PLATE LOAD TEST: Procedure, Interpretation, Bearing Capacity Calculation, and Applications

Table of Contents

Deep Dynamic Compaction (DDC), also called dynamic compaction, is a ground improvement technique in which a heavy steel or concrete tamper is repeatedly dropped from a significant height onto the ground surface. The impact generates stress waves that penetrate the soil and rearrange or densify loose soil particles.

DDC is particularly useful for loose granular, aeolian and collapsible soils, where the problematic soil extends several metres below the ground surface and conventional shallow compaction cannot reach it.

Figure 1: Macro view of untreated porous soil (left) versus a live Deep Dynamic Compaction (DDC) operation demonstrating tamper drops and impact craters on a grid (right).

For large sites, DDC can become an alternative to excavating the entire weak layer, hauling the material away, replacing it with suitable fill and recompacting the replacement soil. The economic advantage can be substantial when the treatment area is large and the collapsible layer is relatively deep.

What is Deep Dynamic Compaction?

In DDC, a large tamper is lifted by a crane and dropped repeatedly from a predetermined height. The impact energy of one drop is approximately:

$$E=W\times H$$

where:

• $E$ = energy per drop
• $W$ = weight of tamper
• $H$ = drop height

If $W$ is expressed in tonnes and $H$ in metres, the result is commonly expressed as tonne-metres (t·m).

For example, a 15-tonne tamper dropped from 20 m produces:

$$E=15\times 20=300\text{ t·m}$$

The energy is transmitted into the ground through the impact. The objective is not simply to compact the soil directly beneath the tamper. The impact generates stress waves and ground deformation that can densify a considerable volume of soil.

DDC is therefore different from conventional surface rolling or vibratory compaction.

Why DDC Works for Collapsible Soils

Collapsible soils can appear reasonably stiff in their natural, relatively dry condition but undergo significant settlement when wetted or loaded. Typical collapsible deposits may contain a loose structure supported by weak particle contacts, apparent cohesion, carbonate bonding or other forms of interparticle structure.

When water enters the soil, the apparent structure can break down. The result can be:

Wetting + loading → collapse of soil structure → rapid reduction in void ratio → settlement

This creates a geotechnical problem when a building, tank, road, pipeline or other structure is constructed over the deposit. DDC attempts to modify the soil before construction so that the loose structure is densified and the potential for future collapse is substantially reduced.

Research based on U.S. case histories found DDC particularly economical for collapsible soils when the problematic material extends deeper than about 3–4 m. Cohesionless and low-plasticity collapsible soils generally respond well, whereas clay layers can absorb impact energy and reduce the effectiveness of the treatment.

The Basic DDC Process

A typical DDC project involves several stages:

1. Pre-treatment investigation
2. Test section
3. Primary high-energy compaction
4. Secondary or intermediate compaction
5. Crater filling and grading
6. Ironing pass
7. Post-treatment testing
8. Verification of settlement and/or bearing requirements

The actual sequence varies according to soil conditions and project performance criteria.

DDC Equipment

The principal equipment consists of a heavy tamper and a crane capable of repeatedly lifting it to the specified drop height. The tamper may be fabricated from steel or concrete and is usually designed to have a relatively large contact area.

Important equipment parameters include:

• Tamper mass
• Tamper shape
• Tamper diameter or contact area
• Maximum drop height
• Crane capacity
• Lifting cycle
• Number of drops per point
• Number of passes

The tamper mass and drop height are selected based on the required depth of improvement and the response of the soil. Large tampers and high drop heights provide greater impact energy, but they also increase equipment requirements, ground vibration and the potential for excessive crater formation.

Tamper Weight and Drop Height

There is no single tamper weight or drop height that applies to every collapsible soil project. A preliminary depth relationship commonly used in DDC design is:

$$D=n\sqrt{WH}$$

where:

• $D$ = approximate depth of improvement, m
• $n$ = empirical coefficient
• $W$ = tamper mass, tonnes
• $H$ = drop height, m

Another commonly presented form in DDC guidance is:

$$D=1.7n\sqrt{WH}$$

The difference between these forms reflects the particular empirical formulation being used. Therefore, the designer should not mix the coefficient from one formulation with the equation from another.

