SITE INVESTIGATION
HINTS ON METHODS OF CHECKING
On site investigations can be divided into two types: direct and indirect.
There are various direct methods of gauging the geo-mechanical properties of the ground, without excavation.
Pressure-meter tests.
Developed by Menard in the 1950's, the test consists of inducing stresses in the ground with an expandable, cylindrical probe, introduced into a hole made beforehand by mechanical drilling (self-drilling pressure meters also exist).
The basic principle of pressure meter tests is therefore the expansion of a cell in a cylindrical cavity, to measure the relationship between the pressure and the deformation of the ground.
Measurements of variations of volume corresponding to variations of pressure are carried out and a pressure-deformation diagram is drawn. This allows the determination of a horizontal pressure Po for the mass in equilibrium, a limiting pressure Pf which corresponds to the end of a pseudo-elastic phase and a pressure Pl corresponding to an equilibrium limit. It also enables the calculation of the equilibrium of the ground bearing with the foundations.
The pressure meter method of calculating the dimensions of the foundations is based on:
The net pressure limit P'l for the evaluation of the bearing capacity at rupture;
The deformation modulus Ep for the subsidence estimate.
From the diagram of the pseudo-elastic phase it is possible to obtain the pressure modulus, according to Menard, from the equation,
EP = 2.66 V < V
We can say, on a somewhat general basis:
That with well consolidated sand and gravel, there are generally Ep values of between 1000 and 2000 kg/cM2; with Pl between 30 and 80 kg/cm 2 ; In soft silty-clayey soil, Ep varies tendentially between 20 and 30 kg/CM2, with Pl contained on average between 3 and 7 kg /CM2.
Penetrometric investigations
This is probably the most commonly used on site tests as they allow numerous precise determinations at a relatively modest cost. Dynamic penetrometric data is used to determine the resistance to penetration offered by predominately granular soils, whereas for fine grain soils, static penetrometric data is used.
There are various types of penetrometer for use in differing situations:
Standard Penetration Test (SPT)
This is a dynamic penetrometric test (rod fixed to a mass of 63.5 kg with a falling height of 76.2 cm) which measures the resistance offered to the penetration of a point fixed to the end of the penetration rod. The number of strikes N measured to obtain 30 cm of penetration can be correlated with numerous geo-mechanical parameters such as relative density Dr; the angle of internal attrition _; the undrained cohesion Cu and others.
Continuous Dynamic Penetrometer
DPSH (super heavy), DPH (heavy), DPM (medium), DPL (light): the initials refer to different types of penetrometer which differ by the weight of the mass and its falling height.
Can be used with or without coating on the rods, but for depths exceeding 15 metres, this becomes indispensable for consistent results. Indicative correlatable information is obtained, with the angle of internal attrition ~, the relative density Dr the undrained cohesion Cu and other parameters.
Static Penetrometer with Mechanical Point CPT
Rods are driven by a continuous hydraulic force. The test supplies qualitative and quantitative stratified information for the calculation of subsidence by the interpretation of values of resistance to the point (qu) and to the advancement of a lateral sleeve (ql).
URETEK Penetrometers
To ascertain the condition of densification of foundation ground, both before and after injection applications, URETEK have medium type dynamic penetrometers (DPM according to the ISSMFE - 1988 procedure). This instrument uses a falling mass of about 30 kg with a fall of 20 cm and is very versatile adapting to use in many awkward situations.
As an example, with the DPM apparatus, the characteristic values of dynamic resistance of well densified gravel and sand are generally high - greater than 30 strokes to achieve a rod advancement into the ground of 10 cm, whereas in less densified granular soil, values of about 15 strokes are obtained.
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