Geotechnical Characterization of the Magdalena River Subsoil in Magangué, Colombia: A Study Using CPTu and SPT Tests

Abstract

This study employs Cone Penetration Tests (CPTu) and Standard Penetration Tests (SPT) to analyze the geotechnical properties of the Magdalena River’s riverbed and banks. While these methods are standard in soil characterization, this research innovatively combines CPTu’s continuous profiling with SPT’s localized sampling to develop a nuanced stratigraphic model of the subsurface. This integrated approach provides a comprehensive view of the soil conditions, which is crucial for understanding sediment variability and stability along the riverbanks. The findings from this methodological integration enhance our ability to predict soil behavior under dynamic riverine conditions, offering valuable insights for erosion control and sustainable river management. The study underscores the practical benefits of synergizing traditional testing methods to address geotechnical challenges in river environments.

1. Introduction

Soil and water represent fundamental natural resources for sustaining life on our planet. Soil provides the nutrients for flora and fauna and is the foundation for global food production. On the other hand, water plays a vital role in maintaining ecosystems and supporting human civilizations. In hydrology, soil and water interact through surface runoff; excess water from precipitation in a watershed drains downhill to feed streams and rivers [1]. The runoff accumulates along the lower areas, forming concentrated flows, which can produce erosion rills or gullies [2]. Erosion involves the general lowering of the riverbed because of the increased capacity of the current to dislodge and transport material from the bed during a flood event [3].

The type of erosion primarily depends on river morphology, and the watershed’s hydrological and geological characteristics influence the channel’s shape. With global warming, extreme rainfall and severe weather are becoming increasingly common, leading to soil nutrient depletion, destruction, and loss of land productivity. This dramatically accelerates erosive processes, resulting in the deterioration of the riverbank and posing a threat to various urban settlements in encroached areas along the active river corridor [4]. Riverbank erosion has become a community-wide phenomenon and is the primary natural hazard. Numerous people face complications yearly from this activity [5]. This occurs due to a combination of hydraulically induced erosion at the banks’ base and subsequent slope failure or collapse. The composition and properties of the soil can also influence this failure [6]. According to a preliminary analysis of the International Research and Training Centre on Erosion and Sedimentation (IRCTES), annual soil erosion in the world’s river basins totals 60 billion tons.

According to a preliminary analysis published by the International Research and Training Centre on Erosion and Sedimentation (IRCTES), annual soil erosion in the world’s river basins amounts to 60 billion tons. Between 5 and 7 million hectares of farmland deteriorate annually. Although the increase in these disasters is often attributed to climate change, there is evidence in Colombia and worldwide that the leading cause is alterations to the Earth’s surface caused by deforestation, agriculture, mining, urban expansion, and infrastructure construction [7]. In the village of Palo de Agua, municipality of Lorica, Colombia, recurrent flood threats and the formation of critical points due to river erosion in the lower Sinú are causes for concern due to the social and environmental vulnerability directly affecting the residents’ quality of life [8]. The village of Amanaven, belonging to the municipality of Cumaribo in the department of Vichada, converges with the Guaviare River, where improper land use and other human activities have accelerated erosion [9]. Additionally, in the municipality of Salamina, the Magdalena Department, more than 5000 homes are on the brink of being swallowed by the Magdalena River due to river erosion. About 78% of the Magdalena River basin is experiencing critical erosion due to deforestation, livestock farming, agriculture, fishing extraction, and energy production [2].

Recent studies have highlighted the critical role of soil management practices in mitigating erosion along riverbanks and estuarine environments. Soil reinforcement techniques using cement and alkaline solutions have shown promising results in enhancing the structural integrity of granite residual soils, which are prevalent in riverine settings [10]. Additionally, using sustainable polymers to reinforce recycled granite residual soils improves the mechanical properties and contributes to environmental sustainability by reducing waste [10]. These reinforced soils demonstrate increased resistance to erosive forces, thereby aiding in stabilizing riverbanks and reducing sediment load in rivers. Furthermore, incorporating fibers into the soil has significantly enhanced shear strength and erosion resistance, providing a robust method for combating soil erosion in flood-prone areas [10,11]. Collectively, these studies underscore the importance of adopting innovative and sustainable soil management practices to preserve riverine ecosystems and prevent the detrimental effects of soil erosion.

Like the cases mentioned above, many more have been intensifying over time, harming ecosystems and human settlements. Hence, there is a need to study these natural phenomena to develop various alternatives to mitigate these processes. Rakib et al. [12] utilized a channel restoration model using satellite images from 1980 to 2021 to assess the morphological characteristics of the Brahmaputra River by measuring erosion and accretion areas. Koohizadeh et al. [13] analyzed the morphological changes of the Bazoft River over thirty years, from 1985 to 2015, utilizing Landsat satellite images. Soil erosion hazards were prioritized using a simple erosion modeling tool, employing the universal soil loss equation and geographic information software [14]. Wang et al. [6] proposed a one-dimensional model to simulate both channel evolution and bank deformation; the calculated erosion and accretion widths closely matched measured data, indicating that the model can accurately simulate flow and sediment transport processes. Geological and geotechnical conditions of the Zirag River dam were investigated and characterized by Birhanu et al. [15] using drilling data, laboratory tests, and core samples; based on their findings, recommendations were made to reduce erosion and filtration problems.

