Investigations in the current project

The investigations at Vatthammar in the current project started in May 1995. First, a number of CPT tests were performed in order to verify the results of the previous investigation and to estimate the variation within the field. The soil conditions were found to be very uniform and the investigations then continued through June with sampling, pore pressure measurements and further soundings and in situ tests.

After the plate loading tests had been performed in September-October, they were supplemented with a study of possible pore pressure variations in wet periods.

Ill CPT tests

The CPT tests penetrated to between 18 and 22 metres using ordinary 5-tonne cones and a drill rig with 7 tonnes pushing capacity. During the soundings, negative pore pressures were recorded down to a depth of about 18 metres below the ground surface. From this level, the recorded pore pressures with only minor variations corresponded to a hydrostatic pressure increase with depth. Also the measured negative pore pressures between 13 .5 and 18 metres depth corresponded to the same hydrostatic pressure line, which indicated that this zone was fully saturated. The stabilised pore pressures at stops in the penetration process and at the end of penetration also corresponded to this line. The dissipation of possible minor excess pore pressures occurred within a few seconds. The results of the tests indicate a certain stratification with alternating thin layers of coarser and more fine-grained soil, Fig. 4.3.3.

The classification of the soil on the basis of the CPT test results required the negative in-situ pore pressures in the upper part of the profile to be taken into account. If this had not been done, large parts of the profile would have been classified as loose to medium sand. When the negative pore pressures were considered, which became possible after they had been measured by using BAT piezometers, the classification changed to mainly medium dense silt. In a layer between 5 and 6 metres depth, the classification became loose silt bordering on stiff clay. This classification refers to the Swedish method proposed by Larsson ( 1992), but the observation regarding the need to take negative in situ pore pressures into account would be valid for most proposed classification methods.

The results of the CPT soundings in the field are very uniform. The combined results of the three tests performed with the ordinary CPT equipment are shown in terms of total cone resistance, qr, in Fig. 4.3.4.

SGI Report No 54 112

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• Seismic cone tests

One CPT test was performed with a seismic cone and the travel time for a shear wave from the ground surface to the cone was measured at stops at every metre of penetration. The initial shear modulus at small strains, G₀, was then evaluated from the shear wave velocity and the bulk density, (Campanella et al. 1986, Larsson and Mulabdic 1991). The evaluated initial shear moduli were found to correspond fairly well to the empirical relations proposed by Hardin (1978), Fig. 4.3.5.

Investigations and load tets in silty soils 115

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Ill Sampling and laboratory tests Sampling

"Undisturbed" samples were taken in two holes down to 10 metres depth by using a Swedish standard piston sampler type St II. Also in this field, samples were taken at every 0.7 metre of depth and all retrieved material was collected in order to obtain a continuous soil profile.

Classification tests

The soil samples were inspected visually and investigated concerning bulk density, natural water content, fall cone liquid limit and plasticity limit. Grain size analyses in terms of sedimentation tests were also performed on a few samples.

SGI Report No 54

116

The visual inspection revealed that the upper 6 metres in the profile consisted of brown- grey layered silt with occasional thin layers of clayey silt or silty clay.

Between 5 and 6 metres depth, the frequency of these clayey layers increased. In the upper 5 metres, the average clay content is small ("" 13 % ) and the silt would barely qualify for the sub-designation clayey. Below 6 metres depth, there is a more uniform grey silt with a clay content less than 10 % in which only a few occasional thin clayey layers were found at about 8 metres depth. The sampling was stopped at 10.4 metres depth, which was considered to be well below any depth that would be influenced by the following plate tests. According to the Swedish classification system, the soil profile was a fairly typical silt deposit, Fig 4.3.6.

The bulk density varied from 2.0 t/m³ in the upper 1.5 metres to about 1.9 t/m³ in the underlying brown-grey silt and 1.8 t/m³ in the grey silt further down. The natural water content was typically about 26 %, the liquid limit about 32 % and the plastic limit about 22 %. These values were fairly constant throughout the profile and according to the Unified Classification System, (ASTM 1992), almost the entire profile would have been classified as clay.

Investigations and load tets in silty soils 117

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Fig. 4.3.6 Soil profile in the test field at Vatthammar.

Oedometer tests

Oedometer tests were performed as both incrementally loaded tests and CRS oedometer tests, both types of tests being performed according to Swedish standard. In both types of tests, the equalisation of generated pore pressures was so fast that no permeability characteristics could be evaluated. Tests of both types were performed on specimens from each sampling level and at some levels, where

SGI Report No 54

118

-the results of -the CRS tests indicated that -the specimen was disturbed more than usual, the CRS tests were doubled. From many of the CRS tests, it was possible to obtain results indicating that the specimens had retained enough of their structure to enable an evaluation of undisturbed in situ properties. For the rest of the CRS tests and all the incrementally loaded tests, it was only possible to evaluate a modulus as a tangent modulus to a more approximate and possibly more disturbed compression curve.

In all, more than 30 oedometer tests were performed. By weighting the results with regard to the shapes of the oedometer curves and possible interpretation, it was possible to obtain a picture of the variation in oedometer modulus as shown in Fig. 4 .3.7. However, it was not possible to estimate any preconsolidation pressure.

Modulus, kPa

Fig. 4.3.7 Estimated variation in compression modulus in the test field at Vatthammar based on oedometer tests.

