What Are the Different Methods of Drainage Installation?

Drainage consolidation method is to install a vertical drainage body such as a sand well (a bagged sand well or a plastic drainage belt) in the foundation, and then load it according to the weight of the building; The method of loading and preloading in advance to discharge the pore water in the soil and gradually consolidate, the foundation is settled, and the strength is gradually increased.

The principle of drainage consolidation is

Drainage consolidation method consists of two parts: drainage system and pressurization system. The drainage consolidation system consists of a vertical drainage body and a horizontal drainage body. Its main function is to change the drainage boundary conditions of the foundation, shorten the drainage distance and increase the way of pore water drainage. When the soft soil layer is near the ground and is thin, or the soil has good permeability and a long construction period, a certain thickness of sand cushion can be laid on the ground without vertical drainage channels. The pore water in the soil is discharged to the sand cushion under the effect of external load, thereby causing consolidation. If the soft soil layer is thick, in order to accelerate the drainage consolidation, vertical drainage bodies such as sand wells should be set in the foundation to form a drainage system together with the horizontal sand cushion. Pressurized system refers to the arrangement of the load applied to the foundation.

Drainage and pressurization systems are always used in combination. If only a drainage system is set up, no consolidation pressure is applied, there is no pressure difference in the pore water in the soil, no infiltration consolidation will occur, and the strength will not increase. If only the consolidation pressure is applied and no drainage body is provided, the pore water will be difficult to discharge, and the consolidation and settlement of the foundation soil will take a long time. Therefore, to ensure the reinforcement effect of drainage consolidation method, from the perspective of construction, the following three links are mainly done: laying a horizontal cushion, setting up a vertical drainage body, and applying consolidation pressure.

The drainage consolidation method is generally applicable to saturated soft clay, dredger soil, loose silt soil, recently deposited soil, organic soil and peat soil foundation.

1. Sand cushion construction

The function of the sand cushion layer is to quickly discharge the seepage water from the soil body into the cushion layer during the pre-compression construction, thereby achieving the consolidation of the soil layer. The quality of the sand cushion will play a decisive role in the reinforcement effect and the preloading time. Selecting the corresponding medium and coarse sand as the sand cushion material can ensure that the sand cushion has good permeability. In construction applications, it should be ensured that its mud content is below 5%, and impurities and organic matter cannot be mixed. When determining the thickness of the sand cushion layer, the infiltration water in the soil layer should be drained in time, generally controlled between 30 cm and 50 cm, which can play the role of a holding layer.

2.Bagged sand well construction

Manholes (30 to 50 cm in diameter), bagged manholes (7 to 12 cm) and plastic drainage boards are the main components of the vertical drainage body. Due to the advantages of smaller cross section, smaller volume, convenient construction and low cost, bagged sand wells have been widely used in road engineering soft soil roadbed treatment. According to drainage requirements, ensure that the bagged sand well woven bag has good water permeability to avoid losing the sand in the bag. In the selection of bagged sand well materials, the bag should have a high tensile strength. Generally, the longitudinal tensile strength is within the range of 18.KN/M, and the tensile longitudinal elongation must be controlled at about 17.6%. To ensure that it can withstand the weight of the sand in the bag and the tensile force caused by bending to the maximum extent, and has good anti-aging ability and anti-corrosion ability. At this stage, polypropylene woven bags are the most widely used. Compared to the permeability coefficient of sand, the permeability coefficient of this bag is higher. In the construction of bagged sand wells, medium and coarse sand are generally used as construction materials. This material has a high water permeability. The total amount of sand with a particle size of more than 0.5 mm must exceed that of the material. At the same time, its mud content should be strictly controlled The amount is generally below 3%. The construction process is as follows:

(1) Site cleaning and stakeout

In the construction of soft foundation drainage consolidation technology for highway engineering, the construction site should be leveled and cleaned in time. Drain the ground water in time in the early stage of construction, and at the same time, pavement hardening treatment should be performed to ensure the smoothness of the engineering access road. When determining the construction location, the relevant design regulations shall be strictly followed to ensure that the deviation is less than 2 cm. The elevation of the site shall be accurately measured and recorded, and the total station shall be used for accurate stakeout measurements in accordance with the requirements of the drawings. At the same time, the construction personnel shall periodically review. In the backfilling of the basement, a straw bag cofferdam is first built along the toe of the slope to the outside. The excavation of the steps should be carried out on the section of the base cross-slope steeper than 1: 5, and the step width is generally between 1 and 2 meters Control, when setting the reverse cross slope of the top surface of the step, its slope should be between 2% and 4%. When cleaning and backfilling the substrate, a slope of 0.3% should be reserved on the base surface, which can facilitate the drainage of the base surface or sand cushion.

(2) Manhole in place and pressure pipe

After completion of the basement operation, the sand rig and casing can be placed. After the sand rig is in place, it should be ensured that the guide rail, casing tube is vertical to the ground and aligned with the pile position. The verticality error is effectively controlled within a reasonable range of 1.5%, and the corresponding length is on the casing The position of the ruler is drawn out, so as to achieve the purpose of effectively controlling the depth of the sand well. There should not be curved casing, non-smooth inner walls, burrs, etc. 8 cm to 10 cm is a reasonable range for diameter control. After the sand rig is in place, the sand rig pressure device and exciter should be started in time, and the casing vibration should be slowly pressed into the soft soil to ensure the pressure uniformity. The pipe pressing operation must be continuously pushed downwards, and no phenomenon such as the pipe being pushed in can occur. When the pressure pipe is close to the weathered rock formation, it is necessary to pay attention to the casing sinking speed. After the sinking speed gradually decreases, the length of the casing must be accurately measured to ensure that the casing length in the weathered rock formation is above 50 cm.

(3) Sand filling bag

Use the inlet roller sand bag to quickly and smoothly feed to the mouth of the casing, at the same time, pull up the tail of the bag, and raise the casing through the shaker. When the height rises to 0.5 meters, release the sand bag to the bottom of the hole Place it. When placing the sand bag in the casing, the sand bag should be prevented from kinking and necking. During extubation, make sure that the sand bag is not taken out or damaged. When a problem occurs, the cause should be analyzed carefully. The soil brought out by the pipe must be cleaned in time. The mortar opening must be formed to ensure that it can communicate with the sand cushion. During the construction process, the irrigation of each sand well must be recorded. The amount of sand.

3.Pre-pressing system construction

Preload is an important part of drainage consolidation method, it promotes the power of free water discharge between particles. The implementation of preloading is mainly reflected in hierarchical loading, and the stability of each stage of loading depends on the increase in strength after the previous stage of preloading. In order to accelerate the consolidation of the foundation, the preloading is combined with gravel cushion and bagged sand wells to treat the foundation. In order to ensure the safety and progress of the construction, observation sections, settlement and displacement are set to guide the construction. Each observation surface is provided with 5 observation points and two level points. Before the embankment is filled, settle plates are installed on the cleaned surface at the center of the road and left and right shoulders according to the design requirements; at the same time, a displacement observation pile is buried 1 meter outside the embankment slope foot. During the pre-compression period, settlement observation should be carried out according to regulations. The pavement can only be paved after the embankment deformation reaches the design expectations, and the excess soil above the roadbed will be unloaded. The embankment will be repaired to the elevation of the roadbed and meet the compaction requirements ^[1] .