The Standard Penetration Test (SPT) is a globally utilized in situ test method for evaluating soil properties through blow counts (N-values), which are influenced by soil characteristics and the energy transferred during hammering. The study by Ji et al. [16] highlights significant variations in N-values due to differences in drop hammer systems, with European and American researchers emphasizing the role of hammer energy transfer efficiency. This study focuses on the quantitative assessment of energy transfer efficiency in China’s SPT systems, finding an average efficiency of 78.7%, suggesting potential improvements in domestic practices. Historically, Schmertmann and Palacios [17] introduced the concept of energy efficiency ratio, indicating its inverse relationship with N-values, with Seed et al. [18] recommending a 60% benchmark for global comparisons. Recent studies underscore hammer energy variability across different systems and its impact on test outcomes [19,20]. Understanding these dynamics is crucial for aligning Colombian practices with international standards and enhancing the reliability of soil evaluations in diverse geotechnical projects. In Colombia, it is essential to establish efficiency criteria for the SPT in extreme conditions, such as on river slopes or flooded areas. Various research efforts have examined the efficiency of the Standard Penetration Test (SPT) in Colombia. Salamanca-Medina and López-Domínguez [21] conducted a comprehensive study correlating SPT results with the Dynamic Probing Super Heavy (DPSH) test. Their findings, based on 129 data points from different geotechnical projects across Colombia, indicate that the SPT remains a reliable method for soil characterization, particularly when supplemented by the DPSH for more diverse soil textures.

Additionally, Ruge et al. [22] introduced the SPT with Torque (SPT-T), which combines SPT with vane shear test (VST) principles to provide enhanced insights into lateral friction and soil structure, proving particularly effective in sedimentary soils. This innovation demonstrates the ongoing evolution and refinement of SPT methodologies to increase their accuracy and efficiency in various soil conditions in Colombia. Finally, the study by Santos and Bicalho [23] highlights the global relevance of these tests and their applications in local contexts, further reinforcing the adaptability and efficiency of the SPT in Colombian geotechnical practices. The cone penetration test, known as CPT, utilizing load cells, was heavily developed in the 1970s and has experienced widespread acceptance due to its precision, continuous data acquisition, and detailed nature of the data collected. Subsequently, with the introduction of pressure transducers capable of measuring pore pressure during penetration, the test became called CPTu. The main measurements recorded by a computer are cone tip resistance (q_c, q_t), sleeve friction (f_s), and pore pressure (U₂). In soft clays and silts and overwater work, the measured q_c must be corrected for pore water pressures acting on the cone geometry, thus obtaining the corrected cone resistance, q_t, as defined by Equation (1):

$$q_{\mathrm{t}}=q_{\mathrm{c}}+U_2(1-a)$$

(1)

where ‘a’ is the net area ratio determined from laboratory calibration with a typical value between 0.70 and 0.85. In sandy soils q_c = q_t.

Unlike SPT tests or standard penetration tests, which measure the number of blows required to penetrate a standard sampler and allow for sample collection for soil classification in the laboratory, the piezocone measures only stress. However, the data obtained allow for soil classification and the definition of parameters based on its behavior in the test through methods and empirical correlations presented by various authors [24,25,26,27,28,29,30,31,32]. The sedimentary environments and layer sequences were controlled by fluctuations in the level of the Magdalena River during the Quaternary and may indicate that each has specific characteristics, perhaps influenced by different rates of sediment disposal.

The CPTu test has been practical in studying the stratigraphy of soils along river banks, as demonstrated by recent publications. The geotechnical exploration of deltaic regions has garnered substantial attention due to their unique soil characteristics and environmental significance. Studies have delved into various aspects of geotechnical behavior within these environments, shedding light on crucial factors influencing soil mechanics and stratigraphy. For instance, research has been conducted on the effects of cone penetration rate in silty soils of the Yellow River Delta, considering diverse clay contents and state parameters [33]. Additionally, investigations have focused on refining soil classification methods in deltaic regions, leveraging the comprehensive CPTu database for enhanced applicability [34]. Geotechnical characteristic assessments of floodplain soils have been meticulously carried out using seismic-CPTu data in Nanjing, China, providing valuable insights into local soil behavior [35]. The strength properties and behavioral classification of silts in the Huanghe River Delta have been thoroughly examined through variable rate piezocone penetration tests, elucidating critical aspects of soil behavior [36].

Furthermore, the formation mechanisms of deltas have been scrutinized for their influence on the geotechnical property sequence of sediments in the late Pleistocene-Holocene period, particularly in the Mekong River Delta [37]. Finally, the strength characteristics of soils in aquatic environments have been evaluated through comprehensive piezocone penetration tests and flat dilatometer tests, offering crucial data for understanding soil behavior in water areas [38]. These multifaceted studies contribute significantly to the scientific understanding of deltaic soil mechanics and stratigraphy, informing engineering practices and environmental management strategies in these dynamic landscapes.

Cone Penetration Testing with pore water pressure measurement (CPTu) has become a cornerstone for detailed subsurface characterization in diverse geological settings, including riverine environments. These settings often present complex sedimentary structures due to dynamic fluvial processes, making understanding subsoil properties critical for various geotechnical and environmental applications. In river deltas and floodplains, the application of CPTu offers a reliable means to investigate the stratification and mechanical properties of sediments frequently affected by deposition and erosion episodes [39]. Studies like those conducted on the sediments of the Yangtze River Delta and the Jiangsu Quaternary deposits [40] underscore the versatility of CPTu in capturing the vertical variability of soil behavior type indices (I_c), which are instrumental in classifying soil types and assessing their liquefaction potential. These indices help delineate soil layers with varying geotechnical properties essential for designing foundations and other structural elements in flood-prone areas. Moreover, the incorporation of Random Field Theory (RFT) in analyzing CPTu data facilitates the probabilistic assessment of soil behavior, taking into account the inherent spatial variability of soil properties, which is particularly pronounced in fluvial deposits due to intermittent sedimentation processes [41,42].

