Consolidation of grout
element tests
Project number Version 403050/3 concept
Date December 2002
By order of HSL-zuid Boortunnel Groene Hart. P.O. Box 147 2350 AC Leiderdorp Delft Cluster partner
P.O. Box 69 Telephone +31 15 269 35 00 Chambre of commerce NL-2600 AB Delft Telefax +31 15 261 08 21 S41146461 Stieltjesweg 2 [email protected] VAT NL80097476B01 NL-2628 CK Delft www.geodelft.nl Date Summary December, 2002 The rheological properties of grout depend on the water content and the chemical reactions in the grouting material. When grout is applied Version Number of pages in a tail void it will have a pressure higher than the pore pressure of final 10 the surrounding soil and this can lead to consolidation of the grout. The consolidation properties and the rheological properties of the grout used for the Green Heart Tunnel were tested in element tests. A standard oedometer test was used to investigate the compression versus stress curve. A consolidation cell of 0.3 m diameter was used to Title / subtitle investigate the consolidation properties and to test the strength of the Consolidation of grout/ grout at various depths. A simple calculation model will be presented element tests for interpretation of the results.
Project name It appears that the stress-strain relation for unconsolidated grout is Groutonderzoek Groene Heart highly non-linear. The consolidation and the resulting removal of water determine the strength of the grout. Project engineer(s) Adam Bezuijen
Project companions
Other Project members
Reference principal
Version Date Made by Initials Checked by Initials ir. A. Bezuijen dr.ir. A.M. Talmon
Project number Date page 403050/3 December 2002 2 Summary
1Introduction 1 2Oedometer tests3 3Consolidation+vane tests4 3.1 Test set-up 4 3.2 Results 5 3.3 Consolidation properties 7 4References 12
Project number Date page 403050/3 December 2002 3 1Introduction
The pressure distribution around a tunnel is of importance for the surface settlement and the tunnel soil interaction. The pressures are partly determined by the grouting process. Injection strategy and grout properties are of importance. In the research project that was started around the Green Heart Tunnel it was decided to measure the grout pressures at various locations on the lining and to determine also the grout properties.
This report deals with grout properties at pressures comparable to the injection pressures in the tail void. It was envisaged that at these higher pressures consolidation of the grout could occur leading to a reduction of the water content in the grout. The experiments described in this report therefore focus on the compressibility of the grout and the consolidation properties.
Two types of experiments have been performed. Two oedometer tests to determine the relation between compression and stress and a consolidation test for a grout layer that has the same thickness as the grout layer that will be applied for the Green Heart Tunnel. In this consolidation experiment the pore water expelled from the sample was measured as a function of time and the applied stress. The development of strength of the grout was measured with a vane during this experiment. Experiments have been performed with grout material supplied by the contractor of the tunnel.
This report describes the set-up of the experiments and presents a first analysis of the results. Suggestions for further analysis will be made.
The content of this report is as follows: After a short description of the grout the report deals with the results of the oedometer tests in Chapter 2. Chapter 3 presents a description of the consolidation tests and Chapter 4 presents the results of these tests. Results are discussed in Chapter 5 and the report ends with Chapter 6, Conclusions and recommendations.
Project number Date page 403050/3 December 2002 1 2The GHT grout
The grout sample was obtained from the normal production for the GHT tunnel. The sample was obtained from the plant in Leidschendam, 30 October at 8:30 and transported to Delft. The experiment started at 10:00.
Density and water content were determined. Based on this the following properties were found for a fresh grout sample.
Property value dimension water content 0.2011 - density sample 1850 kg/m3 density grains 2231 kg/m3 porosity 0.31 -
The initial shear strength was determined with a vane and was approximately 2 kPa, see Figure 6.
Project number Date page 403050/3 December 2002 2 3Oedometer tests
Two grout samples were tested in an oedometer test. Loading steps were determined from possible loading in reality (substracting the hydrostatic pressure). In principle it is also possible to calculate the consolidation properties form an oedometer test. However, the consolidation process appeared to be too quick. Therefore a larger device was used to measure the consolidation properties, as will be described in the next chapter.
Results could be fitted to a power law, see Figure 1. The compressibility appeared to be comparable to the compressibility of the grout that was used for the Sophia Railway Tunnel.
