Figure 1: Soil Profile as per SI(A)

Problem Statement

Taking into account the soil conditions, earthwork requirements and constrained schedule, optimized strategy for soil improvement and fill compaction requirement was identified. Removal and replacement, followed by conventional layer-by-layer roller compaction would prove expensive and prevent meeting the time schedule.

Observing the soil suitability recommendations by Braiek (2017) and Han (2015), two prominent techniques may be implemented as shown in Figures 5 & 6:

1. Rapid Impact Compaction
2. Dynamic Compaction

A brief description with the principle of both techniques is given as follows:

Rapid Impact Compaction: Rapid Impact Compaction is a cheaper and faster alternative to conventional roller compaction, typically implemented for shallow compaction of soil within a thickness of 5m. The technique is implemented on granular or coarse-grained soils, however, it may be implemented for soil types with fine content up to 20%. The principle of Rapid Impact Compaction involves compaction of soil, as a result of energy transferred by the repeated free fall of a hammer, on a set grid spacing. The number of blows, height of fall and the set grid are optimized in real-time, as a function of the behaviour of the ground reaction and penetration, to deliver the most optimum results. A work sequence of the Rapid Impact Compaction technique is shown in Figure 3.

Dynamic Compaction: Dynamic Compaction is a cheaper and faster alternative to Vibro-Compaction (VC), typically implemented for treatment of soils within a thickness of 8-10m. The technique is implemented on granular or coarse grained soils, however, it may be implemented for soil types with fine content up to 30%. The principle of Dynamic Compaction involves compaction of soil, as a result of energy transferred by the repeated free fall of a Heavy Pounder, on a set grid spacing. The number of blows, height of fall and the set grid are optimized in real-time, as a function of the behaviour of the ground reaction and penetration, to deliver the most optimum results. A work sequence of the Dynamic Compaction method is shown in Figure 4.

In an attempt to meet the short project duration, an optimized strategy had to be defined. The project design consisting of the fol-lowing criteria to be achieved by soil improvement works.

Settlement and Bearing Capacity Criteria

An embedment of 1.3m was to be considered for the following foun-dation designs shown in Table 2.

Table 2: Foundations considered for Analysis

Liquefaction

The following liquefaction criteria was to be achieved after soil im-provement works.

• Minimum Factor of Safety against Liquefaction: 1.2
• Peak Ground Acceleration (PGA): 0.07g
• Magnitude (M): 4.0

The assessment of Liquefaction Risk Potential was to be carried out implementing Youd et al. (2001). Within the summary report by Youd et al. (2001), it is explained how the Cyclic Stress Ratio is determined (implementing average shear stress and stress reduc-tion factor by Seed and Idriss (1971) and Blake (1996), both cited in Youd et al. (2001)). Following which Cyclic Resistance Ratio is calculated for a 7.5 magnitude earthquake (using Rauch (1998) as cited in Youd et al. (2001)). A Magnitude Scaling Factor (after Youd et al. (2001)) is applied to the applicable design and the Factor of Safety is assessed, where resistance is larger than the stress pre-dicted.

Fill Compaction – Performance Line

The compacted fill was to meet required density based on project specifications. Minimum relative density of 85% after fill compac-tion works was required (fill thickness of 3m).

Relative density is estimated by reverse calculating the required degree of relative density (Rd) to cone tip resistance (qc) values based on a standard, thereby creating performance lines and com-paring them to the post compaction cone resistance values. For the proposed improvement strategy explained herein, a performance line for qc values has been developed according to three correlation formulas for relative density of 85% as shown in Figures 7 & 8.

Hypothesis

In the current hypothesis section, the authors detail the prelimi-nary scenario, predicted duration and closing to meet the 6-month time duration.

Reviewing data from previous studies, Table 3 summarizes general parameters for both techniques.

Table 3: Dynamic Compaction and Rapid Impact Compaction General Production Parameters

Note:

• Energy = (drop height x weight x number of drops) / soil volume expecting compacted.
• Number of passes should be kept as less as possible, preferably two, but can exceed based on soil conditions (Lukas, 1995).
• The depth of improvement assessment of Dynamic Compaction cannot be applied to Rapid Impact Compaction, instead categorical depth of improvement based on soil type and energy applied should be considered (Han, 2015).

Figure 8: Target Performance Lines (Rd = 85%) for cone tip resis-tance (qc) as minimum

The aim was to attain a minimum of 1,154m² of working days (26 days per month) production, with the added time allotted for earth-work filling works.

Equipment were in vicinity of the site and the mobilization and set up task could be completed in one working week. As per Khan (2019), the rate of production for both techniques may be assumed as follows,

• Rapid Impact Compaction: 75,000 m² / month (per rig under one shift per day)
• Dynamic Compaction: 35,000 m² / month (per rig under one shift per day)

Duration was to be slightly adjusted further for site conditions, soil conditions and other supplementary factors.

In attempt to provide a value engineered design, supplementary pre-investigation works were required. The same would then be reviewed along with the earthwork design, to effectively segregate areas where either technique would be applicable. In addition to the same, where applicable, the techniques would be designed in combination with one another, to boost production and provide a value engineered design.

Method – Ground Improvement

The designed testing regime consisted of varying frequency of Piezocone Penetration tests (CPTu) with respect to the areas desig-nated either for structures or for roads. To simplify the same, a fre-quency of 4,000m² can be considered for each Pre-CPTu as shown in Figure 9.

Based on the results of the pre-investigation campaign by the soil improvement contractor and the earthworks design, the entire project area was segregated into the following soil improvement scenarios as given in Figure 11.