The Geo-Institute guidance gives $n$ values that depend on soil type and saturation. For granular soils, values around 0.5 to 0.6 are suggested for pervious deposits, while lower values are applicable to silts and other less permeable materials.

These equations are preliminary design tools. The final energy requirement should be established and adjusted using field trials and post-treatment testing.

Example

Suppose:

• $W=16$ tonnes
• $H=21$ m
• $n=0.5$

Using the simple relationship:

$$D=0.5\sqrt{16\times 21}$$ $$D=0.5\sqrt{336}$$ $$D\approx 9.17\text{ m}$$

This illustrates why a 16-tonne tamper dropped from approximately 21 m can be considered for several-metre-deep improvement. It should not, however, be interpreted as proof that every 16-tonne/21-m DDC operation will improve soil to exactly 9.2 m.

Energy Per Drop

The simplest measure of impact energy is:

$$E_d=WH$$

For a 16-tonne tamper dropped from 21 m:

$$E_d=16\times 21=336\text{ t·m}$$

In SI units:

$$336\text{ t·m}\approx 3.30\text{ MJ}$$

because:

$$1\text{ t·m}=9.81\text{ kJ}$$

Therefore:

$$336\times 9.81\approx 3296\text{ kJ}$$

or approximately $3.30\text{ MJ}$ per drop.

The actual energy transferred into the ground will depend on the impact system and field conditions. The $W\times H$ value is the gravitational potential energy available before impact.

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Applied Energy Per Unit Area

For large DDC projects, energy is often expressed as average applied energy over the treated area. A commonly used relationship is:

$$AE=\frac{WHNP}{G^2}$$

where:

• $AE$ = average applied energy
• $W$ = tamper weight
• $H$ = drop height
• $N$ = number of drops at each point per pass
• $P$ = number of passes
• $G$ = grid spacing

The South Carolina DOT ground improvement guidance presents this form for calculating average applied energy over a DDC grid. This equation demonstrates why grid spacing is important. If the spacing is reduced, the same drop energy is distributed over a smaller area and the average energy density increases.

DDC Grid Patterns

DDC is generally carried out on a systematic grid. Common arrangements include:

• Square grid
• Rectangular grid
• Staggered grid
• Triangular pattern

For many projects, the first high-energy pass uses relatively wide spacing. A second pass may then target intermediate points between the primary locations.

For example:

Primary grid

●---------●---------●
|         |         |
|         |         |
|         |         |
●---------●---------●
|         |         |
|         |         |
|         |         |
●---------●---------●

A secondary pass can target the centre of the primary grid cells:

Primary points = ●
Secondary points = ○

●---------●---------●
|    ○    |    ○    |
|         |         |
●---------●---------●
|    ○    |    ○    |
|         |         |
●---------●---------●

This allows the energy to be distributed more uniformly through the treatment area.

Typical Grid Spacing

Published guidance indicates that DDC grid spacing can vary considerably with soil type, tamper size and required depth. A ground improvement reference gives approximately 2 to 6 m as a typical range for DDC grid spacing in applicable soils.

A Wyoming collapsible-soil case history used a 20-tonne tamper with a 1.22 m diameter and a 30.5 m drop height. Primary drop points were typically spaced at about 3.05 m by 3.66 m, with secondary points located at the centres of the primary grid cells. Five drops were typically applied at primary points and two at secondary points.

This illustrates an important point:
Grid spacing cannot be selected independently of tamper energy and soil conditions.

Number of Drops Per Point

The number of drops is another important variable. A typical sequence may involve:

• Primary pass: A high-energy tamper is dropped several times at each primary grid point.
• Secondary pass: Additional drops are made at intermediate grid points.
• Ironing pass: A lower drop height or lower-energy operation is used to compact the disturbed near-surface material and reduce surface irregularities.

DDC guidance notes that multiple passes may be necessary, particularly where excessive crater depths develop or where excess pore pressures require time to dissipate.

Published field experience also shows that the benefit of additional blows is not linear. Recent review work reports that much of the improvement often occurs during the early blows, with diminishing improvement from subsequent impacts. Therefore, simply increasing the number of drops is not necessarily the most efficient way to improve the ground.

