What Was the Brief for This Project?

📍 Project details

The grading design on a hillside for a project in northeastern Thailand was split into 2 zones — Zone A, soil and rock cut only, and Zone B, cut combined with fill and an RC retaining wall. SPN Soil Engineering was responsible for designing the cut slopes, the compacted fill specification and the retaining wall, together with a complete testing specification.

The task was to grade a hillside with a soft rock layer beneath the soil, so the cut slopes had to be designed with separate gradients for each material. Because of safety constraints at the site, the drawings specify rock excavation by hydraulic breaker only — no explosives under any circumstances.

Overall plan of hillside grading works, divided into Zone A soil cut and Zone B cut and fill with a retaining wall
Overall grading plan — split into Zone A (soil and rock cut) and Zone B (cut + fill + RC retaining wall) (Drawing: SPN Soil Engineering)

Before going into each part in detail, the section below brings the elements of both zones together in one image, running from the benched cut slopes on the hillside down to the compacted fill area and the RC retaining wall at the edge of the fill.

Conceptual section of hillside grading works: benched cut slopes, compacted fill and an L-shaped RC retaining wall A section combining the elements of Zone A and Zone B in one image. On the left is the hillside cut into benches. The upper slope is in soil at a 1:1 gradient, covered with vetiver grass; the lower slope is in soft rock at a 1:0.5 gradient, protected with shotcrete. Between the two is a 2.00 m wide bench with a reinforced concrete drain on every bench, and each slope segment is no higher than 4.00 m. The dashed line is the original ground before cutting. In the middle is the finished grade level. On the right is fill compacted in layers of no more than 300 mm each. At the edge of the fill is an L-shaped reinforced concrete retaining wall up to approximately 4.20 m high. Behind the wall is a 0.30 m thick gravel drainage layer and 4-inch weep holes at every 3.00 m. Trucks must not drive or park behind the wall. The soil beneath the base must have a bearing capacity of no less than 30 t/m². The bottom strip summarizes the requirements for the cut slopes, compacted fill and retaining wall. Section (2D): benched cut slopes · compacted fill · RC retaining wall Conceptual view · Not to scale Zone A · Soil & rock cut Zone B · Cut + fill + wall Soil cut slope 1:1 (45°) Vetiver grass on slope face Bench, 2.00 m wide Max. 4.00 m per slope Excavated part No trucks driving/parking behind wall Gravel 0.30 m Drainage behind wall RC retaining wall Max. ~4.20 m Weep Hole 4" @ 3.00 m RC bench drain Catches runoff Soft rock cut 1:0.5 Shotcrete vs. weathering Soft rock layer Compacted fill Layers ≤ 300 mm Base soil ≥ 30 t/m² Original ground before grading A Cut slope Soil 1:1, vetiver Soft rock 1:0.5, shotcrete Rock by hydraulic breaker B Compacted fill Moisture ±2% of OMC 90–95% Standard Proctor Field Density ≥ 1 pt/500 m² per layer C RC retaining wall Compacted clay cap vs. seepage Expansion joints every 15 m Plate Bearing Test ≥ 3 pts first
Conceptual section combining the elements of Zone A and Zone B in one image — 1:1 soil slopes, 1:0.5 soft rock slopes, 2.00 m benches with drains, fill compacted in layers of no more than 300 mm, and an RC retaining wall up to approximately 4.20 m high. Figures are from the project drawings; not to scale (Image: SPN Soil Engineering)

How do soil cut slopes and rock cut slopes differ in design?

They differ in gradient and slope-face protection, because different materials can stand at different gradients. Soft rock is stronger than soil, so it can be cut steeper. The design for this project was split into 2 cases.

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ItemCut slope in soilCut slope in soft rock
Gradient1:1 (45°)1:0.5 (steeper than soil slope)
Slope-face protectionVetiver grass planting to prevent rainfall erosionShotcrete to protect against rock weathering and ravelling
Slope height per segmentMax. 4.00 mMax. 4.00 m
Bench between segments2.00 m wide with RC drain2.00 m wide with RC drain

High slopes are divided into benches, and every bench has an RC drain (Concrete Interceptor Drain) that catches surface runoff so it does not accumulate and erode the slopes below. This is the same principle used to prevent slope failure in general (read more at Factor of Safety in slope stability).

