Field tests CPT testsCPT tests

CPT tests have been performed at Points 1, 2, 4 and 6 in the new investigation. The tests were performed with a clay probe, which is a probe with a high sensitivity and resolution but a load capacity limited to 5 MPa in cone resistance. Pre-drilling has therefore been performed through any upper layers of fill or dry crust. No such layers existed at Point 1 and at Points 4 and 6 the crust effects and stiffer upper layers were limited to about 1 metre. At Point 2, which was located at the erosion protection and old access road, pre-drilling down to 2.9 metres had to be done to ensure that all coarse cobbles, stones and remains of old fills had been passed.

The drill rig was anchored by screw augers in the tests on land. The tests here were stopped when the maximum load capacity of the probe was reached in stiff bottom layers. In the test below the river, the drill rig could not be anchored. This test was stopped when the front of the drill rig lifted, which almost coincided with reaching the maximum probe capacity. The stop levels are deeper than those reached in the weight sounding tests and correspond approximately to those reached in “machine sounding tests” in the previous investigations. It is thus unlikely that significantly greater depths would have been reached if a coarser probe and more penetration force had been used.

The results indicate that the soil below the upper layers of sand and silt or fill material consists of relatively homogeneous clay. However, according to the trends in pore pressure generation and tip resistance with depth, which show a number of significant breaks, the character of the clay varies with depth. At greater depths, there appears to be a certain content of shells or other coarser material, and indications of coarse objects or layers are also found. A rather thin embedded stiffer

layer is found at a relatively great depth at all points. The layer is located at a level of –27 metres at Points 4 and 6, and below the slope and under the river it rises to a level of about –22 metres.

The pore pressure registrations indicate that this layer may be draining. The CPT tests have passed through the layer and below there is another layer of stiff clay with a somewhat different character. Further down, stop in penetration occurs in coarse permeable soil, Fig. 90.

The tip resistance in Point 6 was found to be rather low in relation to the overburden pressure and the results at the other points. The test was therefore repeated, but the results of the two tests at this point proved to be almost identical, Fig. 91.

Dilatometer tests

Dilatometer tests were performed at Point 1 out in the river and Point 4 on the excavated terrace. The purpose of these tests was to obtain a supplementary classification, alternative determinations of the undrained shear strength and the overconsolidation ratio and an estimate of the in situ horizontal stresses. The test from the raft in the river reached a depth of only about 19 metres below the water level before the front of the drill rig lifted. This is 8 metres less than the CPT test and is probably a result of the side friction against the coarser drill rods in the stiff clay. The test at Point 4 reached 39 metres depth, which is the same as for the CPT test.

The results from the dilatometer tests confirmed the information obtained from the CPT tests, but fewer details of layers could be observed, Fig. 92.

Field vane tests

Field vane tests were performed at Points 1, 2, 4 and 6 using equipment type SGI and the normal vane size. The test depths were selected with guidance from the CPT test results and in general comprised the whole profile. The exception was the tests from the raft in the river, where friction against the large protecting tubes in combination with the unanchored drill rig meant that the tests had to be stopped at the same level as the dilatometer tests.

Fig. 90.Results of the CPT test in Strandbacken presented using the program CONRAD. a) Point1

Fig. 90.Results of the CPT test in Strandbacken presented using the program CONRAD. b) Point 2

Fig. 90.Results of the CPT test in Strandbacken presented using the program CONRAD. c) Point 4

Fig. 90.Results of the CPT test in Strandbacken presented using the program CONRAD. d) Point 6

Fig. 91. Comparison between the results of the parallel CPT tests at Point 6.

Fig. 92. Results of the dilatometer tests in Strandbacken.

a) Results of dilatometer tests at Point 1, base data

Fig. 92. Results of the dilatometer tests in Strandbacken.

b) Results of dilatometer tests at Point 1, evaluated properties

(τττττ_fu = undrained shear strength evaluated according to SGI Information No. 10, c_u = undrained shear strength evaluated according to the alternative method).

Fig. 92. Results of the dilatometer tests in Strandbacken.

c) Results of dilatometer tests at Point 4, base data

Fig. 92. Results of the dilatometer tests in Strandbacken.

d) Results of dilatometer tests at Point 4, evaluated properties

(τττττ_fu = undrained shear strength evaluated according to SGI Information No. 10, c_u = undrained shear strength evaluated according to the alternative method).

The field vane tests produced fairly even results although the curves versus depth are slightly serrated because of the content of shells and other coarse material in the clay. The tests in Point 4 were doubled in other to check the repeatability of the results. The test series in each point was started with tests at every metre depth. The sequence was then thinned out to tests at every second metre at larger depths. In the doubled test series in Point 4, the deeper test levels were selected in such way that one test series was performed at every second even metre depth and the other at every second odd metre. In this way a more continuous profile was obtained.

Pore pressure measurements

Closed pore pressure measuring systems of BAT type were installed at Points 2, 3, 5 and 6. The stabilised pore pressures in the bottom layers were also read off in the CPT tests. Three systems were placed in each station. The filter tips in each station were placed in such a way that the uppermost tip was located fairly close to the ground surface but safely below the upper free groundwater level. This was intended to enable a monitoring of the variation of this groundwater level. The lowest tips at Points 5 and 6 were placed at a level of about –27 metres, where according to the CPT test results there might be a continuous permeable layer. The lowest tips at Points 2 and 3 were placed at the same level, which here corresponds to the transition between the lower clay layer and the coarse and firm soil below.

The third tip in each station was placed in the central part of the upper clay layer at levels selected from indications in changes in soil properties from the CPT tests.

The pore pressure measuring systems have been read off more or less regularly with the purpose of monitoring seasonal variation and catching extreme values during wet and dry seasons. During the course of the measurements, problems with development of gas have been encountered in some of the tips. These tips have then had to be re-saturated at each measurement after the problem started.

3.7.4 Sampling

Undisturbed samples have been taken with Swedish standard piston sampler type St II at Points 1, 2, 4 and 6. The sampling levels have been selected with guidance from the results of the CPT tests and have in principle comprised the whole penetrated profiles. The material brought up in the cutting edges and spacers has been examined in the field in order to obtain as detailed information about the stratification as possible. Extreme care has been taken when taking, transporting and handling the samples. Disturbed sampling has also been performed to shallow

depths at a point at the crest of the lower slope in order to investigate the thickness of the remaining sand and silt layer at this point.