Note: The chart above represents cross-sections of improvement methodology deployed during actual works. The elevations mentioned, are related to the cross-sections with respect to the existing ground level considered 0.0 for the sake of easier understanding. The Final Ground Level on the project ranged from +7.0m to +10.5m.

The estimated quantities of the scenarios mentioned in Figures 10 & 11 were as follows:

• Dynamic Compaction after fill placement: 104,000m²
• Rapid Impact Compaction after fill placement: 76,000m²
• Dynamic Compaction and Rapid Impact Compaction inCombination

Stage 1: Dynamic Compaction to improve existing soil – 16,000m²

Stage 2: Rapid Impact Compaction for fill compaction – 16,000m²

A preliminary project duration analysis was conducted in-line with the production details previously mentioned, which were further adjusted to site conditions and supplementary factors. The project duration was limited to 6 months as required and a Gannt Chart for the same is presented in Figure 12.

Based on the results of the project duration analysis, it was deter-mined, implementing both Rapid Impact Compaction and Dynamic Compaction, independently and in combination with one another, with just one rig for each technique, the project could successfully be complemented in a little under 5 months. Taking a safe estima-tion of two working weeks for Calibration Works, the preliminary project duration was assumed to be 5.2 months which was in-line with the fast-track 6-months project requirement.

The basis of designing operation parameters for Dynamic Compac-tion and Rapid Impact Compaction techniques are detailed in Table 3. Based on the same, the following were deduced (subject to ad-justment and further calibration during Trial Works).

In the exercise, general parameters based on soil type, expected soil behavior and required depth of influence were assumed, while trying to meet energy requirement for both techniques based on minimum energy required, as mentioned in Table 3.

Dynamic Compaction

$$260.89=\frac{15\times 20.87\times 15\times 2}{{(6)}^2}\text{ (2)}$$

Using a 23 Ton pounder, with 15m height of drop, over a 6 x 6 m² grid spacing, for a total of 15 blows and 2 passes (discounting the ironing pass), the required energy can be attained. Dynamic Com-paction works were further calibrated during Trials with parame-ters in a similar range.

Rapid Impact Compaction

$$150=\frac{60\times 14.51\times 1.1\times 2}{{(3.5)}^2}\text{ (3)}$$

Using a 16 Ton Hammer, with 1.1m height of drop, over a 3.5 x 3.5 m² grid spacing, for a total of 60 blows and 2 passes (discounting the ironing pass), the required energy can be attained. Rapid Im-pact Compaction works were further calibrated during Trials with parameters in similar ranges.

Results and Discussion

Calibration works were carried out for both techniques considered in the study. Three grid spacing were considered for the trials of both techniques as follows,

• Dynamic Compaction
• Grid A : 5.0 x 5.0 m²
• Grid B : 5.5 x 5.5 m²
• Grid C : 6.0 x 6.0 m²

• Rapid Impact Compaction
• Grid A : 3.5 x 3.5 m²
• Grid B : 4.0 x 4.0 m²
• Grid C : 4.5 x 4.5 m²

Successful completion of calibration works identified optimum production parameters shown in Table 4, i.e. parameter with which the required soil improvement can be achieved while avoiding overdesign of operations.

Table 4: Operation parameters adopted for general production works

Note: General production parameters may vary based on soil conditions and soil behavior during actual soil improvement works

Figure 13 (a & b) illustrate the efficiency of applied improvement works during calibration works for both techniques considered.

Soil Improvement works lasted 4.5 month. Verification of design criteria (as mentioned in section 3) were performed following guideline references as mentioned below:

• Bearing Capacity Assessment: Eslaamizaad and Robertson (1996).
• Settlement Assessment: Schmertmann et al. (1978).
• Relative Density: As mentioned in section 3.
• Liquefaction Risk Assessment: Based on the summary report on evaluation of the liquefaction resistance of soils by Youd et al. (2001) as briefly described in section 3.

The achievement of design criteria is shown in Figures 14 to 16.

Following soil improvement works, all design criteria requirements were met by all performed Post CPTu’s, proving the value engi-neered design of combination of Dynamic Compaction and Rapid Impact Compaction for the current project, a success in terms of both, the efficiency in improving the soil and a time efficient strat-egy.

Future Research

Similar to the chart attached in Figure 5 (Braiek, 2017), the authors devised a similar chart to determine value engineering soil im-provement technique based on applicable soil conditions as shown in Figure 17.

Figure 17 includes the inclusion of High Energy Impact Compac-tion (HEIC) and Rapid Impact Compaction (RIC) to the pre-existing chart as cost-effective solutions to granular soils. In terms of future studies, the authors suggest research into a new technique, which may be called Rapid Impact Replacement (RIR). The aim of the RIR technique is to provide a cost-effective solution to the treatment of soft cohesive soils to shallower depths, compared to existing tech-niques in the industry today. The authors envision an evolution of the technique from Rapid Impact Compaction, similar to the evolu-tion of Dynamic Compaction to Dynamic Replacement.

Conclusion

In the current paper, a case study was presented for a fast-track project, which required the application of soil improvement works and fill compaction works in combination. Although a number of techniques exist in the market, in an attempt to meet the strict time duration constraint, the authors devised a value engineered strat-egy, involving the combination of Dynamic Compaction and Rapid Impact Compaction techniques. A review of the strategy, the design and the project duration analysis was presented in the paper. Fol-lowing the same, the results of the campaign were provided, verify-ing the design of the strategy wherein, the project duration require-ment was met, and the required design criteria was achieved.

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