The site investigation carried out with Exploratory / Sampling boreholes includes the execution of, the appropriate number and to the appropriate depths, boreholes according to the Geotechnical Engineer of the project arranged appropriately so that they cover the entire project.
Depending on the type of soil that is found during drilling, undisturbed, semi-disturbed and disturbed soil samples are obtained and also rock cores in the case of a rocky ground. Also when obtaining the samples descriptions of the samples, in situ test results and remarks are presented briefly in the borehole logs.
During drilling Standard Penetration Tests (SPT) are carried out at intervals, according to the Ministry of Public Works Specification of In-situ Soil Mechanics Tests (E106-86). During this test the number of blows required to drive a standard split-spoon sampler into the soil by 150 mm, 3 times successively, is measured. The first 150 mm is considered to be a seating drive. The penetration resistance N, the number of blows per 300 mm, is the sum of the second 150 mm and the third 150 mm of penetration. If the penetration does not reach 150 mm after 50 blows during the seating drive or does not advance by 300 mm after 50 blows following the seating drive, the test is terminated. In this case, the depth of penetration for 50 blows is recorded. The driving of the split-spoon sampler is carried out by a 63.5 kg weight falling from a height of 0.76 m. The weight is lifted each time via a Manila type rope wound twice around a rotating drum (cut-head), according to the Ministry of Public Works Specification.
The undisturbed samples were obtained with a thin-walled sampler with an inner PVC skirt, which was advanced into the ground by pure pressure without rotation. The semi-disturbed samples were obtained during dry coring for the advancement of each borehole. The disturbed samples were obtained with the split-spoon sampler of the Standard Penetration Test.
Packing and storing of the obtained samples is carried out according to the Ministry of Public Works Specifications (ΦΕΚ 363 Β’/24-6-83). More specifically the surface of all cohesive samples is wiped dry to remove free moisture and they are then air-tight wrapped by a double plastic membrane. All the samples are placed in two plastic bags, tied separately, after removing excess air, in order to preserve their natural water content. All the samples are stored in wooden boxes, photographed and shipped to the laboratory for testing.
Field permeability tests (Maag, Lefranc and Lugeon) are also carried out according to the Ministry of Public Works Specification of In-situ Soil Mechanics Tests (E106-86), depending on the drilled strata. Maag test is a variable head test (measuring the speed with which the water surface drops by adding water into the borehole) and is performed in low permeability formations. Maag test is mainly performed in soil formations. Lefranc is a constant head permeability test (measuring the supply of water added to the borehole in order to keep its level steady at a certain point above the ground water level) and is performed in high permeability formations. Finally Lugeon is also a constant head permeability test (measuring the supply of water pressured into the borehole in increasing and decreasing pressure steps) and is performed in relatively low permeability rock formations. This test requires, due to the external pressure exerted on the water being added to the borehole, the use of rubber seals for the isolation of the examined part of the borehole.
Finally, after the drilling of the boreholes is completed and if it is required by the geotechnical design, pumping tests can be performed according to CIRIA REPORT 113 (1986) and the drop of the ground water level in the neighboring boreholes measured in comparison to the borehole in which the pumping is taking place. This test gives out useful data for the estimation of the ground permeability and the radius of influence of the pumping, from which the number and supply of water pumps that will be required in case pumping is needed during construction and life span of the project.
The CPTs (Cone Penetration Tests) are carried out with a GEOCONSULT Ltd. owned, self-propelled, self-anchoring, tracked, heavy duty penetrometer rig (20 ton capacity), manufactured in Italy by FONDECO SRL. The CPTs are executed with a Begemann type cone, with a tip area of 10 cm2, pushed into the ground at a constant penetration rate, capable of measuring cone resistance qc and sleeve friction fs. The measurement of the applied load is carried out with a load cell and a digital readout unit, with a precision of 1 kg (10 N). During the CPTs the penetrometer is anchored to the ground with a specialized anchoring system. The DPSH (Dynamic Penetration Super Heavy) tests are carried out with a solid cone, with a tip area of 20 cm2, which is wider than the rods in order to reduce the skin friction along them. During the DPSH test the number of blows required to drive the cone into the soil by 10 cm is measured (N10). The driving of the DPSH cone is carried out by a 63.5 kg weight falling freely from a height of 0.75 m, through an automatic release mechanism.
Inclinometer tests are carried out in order to locate the depth at which the slip surface is located and for monitoring possible horizontal ground movement, especially after heavy rainfall. Inclinometer casings are installed inside drilled boreholes. These are plastic pipes with 70 mm external diameter and 10 mm thick, that have four internal grooves arranged in equal distances at the inner perimeter of the casing. The inclinometer casings are installed inside the drilled borehole down to the desired depth and the space between the walls of the borehole and the casing is grouted using appropriate grout mixture. The grouting is carried out from bottom to top of the borehole using a flexible water leveling plastic hose that for this purpose is lowered to the bottom of the borehole. The type of grout mixture varies depending on the ground conditions in each borehole. On the surface, in order to protect the casings, a protective head is installed, with a cap with a lock at the top.
The inclinometer tests are carried out with a GEOCONSULT Ltd. owned inclinometer manufactured in the USA by SLOPE INDICATOR. The main parts of the equipment are the probe that is lowered inside the borehole, the digital readout unit and the control cable that connects the probe to the digital readout unit. The probe has micro sensors at the top and the bottom, that measure with grate accuracy angular divergence between them. The reading is carried via the control cable to the digital readout unit at the surface. The probe has wheels at the top and the bottom, that are placed in the grooves of the casing, and connected via the control cable it is lowered inside the borehole. The readings are taken every in 0.50 m vertical increments. Comparing the readings of consecutive inclinometer tests, that are carried out in regular intervals, possible ground horizontal micro-movements are recorded in reference to depth.