Zone A soil and rock cut plan showing benched cut slope lines on the hillside
Zone A soil and rock cut plan — benched cut slope lines stepping down with the hillside gradient (Drawing: SPN Soil Engineering)
Section of benched cut slopes: 1:1 soil slope with vetiver grass, 1:0.5 soft rock slope with shotcrete, 2 m wide bench
Cut slope section — 1:1 in soil with vetiver grass, 1:0.5 in soft rock with shotcrete, no more than 4.00 m high per segment (Drawing: SPN Soil Engineering)
Detail of the reinforced concrete drain on a bench of the cut slope
Detail of the RC drain (Interceptor Drain) on a bench — catching surface runoff at every level before it flows down to the slope below (Drawing: SPN Soil Engineering)

How is the quality of compacted fill controlled?

It is controlled by specifying fill material properties, compacted layer thickness, moisture content and field density testing on every layer. The fill side of Zone B uses fill slopes of 1:1.5 for heights of 0–4 m (and 1:1 for low fills not exceeding 2 m). The key to the work is quality control of both the material and the compaction of every layer, because fill that is not compacted to standard is a leading cause of later fill settlement problems.

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Control itemProject specification
Compacted layer thicknessMax. 300 mm per layer (after compaction)
Moisture during compaction±2% of optimum moisture content (OMC)
Dry density90–95% of Standard Proctor (depending on location)
Fill material propertiesLL ≤ 40, CBR ≥ 4%, max. particle size 75 mm
Material tests (Sieve, Atterberg, Compaction, CBR)3 samples per 5,000 m³ of fill
Field density test (Field Density)Min. 1 point / 500 m² per layer

Field density testing is done by the sand cone method or equivalent (see the test method at Field Density Test: Measuring In-Place Soil Density). Any layer that fails must be recompacted until it passes before the next layer is placed.

Zone B cut and fill plan with the reinforced concrete retaining wall line
Zone B plan — cut area combined with compacted fill, and the RC retaining wall line supporting the fill level (Drawing: SPN Soil Engineering)
Zone B grading detail showing a 1:1.5 fill slope section and compacted layer details
Zone B grading detail — 1:1.5 fill slope with layer-by-layer compaction requirements (Drawing: SPN Soil Engineering)

How was the RC retaining wall designed?

The designer chose an L-shaped reinforced concrete cantilever retaining wall (RC Cantilever Retaining Wall) where the fill level differs greatly from the adjacent area. The stem is up to approximately 4.20 m high (4.80 m including the 0.60 m thick base), and the base is about 4.65 m wide — a type well suited to heights of 3–6 m (compare its pros and cons with other types at Which Retaining Wall Should You Choose? Comparing 7 Types).

  • Main reinforcement: DB25 @ 0.200 m and DB20 @ 0.250 m (SD40), according to moment location
  • Sub-base: lean concrete 0.05 m thick on compacted sand 0.10 m thick
  • Drainage behind the wall: 1-inch gravel layer 0.30 m thick + 4-inch weep holes every 3.00 m + compacted clay cap on top to prevent water seeping in
  • Expansion joints: every 15 m, with RB25 dowels 500 mm long and 15 mm thick joint sealant
  • Materials: concrete compressive strength 280 ksc (cylinder) · SR24 round bars (TIS 20-2559) · SD40 deformed bars (TIS 24-2559)

Another notable requirement is that the drawings define a zone where trucks must not drive or park behind the wall, because surcharge from trucks would increase the lateral earth pressure beyond the design value. Small details like this are what make a wall safe on site, not just on paper.

