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Земляные работы

Method Statement: Backfilling, Leveling, and Soil Compaction in a Trench with a Utility Collector

This method statement regulates the professional procedures for layered backfilling, leveling, and compaction of cohesionless and cohesive soils in trenches up to 3 m deep containing an installed utility collector with a 1.8 x 1.9 m cross-section. The document is adapted for international projects and contains strict requirements for mechanization, quality control (target compaction factor up to 0.98), and labor organization.

6 sections 22 figures

Materials

Cohesionless soil Group I (fine/medium sand, optimal moisture content ±20%)
Cohesive soil Group II (loam, clay, optimal moisture content ±10%)
Cleaned sand (for bedding cable lines and casings, fraction 0.5–2.0 mm)
Lawn grass seeds (Kentucky bluegrass, creeping red fescue) for land reclamation
Fuels and lubricants (diesel fuel, hydraulic oil, lubricating greases)

Equipment

Hydraulic grading excavator with a telescopic boom (digging radius up to 6.8 m, bucket 0.63 m3)
Crawler bulldozer with an operating weight of 7–10 tons (blade width 2.5–2.6 m)
Dump truck with a payload capacity of 4.5–10 tons (body volume 3–8 m3)
Heavy-class reversible vibratory plate (weight 200–400 kg, productivity up to 750 m2/h)
Electric or petrol vibro-tamper (trench rammer, productivity approx. 50 m2/h)
Excavator-mounted hydraulic hammer with a tamping plate
Self-propelled vibratory soil compactor roller (operating weight 6–15 tons)
Dynamic penetrometer or a set of cutting rings for soil density control

1. General Provisions and Scope of Application

The method statement is developed for earthworks on a typical 50 m long work section. The subject is a trench up to 3 m deep, in which a utility collector with dimensions of 1.8 m (width) by 1.9 m (height) is installed. The works are carried out using cohesive (clay, loam) and cohesionless (sand, sandy loam) soils, provided their optimal natural moisture content is maintained. Prior to backfilling, all underground structures must undergo technical acceptance, waterproofing tests, and geodetic control, followed by the signing of the corresponding certificates of concealed works.

Backfilling of trenches for underground utilities must be carried out strictly following the laying of pipelines and network devices. At the same time, it is critically important to take preventive measures against the displacement of the laid elements along the longitudinal and transverse axes, as well as against mechanical damage to the waterproofing and anti-corrosion coatings. The minimum distance from the trench slope line to the beginning of the soil stockpile along the edge must be at least 0.7 m for trench depths up to 3 m, and at least 1.0 m for depths exceeding 3 m.

The backfilling process is divided into two key stages: manual (or lightly mechanized) tamping of the so-called 'haunches' (zones between the collector wall and the trench) and subsequent mechanized backfilling of the main trench section. The use of heavy construction equipment in the area directly above the pipeline or collector is strictly prohibited until a protective cushioning layer of the required thickness has been formed.

Fig. 1 — Typical cross-section of an underground pipe installation detailing engineered bedding, haunching, and protective backfill zones

Completely finish the collector installation, verify the waterproofing, and remove all auxiliary materials and construction debris from the trench.
Execute the certificates of concealed works and obtain written permission from the technical supervision authority to commence backfilling.
Cut and stockpile the topsoil in designated areas on the construction site.
Prepare soil stockpiles (cohesive or cohesionless) while verifying their particle size distribution and moisture level.

2. Layered Backfilling Technology

The formation of the soil mass in the trench begins with the manual backfilling and tamping of the haunches. The haunches are backfilled evenly on both sides of the collector in layers not exceeding 0.15–0.25 m in thickness. Simultaneous backfilling from both sides is a mandatory prerequisite to prevent lateral displacement of the structure. The initial protective layer above the top of the pipeline or collector must be at least 0.2 m thick when using manual tamping. During the winter period, for fragile utilities (plastic, ceramics), the thickness of this layer is increased to 0.5 m.

For metal and reinforced concrete pipelines, the minimum thickness of the protective layer compacted by light mechanized tools is 0.25 m, and for plastic ones, it is 0.4 m. If the trench crosses existing or planned roadways, backfilling for the entire depth is carried out exclusively with sand compacted to a factor of K=0.98. When laying cable lines, a 0.1 m thick bedding of clean fine sand is formed at the bottom of the trench, and the cable is covered with an identical layer (0.1 m) before placing the main backfill soil.