Crater Formation

One of the most visible effects of DDC is the formation of craters. A large tamper may penetrate the ground significantly during the first impacts. The crater depth depends on:

• Tamper weight
• Drop height
• Soil density
• Soil moisture
• Grain size
• Groundwater
• Number of drops
• Existing ground condition

The craters are generally filled between passes or after completion of high-energy compaction. The volume of material required to fill the craters should be included in project planning and cost estimates.

Ironing Pass

The high-energy phase can leave the ground surface irregular. An ironing pass is therefore commonly used near the end of the treatment. The purpose is to:

• Compact the shallow disturbed zone
• Reduce surface irregularities
• Close large craters
• Improve near-surface uniformity
• Prepare the site for subsequent construction

The Geo-Institute describes the low-energy ironing phase as a possible final stage after high-energy compaction and subsequent crater filling/leveling.

SPT N-Value Before and After DDC

The Standard Penetration Test (SPT) is frequently used to assess changes in soil density. Before DDC, boreholes are drilled and SPT tests are conducted through the proposed treatment depth. The initial values establish the baseline condition.

After DDC, verification borings and SPT tests are performed at selected locations. The objective is not simply to obtain a higher N-value. The real engineering objective is to demonstrate that the treatment has achieved the specified:

• density
• strength
• bearing capacity
• settlement performance
• resistance to collapse
• liquefaction resistance, where applicable

SPT N-values should therefore be interpreted with appropriate corrections and with consideration of soil type.

Typical SPT Improvement

There is no universal relationship such as:
"DDC always increases SPT N by 5 blows."

The increase depends on initial density, soil type, moisture, fines content, impact energy and depth. Published case histories show that improvement generally decreases with depth.

One field study reported corrected SPT improvements of approximately 50–140% in the upper 4 m, 25–50% to approximately 11 m, and 10–25% below about 15 m. The same source reported that the strongest improvement occurred around 1–4/5 m depth and that the effect became marginal at greater depths.

For collapsible soils specifically, a published U.S. case-history study found that DDC can successfully treat cohesionless and low-plasticity collapsible soils, but clay layers can significantly reduce compaction effectiveness.

Therefore, a realistic article should report measured project-specific N-values, rather than applying one fixed improvement factor to all DDC projects.

Illustrative SPT Improvement Profile

Consider a hypothetical collapsible sand deposit with the following pre-treatment SPT values:

Depth below ground	Pre-DDC N	Post-DDC N
1.5 m	6	18
2.5 m	8	20
3.5 m	10	22
4.5 m	11	20
5.5 m	9	17
6.5 m	8	15
7.5 m	7	13

This table is illustrative, not a field case history. The pattern demonstrates a common behaviour: the largest improvement occurs in the shallower portion of the treated zone, while the increase becomes smaller with depth. For design documentation, actual project measurements should replace illustrative values.

A Published Wyoming Case History

One useful case history comes from Wyoming, where collapsible alluvial soils affected sections of Interstate 25. Because the collapsible layers were more than 20 ft thick, the Wyoming Department of Transportation selected DDC as the principal treatment method on several highway sections.

The reported work covered more than 850,000 ft² and involved more than 156,000 tamper drops. The typical equipment consisted of:

• 20-tonne tamper
• Approximately 1.22 m diameter
• 30.5 m drop height

The primary grid spacing was approximately 3.05 m by 3.66 m, with secondary drops between primary points. This is a useful example because the project dealt directly with thick collapsible soil rather than simply loose fill.

Southern Delivery System Water Treatment Plant

The Southern Delivery System (SDS) in Colorado provides another useful example of DDC being selected as a ground improvement solution. The Edward W. Bailey Water Treatment Plant was constructed as part of the Southern Delivery System. The project site presented challenging soil conditions, including collapsible aeolian deposits.

Project documentation states that extensive ground improvement would have been costly using conventional methods, and the project team selected DDC for the sedimentation drying basin area. The reported saving from using DDC was more than $1 million. The broader SDS program included a new water treatment plant and extensive pipeline infrastructure.

The case is useful from a construction-management perspective because the benefit was not simply a higher soil strength value. The DDC alternative reduced the quantity of conventional ground improvement required over a large footprint.

DDC Versus Over-Excavation

The conventional solution for a shallow or moderately deep collapsible layer is often:

1. Excavate the problematic soil.
2. Transport or stockpile the excavated material.
3. Import suitable fill if necessary.
4. Place the replacement soil in controlled layers.
5. Compact each layer.
6. Test the compacted fill.
7. Continue until the required elevation is reached.