350
300 sample A 250 sample B 200
150
stress (kPa) 100
50
0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 vertical strain (-) Figure 1: Results oedometer test fitted to a power law.
The following fit function was used: σ ε b 'v a (3.1)
With:
’v : the vertical effective stress on the sample (kPa) the vertical strain (-) a : coefficient (kPa) b : coefficient (-)
The following values were found for the coefficients: Sample a b (kPa) (-) A 1.89*106 4.22 B 2.68*106 4.21
Project number Date page 403050/3 December 2002 3 4Consolidation+vane tests
44.1 . 1Test set-up The principle for the test set-up for the consolidation and vane tests is shown in Figure 2. Figure 3 shows a picture of the set-up.
0.284 vane 0.284
air pressure plunger 0.2 grout 0.2 grout dimensions in metres 0.1 sand 0.1 sand
drainage
Figure 2: Principle test set-up . The left-hand side showed the situation during the vane tests, the right-hand side the set-up during consolidation of the grout.
vane
Load cell
Vessel to collect container expelled water
drainage
Figure 3: Test setup.
The test aims to simulate a part of a consolidating grout layer. The grout thickness is comparable to the thickness of the grout layer in the tail void of the tunnel. The sand simulates the soil and the plunger on top of the grout the lining. Only one side drainage of the grout is allowed, as is the case in the tail void. After some minutes of consolidation at a pressure of 1 bar the pressure was released and the plunger on top of the grout was removed. The vane was placed at various depths and the strength of the grout was tested. The highest shear strength that could be measured was around 10 kPa. At higher shear strengths it was not possible to penetrate
Project number Date page 403050/3 December 2002 4 the vane into the grout. The top plate was placed on the grout again after the vane test and consolidation was allowed for another prescribed time.
The whole system was saturated before the start of the test and the expelled water from the grout was measured continuously. The amount of water is equal to the decrease in grout volume. The sand underneath the grout was compacted to the maximum density and therefore hardly deforms during the test. The sand used was steep graded sand with a d50 of 442 m, see Figure 4. The sand has a comparable grain size as the Pleistocene sand through which the Green Heart tunnel will be bored to study possible penetration of the grout into the sand.
100 90 80 70 60 50 40 30 20
Percentage passing the sieve 10 0 0.1 1.0 diameter (mm)
Figure 4: Sieve curve of the sand used in the sand.
The applied stress on the plunger is not exactly equal to the stress exerted on the sample due to friction between the top plate and the container walls. It was determined from tests without samples that 9.96 kPa of air pressure was necessary to overcome the friction. Further on the measurement data have been corrected for this friction.
44.2 . 2Results Figure 5 shows a result of the test. The amount of expelled water and the air pressure above the plunger as a function of time. Clearly the consolidation phases and the ‘vane’ phases could be distinguished. During the latter there was no drainage and a low pressure. Unfortunately a leakage occurs between the plunger and the side walls of the container after 2.6*104 s. After that time the expelled water is also caused by the leakage and air coming trough the sample. Therefore the results after 2.6*104 s will not be used.
Project number Date page 403050/3 December 2002 5 1.8 110 1.7 100 1.6
1.5 90 1.4 1.3 80 1.2 70 1.1 1.0 60 0.9 50 0.8 0.7 40 0.6 30 grout pressure (kPa) 0.5
weight pore expelled water (kg) 0.4 20 0.3 weight expel led pore water 0.2 10 air pressure 0.1 0 0.0 -0.1 -10 0·104 1·104 2·104 3·104 4·10 4 5·104 6·104 7·104 8·104 9·104 time (s) Figure 5: Results grout consolidation test.
After noticing the leakage it was decided to perform the test once again but without the vane tests. However, there was also leakage in this test. Even after a shorter period than the first test. It was decided to improve the set-up to minimize the change of leakage. Since there was no grout anymore available this improvement came too late for this test and the result of the first test will be reported here.
The shear strengths found in the vane tests are shown in Figure 6. The results show that after applying the consolidation pressure for 20 minutes approximately half of the grout was consolidated and that after 60 minutes most of the grout was consolidated.