3D view of the reinforced concrete retaining wall installation sequence and compacted fill behind the wall
3D view — installation sequence of the RC retaining wall and layer-by-layer compaction of the fill behind it (Drawing: SPN Soil Engineering)
Reinforced concrete retaining wall details: section, reinforcement and drainage behind the wall
RC retaining wall details — section, reinforcement, drainage system and expansion joints (Drawing: SPN Soil Engineering)
Enlarged section of the RC retaining wall showing the gravel drainage layer behind the wall, weep hole pipes and clay cap
Enlarged section — 0.30 m thick gravel drainage layer + 4-inch weep holes @ 3.00 m + compacted clay cap (Drawing: SPN Soil Engineering)
3D view of the reinforcement layout inside an L-shaped reinforced concrete retaining wall
3D view of reinforcement layout in the L-shaped retaining wall — main bars DB25/DB20 (SD40) (Drawing: SPN Soil Engineering)

What investigation and testing is needed before construction?

The bearing capacity of the soil beneath the wall base must be proven by 2 methods together, because all design figures rest on the assumption that the soil beneath the wall base can carry no less than 30 t/m².

  • Soil investigation with at least 3 boreholes along the wall alignment, to confirm the soil layers and actual bearing capacity (for how the number of boreholes is set, see How many boreholes, and how deep?)
  • Plate Bearing Test (ASTM D1195/D1196) at at least 3 points, testing the bearing capacity of the soil at the actual foundation level, with settlement not exceeding 25 mm per the project specification

If the test results are below the design values, the engineer must revise the foundation design or improve the ground first. This step is the final safety checkpoint before concrete is poured. The 3D view below summarizes the test locations, both before wall construction and during fill placement on the graded area.

Conceptual 3D model of the graded hillside area: benched cut slopes, compacted fill area and RC retaining wall line A 3D view of a hillside soil block cut away to show the front section. At the back left is the hillside. The upper slope is in soil at a 1:1 gradient with rows of vetiver grass. Below it is a 2.00 m wide bench with a concrete drain running along its full length. The lower slope is in soft rock at a 1:0.5 gradient, protected with shotcrete. Then comes the flat graded area: the inner part is cut and the outer part is compacted fill. Along the front right edge of the fill is the reinforced concrete retaining wall, with weep holes spaced every 3.00 m. The red strip behind the wall is the no-truck driving or parking zone. Markers B and P in front of the wall represent soil boring and Plate Bearing Test locations, at least 3 of each along the wall before construction. Markers D on the fill represent field density test points. The panel on the right summarizes testing before and during construction. 3D model: graded hillside area Not to scale D D B B B P P P Soft rock layer Soil 1 Soil slope 1:1 Vetiver grass cover 2 2.00 m bench + drain Catches runoff 3 Soft rock 1:0.5 Shotcrete 4 Compacted fill Layers ≤ 300 mm · 90–95% 5 RC retaining wall Max. ~4.20 m · weep holes @ 3.00 m No trucks driving/parking behind wall Verification tests Before wall construction B Soil boring ≥ 3 holes along the wall P Plate Bearing ≥ 3 pts at actual base level Settlement ≤ 25 mm Base soil must carry no less than 30 t/m² During fill compaction D Field Density Test ≥ 1 pt / 500 m² per layer Fail = recompact Fill: 3 samples per 5,000 m³ of fill Figures are specific to this project Conceptual · marker locations are examples, not a layout
Conceptual 3D model of the graded area — markers B and P are soil borings and Plate Bearing Tests, at least 3 of each along the wall before construction; markers D are field density test points during filling. Marker locations are examples, not the actual layout (Image: SPN Soil Engineering)

Summary: lessons from hillside grading works

Grading works on a hillside must be designed as a single system with no part left out. This case study shows all the elements: cut slopes designed separately by material (soil 1:1 / soft rock 1:0.5) → benches with drains at every level → fill with quality control on every layer → RC retaining wall with drainage → verification testing before construction.

Note that the figures in this article (slope gradients, wall dimensions, reinforcement spacing and test criteria) are design results specific to this project, based on the actual soil and rock conditions on site, and cannot be applied directly to other projects. Every hillside project must be designed afresh by an engineer using soil investigation data from that site.

SPN Soil Engineering provides a full range of services, from soil investigation and field soil testing to geotechnical engineering design (slopes, retaining walls and site grading), with more than 20 years of experience and projects nationwide. Consult us about your project free of charge.