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DELAYED RISE WtTH GRAOuAL SWELLING OF SOIL DURING RISE
CONSOLIDOMETER USEO AS PERMEAMETER
100 200 240 400 500 600
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ATLANTIC MuCK - PANAMA IMM EDIATE FALL SOFT ORGANIC CLAY
SAMPLE: D=4.25,,, L=087,, OELAYE D FALL VERT. LOAD 0.115KG./CM.2
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Fig. 10. Volume changes during laboratory permeability tests
pressure indicated by the standpipe level. With further fall in this level and de- crease in pore-water pressures, a reconsolidation of the soil takes place with a con- sequent deficiency in rate of flow from the stand pipe. The curvature of the equaliza- tion diagram decreases; the diagram becomes fairly straight and may even acquire a slight convex curvature as it approaches the normal diagram, obtained when there is no change in void ratio of the soil. However, the ultimate shape and slope of the diagram could not be determined from the results of tests so far performed, since these results were influenced by very small temperature changes in the laboratory.
When the water level in the standpipe is raised and maintained in its upper position until the initial swelling of the soil sample is completed and then allowed to fall -- Cases B-2 in Figs. 9 and 10 -- a gradual re-consolidation of the soil takes place during the actual test, and an equalization diagram which lies above the normal diagram is obtained, but its lower part is more or less parallel to the lower part of the diagram for immediate fall.
Similar diagrams were obtained by rising head tests. When the water level in the U-tube is lowered and immediately thereafter allowed to rise, Case A-1 in Figs. 9 and 10, the soil will be subjected to an initial consolidation with a consequent increase in rate of flow to the U-tube, but this volume decrease of the soil will later be eliminated by a swelling and a corresponding deficiency in rate of flow to the U-tube. The resulting equalization diagram has a concave curvature and lies below the normal diagram. When the water level in the U-tube is maintained in its lower position until the initial consolidation is completed and then allowed to rise, a gradual swelling of the soil takes place; the rate of flow to the U-tube is decreased, and the equalization diagram lies above the normal diagram.
All the above mentioned tests were repeated several times with both undis- turbed and remolded soil, and the results obtained were all similar to those shown in Fig. 10. A slight sudden drop in head ratio in case of immediate fall -- or rise -- is probably due to a small amount of air in the system. As already indicated, the shape of the lower part of the diagrams was influenced by small amounts of leak- age and evaporation and by temperature changes. The temperature in the laboratory did not vary more than 1.5° F from the mean temperature, but even such small var- iations are sufficient to cause conspicuous irregularities in the test results when the active head is small. However, it is believed that the results are adequate for dem- onstration of the consolidation and swelling of the soil during permeability tests and of the resulting general shape of the equalization diagrams.
Volume changes of gas in soil
The influence of gas bubbles in an open or closed pressure measuring sys- tem is summarized in Fig. 1 and discussed briefly on pages 6 and 7. Whereas such gas bubbles may cause a change in both the ultimate indicated pressure and the time lag or slope of the equalization diagram, they will not materially influence the shape of the latter, since changes in pressure and volume of the gas bubbles occur nearly simultaneously with the changes in hydrostatic pressure within the system. On the other hand, when the gas bubbles are in the soil surrounding the well point and their volume and the water content of the soil are changed, there will be a time lag be- tween changes in hydrostatic pressure in the system and corresponding changes in pressure and volume of the gas bubbles, and this time lag will cause a change in both slope and shape of the equalization diagrams. The general effect of the gas bubbles is an increase in the apparent compressibility of the soil, and the equaliza- tion diagrams should be similar to those shown in Fig. 10.
The change in volume of the gas bubbles, when the piezometer level is low- ered or raised, and probable resulting equalization diagrams are shown in Fig. 11.
This figure and the following discussion are essentially a tentative interpretation of the results of the laboratory permeability tests and the field observations shown in Figs. 10 and 17.
27
PIEZOMETER A- 1 IMMEDIATE FREE RISE
IL LOWEREO 10
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NOTE: 01AGRAM5 FOR ILLUSTRATION ONLY AND DO NOT REPREsENT ACTUAL TESTS.