After the test a vertical cut was made through the grout sample and the sand underneath to investigate whether or not the grout was penetrated into the sand. This appeared not to be the case, see Figure 7, which shows a clear boundary between the grout and the sand. The ph of the pore water in the sand was increased due to the basic nature of the grout. From these results it can be concluded that soluble parts of the grout are expelled with the pore water during consolidation, but that non-soluble parts of the grout remain in the grout.
24 hours after the beginning of the test the strength of the grout was determined by a pocket penetrometer. The values found are summarized in Table 1.
Table 1 : Measured undrained shear strenth 24 hours after the start of the test. measurement Just below plunger 130 mm above sand Cu [kPa] Cu [kPa] 1 49.1 42.4 2 44.7 42.4 3 48.0 44.7 average 47.3 43.2
Project number Date page 403050/3 December 2002 6 0 hour 20 1 hour 1.5 hour 18 2.5 hour 3.5 hour 16
14
12
10
height (cm) 8
6
4
2
0 0 2 4 6 strength (kPa) Figure 6: Results vane tests.
Figure 7: Sand-grout transition after the test.
The measured expelled pore water, as shown in Figure 5, is used to determine the consolidation properties. The procedure used will be explained in the next paragraph.
44.3 . 3Consolidation properties
4.3.1 calculation method To determine the consolidation properties (permeability and compression of the grout) it is necessary to use some consolidation theory. The compression tests have shown that there is highly non-linear stress-strain behaviour in the grout sample. Therefore linear consolidation theory doesn’t look so appropriate to evaluate the
Project number Date page 403050/3 December 2002 7 measurements. Based on other measurements a different theory was developed, dealing with the grout mixture as if it was a sand-water mixture.
The principle of the theory that will be outlined below is as follows: Assume that the fresh grout behaves as a sand-water mixture. There is no grain stress in the mixture and thus no grain contacts. As consolidation starts, the water is expelled from the grout until the grains get in contact. When there is contact between the grains, these form a rigid formation and there is no further settlement. This assumption is a simplification from reality. In reality the deformation will also depend from the grain stress as can be seen in Figure 1. However, it can be seen as a first approximation. Using this assumption it follows from the theory that in a consolidation situation, as applied in the test, there will be a sharp transition between consolidated and non- consolidated grout. The non-consolidated will have the same water content as at the beginning of the test, but from the drainage boundary there will be a ‘front’ of consolidated grout that moves into the sample as a function of time, see Figure 8. F
grout
v=dx/dt ni
ne x
R
Figure 8: Sketch of the movement of the front in a consolidation test. R is a flow resistance. This resistance is of importance to calculate field situations, but can be neglected to describe the tests.
The movement of the front is described in [1], leading to the following expression: 1 n x 2k e φt (4.1) ni ne Where: x : is the movement of the front (m) k : the permeability of the consolidated grout (m/s) ni : the starting porosity in the grout (-) ne : the final porosity (-) : difference in piezometric head in- and outside the grout (m) t : time (s)
The relation between x and the specific discharge follows from continuity: 1 n dx q e (4.2) ni ne dt Thus the relation between the volume of expelled pore water (V) and x can be written as:
Project number Date page 403050/3 December 2002 8 1 n V πR 2 e x (4.3) ni ne where R is the sample radius (m).
Normally the V is measured, see Figure 5 and ni and ne are determined for and after the test. However, due to the leakage and the air in the sample it was not possible to determine ne. In principle it can be determined from the oedometer tests, but earlier measurements have shown that this is not very accurate. So various methods are used to estimate the final porosity: 1. The movement of the pressure plate was measured, from this the final porosity was determined. 2. The results of the oedometer tests were used (taking the average of the two tests). 3. The amount of water expelled from the grout as a function of time was fitted to the theoretical curve. According to Equation (4.2) and (4.3) x and thus the discharge is proportional to the square root of time. Using only the time that there is a pressure on the grout it can be investigated until what time this relation is valid. It is known from earlier measurements that x is equal to the top of the sample when the measurements deviate considerably from this curve.
The third method can be explained a bit further by Figure 9.
1.2 fit 1.0 measurement 0.8
0.6
0.4
0.2
weight of expelled water (kg) 0.0
-0.2 0 5,000 10,000 15,000 20,000 25,000 time (s) Figure 9: Measurements compared with fit.