This approach is reliable but becomes increasingly expensive as treatment depth and site area increase.

For example, assume:

• Treatment area = 20,000 m²
• Problematic layer = 6 m thick

The volume requiring treatment is:

$$V=A\times d$$ $$V=\mathrm{20,000}\times 6=\mathrm{120,000}{\text{ m}}^3$$

That means approximately 120,000 m³ of soil would have to be excavated if the entire problematic layer were removed. The actual project quantity can be substantially higher once excavation side slopes, working space and unsuitable material are considered.

Cost Components of Over-Excavation

The apparent excavation rate is only one part of the total cost. A realistic estimate should include:

$$C_{OE}=C_{exc}+C_{haul}+C_{disposal}+C_{fill}+C_{comp}+C_{testing}+C_{dewatering}$$

where:

• $C_{exc}$ = excavation
• $C_{haul}$ = transportation
• $C_{disposal}$ = disposal or stockpiling
• $C_{fill}$ = replacement material
• $C_{comp}$ = compaction
• $C_{testing}$ = quality control/testing
• $C_{dewatering}$ = dewatering, where required

The cost can increase rapidly when suitable replacement soil has to be imported from a distant source.

Cost Components of DDC

DDC has a different cost structure. Typical components include:

• Mobilization
• Crane and tamper
• Operator and crew
• Grid layout
• High-energy passes
• Secondary passes
• Ironing pass
• Crater backfilling
• Leveling
• Verification testing
• Monitoring

The Geo-Institute's published cost information gives an indicative DDC range of $10–$25 per square yard for projects exceeding 50,000 square yards, excluding some separate costs such as mobilization and crater backfill. It also notes that price is affected by tamper mass, grid spacing, crater depth, groundwater and production rate.

Older FHWA ground-modification guidance similarly shows that DDC cost depends strongly on tamper size, with larger tampers requiring more substantial lifting equipment. These figures should be treated as historical/indicative U.S. cost information rather than current construction rates in India.

The Common "$5/m³" DDC Figure

Some ground-improvement references give an approximate DDC cost of around $5/m³ under particular project assumptions. One ground-modification summary lists DDC as a low-cost method with an approximate cost of $5/m³.

This figure should not be directly compared with a current Indian rate for excavation and replacement. Construction cost is highly dependent on:

• Project size
• Equipment availability
• Mobilization distance
• Tamper size
• Grid spacing
• Number of drops
• Treatment depth
• Soil conditions
• Groundwater
• Fill source
• Haul distance
• Disposal distance
• Labour and fuel costs

For an actual project estimate, local quotations and project-specific quantities should be used.

Worked Economic Comparison

Consider a hypothetical site with:

• Area = 20,000 m²
• Collapsible layer = 6 m
• Problematic soil volume = 120,000 m³

Assume, for illustration:

Option A: Over-excavation
Suppose the combined excavation, haulage, disposal, replacement fill and recompaction cost is:
₹2,400/m³

Then:

$$C_{OE}=\mathrm{120,000}\times \text{₹}\mathrm{2,400}=\text{₹}\mathrm{288,000,000}$$

or:
₹28.8 crore
This is an illustrative calculation only.

Option B: DDC
Suppose a project-specific DDC quotation works out to:
₹500/m²

For 20,000 m²:

$$C_{DDC}=\mathrm{20,000}\times 500=\text{₹}\mathrm{10,000,000}$$

or:
₹1.0 crore

This large difference illustrates why DDC can become attractive for large-area ground improvement. However, this comparison is incomplete unless the DDC scope includes all associated costs such as:

• Mobilization
• Crater filling
• Leveling
• Verification testing
• Additional treatment passes
• Drainage measures
• Working platform
• Any required surcharge or preload

Similarly, the excavation option must include all actual excavation, disposal, replacement and compaction costs.

Why DDC Can Be Economical

The key economic difference is that DDC treats the existing soil in place. Over-excavation removes the soil and creates several additional material-handling operations.

Conceptually:

Over-excavation
Excavate → Load → Haul → Dispose → Import → Place → Compact → Test

DDC
Drop → Densify → Fill craters → Level → Test

This reduction in material movement is the main reason DDC can become economically attractive on large sites.