Further backfilling of the trench over the protective layer is performed by grading excavators and crawler bulldozers. The following layer thicknesses are permitted for placement (under heavy hydraulic hammers and vibratory rollers): for sand — up to 0.7 m; for sandy loams and loams — up to 0.6 m; for clay — up to 0.5 m. The bulldozer works the soil stockpile using frontal and oblique passes, moving the soil into the trench in consecutive sections, which minimizes the travel distance of the loaded equipment.

Fig. 1 — Sequential schematic of trench backfilling operations detailing protective utility pipe cover placement and main trench filling procedures — Fig. 2 — Sequential schematic of trench backfilling operations detailing protective utility pipe cover placement and main trench filling procedures

Perform layered (max 0.25 m) backfilling of the haunches on both sides of the collector using a grading excavator or manually.
Place a protective soil layer above the collector roof (0.2–0.5 m depending on the pipe type and season) without using heavy equipment.
Proceed with layered backfilling of the upper trench zone using a bulldozer, distributing the soil in layers of 0.5–0.7 m (depending on the soil type).

3. Mechanized Compaction Methods and Regimes

Soil compaction must be performed at its optimal moisture content: the tolerance is ±10% for cohesive soils and ±20% for cohesionless soils relative to the optimum Proctor moisture content. In confined spaces and haunches, electric tampers (productivity around 50 m2/h) or reversible vibratory plates (productivity up to 750 m2/h) are utilized. When operating tamping equipment, compaction starts from the collector wall and proceeds towards the trench slope. Each subsequent pass of the compaction machine must overlap the track of the previous pass by a minimum of 0.1–0.2 m.

For layered compaction of cohesionless soils, vibration and vibro-tamping methods are recommended. Low-cohesive and cohesive soils require static rolling, impact tamping, or combined methods. For a layer thickness of 20–25 cm, light electric tampers are used, whereas for 40–75 cm layers, heavy vibratory plates and excavator-mounted hydraulic tamping hammers are employed. The upper layers (to a depth of 1.0–1.2 m from the ground surface) are compacted by self-propelled vibratory soil rollers weighing 6–15 tons.

When working in sub-zero temperatures (in winter), the compaction of frozen soil is prohibited. Thawed soil must be compacted to a factor of K=0.98 before it freezes. The time until freezing commences depends on the ambient temperature: during moderate frost, it is 90–120 minutes, while during severe frost, it drops to 20–30 minutes. This necessitates a high intensity of work and immediate compaction of each placed layer.

Fig. 1 — Cross-sectional details of utility trenches showing pipe placement, bedding layers, and backfill specifications. — Fig. 3 — Cross-sectional details of utility trenches showing pipe placement, bedding layers, and backfill specifications.

Check the moisture content of the backfill soil; moisten or dry it to optimal values if necessary.
Compact the soil in the haunches using manual electric tampers (in 15–25 cm layers), moving from the collector towards the trench walls.
Compact the main layers with heavy vibratory plates or soil rollers (in 40–75 cm layers) with a 0.1–0.2 m track overlap.
In winter conditions, place the soil in short sections and compact it within 30 to 120 minutes to prevent freezing.

4. Quality Control and Acceptance Criteria

Quality control of earthworks is conducted continuously and includes checking the particle size distribution of the soil, its moisture content, and the achieved density. The degree of soil density is monitored by taking samples (using the cutting ring method or dynamic probing) and comparing the dry unit weight of the soil skeleton with the maximum standard density. The minimum allowable dry unit weight is: 1.7 t/m3 for fine sands, 1.65 t/m3 for sandy loams, 1.6 t/m3 for loams, and 1.5 t/m3 for clays.

Density testing is carried out in inspection test pits, which are excavated along the trench axis with a spacing of no more than 50 m. Sampling is performed at fixed depths: 0.3 m, 0.5 m, 0.9 m, 1.2 m, and 1.5 m from the surface of each tested tier. For trench sections crossing roadway carriageways, the compaction factor (K) must be strictly no less than 0.98 throughout the entire depth. In areas free from traffic loads, a factor of K=0.95 is permissible.

At points where the trench intersects with active perpendicular utilities (cables, pipes), a sand cushion is formed unless protective casings are specified in the design. Sand is placed up to half the diameter of the intersecting pipe and 0.5 m on each side of its axis, with a sand prism slope steepness of 1:1. Compaction quality at these nodes is verified with 100% coverage, after which a certificate of concealed works is drawn up with the participation of technical supervision.

