First, the choice of tamping machine
Because the thickness of collapsible loess in this section of subgrade is generally 3-6m, the influence depth is required to be greater than 6m. The influence depth is calculated as follows:
H=a？ W? h
Where H- reinforcement influence depth (m)
W- hammer weight (KN)= ton? L0 (kn/ton)
H—— falling distance (m)
A coefficient is 0.35.
According to the above formula,
Let H=6m, w? h=2938？ 78KN？ Meter (short for meter))
When the influence depth is 5m, w? H is 2000KN? Meter (short for meter))
Therefore, if the falling height of 20t hammer is 15m and the impact depth is 6m| test | large |, the requirements can be met, but the falling distance is generally not less than 6m, usually 10- 15m. Obviously, 10T hammer can't meet the requirement of 6m impact depth. When the reinforcement depth is 4.5m, the falling distance of 20m and 10T hammers can't meet the design requirements. Therefore, the subgrade section requires that the dead weight of the rammer is 20T and the falling distance is 6- 15m to meet the requirements.
Second, the test section commencement report
1. The commencement report of the test section shall be submitted to the supervision engineer for approval according to the requirements of the sectional project report;
2. The length of the test section is generally 50-100m;
3. Prepare the layout of tamping points, measure the tamping area required for lofting, and ensure the design requirements;
4. The test section should be carefully cleaned to remove impurities such as grass roots, tree roots and construction waste, so that the foundation surface is smooth. It is required to level with a grader and crush with a roller 1-2 times. The scarp shall be subject to the arranged dynamic compaction points, and the dynamic compaction before filling must meet the design area.
5. Before the test, investigate whether there are buildings, structures, underground facilities, high-voltage lines, cables, water supply pipes, etc. Around the construction section;
6. Arrange the main tamping point and the auxiliary tamping point, and the inclination angle of the bottom of the hammer should be less than 30? , or filling leveling;
7. Leveling datum points shall be arranged, 30m away from the influence range of dynamic compaction line, and the number of datum points shall ensure that the observation route of tamping point settlement is less than 60m and the maximum is 80m. It is best to use concrete pouring, the depth is not less than 50cm. Ramming point elevation observation should be accurate to 65438±0mm；;
8. Relevant parameters required for compactness of test section
(1) tamping energy (KN? m)；
(2) The spacing of tamping points shall be determined according to the design;
(3) tamping times;
(4) the interval between two adjacent tamping (generally more than 2 hours);
(5) Compaction width and influence range;
(6) Compaction depth and influence range;
(7) Obtain the total settlement, the settlement of the last two blows and the first blow, the minimum tamping distance a, the interval between two times b and the degree of compaction c through the test section, and confirm whether the tamping times specified in the design can meet the requirements of settlement and degree of compaction.
9. Total settlement
(1) Before dynamic compaction, after leveling, rolling and lofting, confirm the preloading elevation of each tamping point and the preloading elevation of the test section.
(2) After primary tamping, secondary tamping and full tamping, besides measuring the settlement of tamping points, it must also be leveled with a grader. The transverse and longitudinal slopes are the same as before compaction. After the roller is statically pressed again, the settlement of each tamping point and the total settlement of subgrade are detected.
10. tightness detection
(1) Before dynamic compaction, the test section shall be tested, including particle analysis, liquid limit, shrinkage limit, collapsible loess category, optimum water content, maximum dry density, CBR value if necessary, and the water content of the test section shall be determined in time.
(2) Before dynamic compaction, select the necessary compaction observation points at the measuring section to measure the compactness of 1m and 0.5m, and the compactness of the three layers after compaction.
1 1. Confirm the compaction effect.
(1) Whether the maximum settlement of collapsible loess given according to the drawing is consistent with the measured total settlement of subgrade test section.
(2) Confirm the compactness of each layer before and after tamping.
Three. Dynamic compaction foundation quality
After dynamic compaction, the compaction degree of this section of subgrade reaches 93%, which can basically meet the design requirements and specification requirements.