01. Waterproofing of Retaining Structures

(1) Water-stopping Characteristics of Various Retaining Structures

        Vertical earth-retaining structures are generally installed around deep foundation pits, known as retaining structures. Retaining structures are a prerequisite for ensuring the safe excavation of deep foundation pits. There are many structural forms used for deep foundation pits, with varying construction methods, techniques, and machinery. The water-stopping effects achieved by different construction methods also differ.

(2) Waterproofing Precautions for Diaphragm Wall Construction

Some foundation pit projects adopt diaphragm wall structures. Diaphragm walls have a relatively good waterproofing effect, and their construction technique is similar to that of bored piles. The main points to note are as follows:

1. The key quality control point for waterproofing lies in the treatment of joints between adjacent wall panels. If the key construction points of joint treatment are mastered, good waterproofing results will be achieved.

2. After trenching, the end faces of adjacent concrete should be cleaned and brushed. Brushing should reach the bottom, and the number of brushings should be no less than 20 times, until no mud remains on the brushing tool.

3. Before lowering the reinforcement cage, install small conduits at the end of the cage along the wall direction. Strictly control the joint quality during installation to prevent grout leakage from blocking the conduits. During foundation pit excavation, if water seepage is found at wall joints, grout through the small conduits.

(3) Waterproofing Focus for Cast-in-Place Pile Construction

Some retaining structures adopt the form of bored cast-in-place piles plus high-pressure jet grouting pile curtains. Controlling the construction quality of the high-pressure jet grouting pile water-stopping curtain is the key waterproofing point during construction. During the construction of the water-stopping curtain, it is necessary to strictly control the pile spacing, grout quality, and jetting pressure to ensure a sealed waterproof belt is formed around the cast-in-place piles, thereby achieving good waterproofing results.

Waterproofing of Retaining Structures

02. Foundation Pit Excavation Control

During foundation pit excavation, water leakage from retaining structures may occur due to improper treatment of structural nodes and other reasons. To avoid accidents caused by water seepage from retaining structures, the following points should be noted during excavation:

1. During excavation, blind excavation is strictly prohibited. Pay close attention to changes in water levels outside the foundation pit and water seepage conditions of the retaining structures. If water inrush occurs during excavation, backfill the water inrush location promptly to prevent expansion and instability. Excavation may continue only after the situation is treated by appropriate methods.

2. Timely treatment of small-scale water seepage. Clean the concrete surface, seal the wall surface with high-strength rapid-setting cement, and use small conduits for drainage to prevent the seepage area from expanding. After the sealing cement reaches strength, seal the small conduits with a grouting machine that has grouting pressure.

Foundation Pit Excavation Control

03. Waterproofing of Main Structures

Waterproofing of the main structure is the most important part of deep foundation pit waterproofing. By controlling the following aspects, the main structure can achieve good waterproofing results.

(1) Concrete Quality Control

Concrete quality is the prerequisite for ensuring structural waterproofing. The selection of raw materials and the design of mix proportions are the supporting conditions for ensuring concrete quality.

Aggregates entering the site shall be inspected for mud content, clay lump content, elongated and flat particle content, and particle gradation in accordance with the Standard for Technical Requirements and Test Methods for Sand and Crushed Stone for Ordinary Concrete. Under the premise of meeting strength and workability requirements, a sand ratio as low as possible should be adopted so that the concrete contains sufficient coarse aggregate. The concrete mix proportion shall meet the strength requirements of the concrete structure design and durability under various environments, and the concrete mixture shall have flowability and other working properties suitable for construction conditions. The concrete mixture should be uniform, easy to compact, and resistant to segregation, which is the prerequisite for improving concrete quality. Therefore, the workability of concrete must be fully ensured.

(2) Construction Control

 1. Concrete treatment. Construction joints are formed at the interface between old and new concrete. Chipping treatment effectively increases the bonding area between old and new concrete, which not only improves the continuity of the concrete but also benefits the bending and shear resistance of the wall. Before concrete pouring, apply a neat cement slurry first, then coat with cement-based permeable crystalline waterproofing material. Cement-based permeable crystalline material can effectively bond the gaps between concrete and prevent the intrusion of external water.

2. Installation of steel plate waterstops. The steel waterstop plate should be embedded in the middle of the poured concrete structural layer, with the bent ends at both ends facing the water-facing side. For the construction joint waterstop steel plate at the post-cast strip of the exterior wall, it should be placed in the middle of the concrete exterior wall, installed vertically, and welded tightly in a closed loop with each horizontal waterstop steel plate. After determining the horizontal elevation of the horizontal steel plate waterstop, a through line should be pulled along the upper edge of the steel plate waterstop based on the building's elevation control points to keep the upper edge straight.