The research on the Cimanuk River Delta further illustrates the application of CPTu in understanding the geotechnical properties of riverine sediments, where identifying different facies types helps predict the geotechnical behavior of the deltaic environment [43]. This kind of detailed subsurface profiling enabled by CPTu is vital for developing regional geotechnical models and planning and maintaining infrastructure in river deltas. Furthermore, the variability in results obtained from CPTu testing across different river systems highlights the influence of local geological conditions on soil behavior. For example, the variations in I_c profiles across different geologic formations in the Jiangsu study [40] indicate that local depositional history significantly affects soil properties, which can vary considerably even within a small geographic area. This spatial variability challenges the traditional deterministic approaches to soil characterization and underscores the need for a more nuanced understanding of soil behavior under varying environmental conditions.

Integrating CPTu data with geological and sedimentological analyses provides a more comprehensive understanding of the subsurface conditions. This multidisciplinary approach is crucial for effective risk management and designing resilient infrastructure in areas susceptible to natural disasters such as floods and earthquakes. The ongoing development of probabilistic models and statistical tools further enhances the predictive capabilities of geotechnical investigations, making it possible to anticipate and mitigate potential geotechnical failures before they occur [44]. The application of CPTu in riverine environments exemplifies the advancement in geotechnical exploration techniques, offering a robust framework for understanding the complex interplay between geological processes and material properties. The continuous refinement of these techniques, coupled with advanced statistical modeling, promises to significantly improve the safety and sustainability of engineering practices in fluvial landscapes [23,26,45,46,47,48,49,50].

The geotechnical characterization of subsurface conditions using tests such as CPTu and SPT has been widely applied in slope stability studies, risk assessments, and infrastructure design in regions with sedimentary soils. Specifically, the Magdalena River in Colombia is of particular interest due to its complex fluvial dynamics and the associated risks of erosion and landslides, which impact both urban settlements and agricultural and industrial activities. Numerous studies have demonstrated the importance of assessing soil’s mechanical and stratigraphic properties to mitigate geotechnical instability and fluvial erosion. For example, recent research in the Yellow River Delta and other geotechnically significant areas has emphasized the utility of CPTu tests in characterizing sediment profiles, improving empirical correlations, and predicting soil responses to loading conditions and environmental changes [39]. In this context, our study aims to contribute to understanding sediment stratification and the mechanical properties of subsurface layers along the Magdalena River, providing critical data for designing mitigation measures against erosion and slope failures in the region.

Thus, the objective of this study was to characterize the different subsurface layers of the Magdalena River (Northern Colombia) through the execution and analysis of four (4) cone penetration tests (CPTu) and four (4) standard penetration tests (SPT). The aim was to determine the stratigraphy present in the Magdalena River’s riverbed and riverbank and its geotechnical properties to characterize the area for a possible coastal protection structure in the future. This is to mitigate the high risk of erosion and scouring caused by the river channel. The originality of this study is encapsulated in its detailed geotechnical characterization of the subsurface stratigraphy along the Magdalena River’s left bank in Magangué, Colombia, utilizing an integrated methodology involving Piezocone Penetration Testing (CPTu) and Standard Penetration Testing (SPT). This research advances the understanding of sediment stratification and mechanical properties by correlating CPTu-derived parameters, such as cone tip resistance (q_c), sleeve friction (f_s), and pore pressure (U₂), with SPT N-values to elucidate the geotechnical behavior of alluvial deposits. The innovative aspect lies in applying these in-situ testing techniques to develop a robust subsurface profile, providing critical data for designing and implementing future erosion mitigation structures. Understanding the geotechnical properties of riverbeds not only aids in safe infrastructure development but also ensures sustainable management of natural resources, minimizing environmental impact.

2. Description of the Study Area

The study area is located in a stretch of the Magdalena River, on the left bank, converging with the municipality of Magangué, Bolívar. The Magdalena River in this stretch exhibits meandering characteristics, with a semi-straight stretch at the end of the meander, as shown in Figure 1, where specific hydrodynamic behavior occurs due to flow stresses, generating sudden changes on the riverbed and banks. The intervention study area is part of a paleochannel that used to circulate within the marshy system, meaning that a sector of the municipality of Magangue is situated in an area of alluvial deposits due to the land use that the study area had before being settled as an urban area. The closure of the meander, mainly on its southern flank, exerts more significant pressure on its closure zone or crest, with significant fluvial erosion processes occurring on its left bank, which has noticeably worn down the existing natural alluvial bars and levees [3].

The geology of the Magdalena River area near Magangué is characterized by a predominance of recent alluvial deposits formed by the sedimentation of sands, silts, and clays transported by the river. These sediments have accumulated due to the active fluvial dynamics of the Magdalena River, especially in the Momposina Depression, where Magangué is located. Additionally, the region features older sedimentary formations, including sandstones and shales dating back to the Quaternary period. The influence of fluvial processes has significantly shaped the landscape, creating a dynamic and varied geomorphological environment [51,52]. Locally, in the City of Magangué region, different types of lithological materials occur between rocks and unconsolidated deposits. The clastic sedimentary rocks of the Neogene, on which a residual sandy silty and clayey soil, active alluvial deposits of the main bed of the Magdalena River, deposits of alluvial dikes and fluvial-lacustrine deposits are developed of floodplains. Alluvial dams form longitudinal structures that protect the active river channel from overflows, which occur in floodplain and swamp areas, mainly composed of clay and silt. These structures are low in height and have medium resistance to river erosion but are very fragile due to anthropogenic activity.

3. Methodology

The cone penetration test (CPTu) involves inserting a conical tip of known dimensions into the ground at a constant speed (20 mm/s). The conical probe is pushed into the soil by hydraulic jacks that generate sufficient pressure to achieve penetration. The CPTu test was conducted based on ASTM D-5778 standard [53], which provided data on depth, cone penetration resistance (q_c), sleeve friction resistance (f_s), pore pressure (U₂), and inclinometer readings at each centimeter of penetration. For the CPTu tests, a TG 200 kN Model 2013 tracked drilling rig manufactured by Pagani Geotechnical Equipment, Caledasco, Italy, was used. A BERETTA T-46 drilling rig was utilized for installing casing and confinement pipes. Figure 2 shows the location of the executed cone penetration tests, while

Table 1. Description of the points where the CPTu tests were carried out.

