Background

Reviewing the data and details of the project, it was theorized the deformation occurred due to the presence of water sensitive soils in the fill profile imported locally, knowing the plant was situated in a location of North of Riyadh known for its presence of collapsible soils [1]. Collapsible soils are bonded soils with an open structure that where the particles are in metastable condition. Once collapse agents are active, the bonds break, resulting in a substantial decrease in shear strength, disintegration of the soil skeletal structure and the subsequent deformation [2]. As per [3, 4] collapse instigated requires two agents/conditions, i.e. wetting and loading in combination, application of either one without the other will not result in a collapse of the full potential.

Collapsible Soils are majorly of two types, Aeolian Soils and Residual Soils. In the case of collapsible soils in Riyadh, the properties exhibited are more in line with those of Aeolian soils and they are granular soils with little SILT and medium to well cementation. The cementation found is in varying degrees, but in more common cases, the soil is well cemented with stiffness in the range of dense to very dense state, when in a dry state. The same soil exhibits substantial collapse upon wetting and a present load. An example of collapse potential for a site located in Riyadh is given in (Figure 1) [5, 6].

Figure 1: Example of Collapsible Soil potential observed at a site in Riyadh (ACES, 2019)

Another way to identify collapsible soils is through their uncharacteristically low dry density. A preliminary direct test using field density tests can be a convenient way for early identification of presence of collapsible material in the geo-profile [7]. produced a chart to evaluate collapse potential on the basis of 2 parameters, the dry density and the liquid limit of the sample tested, as a function of specific gravity [8]. later verified the accuracy of the chart, however it should be noted that the method does not guarantee collapse nor the degree of potential collapse; it is primarily used for preliminary identification. (Figure 2)

Geotechnical solutions* to treat collapsible soil include the following [2, 6],

(*) The above-mentioned methods are solutions to treat collapsible soils, alterations to Foundation and Foundation types are not considered in the current study.

Problem Statement

Following a heavy rainfall event and limited ponding, geological failure of excessive settlement occurred beneath the already casted concrete pavement. Substantial settlement of more than 15cm in most places were observed. At most, the recorded settlement was 17.5cm, indicating a non-uniform mechanism of deformation. (Figure 3)

Reviewing remediation measures, geotechnical analysis of the soil was overlooked initially as the soil was supposedly compacted to 95% modified proctor dry density. Upon further internal discussions and desk-study, geotechnical analysis of the soil was considered.

A concern with any activity that was to be conducted in the current site, is the presence of an underground concrete tunnel approximately 10m below the area of concern. (Figure 4).

Hypothesis

Preliminary understanding was that the geo-disaster occurred as result of heavy rainfall and ponding (see Figure 5). Therefore, design and earthwork plans were made to prevent ponding and assist in drainage.

The initial remediation works involved the following steps with some difficult to achieve criteria as given below.

Realizing the problem to be of a geotechnical nature. The Contractor of the project approached a specialist soil improvement contractor for their input and suggestive action in ensuring a future heavy rainfall event does not pose a potential hazard.

Method – Ground Improvement

Reviewing data provided and pre-existing knowledge of collapsible soils in vicinity, the geotechnical contractor suspected something a-miss, considering the soil had been compacted to a 95% M.D.D. The potential of collapse should be negligible compared to original collapse in-situ [1], additionally the dry density would’ve been uncharacteristically low. Soil investigation tests involving Cone Penetration Tests (CPTs) were carried out to better understand the geological conditions on site. Light green colour signifies the presence of SILT layers while dark green signifies CLAY layers. (Figure 7 and 8)

Of the techniques implemented in the field of compaction, roller compaction is one of the more established and conservative technique that exists presently. However, applicability of the technique is greatly influenced by the material expecting compaction. Roller compacting cohesive material requires different roller types, very stringent control of moisture and is a considerably difficult operation compared to select fill of granular material (Han, 2015). In a general sense, the works conducted was greatly incompatible with the material placed, in terms of operation and equipment employed. Appropriate Quality Assurance and Quality Control would prevent the occurred geo-disaster, as anomalies would have been realized at the following stages.