When DDC is Preferable

DDC is particularly attractive when:

• The problematic layer is relatively deep.
• The soil is granular or low-plasticity.
• The treatment area is large.
• Excavation would require substantial soil removal.
• Disposal areas are distant.
• Imported replacement fill is expensive.
• Groundwater does not prevent effective impact.
• Sufficient working space is available.
• Ground vibration can be managed.

Research on U.S. collapsible-soil case histories identifies DDC as an economical approach particularly where collapsible soil extends deeper than about 3–4 m.

When Over-Excavation May Be Better

Over-excavation can remain preferable when:

• The problematic layer is shallow.
• The treatment area is small.
• The soil is unsuitable for DDC.
• Thick clay layers are present.
• The required improvement is very shallow.
• There are vibration-sensitive structures nearby.
• Excavated soil can be reused conveniently on site.
• Suitable replacement material is readily available.
• The excavation can be performed safely without major groundwater problems.

DDC is not a universal replacement for excavation.

Limitations of DDC

Several limitations need to be considered during design.

1. Clayey layers
Clay can absorb impact energy and reduce the depth and degree of improvement. U.S. case-history research found substantially better performance in cohesionless and low-plasticity collapsible soils than in clayey layers.

2. Groundwater
High groundwater levels can change the response of the soil and may reduce the effectiveness or alter the construction sequence.

3. Vibration
Heavy impacts generate ground vibrations. Nearby buildings, pipelines, underground utilities, railway tracks, and sensitive equipment may require vibration monitoring and exclusion distances.

4. Noise
Repeated high-energy impacts generate considerable noise and may create restrictions in populated areas.

5. Surface settlement
DDC deliberately produces ground deformation. The expected surface settlement must therefore be incorporated into grading and elevation calculations.

6. Crater filling
Large craters may require significant quantities of suitable material for backfilling and leveling.

Quality Control During DDC

DDC should be treated as a performance-based ground improvement operation rather than simply a sequence of tamper drops. Important field observations include:

• Crater depth: Crater depth after each set of blows provides an indication of ground response.
• Ground heave: Heave between impact points may indicate lateral soil movement.
• Surface settlement: Survey measurements help determine the amount of densification achieved.
• Pore-water pressure: In saturated or partially saturated fine-grained soils, pore pressure monitoring may be necessary.
• Vibration: Seismographs can be used near sensitive structures.
• SPT/CPT verification: Post-treatment investigation is required to demonstrate the improvement achieved.

Pre- and Post-DDC Testing

A good DDC program normally begins with baseline testing. Typical investigations include:

• Boreholes
• SPT
• CPT/CPTu
• Laboratory classification
• Density testing
• Moisture content
• Collapse testing
• Groundwater observations

After treatment, the same or comparable investigation methods are used to evaluate improvement. The comparison should ideally consider:

$$\text{Improvement ratio}=\frac{N_{post}}{N_{pre}}$$

or percentage improvement:

$$I_N=\frac{N_{post}-N_{pre}}{N_{pre}}\times 100$$

For example, if $N_{pre}=8$ and $N_{post}=20$, then:

$$I_N=\frac{20-8}{8}\times 100=150\%$$

Again, this is a mathematical illustration, not a prescribed DDC performance criterion.

SPT Interpretation Requires Care

Raw SPT N-values should not automatically be compared without considering:

• Hammer energy
• Borehole diameter
• Rod length
• Sampler configuration
• Overburden pressure
• Equipment type

For engineering interpretation, corrected values such as ${\left(N_1\right)}_{60}$ may be more appropriate where applicable. This is especially important when comparing pre- and post-treatment investigations performed by different drilling crews or equipment.

Designing a DDC Test Section

Before treating a large site, a trial area is often highly useful. A test section can be used to determine:

• Appropriate tamper weight
• Drop height
• Grid spacing
• Number of drops
• Number of passes
• Crater depth
• Surface settlement
• Improvement depth
• Post-treatment SPT/CPT response

The test section reduces uncertainty before full-scale production. A practical approach is:

Select trial grid → Apply planned energy → Measure response → Perform post-treatment testing → Adjust energy/grid → Finalize production procedure

This is usually more reliable than relying entirely on an empirical equation.