Point	Co-Ordinates		River Depth (m)	Total Depth below River (m)
	Latitude	Longitude
CPTu-01	9°13′21.12″ N	74°45′5.04″ O	11	31.2
CPTu-02	9°13′21.04″ N	74°45′4.43″ O	15.6	45
CPTu-03	9°13′20.17″ N	74°45′7.44″ O	9.4	54.8
CPTu-04	9°13′20.71″ N	74°45′11.45″ O	10.2	50.9

presents the co-ordinates, execution dates, water table height, and total depth reached. The selection of test locations for the CPTu tests was informed by historical observations of heightened erosion and riverbank instability during rainy periods at these specific points along the river. This choice was made to investigate the geotechnical responses of soils prone to such environmental influences, aiming to enhance our understanding of their behavior under varying conditions, particularly in relation to rainfall-induced changes in soil properties and stability. Thus, these locations were strategically chosen to capture critical aspects of the local geomorphological dynamics, ensuring the relevance and applicability of the study’s findings to the broader context of riverbank stability and soil mechanics. Figure 2 presents the location points of CPTu test area. In addition, Figure 3 shows the photos of the execution of CPTu tests on the Magdalena River. As seen in Figure 3, the machinery was supported on a small boat leveled at the river’s edge and stabilized with tensioners so that it did not move during the penetration of the cone. The use of advanced CPTu testing in our study aligns with sustainable engineering goals by reducing the need for extensive ground disturbance, thereby preserving the river’s natural state and reducing the environmental footprint.

The Standard Penetration Test (SPT) aims to determine the compactness and bearing capacity of the soil. This test involves counting the number of blows (N) required to drive a sampler into the soil at different depths [16]. The method used for conducting the tests was percussion and washing, obtaining samples from each encountered stratum at least every 0.50 and 1.0 m depth.

Table 2. Details of the location of SPT points.

Point	Co-Ordinates		Water Table (m)	Depth (m)
	Latitude	Latitude
SPT-01	9°13′20.25″	74°45′6.73″	1	15
SPT-02	9°13′20.77″	74°45′11.26″	1	17.5
SPT-03	9°13′20.82″	74°45′3.05″	1	17.5
SPT-04	9°13′20.91″	74°45′1.52″	1	15

shows the co-ordinates, execution dates, water table height, and total depth reached during SPT drilling. In addition, Figure 2 presents the location points of SPT test area. During percussion, samples were removed with Shelby tubes to perform geotechnical characterization tests. The natural moisture content and Atterberg limit tests followed the American standard D2216 [54] and ASTM D 4318 [55]. For their part, the samples’ unit weight tests were carried out per American standard ASTM D2167 [56]. Other tests were carried out with samples, such as unconfined compressive tests that followed the American standard D2166 [57].

Figure 4 illustrates the testing setup for the Standard Penetration Test (SPT) and the Cone Penetration Test with pore pressure measurements (CPTu) conducted on a riverbank and riverbed. The SPT was performed on the riverbank, reaching a depth of 15 m, while the CPTu was conducted in the riverbed, extending down to a depth of 55 m. This setup allowed for a comprehensive assessment of subsurface conditions in both the bank and river bed, providing valuable geotechnical analysis data.

4. Results

The experimental plan for the field tests was primarily based on conducting four (4) Cone Penetration Tests with pore pressure measurements (CPTu) performed with a controlled speed of 2 cm/s to depths exceeding 50 m. Additionally, four (4) Standard Penetration Tests (SPT) with extraction of undisturbed samples using Shelby tubes for geotechnical and mechanical characterization. The SPT tests were carried out on the left bank of the Magdalena River, while the CPTu tests were conducted directly on the riverbed, as presented in Figure 4. Based on the above, the following chapters present the characterization of the soil strata on the riverbank and riverbed and the geotechnical and mechanical properties of the soils within these strata.

4.1. CPTu Analysis

Figure 5 presents the results of CPTu drilling in the 1-, 2-, 3- and 4-point locations indicated in Figure 2. Figure 5 shows the corrected cone tip resistance (q_t), the friction ratio (R_f), the excess of pore pressure, and the hydrostatic pressure presented (Uo). In the CPTu-1 test, the riverbed was found at a depth of 11.0 m. From there, down to 20 m depth, a layer of soft to medium stiff clay was encountered. Subsequently, from there to the exploration depth of 31.2 m, clays and silts of very stiff to hard consistency were found, except for a 2.1 m thick intercalation where a firm clay was encountered (between 24.3 and 26.4 m). Moreover, from 22.5 m onwards, peaks in cone tip resistance (q_t) associated with sand were observed, indicating that the clays/silts are sandy or contain sand lenses.

In the CPTu-2 test, the riverbed was found at a depth of 15.6 m. From there, down to 20 m depth, a layer of firm clay was encountered. Subsequently, down to the exploration depth of 45.0 m, clays and silts of very stiff to hard consistency were found. Similar to what was observed in CPTu-1, peaks in cone tip resistance (q_t) associated with sand were observed from 21.0 m onwards, indicating that the clays/silts are sandy or contain sand lenses. Thus, based on the information obtained from the CPTu tests, it can be affirmed that the site contains deposits of clay ranging from soft to firm consistency down to a depth of 20.0 m. These deposits exhibit undrained shear strength (S_u) values ranging from 21.9 to 58.9 kPa and cone tip resistance (q_t) values ranging from 0.6 to 0.9 MPa. From 20.0 m depth down to the maximum exploration depth (45.0 m), clays and silts with the presence of sand of very stiff to hard consistency are encountered, with S_u values ranging from 135.6 to 317.3 kPa and q_t values ranging from 2.4 to 6.8 MPa.