Due to constraints in soil type and allowable vibrations (underground concrete tunnel), Vibro-Replacement or Stone Columns was selected as the most applicable technique, due to its flexibility with soil type, deep depth of improvement and limited vibrations compared to Dynamic/Impact techniques. More importantly Stone Columns’ efficiency is exemplified in mitigating collapse potential as suggested by [9] study on samples collected from collapsible soils from Borg Al-Arab area, Alexandria, Egypt [Han, 2015, 10].

Majority of the previous studies reviewed implemented models or simulations to verify the efficiency of stone columns in treating collapsible soils. Stone columns can be constructed in two ways, the dry method or the wet method (the dry method is not applicable in all locations, due limitations in penetration). As suggested by [2] and internal analysis by the contractor’s technical team, by performing the wet method of penetration a better induction of pre-collapse was predicted via simulation and a large collapse was not envisioned. Vibro-flotation can greatly enhance pre-collapse mechanism; however, the operation should be performed vigilantly, as additional settlement may occur if the layers are in efficiently saturated [2].

Apart from the induced collapse and partial replacement, the stone columns created were end bearing i.e. stone columns executed were until refusal. (Figure 9)

The steps taken in the process of design were as follows (there is no fixed standard for the design of stone columns in collapsible soils, researchers may choose their own strategy depending on the respective site conditions):

The soil at the site was determined to possess collapsible potential of the medium category. The decision to validate the integrity of the soil during stone column construction is based on both the expected soil behaviour and judgment of the engineer. Unfortunately, Plaxis does not take into account matric suction which is basically one of the governing criteria in the unsaturated to saturated behaviour of the soil. Implementing the Soil Water Characteristic Curve, the geotechnical designer modelled the effect of inundation in different stages, each representing a state of saturation and respectively adjusted geotechnical properties of the soil layers. Cavity Expansion method was used to model the installation of a single column to effectively simulate the generation of pore pressures in the soil layers and subsequent behaviour. The created model was made with multiple assumptions so there was a need for a trial. The decision was of the design engineer to give a judgment if the generated in-house model and results were acceptable or a geotechnical failure could be foreseen. Based on the fact that the soil was of medium collapsible potential and the favourable simulation results, the final decision was that the soil would have sufficient lateral shear strength to allow the construction of ordinary stone columns, subject to a trial column construction. A trial with the installation of a single column was made prior to the actual production works.

Design of the columns were carried out as per DIN 4017-1:2006- 3 [11] for the estimation of Bearing Capacity and [12] for the calculation of estimated settlement of end-bearing stone columns. The design was subsequently verified using Elastic Theory [13]. A summary of the composite geotechnical properties and the results of preliminary analysis are given below. [Table 1]

Table 1: Composite Soil Geotechnical Properties

In relation to Table 1, the abbreviations and symbols are described below,

(Figure 11) It should be noted, adequate analysis and numerical modelling should be undertaken prior to construction. As mentioned in Section 2, if substantial loss of lateral shear strength is envisioned, alternative techniques may be reviewed, such as geo-synthetic encased stone columns. Further studies are encouraged involving implementing Stone Columns in the treatment of collapsible soils, as presently the topic is to a certain extent unexplored. (Figure 12) gives an insight on the available research/study that exist presently.

Stone Column were designed on the comparably worse soil conditions as given below. [Table 2]

Table 2: Soil Profile considered for Improvement

The design criteria to be achieved following construction was as given below,

• Allowable Bearing Capacity > 100kPA
• Allowable Settlement limit < 50mm

In line with the requirement and conditions, Stone Columns of the configuration mentioned in [Table 3] were designed.