Fig. 1 — Cross-section detail of underground mechanical excavation or drilling showing sequential stratification and central material removal shaft. — Fig. 4 — Cross-section detail of underground mechanical excavation or drilling showing sequential stratification and central material removal shaft.

Take soil samples from the stockpile for laboratory confirmation of particle size distribution and plasticity index.
After compacting the layer, excavate inspection test pits (1 test pit for every 50 m of trench length).
Measure the dry unit weight of the soil at depths of 0.3, 0.5, 0.9, 1.2, and 1.5 m.
Upon achieving K=0.98 (under roads) or K=0.95 (lawns), issue the certificate of concealed works.

5. Safety Requirements and Environmental Protection

Only certified personnel over 18 years of age, who have undergone health and safety induction and hold an Electrical Safety Qualification Group II, are permitted to perform mechanized earthworks and operate power tools. All applied equipment must undergo regular technical inspections. When unloading imported soil, dump trucks must not approach the trench edge closer than 1.0 meter. The presence of personnel within the operating radius of the excavator boom or bulldozer blade is strictly prohibited.

The descent of workers into the excavation and their ascent must be carried out exclusively via standard utility ladders installed outside the hazardous operation zones of the machinery. During unilateral backfilling of haunches adjacent to retaining walls or freshly laid foundations, work may only commence after confirming the structure's resistance to lateral earth pressure. Continuous monitoring of the trench slope conditions is required; upon detection of longitudinal cracks, works must be halted immediately until the walls are reinforced.

From an environmental perspective, the use of machinery exceeding permissible emission or noise levels in urban environments is not allowed. The fertile topsoil excavated prior to the commencement of works is stockpiled separately for subsequent land reclamation. Following the completion of tamping, the area is subject to grading and landscaping. Sowing lawn grasses (Kentucky bluegrass, creeping red fescue) or planting trees with dense crowns is recommended, taking into account the layout of the underground utility protection zones.

Fig. 1 — Mechanized joint sealing process for airfield concrete pavements using self-propelled sealers — Fig. 5 — Mechanized joint sealing process for airfield concrete pavements using self-propelled sealers

Install standard protective fencing around the trench, place warning signs, and provide nighttime illumination.
Place signal poles at a distance of 1.0 m from the edge to restrict the dump truck approach zone.
Conduct a targeted safety briefing for the crew when working in intersection zones with gas pipelines or live cables.
Restore the fertile soil layer and perform hydroseeding of lawn grasses upon completion of the construction cycle.

6. Labor Organization and Resource Provision

To ensure an uninterrupted technological workflow, the work is organized by multi-skilled crews. For backfilling and compacting cohesionless soil (Group I), an 8-person crew is formed: one excavator operator (Grade 6), one assistant operator (Grade 5), one bulldozer operator (Grade 5), and five earthworker/hand tool operators (Grades 1-3). When working with denser cohesive soil (Group II), the number of earthworkers is increased, bringing the total crew size to 9 personnel.

Delivery of backfill soil is carried out by dump trucks with a payload capacity of 4.5 to 10 tons. The productivity of manual electric tampers (trench rammers) is approximately 50 m2/h, whereas heavy reversible vibratory plates can compact up to 750 m2/h with a layer thickness of up to 60 cm. The works schedule must account for technological breaks for equipment repositioning and geodetic control of each tier.

The consumption of fuels and lubricants is standardized per 1 hour of equipment operation. For the hydraulic grading excavator and crawler bulldozer, the consumption of diesel fuel, motor, hydraulic, and transmission oils is factored in. The exact equipment requirement is determined by the Method Statement based on the backfill volumes, soil delivery logistics distance, and the mandated commissioning deadlines of the utility network.

Fig. 1 — Cross-section A-A of trench excavation and backfilling process for a rectangular concrete conduit using an excavator — Fig. 6 — Cross-section A-A of trench excavation and backfilling process for a rectangular concrete conduit using an excavator

Distribute the earthworker teams for simultaneous symmetrical manual tamping of the haunches over the 50 m work section.
Synchronize the supply of soil dump trucks with the working cycle of the grading excavator to prevent downtime.
Ensure the timely rotation of vibratory plate and electric tamper operators to comply with local vibration exposure standards.

