Results of the load tests

The load tests were performed as planned without any serious mishaps. The only significant practical problem was heavy rain during the first test, which was a nuisance but did not affect the test results in any way other than that a rising ground water level had to be taken into account. Some minor problems with maitaining a constant load were also encountered. These problems were mainly related to movements in the reaction system.

• 0.5 x 0.5 metre plate

The loading of the smallest plate was performed in steps of 10 kN up to 60 kN total load and then in steps of 20 kN. Each step was applied for 2 hours. In order to check the elastic properties, unloading and reloading cycles were also performed with durations of each step of only 1 or 2 minutes.

At the end of the load step with 120 kN load, the settlements had reached 60 mm and an unload-reload cycle was performed which also enabled additional distance plates to be placed on top of the jack in order to increase the possible stroke. The

Investigations and load tets in silty soils 93

test was ended at a load of 180 kN when the settlements had reached 140 mm and new difficulties with the stroke capacity of the jack developed. The plate went straight down without any tilting, Fig. 4.2.29.

Vertical load, kN plate in the test site at Vagverket.

The time -settlement curves for each load step formed straight lines in a settlement - log time plot, Fig. 4.2.30. Any deviations from this pattern are related to difficulties in maintaining a perfectly constant load.

The evaluated slopes of these lines constitute a measure of the creep rate in the load steps. The creep rates increase continuously with increasing load. The curve, however, is smoothly rounded and it is difficult to locate any particular point where there is a significant change in behaviour and thereby an indication of a creep failure load, Fig. 4.2.31.

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Time, minutes

Fig. 4.2.30 Settlement - log time curves measured in the load test on the 0.5 x 0.5 metre plate in the test field at Vagverket.

Fig. 4.2.3 I Rate of creep settlements versus load measured in the load test on the 0.5 x 0.5 metre plate in the test field at Vagverket.

Investigations and load tets in silty soils 95

Under the assumption that the pattern in the measured time-settlement curves continues, the measured creep settlement rates enable an extrapolation of the measured settlements after 2-hours to expected settlements after 10 years, which is the time perspective normally considered in settlement predictions. Both the measured 2 hour curve and the extrapolated 10 year curve for settlement versus load are smoothly rounded without any sign of a bearing capacity failure for loads up to 180 kN, Fig.4.2 .32. However, the settlements at this stage are very large and all criteria for acceptable settlements were passed at much lower loads.

Load, kN

0 20 40 60 80 100 120 140 160 180

0

50 E E

+J' 100

C Q)

E Q)

E 150

Q) U)

-<>-2 hours

200 --a-1 0 years

250

Fig. 4.2.32 Measured and extrapolated load - settlement curves in the load test on the 0.5 x 0.5 metre plate in the test field at Viigverket.

The time dependence of settlements in coarser soils is normally estimated by assuming that the calculated settlements for a certain time can be adjusted to another time by applying the formula

S

₁

=

S₀ (1+c·log-) t ta

where S₁

=

settlement at time t Sa= settlement at time ta c

=

constant

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In load tests, the constant c can be calculated in two ways, either versus the total settlement or versus the settlement in the current load step. When this is done for the results from the current load test, the factor c calculated on the basis of total settlements is found to be about 0.07 and calculated on the basis of settlements in the load step it becomes about twice this value or 0.13, Fig. 4.2.33. The values are somewhat sensitive to the time used as a reference, which in this case was 1 year.

0.18

Fig. 4.2.33 Relative creep settlement rate versus load in the load test on the 0.5 x 0.5 metre plate in the test field at Vagverket.

The measured excess pore pressures were small. At application of the 10 kN load steps, the measured pore pressure momentarily increased by about 1.5 kPa. When the load steps were increased to 20 kN, the pore pressure increases became about 3 kPa. The excess pore pressure dissipated within about 1 hour. The dissipation was partly masked by the simultaneous change in ground water level, which rose about 0.4 m, corresponding to 4 kPa during the test, Figs. 4.2.34 and 35.

On the basis of the pore pressure measurements, the load test can thus be considered a drained test and the selected time for load application was found to be well suited.

Due to the very large settlements of this plate, the measuring range of the settlement gauges with depth was exceeded and the distribution of settlements with depth has not been evaluated for this plate. After this experience, the settlement gauges for the two remaining plates were screwed down further to allow for larger settlements.

Investigations and load tets in silty soils 97

7.00

Fig. 4.2.34 Measured pore pressure changes and change in ground water level during the load test on the 0.5 x 0.5 metre plate in the test field at

Fig. 4.2.35 Measured pore pressure changes corrected for changes in ground water level in the load test on the 0.5 x 0.5 metre plate in the test field at Vagverket.

98 SGI Report No 54

• 1 x 1 metre plate

The loading of the 1 x 1 metre plate was performed in steps of 40 kN up to a total load of 200 kN and then in steps of 50 kN. Each load was applied for 3 hours. Two unloading-reloading cycles were performed at 300 and 350 kN load respectively.

The test was terminated at 400 kN load when the average total settlement was 135 mm. This plate started to tilt and the tilting gradually increased, although not alarmingly so. When the test ended, one of the sides had settled 110 mm and the opposite side with the largest settlements 160 mm, Fig. 4.2.36.

Load, kN

The measurements of the deep settlement gauges showed that the settlements started just below the plate and then spread downwards with increasing load but only just reached the level 2.1 m (= 2.lb) below the plate, Fig. 4.2.37.

Also in this case, the settlement-log time curves for the load steps formed fairly straight lines and any deviations can be attributed to load variations, Fig. 4.2.38.

The curve for the creep rate versus load formed a smooth curve and also in this case it would be difficult to evaluate any point with a significant change in creep behaviour, Fig. 4.2.39.

Investigations and load tets in silty soils 99

Fig. 4.2.37 Measured settlements at - -Average settlement of the plate

different levels below the - -0.25 m below the plate

plate in the I x I metre plate - -1.0 m below the plate

in the test site at Vagverket.

- -2.0 m below the plate

0

Fig. 4.2.39 Rate of creep settlements versus load measured in the load test on the I x I metre plate in the test field at Vagverket.

Both the measured average 3-hour settlements and the extrapolated 10-year settlements show smoothly rounded curves versus load without any indication of an imminent bearing capacity failure before the test was stopped at 400 kN load, Fig. 4.2.40. However, as in the previous test, all normal criteria for acceptable settlements had been passed before then.

Load, kN

Fig. 4.2.40 Measured and extrapolated load -settlement curves in the load test on the I x I metre plate in the test field at Vagverket.

Investigations and load tets in silty soils 101

The parameter c describing the creep rate in relation to total settlement or in relation to settlement in the time step was found to be about 0.07 and 0.13 respectively, which is identical to the results in the previous test, Fig. 4.2.41.

0.16

Fig. 4.2.41 Relative creep settlement rate versus load in the load test on the I x I metre plate in the test field at Viigverket.

The piezometer below the plate recorded pore pressure increases of about 4.5 kPa during the first four load steps. These excess pore pressures totally dissipated within an hour and a half, Fig. 4 .2.42. The piezometer ceased to function at the start of the fifth load step, but the responses in the first four steps are uniform and can be considered reliable. The tests, which had a duration for each load load step of 3 hours, can therefore be considered fully drained.

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6T

Fig. 4.2.42 Measured pore pressure changes corrected for changes in ground water level in the load test on the I x I metre plate in the test field at Vagverket.

In document LARSSON ROLF (Page 95-105)