Fix the steel plate by welding with reinforcing bars, and weld diagonal reinforcing bars to the top formwork rods for fixation. Weld short reinforcing bars below the steel plate waterstop to support the steel plate. Their length should be based on the thickness of the concrete wall reinforcement mesh and should not be too long, to prevent the formation of seepage channels along the short reinforcing bars. Short reinforcing bars are generally spaced no more than 200 mm apart, one on each side. Too small a spacing increases cost and workload, while too large a spacing makes the steel plate waterstop prone to bending and deformation under vibration during concrete pouring.

Steel plate joints shall be welded. The overlap length of two steel plates shall not be less than 50 mm, and both ends shall be fully welded. The weld height shall not be less than the thickness of the steel plate. Trial welding should be carried out before welding to adjust the current parameters. Excessive current may burn or even penetrate the steel plate, while insufficient current makes arc striking difficult and the weld not firm.

3. Installation of water-swelling waterstop strips. Before laying the water-swelling waterstop strip, sweep away floating slag, dust, debris, etc., to expose the hard base. For post-cast strips and horizontal construction joints, unfold the water-swelling waterstop strip along the extension direction of the construction joint and directly adhere it to the middle of the construction joint using its own adhesiveness. The overlap at joints shall not be less than 5 cm, and no break points shall be left. For vertical construction joints, a shallow positioning groove should be reserved first, and the waterstop strip should be embedded in the reserved groove. If no groove is reserved, high-strength steel nails may also be used for fixation, and the strip can be directly adhered to the construction joint interface using its self-adhesiveness, evenly pressed against the release paper. After the waterstop strip is fixed, tear off the release paper and pour the concrete.

4. Concrete vibration. The vibration time and method of concrete must be correct, and vibration must be dense without over-vibration or missed vibration. During vibration, mortar splashing should be minimized, and mortar splashed on the inner surface of the formwork should be cleaned promptly. Vibration points should be distributed from the middle to the edges, with uniform rod placement, layer-by-layer overlapping, and continuous pouring throughout every part of the concrete placement. The vibration time at each point should be based on the concrete surface becoming covered with laitance, flat, and no longer emitting air bubbles, usually 20–30 s, to avoid segregation caused by over-vibration.

Concrete pouring should be carried out continuously in layers. When using an insert-type vibrator, insert quickly and pull out slowly. Insertion points should be evenly arranged in a quincunx pattern. When vibrating the upper layer of concrete, the vibrator should be inserted 5–10 cm into the lower layer of concrete so that the two layers bond firmly. The vibration sequence should be in the direction opposite to the concrete flow as much as possible, so that the vibrated concrete no longer takes in free water and air bubbles. During vibration, the vibrator must not touch embedded parts or formwork.

 5. Curing. After concrete pouring is completed, it should be covered and watered within 12 h to keep the concrete moist. The curing period is generally no less than 7 d. For parts that cannot be cured by sprinkling water, curing agents should be used, or a protective film may be sprayed directly on the concrete surface after formwork removal. This not only eliminates the need for curing but also improves durability.

Waterproofing of Main Structures

04. Foundation Pit Excavation Control

Although deep foundation pit waterproofing is mainly based on concrete self-waterproofing, the laying of waterproof layers also plays a crucial role in deep foundation pit waterproofing projects. Strictly controlling the construction quality of the waterproof layer is a key control point in waterproofing construction.

(1) Base Surface Treatment

 Before laying the waterproof layer, the base surface should be effectively treated, mainly for flatness and water seepage. For water seepage on the base surface, plugging methods should be adopted. The treated base surface must be clean, free of contamination, water droplets, and seepage.

(2) Quality of Waterproof Layer Laying

1. Waterproof membranes must have factory qualification certificates, and only products with qualified quality may be used. The waterproofing construction base must be flat, dry, clean, firm, and free from sanding and peeling.

2. Before applying the waterproof layer, the internal and external corners of the base should be treated and made into arc shapes. The diameter of internal corners should preferably be greater than 50 mm, and the diameter of external corners should preferably be greater than 100 mm.

3. Waterproof layer construction must be carried out in accordance with specifications and design requirements.

4. Treatment of construction joint locations: determine the concrete pouring height and carry out waterproof reinforcement treatment at construction joint locations.

5. After the base waterproof layer is laid, construct the protective layer promptly to prevent the waterproof layer from being burned or punctured during reinforcement welding and damaged during concrete vibration.