Standard penetration test, the application scope and function of standard penetration test

Precautions to be taken when conducting a standard penetration test:

It should be ensured that the cutting edge (standard cutting edge) of the penetrator boot should be kept intact, and when there is damage such as notched or curled edge, its single length is greater than 5mm, or the total length is greater than 12mm, it should be discontinued;

The seams of the split penetrator (two halves of the cylinder) should be straight and tight, and the use should be stopped when there is distortion, expansion, staggered seam and other deformations;

The weight of the standard hammer is 76cm, and the height of the free drop hammer of the guide rod is 76cm, and when the error of the drop weight and the drop distance of the guide rod exceeds the allowable range, it should be stopped;

Pay attention to the perpendicularity of the drill pipe and the guide rod to prevent shaking in the hole;

The diameter of the drill pipe must be 42;

The hammering speed should not exceed 30 hammers per minute, and the automatic hammer drop device should maintain normal drop and lifting performance, and should not have a lifting effect on the guide rod.

Repeat the next standard test, generally at a greater interval than the test is performed.

For drilling holes with casing wall protection, the test depth should be less than 75cm below the casing mouth for standard penetration test.

Pay attention to the drilling process and hole cleaning quality, and record the number of hits of the penetrator starting to penetrate 15cm to judge whether there is residual soil or disturbance degree at the bottom of the hole;

For the test section (i.e., the part of penetration 15 45cm), it is required to measure the cumulative penetration amount after each hammer stroke, and when the penetration amount is less than 2cm, the number of hammer strokes per penetration of 10cm is recorded. The relationship curve between the number of hammer strokes and the cumulative penetration amount was drawn to analyze whether the soil layer was uniform, and finally the number of hammer strokes in the 30cm test section was selected as the n value to be recorded.

The purpose of the standard penetration test is to judge the compactness of the sand or the consistency of the cohesive soil by the measured standard penetration hammer number n, so as to determine the allowable bearing capacity of the foundation soil; Evaluate the vibratory liquefaction potential of the sand and estimate the bearing capacity of the monopile; And can determine the soil profile and take disturbed soil samples for general physical tests.

The in-situ test method should be selected according to the geotechnical conditions, the requirements of the design parameters, the regional experience and the applicability of the test method. The standard penetration test is suitable for sandy, silt and general cohesive soils. In order to ensure the quality of the standard penetration test hole, rotary drilling is required to reduce the disturbance to the subsoil of the hole as much as possible.

When drilling to 15cm above the test elevation, stop drilling.

This is an in-situ assay method. Determine some of the characteristics of the foundation under the condition that the actual situation of the site is basically not changed. It can be used to determine the bearing capacity of the foundation on site. It can also be used to determine the liquefaction resistance of sandy soils.

The scope of application and function of the standard penetration test are as follows:

1) Take disturbed soil samples, identify and describe soil types, and name them according to the results of particle analysis.

2) According to the standard number of penetrations, the compactness of sand, the state of silt and load-bearing soil, the strength parameters of soil, the deformation modulus, and the bearing capacity of the foundation are evaluated by using regional experience.

3) Estimate the ultimate bearing capacity of a single pile and determine the possibility of pile sinking.

4) Determine the liquefaction possibility of saturated silt and sandy silt and the liquefaction grade of the field.

Under the hammer blow of a certain hammer weight and a falling distance, the number of strokes required for the probe to penetrate into the core crack of 30 cm is specified to determine the compactness of the sand or the consistency state of the cohesive soil, determine the allowable bearing capacity of the foundation soil, infer the shear strength of various types of soil, estimate the deformation modulus of the cohesive soil and evaluate the vibration liquefaction of the sand. At the same time, according to the difficulty of penetrating the beam pants, the soil layers of different properties can be qualitatively divided and the uniformity of the soil body can be evaluated, the filling quality can be checked, the position of the sliding zone and the soil hole can be explored, and the burial depth of the bedrock surface or gravel soil layer can be determined.

The standard penetration test is a method for determining the bearing capacity of the foundation of sand or cohesive soils in the field. The specific application scope and function are as follows: Wax Balance

The Standard Penetration Test (SPT) is a type of dynamic penetration test, which has been included in the Chinese National Code for the Design of Industrial and Minhong Nuclear Building Foundations. It uses a certain hammering function (hammer weight, falling distance 76cm), and drives a certain specification of the split pipe penetrator (the outer diameter of the split pipe is 51mm, the inner diameter is 35mm, the length is greater than 457mm, the lower end length is 76mm, the cutting angle is 18 ° 20 °, the pipe shoe with a thick end of the cutting edge, and the upper end is connected to the drill pipe) into the soil at the bottom of the drilling hole wheel, and according to the penetration impedance in the soil, the change of the soil layer and the engineering properties of the soil are judged. The purpose of the standard penetration test is to determine the bearing capacity of the foundation of sand or cohesive soil in situ.

Steps:1Use a drilling rig to drill to the soil layer that needs to be tested by standard penetration, and after cleaning the hole, replace it with a standard penetrator and measure the depth size.

2.The penetrator is vertically driven into the test soil layer, first driven into 15cm, regardless of the number of strokes, and continues to penetrate into the soil for 30cm, and the number of hammer strokes is recorded, which is the number of standard penetrating strokes. 3.

The penetrator is proposed, the soil sample in the penetrator is taken out, the identification description and record are carried out, and then the drilling tool is replaced to continue drilling to the next depth that needs to be tested, and then the above operation is repeated, and the test can generally be carried out every interval. 4.The same soil layer should be tested several times, and then the average value of the number of hammer strokes should be taken.

Notes:1Pay attention to the quality of the drilling process and hole cleaning, and record the number of hits that the penetrator starts to penetrate 15cm to judge whether there is residual soil or the degree of soil disturbance at the bottom of the hole; 2.

The drill pipe and guide rod are perpendicular to prevent shaking in the hole. 3.For the test section, it is required to determine the cumulative penetration amount after each hammering of the weibu.

When the amount of penetration is less than 2 cm in one penetration, the number of hammer blows per 10 cm of penetration is recorded.