates Since August 1999

Figure 5-15 through 5-23 show the results of the displacements measured by the magnet plates in the middle array section. The magnet plates were placed in level zero and at every other block interface level. All four readings are measured from level zero. That will null the zero level reading as shown in figure 5-15. From this figure it can be seen that the distances between the magnetic plates is approximately 1.6m. The creep is difficult to observe in the scale of figure 5-15. More readings were obtained during the construction period.

Figure 5-16 shows the values of the gaps between the blocks in the vertical direction for the middle array. The values shown are for two layers that mean it equals to two gaps plus the thickness of the magnetic plate. All values are positive except for the top layer a negative value is recorded. The gaps are calculated by subtracting the nominal thickness of the layers from the thickness between the layers recorded by the sensors. The gaps are not of the same values between the layers. Negative values are expected as the blocks have a dimension tolerance of ±0.5 percent as was mentioned before. As the magnet is almost 1cm in thickness and the total gap in this array equals to 103mm, the air gap will be around 60mm.

Figure 5-17 shows the settlement in the middle array for portions of the geofoam measured from the level zero. The total settlement shown is for both the immediate deformation and the creep settlement. The maximum value reached is approximately 80mm. From the equation in figure 3-23 the initial modulus of this 18kg/m³ minimum density about 4.3 MPa, hence the intermediate strain is equal to 0.64% for a stress of 27.5kPa resulting in an immediate settlement of 41mm between levels zero and eight. The gaps are not completely closed by the summer of year 2000 and there is still non-uniform stress distribution because of the existence of the gaps.

The settlement calculated since the end of construction reached 11mm as shown in figure 5-18. The main trend is that the curves tend to level. Figure 5-19 shows the settlement occurring in the different intermediate portions of the embankment. All layers experienced total settlement between 10 and 26mm by June 2000. The settlement was large during the construction period and tends to level by the year 2000.

Figure 5-20 shows the strain for different portions of the middle array measured from the bottom level of the bottom foam layer. A 1.4% strain was reached by June 2000 for all the portions. Figure 5-21 shows less than 1.5% strain developed in all segments. Some portions experienced 0.7% total strains, and closer to estimates for no gaps between the layers. Strains of less than 0.2% developed since the end of construction in all intermediate portions of the middle array as shown in figure 5-21.

Strain rates for different intermediate portions are shown in figure 5-22. During construction the values of strain rate was up to 60% per year. This is not actually the strain inside the foam rather than closing the gaps between the foam layers. After the end of construction the strain rates tend to zero average value (figure 5-23).

Figure 5‑14 Elevations of the Different Levels in the Middle Array

Figure 5‑15 Gap Heights in the Middle Array

Figure 5‑16 Middle Array Creep for Different Portions

Figure 5‑17 Middle Array Creep for Different Portions since August 1999

Figure 5‑18 Creep in the Middle Array for Intermediate Portions

Figure 5‑19 Middle Array Strains for Different Portions

Figure 5‑20 Middle Array Strains for Intermediate Portions

Figure 5‑21 Middle Array Strain Rates for Intermediate Portions

Figure 5‑22 Intermediate Portions Middle Array Strain Rates since August 1999