Berner Nicolaslaboratory 2December, 12 2006

Berner NicolasLABORATORY 2december, 12 2006

Brethaut Yoann

Gafsou Blaise

Lochmatter Samuel

Vadoze Zone Hydrology

Introduction

In this laboratory, we have to determine the saturated hydraulic conductivity (Ks), by measurement of the constant head method. In the first case, we have a one-layer soil column, and three series of data.

Then we measure another hydraulic conductivity (Ks) through an hydraulic head distribution within a two-layer soil column.

Discuss of saturated hydraulic conductivity (Ksat) and methods of measurement

Ks(θ) = a + eb.θ, where a,b are specific constant of the soil.

And we have Ksat = Ks(θs) = a + eb.θs.

MANQUE ENCORE UN PEU DE TEXTE SUR KSAT, KéKE C’EST ET COMMENT LE DETERMINER

Plot of the calculated hydraulic conductivity Ks vs. Time

We have calculated the Ks coefficient three times for the one-layer soil column with different height of the bubbling tube as describe in the tab XX.

For these three heights, we are able to determine several Ks between each piezometrical measure’s point with Darcy’s law, as describe here :

Ks = - Jw * Δz / Δψh where ψh = ψm + ψp + ψz = ψp + ψz .

We observe that the approximate value of Ks 23 is the same for the different ABC case. We can do the same observation for Ks 13, but the concordance isn’t well as before. In fact, there is a big difference between the three Ks 12.

By looking the previous graph, we remarka general trend :Ks 23 is smaller than Ks 12, so it result that Ks 13 is situated between both.

Theoretically, we should obtain a same value for all this Ks, because we have experimentations over an one-layer soil column, but we don’t.

A first global explanation is that there was air in the system for the two first experimentations, so the continuity pressure wasn’t assured. Consequently, we cannot use the Darcy’s law with our measurements, which aren’t more representative.

Another explanation could be our inability of good precision during measurement. The water level is not easy to read exactly at 10 ml in the graduate cylinder, water outflow making move level.

Secondly, Ks 12 is different than Ks 23 because we think that the piezometers gave us a wrong value. It’s also possible that some preferential path have been formed in the section 12.

For the next graph, we determined the Ks effective using the following equation:

Keff = Σ Li / Σ ( Li / Ksi ).

We use Ks 12 and Ks 23 in this formula and the result should be equal to Ks 13. This relation is respected : Ks eff = Ks 13 for the three experiment.

Because we don’t obtain the same result for Ks 12 and Ks 23 alone, we can supposed that the piezometer 2 give us a wrong value, Ks eff calculated corresponding exactly to Ks 13. It assume that Ks 12 and Ks 23 are wrong calculated. However, when we determine Ks eff, this false measurement at piezometer 2 disappear, because the distance between piezometer 12 and 23 is the same.

Once again, there isn’t a good exactitude between this three Ks, for the same reasons probably, as previously said. In fact, analysing Ks eff of the first experimentation, we see a big fluctuation of its value vs. time. It’s the sign there was air in the system and the inflow water pressure fluctuated. Ks eff of the two other soils are more constant during the time, it’s probably another reason of their non-exactitude.

For the three experimentation,

Plot a potential diagram of the two-layer experiment

Compute the Ksat and Ks for each layer

Discuss Ksat vs. If you thought it was a one-layer  Ks-eff

For the 3 first case, this law is respected. But with the two-layer soil column, because there was air in the system we cannot calculate Keff . Probably if we had results for this last case, there would be confirme this relationship between Keff and Ki .

We’ve effected the experiment as described in the procedure and in the following figure.

For different situation we obtain the following results :

With this different parameters

We can measure and calculate

First we have had a problem, there was air in the tube of arrival water. The pressure was’nt at saturation.

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