In the CPTu-1 test, a firm clay (S_u = 68.1 kPa and q_t = 1.6 MPa) was encountered in a 2.1 m thick intercalation within the material of higher resistance encountered from 20 m onwards. This indicates that significant intercalations of low resistance can be encountered even with underlying materials of hard consistency. The provided undrained shear strength (S_u) values were obtained from a correlation with test results, following Robertson’s methodology.

In the CPTu-3 test, the riverbed was found at a depth of 9.4 m. From there, down to 17.2 m depth, a firm clay layer was encountered, exhibiting an undrained shear strength (S_u) value of 60.4 kPa and cone tip resistance (q_t) of 0.8 MPa. Underlying the clay layer, medium-dense sands were encountered, with estimated Standard Penetration Test (N₆₀) values ranging from 19 to 24. These sands extend to a depth of 28 m and are underlain by hard clay, which extends down to the exploration depth of 54.8 m; these exhibit S_u values ranging from 290.4 to 560.6 kPa, and q_t values ranging from 4.5 to 9.0 MPa.

In the CPTu-4 test, the riverbed was found at a depth of 10.2 m. From there, down to 15.4 m depth, a layer of moderately stiff clay was encountered, which exhibits an undrained shear strength (S_u) value of 43.8 kPa and cone tip resistance (q_t) of 0.8 MPa. Underlying the clay layer, dense sands were encountered, with estimated Standard Penetration Test (N₆₀) values ranging from 29 to 50. These sands extend to a depth of 40 m and are underlain by hard clay, which extends down to the exploration depth of 50.9 m; these exhibit S_u values ranging from 197.0 to 478.4 kPa, and q_t values ranging from 6.0 to 8.3 MPa.

Analyzing the results of the four conducted CPTu tests in the riverbed, a distinctive stratigraphic profile emerges, showcasing a diverse composition of clay layers interspersed with sandy intervals at varying depths. The stratigraphy delineates a sequence starting with soft to medium stiff clay formations extending to a depth of 20 m, transitioning into layers of very stiff to hard clays and silts. The recurring observation of sand peaks from 22.5 m depth onward is noteworthy, suggesting the presence of sandy lenses within the clay matrix.

The comparative analysis of Cone Penetration Test (CPTu) data from the present study with findings from the Yangtze River Delta by Cai et al. [40] and the floodplain of the Long River by Liu et al. [50] reveals complex patterns in cone tip resistance, lateral resistance, pore water pressure, and Soil Behavior Type Index (I_c) across various soil strata. In the current study conducted along the Magdalena River, cone tip resistance (q_t) was observed to range from 0.6 to 6.8 MPa, with notable increases in layers interspersed with sand at depths exceeding 22.5 m, indicating significant subsurface heterogeneity and the presence of sand lenses within clay matrices. In contrast, Cai et al. [50] reported cone tip resistance values between 0.2 MPa and 1 MPa within clay up to 15 m deep, followed by silty sand up to 35 m at the Chongqi Bridge site along the Yangtze River Delta. Their observations also included a drastic decrease in lateral resistance from 50 kPa to 5–10 kPa within the top 15 m and average pore water pressures around 300 kPa, reflecting a geotechnically more straightforward context compared to the Magdalena River site, where resistance and water pressures exhibit more significant variations, indicative of more complex stratification. Moreover, Liu et al. documented the presence of silty sand and sandy silt up to 17 m, transitioning to sandy silt deposits up to 30 m at the Nanjing Yangtze River Bridge site, with I_c values ranging from 2 to 3.5 and lateral resistances reaching 100 kPa in silty sand strata. They also recorded pore water pressures ranging from 130 kPa to 800–1000 kPa at depths reaching 30 m, which aligns with the more extreme conditions observed along the Magdalena River, where dynamic interactions between the solid and fluid soil phases are evident. This detailed comparative analysis highlights the variability and complexity of soil behavior along the Magdalena River, demonstrating resistance and pressure patterns that are similar yet more pronounced compared to those observed by Cai and Liu along the Yangtze River. These insights underscore the necessity for precise and tailored geotechnical analysis and structural design to address the diverse and dynamic fluvial environments encountered effectively.

The study considers the spatial variability of soil properties, as evidenced in Figure 4, which showcases vertical and horizontal variability. This variability critically influences soil behavior under load and its interaction with structural elements, necessitating advanced analytical approaches for accurate geotechnical predictions. Incorporating insights from recent research, such as Chen et al. [58] on the impact of tunneling operations on soil arching effects and Cheng et al. [59] on the behavior of helical anchors in variable clay, the study leverages extensive deformation finite element analysis to account for non-linear interactions between soil properties and structural elements. These methodologies enhance the design strategies for foundations and underground structures by ensuring careful consideration of soil heterogeneity, thus improving the safety and performance of engineered structures.

4.2. Soil Classification with CPTu Data

One of the applications of the CPTu test is to classify soil types based on their behavior and thus obtain a profile of the entire soil. This is based on measurements taken in the field. However, it cannot be expected that the piezocone will provide exact predictions of soil type based on physical characteristics such as grain size distribution. Instead, it guides soil’s mechanical characterization (strength, stiffness) [31]. The soil characterization through this test is known as “Soil Behavior Type” (SBT). The normalized cone resistance (Q_tn) is expressed in a non-dimensional form, considering the in-situ vertical stresses for SBT characterization. The Q_tn is defined as (Equation (2)):

$${\mathrm{Q}}_{\mathrm{t}\mathrm{n}}=(q_{\mathrm{t}}-{\sigma }_{\mathrm{v}\mathrm{o}})/{\sigma }_{\mathrm{v}\mathrm{o}}^{\prime }$$

(2)

where ${\sigma }_{\mathrm{v}\mathrm{o}}^{\prime }$ is the effective stress and ${\sigma }_{\mathrm{v}\mathrm{o}}$ is the total stress of soil.