In this figure the measurement data are selected for the time that there was pressure on the plunger and the measurements without pressure are deleted. The measurements until 5000 s are fitted tot a power law: W at b (4.4) where W is the weight of the expelled water, t the time in seconds with pressure on the sample and a and b coefficients. The figure shows the measurements and a fit that was made for the first 5000 s of the experiment. This part has a good fit to the curve and found a value of 0.492 for the coefficient b, close to the theoretical value of 0.5 that results from Equation (4.1). It can be seen that the curve starts to deviate at approximately t = 3800 s. This means that at that time there is grain stress all over the sample. Since the description is only an approximation there can be some deviations at the end of the test. Therefore it is assumed that without leakage there would have been a discharge of aprox. 0.9 kg to be reached in 7000s. For the fit shown the value of the coefficient “a” was 0.00117 using kg for the amount of expelled pore water and seconds for the time. Only this third model can be used to calculate the permeability.
Project number Date page 403050/3 December 2002 9 Using all 3 methods we came to the results of Table 2.
Table 2: Final porosity determined with different methods and .
Method expelled pore water ne k (kfg) (-) (m/s) 1 0.807 0.245 2 1.15 0.222 3 0.900 0.239 2.4*10-8
The starting porosity was determined from the density and water content of the grout. The difference between the methods is not very large. A value of 0.24 for the final porosity was chosen.
The permeability of the grout after consolidation as presented in Table 2 was determined with the formulae presented (according to the theory used there is no flow in the grout before consolidation, thus it is not possible to determine the permeability of that grout).
4.3.2 Consequences The test shows that the grout will consolidate. With a continuous overpressure of 1 bar the grout will loose nearly 10% of is original thickness due to consolidation. In reality the overpressure is not constant, but the result presents an order of magnitude. How much water is expelled also depends on the elasticity properties of the surrounding soil. If the soil is very stiff a small decrease in grout volume will lead to a considerable decrease of the stress on the sample and the consolidation will stop. In a less stiff soil the pressure will not drop very fast and consolidation will proceed further.
The speed of the consolidation is also determined by groundwater flow. As explained in [1], the groundwater flow will have an influence if the permeability of the soil is less than 70 times the permeability of the grout. Normally the Pleistocene sand is more permeable and will not influence the consolidation of the grout unless the permeability of the soil is reduced by bentonite from the tunnel face.
With a limited influence of the bentonite in the soil and an applied excess pore pressure of about 1 bar, grout will consolidate within 2 hours. This time is considerable less than the time necessary for the hardening of the grout material.
44.4 . 4Penetration of grout in the sand No penetration of grout material in the sand was observed. There was a sharp 1 grain separation between the grout and the sand. However, the Ph of the pore water in the sand was changed. This showed that no solid particles penetrate into the sand, but the pore water and solvable parts of the grout do penetrate.
Project number Date page 403050/3 December 2002 10 5Conclusions and recommendations
From these grout experiments we came to the following conclusions: 1. The equipment developed in suitable to perform consolidation measurements on the grout. In the tests described in this report there was a leakage problem, but this is solved in the present set-up. 2. The combination of the special developed consolidation test with the standardized oedometer tests can be used to determine both the consolidation and compression properties of the grout. 3. The shear strength of the grout at the beginning of the test is relatively high (2 kPa). 4. The strength development of the grout in the tail void is not determined by the hardening parameters, but by the consolidation of the grout. 5. At an excess pressure of 1 bar the volume loss of the grout is nearly approximately 8%. This volume loss will lead to a reduction of the soil stresses around the tunnel. 6. The theory presented allows calculating the amount of compression and the consolidation rate of the grout for different grout injection pressures.
With the results of these experiments we came to the following recommendations: 1. To compare the results of these experiments with the results of field tests where the grout pressure is measured and the excess pore pressure in the surrounding soil due to consolidation. 2. To perform some more tests to determine the relation between the porosity of the grout and its permeability. 3. To develop a model that describes the interaction between the soil and the consolidating grout based on the results of the tests described in this report.
Project number Date page 403050/3 December 2002 11 6References
[1] Bezuijen A., Talmon A.T. Groutdrukmetingen in de Sophia spoortunnel/Metingen en analyse beide buizen. DC/COB rapport, December 2002.
Project number Date page 403050/3 December 2002 12