DDC Design Example

Consider a collapsible sandy deposit requiring improvement to approximately 7 m depth. Assume a preliminary design uses:

• Tamper mass = 14 tonnes
• Drop height = 21 m
• Primary grid = 4 m × 4 m
• 5 drops per primary point
• Secondary grid at intermediate points
• 2 drops per secondary point
• Final ironing pass

Energy per primary drop:

$$E_d=14\times 21=294\text{ t·m}$$

Energy per primary point:

$$E_p=294\times 5=1470\text{ t·m}$$

For a 4 m × 4 m grid:

$$A_g=16{\text{ m}}^2$$

The primary-pass energy density is therefore:

$$AE=\frac{1470}{16}=91.9{\text{ t·m/m}}^2$$

This is only the primary-pass contribution. Secondary passes and ironing energy must be added when calculating the total applied energy. The actual production energy would then be adjusted based on field performance.

DDC and Collapsible Soil Mitigation

The engineering objective is to ensure that the soil remaining below the structure does not undergo unacceptable collapse after construction. The treatment should therefore be designed around a performance criterion such as:

• Maximum allowable post-construction settlement
• Required corrected SPT value
• Required CPT resistance
• Required relative density
• Required collapse potential
• Required bearing capacity
• Required liquefaction resistance

The DDC energy should be considered a means to achieve the required ground condition, rather than the final design criterion itself.

DDC vs Over-Excavation: Engineering Comparison

Factor	DDC	Over-excavation
Principle	In-situ densification	Removal and replacement
Suitable depth	Several metres and potentially deeper	Usually more attractive for shallow layers
Soil movement	Low	Very high
Imported fill	Usually limited	Often substantial
Disposal requirement	Low	Potentially high
Large-area treatment	Attractive	Can become expensive
Vibration	Significant	Generally much lower
Noise	High	Moderate
Quality control	SPT/CPT, settlement, energy monitoring	Density and layer testing
Groundwater sensitivity	Can be significant	Can require dewatering
Clayey soil	Often less effective	More predictable
Granular/collapsible soil	Often effective	Effective
Mobilization	Specialized crane/tamper	Conventional excavation equipment
Cost behaviour	Strongly influenced by area and energy	Strongly influenced by volume and haul distance

Practical Design Checklist

Before selecting DDC for collapsible soil, the geotechnical engineer should establish:

• Site investigation: Soil profile, Depth of collapsible layer, Groundwater level, Grain-size distribution, Fines content, Plasticity, Initial density, Initial SPT/CPT values, Collapse potential
• DDC parameters: Tamper weight, Drop height, Tamper geometry, Grid spacing, Number of drops, Number of passes, Target applied energy, Expected crater depth
• Verification: Post-DDC SPT/CPT, Settlement monitoring, Ground survey, Collapse testing where required, Vibration monitoring, Acceptance criteria
• Cost: Mobilization, DDC operation, Crater backfill, Imported fill, Testing
• Excavation alternative: Haul distance, Disposal, Programme duration

Final Takeaway

Deep Dynamic Compaction is particularly useful when a large area contains loose or collapsible soil extending several metres below the ground surface. Instead of removing the soil, transporting it and replacing it with compacted fill, DDC uses repeated high-energy impacts to densify the existing soil in place.

The basic physics begins with the energy of each drop:

$$E=W\times H$$

but the actual performance depends on much more than drop energy. Tamper weight, drop height, grid spacing, number of drops, number of passes, soil type, groundwater and initial soil density all interact.

Field verification is therefore essential. Pre- and post-DDC SPT/CPT investigations can demonstrate the change in ground condition, but the improvement should be evaluated using corrected test values and the project's actual performance requirements.

For large areas of deep collapsible soil, DDC can have a major economic advantage over full-depth over-excavation because it avoids much of the excavation, hauling, disposal and replacement-fill operation. The Southern Delivery System Water Treatment Plant provides a useful project example where DDC was selected for difficult soil conditions and reportedly saved more than $1 million compared with conventional ground improvement.

The important engineering principle is therefore simple:
If the problematic soil is deep, extensive and suitable for dynamic compaction, densifying it in place can be considerably more efficient than removing the entire layer.