Figure 6 provides the SBT classification based on Robertson’s methodology. The soil classification is associated with the normalized cone resistance and normalized friction ratio. In addition, the normalized cone resistance is associated with the parameter B_q. The B_q relationship in electrical piezocone tests refers to the relationship between the effective stresses and the water pressure in the soil. In overconsolidated clays, the pore pressure may be higher than expected due to a more intense or rapid past-loading process, meaning that the measured pore pressure may be higher than the initial pore pressure for a given loading. This could result in lower B_q values than normally consolidated clays at the same depth, as pore pressure significantly influences effective stresses. Additionally, over-consolidated clays may exhibit more complex and less compressible behavior than normally consolidated clays. Therefore, in normally consolidated clays, B_q values are expected to increase with depth, indicating a more significant influence of effective stresses on pore pressure. In contrast, B_q values may be lower in over-consolidated clays due to higher residual pore pressures and a more significant influence of pore pressure on soil stresses. In the present study, the B_q values vary up to the first 15 m depth, exhibiting both values close to 1. This implies the presence of soil layers at these depths that are normally consolidated and others exhibiting small over-consolidated behavior.

Figure 7 presents the estimation of subsurface shear strength values using CPTu data. It depicts the internal friction angle of the soil (ϕ′), undrained strength (S_u), unit weight of the soil (γ), and the I_c index. Soil total unit weights (γ) are best obtained by taking relatively undisturbed samples (e.g., thin-walled Shelby tubes; piston samples) and weighing a known volume of soil. When this is not feasible (as a present study), the total unit weight can be estimated from CPT results in concordance with Equation (3):

$$\mathsf{\gamma }/{\mathsf{\gamma }}_w=0.27\left[\log \mathrm{Fr}\right]+0.36\left[\log \right(q_{\mathrm{t}}/p_{\mathrm{a}}]+1.236$$

(3)

where ${\mathsf{\gamma }}_w$ is the unit weight of water. The friction angle ϕ′ is estimated as (Equation (4)):

$${\varphi }^{\prime }=0.37\left[\log \right(q_{\mathrm{c}}/{\sigma }_{\mathrm{v}\mathrm{o}}^{\prime })+0.29]$$

(4)

Theoretical solutions have provided valuable insight into the relationship between cone resistance and undrained strength S_u. All theories result in a relationship between cone resistance and S_u of the form (Equation (5)):

$$S_{\mathrm{u}}=(q_{\mathrm{t}}-{\sigma }_{\mathrm{v}\mathrm{o}})/N_{\mathrm{k}\mathrm{t}}$$

(5)

N_kt is a preliminary cone factor value; typically, N_kt varies from 10 to 18. To simplify the application of the CPTu SBT chart shown in Figure 6, Figure 7, Figure 8 and Figure 9, the normalized cone parameters Q_tn and Fr can be combined into one Soil Behavior Type index, I_c, where I_c is the radius of the essentially concentric circles that represent the boundaries between each SBT zone. I_c can be defined as follows (Equation (6)):

I_c = ((3.47 − log Q_tn)² + (log Fr + 1.22)²)^0.5

(6)

In addition, G_o (stiffness of soil) was estimated using the following expression (Equation (7)):

$$G_{\mathrm{o}}=(q_{\mathrm{t}}-{\sigma }_{\mathrm{v}\mathrm{o}})\times 0.0188\times {10}^{0.55I\mathrm{c}+1.67}$$

(7)

There is no singular value for undrained shear strength, S_u, as the loading direction, soil anisotropy, strain rate, and stress history influence the undrained behavior of soil. Typically, the undrained strength in triaxial compression exceeds that in simple shear, which in turn exceeds that in triaxial extension. Therefore, the value of S_u to be applied in the analysis depends on the specific design problem. Generally, the loading direction in simple shear often represents the average undrained strength. Interpreting the results of all in-situ tests will inevitably require some empirical content due to the influence of anisotropy and strain rate, along with the potential effects of sample disturbance. The internal friction angle was calculated at 40 degrees for uncemented fine-grained soils. However, consolidated triaxial tests conducted on high-quality samples are the most reliable method for determining fine-grained soils’ effective stress friction angle.

Using the results of the piezocone test, a parameter called Soil Behavior Index I_c can be defined. Thus, the type of soil can be classified: gravelly sands I_c (<1.31—zone 7); sands to silty sands 1.31 < I_c < 2.05—zone 6); sand mixtures (2.05 < I_c < 2.60—zone 5); silt mixtures (2.60 < I_c < 2.95—zone 4); clays (2.95 < I_c < 3.60—zone 3); organic soils (I_c > 3.60—zone 2). As presented in Figure 10, the sands are mainly classified as clays and slimes, except for some arena strata found in the CPTu-04.

Taking as a guideline the boundaries defined by Schnaid et al. [8], theoretical results of q_c and G_o for sands and clays were produced based on formulations developed from calibration chamber tests. Formulations were selected that allowed the determination of these parameters based on the intrinsic properties of these materials, such as friction angle and relative density for sands and OCR for clays. Thus, by varying the values of these properties, it was possible to identify changes within the G_o/q_c versus Q_tn space, attributing a behavioral trend similar to the analysis performed from laboratory tests. Evaluating the test results from different types of deposits, it was observed that between the regions defined for sandy and clayey soils, there exists a transitional region corresponding to intermediate silty soils, following a smooth transition and with the same trend as the boundary lines. Thus, it was found that the slope of the boundary lines is a combination of void ratio, mean stress, and soil critical state parameters and is not influenced by the soil type. Vertical lines were introduced to delineate transitions between different materials, which, like the lower boundary line for clays, are empirical, established from field test results, and are used to complement the classification system [60,61].