Investigations and load tets in silty soils 119

Triaxial tests

A number of drained triaxial tests were performed on "undisturbed" samples from Vatthammar. The samples were reconsolidated to the estimated in situ effective stresses and the tests were performed with high back pressures in the pore water to ensure full water saturation. The measured shear strengths in the form of the effective friction angles varied between 34° and 36°.

111 Pore pressure measurements

The negative pore pressures in the upper non-saturated soil layers were measured by BAT piezometers installed at 2.5, 5.0 and 7.5 metres depth. The results of the laboratory tests had shown that the average degree of saturation in the upper non­

saturated zone was about 85 %, Fig 4.3.8. The negative pore pressure should then correspond to the negative pressure corresponding to this degree of saturation on the water retention curve ( or pF curve) for the soil.

The measured negative pore pressures (or matric suction) ranged from 25 k:Pa at 2.5 metres depth to 46 kPa at 7.5 metres depth. These values remained stable for some time and then slowly decreased with time. The latter effect may be assumed to be associated with diffusion of air in the soil through the filter into the pressure chamber in the piezometer as explained by Oberg (1997).

Later investigations performed by Oberg (1977) in an adjacent field, with more piezometers installed closer to the ground surface, showed that the matric suction for the whole unsaturated zone would probably be in the range of 40 to 50 kPa, Fig 4.3.9.

111 Field vane shear tests

A number of field vane shear tests were attempted. However, a normal failure stress value was only obtained at superficial depths of 0.5 and 1.0 metre. At these depths, shear strength values of about 50 kPa were obtained. Further down, between 1.5 and 2.5 metres depth, a form of yield stress of around 50 k:Pa could be observed, whereupon the soil behaviour was strain hardening and the shear stress increased continuously with further rotation of the vane up to the maximum capacity of the recording instrument. Use of a smaller vane was then tried at 3 m depth. In this case, the applied shear stress rose to 140 k:Pa, whereupon the stress­

strain curve started to show an irregular pattern of repeated loss and recovery of stress. As this was again close to the maximum capacity of the instrument, it was difficult to judge whether this reflected behaviour of the soil or of the equipment.

No further field vane tests were performed.

SGI Report No 54

120

Degree of saturation, S,, %

Fig. 4.3.8 Degree of saturation as calculated from the laboratory tests on samples from the upper part of the soil profile at Vatthammar.

Pore water pressure, kPa

Fig. 4.3.9 Measured pore water pressures and assumed normal in situ pore pressure conditions in the test field at Vatthammar.

Investigations and load tets in silty soils 121

1111 Dynamic probing test

One dynamic probing test was performed, mainly in order to estimate the thickness of the penetrable soil layers and to obtain a reference for the penetration ability of other methods. The results also offered an opportunity to evaluate methods of estimating bearing capacity and settlements based on results from such tests. The dynamic probing test was performed according to the Swedish HfA method (see Chapter 4.1 ). The tip penetrated down to 26 metres below the ground surface. This penetration can be compared to the depth of about 20 metres achieved by the CPT test. The previous weight sounding tests had been stopped at 15 metres depth below the ground surface, but it would have been possible to proceed further by using a combination of an excessive amount of rotation and occasional blows.

The blow count in the HfA tests indicated that the soil in the profile was medium dense to dense throughout the profile, Fig 4 .3 .10.

1111 Dilatometer tests

Dilatometer tests were performed at two points down to a depth of 16 metres below the ground surface. If required, the tests could probably have been continued for a couple of metres but this was not attempted. The test results from the two points were very consistent and confirmed the picture of very uniform conditions over the test field area. In evaluation of the test results, it was observed that the negative in situ pore pressures had to be taken into account unless the soil should be classified as considerably coarser than it actually is, Fig. 4.3 .11.

According to the results of the dilatometer tests, the soil was mainly classified as a coarse silt changing to fine sand at depths below 8 metres. A somewhat weaker and finer layer between 5 and 6 metres depth can be observed, but the soil continues to be classified as silt. In the test starting from the ground surface, a few levels just below the stiffer upper crust became classified as silty clay, but this changed to clayey silt in the next test where pre-drilling of the first metre was performed. The soil in the profile was classified as medium dense to dense throughout the profile.

Dilatometer moduli evaluated from tests in soils which are classified as silts or sands are assumed to be directly applicable in settlement calculations and the values obtained at Vatthammar could thus be used without any correction or supplementation.

SGI Report No 54

122

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Investigations and load tets in silty soils 123

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1111 Pressuremeter tests

Pressuremeter tests were performed at one point using the equipment and proce­

dures developed by Menard (Baguelin et al. 1978). The borehole was drilled with the hollow screw auger, but in this case only air was let down through the drill rods and the hollow stem of the auger. The tests were performed down to 10 m depth and the hole, which was very smooth, stood open by itself without any internal support because of the negative pore pressure in the soil. The results of the tests also indicated a very good quality of the hole with smooth and distinctly S-shaped stress-strain curves and relations between evaluated pressuremeter moduli and net limit pressures in the high range of the empirical relations given by Baguelin et al.

The evaluated pressuremeter parameters are shown in Fig. 4 .3 .12. Tests were only performed at one point because of the very good quality of the hole and the very homogeneous properties over the test area found in the other tests. This later proved to be an error of judgement, because the relatively high values measured in a single test at 2 metres depth became a very large influence on the calculated settlements, particularly when using the method proposed by Briaud (1995).

Possible, somewhat lower values just above and just below this level would significantly alter the evaluation of the prediction method.

In document LARSSON ROLF (Page 114-128)