Fig. 1 — Layout of trench backfilling operations using a bulldozer and dump truck — Fig. 7 — Layout of trench backfilling operations using a bulldozer and dump truck

Fig. 1 — Cross-section detail of trench backfilling operation over a subterranean concrete utility conduit using a bulldozer — Fig. 8 — Cross-section detail of trench backfilling operation over a subterranean concrete utility conduit using a bulldozer

Fig. 1 — Plan view of parallel reinforced concrete retaining walls or trench foundations with internal channel — Fig. 9 — Plan view of parallel reinforced concrete retaining walls or trench foundations with internal channel

Fig. 1 — Cross-section detail of a precast reinforced concrete channel installation in a trench with bedding and backfill specifications — Fig. 10 — Cross-section detail of a precast reinforced concrete channel installation in a trench with bedding and backfill specifications

Fig. 1 — Plan view of mechanized leveling and compaction pattern for granular sub-base material in a confined trench — Fig. 11 — Plan view of mechanized leveling and compaction pattern for granular sub-base material in a confined trench

Fig. 1 — Cross-section of a trench with a reinforced concrete utility tunnel and layered backfill compaction — Fig. 12 — Cross-section of a trench with a reinforced concrete utility tunnel and layered backfill compaction

Fig. 1 — Cross-sectional details of trench excavation and backfill layers for buried rectangular structures (SVP12,5, SVP25, SVP63,1) — Fig. 13 — Cross-sectional details of trench excavation and backfill layers for buried rectangular structures (SVP12,5, SVP25, SVP63,1)

Fig. 1 — Typical cross-sections of open-cut trenches for underground rectangular utility structures showing excavation profiles, structural placement, and stratified backfill compaction zones. — Fig. 14 — Typical cross-sections of open-cut trenches for underground rectangular utility structures showing excavation profiles, structural placement, and stratified backfill compaction zones.

Fig. 1 — Trench backfilling schematics illustrating various layer compaction thicknesses and sequences around a box structure — Fig. 15 — Trench backfilling schematics illustrating various layer compaction thicknesses and sequences around a box structure

Fig. 1 — Typical trench cross-sections for precast concrete box culverts showing excavation profiles, conduit placement, and specified backfill layering. — Fig. 16 — Typical trench cross-sections for precast concrete box culverts showing excavation profiles, conduit placement, and specified backfill layering.

Fig. 1 — Assembly drawing of a manual dynamic penetrometer showing key components including the guiding rod, anvil, drop weights, and handle — Fig. 17 — Assembly drawing of a manual dynamic penetrometer showing key components including the guiding rod, anvil, drop weights, and handle

Fig. 1 — Compaction curves showing bulk density of soil skeleton versus number of impacts — Fig. 18 — Compaction curves showing bulk density of soil skeleton versus number of impacts

Fig. 1 — Soil compaction mold assembly and dynamic impact apparatus for geotechnical testing — Fig. 19 — Soil compaction mold assembly and dynamic impact apparatus for geotechnical testing

Fig. 1 — General assembly and key components of a lightweight dynamic cone penetrometer for soil testing — Fig. 20 — General assembly and key components of a lightweight dynamic cone penetrometer for soil testing

Fig. 1 — Graph illustrating the relationship between the number of compaction blows and the compaction coefficient for various conditions — Fig. 21 — Graph illustrating the relationship between the number of compaction blows and the compaction coefficient for various conditions

Fig. 1 — Nomogram for determining soil compaction coefficient based on the number of penetrometer impacts and relative soil moisture content — Fig. 22 — Nomogram for determining soil compaction coefficient based on the number of penetrometer impacts and relative soil moisture content

Tips & Recommendations

The movement of heavy tracked equipment (bulldozers, excavators) directly above the axis of the collector or pipeline is strictly prohibited until a compacted protective soil layer at least 0.5–0.7 m thick has been established.

During the simultaneous backfilling of the collector haunches, ensure strict symmetry of placement and tamping on both sides (height differential no more than 10–15 cm). This will prevent lateral displacement or deformation of the engineering structure.

Compacting frozen soil is unacceptable. During the winter period, operate in short sections, ensuring each layer is compacted to the required density within 30–90 minutes after excavation from the stockpile, preventing it from freezing.

Optimize the bulldozer's route: oblique passes (at an angle to the trench) allow for more efficient soil retrieval from the stockpile and reduce the working stroke cycle time.