Most empirical correlations used to interpret in-situ tests assume that the soil behaves similarly to the soils on which the correlation was initially based, typically unaged and uncemented silica sands. However, applying these correlations to sands other than unaged and uncemented ones can lead to incorrect interpretations. Therefore, it is crucial to determine if the soils are well-behaved. Combining shear wave velocity and cone resistance measurements offers a way to identify these unusual soils. Cone resistance is a good indicator of soil strength, as the cone induces significant strains, leading to failure in the soil adjacent to the probe. Meanwhile, shear wave velocity directly measures small-strain soil stiffness (G_o), as it’s taken at very small strains. Thus, Figure 8 presents the estimation of G_o for CPTu data. The small-strain soil fitness varies depending on the density of the soil. For example, sand material was characterized by q_c data up to 30 m. On average, the soil value at this depth was 21 kN/m³ (Figure 8).

According to Schnaid and Yu [62] and Schnaid et al. [63], the value of state parameter $\psi$ in sands can be obtained from a theoretical equation dependent on the G_o/q_t ratio. It is intended to investigate the applicability of this equation for use in mining tailings (non-plastic silts). Considering the results of cone tests and tests with seismic readings, Schnaid and Yu [62] presented a theoretical correlation calibrated through tests conducted in a calibration chamber for determining the state parameter of clean sands, as presented by Equation (8).

$$\psi =\mathsf{\alpha }{\left({\displaystyle \frac{p^{\prime }}{p_{\mathrm{a}}}}\right)}^{\beta }+\mathsf{\chi }\ln \left({\displaystyle \frac{G_{\mathrm{o}}}{q_{\mathrm{c}}}}\right)$$

(8)

where $\mathsf{\alpha }$ = −0.520, $\beta$ = −0.07, and $\mathsf{\chi }$ = 0.180 are average coefficients obtained from the data of tests in the calibration chamber, and p′ is the average effective stress acting on the material. Thus, Figure 9 presents the relationship between G_o/q_t for the CPTu results, whose outcomes were compared to values in the calibration chamber for a stress range between 50 and 5000 kPa. It is observed that for a certain stress level, the ratio of G_o/q_t decreases with the reduction of the state parameter value $\psi$; that is, G_o/q_t decreases with the increase in the relative density of the material. As presented in Figure 9, the data recorded on the CPTu tests demonstrate that soils are classified as more dilative material than retractive. In this context, the material is susceptible to liquefaction, as explained by Schnaid et al. [62,63].

The piezocone test best portrays the stratigraphy of the soil layers, making it possible to detect the different soil layers [40] more accurately. Typically, the cone tip resistance (q_t) is high in sands and low in clays, and the friction ratio (R_f = f_s/q_t) is low in sands and high in clays [47]. In reality, it is not expected to define the soil type from the CPT, but rather the behavior of the soil type (Figure 6). Another essential factor that must be highlighted is the better characterization of thin layers (a few centimeters), which is not always possible in an SPT-type test (measurements and collections generally take place every 1 m of drilling) [49]. Thus, Figure 10 presents the unified subsurface profile of the river based on the SBT classification and samples extracted from various depths. Five soil strata were primarily detected. The first stratum encountered at the riverbed is a layer of firm clay, followed by loose and dense sands, stiff silts/clays, and dense silts.

4.3. SPT Analysis and Other Geotechnical Properties

Figure 11 presents the results of Standard Penetration Test (SPT) assays conducted on the riverbank (left bank). During field tests, samples were retrieved using Shelby tubes for natural moisture content, Atterberg limits, and grain size analysis. Additionally, shear strength tests were conducted on undisturbed samples during extraction, as most exhibited slumping due to high water content. The groundwater level was found at a depth of 1m, the Magdalena River level. As depicted in Figure 11, N₆₀ blow count values are plotted against depth. Given that the riverbank material historically consisted of sediment deposits, the profile is analyzed based on visual characterization and sample extraction data. Accordingly, N₆₀ values are consistent across all depths and test points. Thus, the material is deemed homogeneous with plasticity, classified as a low-plasticity clay or silt with a reddish color. For the first 5 m of depth, a low-plasticity clayey material was encountered with an N-value of 4 blows. Close to depths of 5 to 6 m, a more resistant clayey material was found with N-values of up to 10. Finally, the SPT values remained constant for depths greater than 6 m, indicating a homogeneous clayey material until reaching the riverbed at approximately 11–12 m depth.

The geotechnical properties of the soil were determined in the laboratory using samples extracted from the Shelby tubes at various depths. Figure 12 presents the results and variation of the natural unit weight of the soil, as well as the Atterberg limits and natural moisture content of the samples at different depths. On average, the natural unit weight values are 21 kN/m³, indicating the homogeneity of the strata. Additionally, the average natural moisture content values are around 42%. Similarly, the calculated Liquid Limit (LL) for the soils is close to 38%, approaching the threshold for exhibiting liquid behavior in the field.

The proximity to the liquid limit indicates that the soil is saturated with water. This can significantly affect soil stability, especially under loading, vibration, or changes in water pressure, leading to loss of strength and the possibility of landslides, liquefaction, or other types of failure [19]. This may require special considerations in design and construction to ensure structures’ stability, such as using soil improvement techniques or implementing adequate drainage systems. High soil moisture can increase the risk of riverbank erosion, especially during heavy rain events or river floods [64]. This may affect the stability of structures near the shore and require additional protective measures. The identification of high-moisture content soils along the riverbanks suggests the potential for sustainable erosion control measures, such as bioengineering with native vegetation, which can stabilize the soil and enhance biodiversity.