A critical aspect of the operation is controlled vibrations with respect to the underground concrete tunnel. British Standards Institution

Table 3: Stone Columns’ Design Parameters

(1993) recommends working limits for allowable peak particle velocity as given in Table 4

Table 4: Vibration Monitoring Limits suggested by British Standards Institution (1993)

The maximum allowable limit was set at 20mm/s. Tests were conducted during Trial Works at 3 different locations as follows.

Results and Discussion

Prior to actual operations, trial works were carried out for specific columns with vibration monitoring in parallel at the locations specified in the previous section. Testing was carried out using NOMIS Mini graph 7000.

Results of the monitoring were satisfactory, with the highest recording below the 20mm/s PPV limit with the results as given in Table 5.

Table 5: Soil Profile considered for Improvement

Stone Column construction was carried out using the wetmethod followed by a successful trial as discussed in Section 5. The entire operation was a time effective solution. The complete setup, from mobilization until completion, lasted 18 working days, with the actual works completed in just a total of 7 working days as shown in (Figure 14).

To put things in perspective, for the conservative approach, after the demolition of the existing concrete pavement, all the material would have to be excavated, removed and replaced. The replaced material would have to be of higher quality i.e. AASHTO A-1-a, to be compatible with Roller Compaction. Apart from a slower production, the conservative approach would also prove as an expensive alternative.

Following the construction of the columns, quality control Plate Load Test (PLT) was performed. The maximum applied load was 1.5 times the design load. The results of the test were satisfactory and in line with the design criteria as illustrated in (Figure 15).

With the help of a load test, the designer is able to verify the column has mobilized and was not affected due to the presence of collapsible soil and in-return reinforced the soil profile and mitigated the risk of collapse potential.

Future Research

The authors suggest the involvement of geotechnical engineers involved in Horticulture to provide their valuable input and expand existing knowledge further regarding collapsible soils. Horticulture engineers are quite well versed with subject matters such as unsaturated soil mechanics, soil-water partial saturation characteristics and behaviour, infiltration and evaporation of groundwater, etc., all of which are closely related to understanding collapsible soils.

Another suggestion is a subject for future research, involving verifying the efficiency of impact-based compaction to highly cemented collapsible soils. The presence of collapsible soils with strength that are very high in strength (NSPT > 50), usually mistaken for a type of rock or intermediate geomaterials, are usually identified in Riyadh. Compaction is a cost-effective method in treating collapsible soils; however, most the research available presently includes soil of medium to medium-well cemented soils, examples given below.

The efficiency of compaction decreases significantly with the presence of stiff geo-material (Han, 2015). The research could include a comparative analysis of prewetting in combination to compaction and no pre-treatment compaction, with the level of efficiency verified for both operations.

Conclusion

The current paper emphasized the vulnerability of inadequate poor execution and quality control to a well-designed plan, demonstrated in the case study with the occurrence of a geodisaster event. Layer wise roller compaction is the more conservative approach and in the case of the current project with fill thicknesses greater than 10m, it was likely even a more expensive option. Investigations conducted revealed the presence of largely unsuitable material (very high fine content) for the type of compaction carried out. The operation was unsuccessful due to insufficient quality of works.

Apart from the above, the geo-disaster failure mechanism indicated soils possessing collapsible potential present in the case study area. The most applicable ground improvement technique was chosen as Stone Columns (or Vibro-Replacement) due to its flexibility with applicable material, comparably lower vibration propagation compared to alternative techniques. Studies regarding proven efficiency were limited, with a number of researchers opinionated with varying degrees of success. Taking the recommendations of most researchers in the type of operation to be adopted (Wet-Method), the modifications to general operations in the case of suspected collapsible material (wetting used for penetration stage to be extended and careful vigilance to ensure uniform wetting) and the design of the final columns (columns to be end-bearing in design), ensured mitigating risk of suspected collapsible soils, usually found in Riyadh (KSA).

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