DELAYED oBsERVATIONS
OELAYED RISE HR
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TANGENT PARALLEL TO A-0
IMMEOIAT E RISE AND OBS,
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o TIME (LINEAR SCALE) 7
Fig. 11. Influence of volwne changes of gas in soil
/EL RAISED B-1 IMMEOIATE FREi FALL
—
When the piezometer level suddenly is lowered and immediately thereafter allowed to rise, Case A-1, the pressure in the pore water is decreased, and the gas bubbles tend to expand and force an excess amount of water into the well point; that is, the initial rate of rise of the piezometer level will be increased and the equali- zation diagram, A-C, will have a steeper slope than the normal diagram, A-B, and a concave curvature . It is emphasized that the normal diagram, A-B, corresponds to the condition of no volume change of the gas bubbles and not to complete absence of gas bubbles in the soil. Even when the volume of the gas bubbles does not change, the presence of these bubbles will decrease the effective permeability of the soiland increase the time lag of the piezometer. As the piezometer level rises, the differ- ence between the pressures in the gas bubbles and the surrounding pore water de- creases. At the time Tr these pressures are equalized, and the rate of excess in- flow ceases; that is, the tangent to the equalization diagram, A-C, at the time Tr should be parallel to the normal diagram, A-B. With a further rise in piezometer level, the pore-water pressure around the well point increases; the volume of the gas bubbles decreases , and there will be a deficiency in inflow of water. The curva- ture of the equalization diagram decreases and may eventually become zero or, per- haps, even change to a slight convex curvature as the volume of the gas bubbles ap- proaches its original value.
If the observations were started at the time of reversal of the volume changes, T r, the volume of the gas bubbles would decrease throughout the observations; there
would be a deficiency in the rate of inflow, and the equalization diagram, A -C’, would be above the normal diagram. A similar but higher-lying diagram, A-D, would be obtained if the piezometer level is not allowed to rise immediately after lowering but is maintained in its lower position until the initial swelling of the gas bubbles is completed, Case A-2. The two diagrams A-C and A-D should ultimately become parallel, and the normal diagram is a straight line between these limiting diagrams and is tangent at ‘CA” to diagrams A-c’ and A-D.
When the piezometer level suddenly is raised and immediately thereafter is allowed to fall, Case B-1 , the volume of the gas bubbles at first decreases with a consequent excess outflow of water from the piezometer. Later on the gas bubbles expand until their original volume is attained, and during this period there will be a corresponding deficiency in rate of outflow. The resulting equalization diagram is similar in form to A-C for Case A-1. When the piezometer level is maintained in its upper position until the initial contraction of the gas bubbles is completed and then is allowed to fall, an equalization diagram similar to A-D is obtained.
Normal operating conditions
The discussions in the foregoing sections concern mainly time lag tests dur- ing which the piezometer level suddenly is changed whereas the general ground- wave r level or pore-water pressure remains constant. In normal operation the ground-water pressure changes first, and the piezomete r level follows these changes with a certain pressure difference or time lag. When the ground-water level or pore-water pressure changes, the void ratio of the soil and the volume of gas bubbles below the ground-water level also tend to change, but the rate of such changes gen- erally decreases in the immediate vicinity of a well point or intake for a pressure measuring installation on account of the pressure difference and time lag. However, all changes progress in the same direction and there is no initial increase in void ratio and water content followed by a decrease -- or vice versa -- as in the case of time lag tests.
In general, normal operating conditions resemble in most cases those of de- layed fall or rise, or rather delayed observations, shown in Figs. 10 and 11. lt IS
brobable that the tame lag during normal ofieratlng conditions corresflonds to an
equullzatzon daaqram which, for firact~cal purposes, may be represented by a stratght l]ne through the ortg~n of the d~a<rram and parallel to the louer porttons of the .{lo}~rams obtained In time lag tests. However, sufficient experimental data for veri - ficationof the suggested approximation -- especially comparative tests during rapidly changing ground-water pressures and with several pressure measuring installations having widely different basic time lags -- are not yet available.
As indicated by permeability tests of the type shown in Fig. 10, it is probable that the influence of swelling or consolidation of the soil is very small or negligible when observation wells or open piezometers are used in ground-water observations,
29
but it is also possible that such changes in void ratio may cause appreciable distor- tion of the equalization diagrams and increase in actual time lag when pressure gages or cells with a small basic time lag are used and the soil is relatively compressible.
On the ofier hand, gas bubbles in the soil around a well point may cause considerable distortion of the equalization diagrams and increase in actual time lag even for open piezometers; see Fig. 17. Accumulation of gas in the pressure measuring system causes no curvature of the equalization diagram but materially decreases its slope and increases the effective time lag under normal operating conditions.