It is possible to observe that both types of field tests (SPT and CPTu) provide similar results regarding soil type and resistance variation with depth. However, the electrical cone test provides more detailed information on punctual variations in penetration resistance and the transition of properties between soil horizons [61]. The potential of CPTu in spatially identifying subsurface variability in sedimentary deposits is recognized, and this potential is equally relevant in identifying vertical and spatial heterogeneity in cohesive-frictional soil resistance properties [65].

Regarding the mechanical resistance tests, obtaining samples suitable for performing unconfined compression (q_u) in SPT-02 and SPT-03 at a depth of 5 m was possible. The strength q_u was calculated on average as 167 kPa.

The analysis in Magangué shows similarities with studies conducted in other fluvial contexts. Both studies describe soils ranging from soft to firm clays to dense sands, using normalized parameters like Q_tn and I_c to define soil type and behavior [66]. Moreover, they share the use of empirical correlations to determine undrained shear strength (S_u) from CPTu results. The Magangué analysis mentions the relationship between q_t and Suhrough a cone factor (N_kt), which is also addressed in comparable studies [67].

In terms of specific values, similar studies found cone resistance (q_t) values ranging between 1.0 and 10.0 MPa for sandy soils and lateral resistance (f_s) values ranging between 50 and 150 kPa [68]. These values are consistent with those in Magangué, where dense sands showed q_t values from 6.0 to 8.3 MPa and f_s estimated in similar ranges. For clays, other studies report q_t values from 0.5 to 2.5 MPa and f_s values from 20 to 80 kPa, aligning with the values obtained in the Magangué tests for soft to medium clays, with q_t values from 0.6 to 0.9 MPa and f_s values from 30 to 70 kPa [12,22,23].

Both studies consider the influence of penetration rate and soil saturation on CPTu results, noting how these factors affect the accuracy of pore pressure and cone resistance measurements [33]. However, notable differences are observed in exploration depth. While the Magangué analysis reached depths up to 54.8 m, other studies focused on shallower depths, up to 31.2 m [37]. This implies differences in soil conditions and characteristics at greater depths, which can affect the interpretation of results. Additionally, in Magdalena River, significant variations in B_q values were observed, indicating the presence of both normally consolidated and overconsolidated soil layers. This variability is not discussed in as much detail in other studies, which may reflect differences in data interpretation approaches [33]. Finally, some studies specifically mention soil preparation through mud consolidation, a procedure not mentioned in the Magangué study, which can influence the direct comparability of the results [37]. There are similarities in methods and approaches for soil classification using CPTu tests between the Magangué analysis and the reviewed studies, but there are notable differences in exploration depth and soil preparation. These differences can influence the interpretation of results and the final soil classification. The findings underscore the necessity for sustainable river management practices that prevent erosion and contribute to the resilience of riverine ecosystems against climate change. Soil erosion along riverbanks is a critical environmental issue affecting land stability and aquatic ecosystems. Recent studies have highlighted the multifaceted nature of soil erosion processes, particularly in riverine environments. Li et al. [68] discuss the mechanical erosion mechanisms driven by hydrodynamic forces in the Yellow River, noting that the variability in sediment composition significantly influences erosion rates and patterns. Furthermore, Zhou et al. [19] explore the impact of anthropogenic activities on soil erosion, emphasizing how land use changes enhance erosion susceptibility by altering soil structure and composition. These studies underscore the complexity of soil erosion, influenced by natural processes and human interventions, highlighting the urgent need for integrated management strategies to mitigate its impacts on river ecosystems and adjacent communities.

5. Conclusions

Following the results and analysis of the field tests conducted on the left bank of the Magdalena River near Magangué (Colombia), the following conclusions can be drawn:

• This study has identified several new insights into the geotechnical characteristics of the riverbed, highlighting complex subsurface stratigraphy revealed by piezocone tests down to 50 m. Notably, delineating extensive firm-to-hard material layers and significant sand deposits enhances our understanding of sediment distribution along the riverbank. Consistent measurements of tip resistance and pore pressure, compared to lateral friction, underscore the reliability of these parameters in predicting soil behavior under environmental stress. Furthermore, the high moisture content near the liquid limit in riverbank materials emphasizes the elevated risk of erosion and slippage, necessitating targeted mitigation strategies to ensure the resilience of riverbank infrastructure against environmental changes.
• Piezocone tests confirmed an extensive layer of firm clay followed by layers of dense sand, silts, and firm clays. The most extensive layer identified was the firm-to-hard material layer, up to 25 m thick. To the north, on the left side of the river, an extensive deposit of sand was found, with a thick 28-m layer detected during the CPTu test.
• The repeatability of the measured parameters, particularly concerning tip resistance and pore pressure, exhibited more excellent stability and consistency than lateral friction. This observation is consistent with previous findings in geotechnical characterization studies of riverbeds subjected to high sediment transport.
• Classification-based SBTn charts confirmed the substantial presence of clays in the investigated material, supported by grain size analysis conducted on soil samples collected during piezocone testing. This geotechnical identification is crucial for understanding the nature and properties of the soil, significantly informing the planning and execution of geotechnical engineering projects in riparian areas.
• Estimates based on CPTu results indicate that materials with higher friction tend to dilate and retract. This observation sheds light on the behavior of subsurface materials under different conditions, which is crucial for understanding their stability and potential response to environmental changes.
• The material from the Magdalena Riverbank exhibits a high degree of moisture, almost at the liquid limit. This poses a potential problem of slippage and erosion in the future due to changes in river currents. Understanding these risks is essential for developing strategies to mitigate potential hazards and protect the surrounding area. This study contributes to sustainable development by providing essential data for the design of resilient and environmentally sensitive riverbank protection works.