IntroductionDevelopment of excess pore water pressure (EPWP) plays a significant role in geotechnical design in soft cohesive soils (e.g., Croney and Coleman 1961; Dunlap et al. 1978; Hiff 1956; Hunt 2005; Strout and Tjelta 2005) and piles in particular (e.g., Li 2019; Liu et al. 2012; Pestana et al. 2002; Tang et al. 2003; Xu et al. 2006). The mobilization of EPWP due to the installation of displacement piles can impact the stability of neighboring structures, as it changes the effective stress in the surrounding soil. Proper monitoring and control of EPWP can prevent excessive soil deformations and ground heave, impair slope stability, and prevent displacements of nearby foundations and structures in soft clays (Tefera et al. 2013).The most important mobilization of EPWP can be expected for driven piles due to the induced large soil displacement and high number of impacts applied during their installation (Eigenbrod and Issigonis 1996; Ng et al. 2013; Pestana et al. 2002; Randolph et al. 1979). It can be also important in the case of jacked piles (Gavin et al. 2010; Rahardjio 2017) and screw auger piles (Larisch 2014; Prezzi and Basu 2005). Significant mobilization of EPWP can be also related with the installation of stone columns in soft clays (Castro and Sagaseta 2012; McCabe et al. 2009). Examples of key EPWP measurements are described below. Xu et al. (2006) analyzed water pressure mobilized due to the installation of open pipe piles. They found that EPWP is lower in slightly overconsolidated clay than in preconsolidated clay. Simonsen and Sørensen (2016) recorded pore water pressure changes in very high plasticity stiff clays adjacent to a large group of driven steel H piles. In heavily overconsolidated clays, they reported the extent zone of EPWP up to 100 pile radii and dissipation time larger than 200 days. Rahardjio (2017) observed high EPWP even 5 months from the installation of large group of jacked piles. Lande et al. (2020) reported approximately 8 kPa EPWP remaining in soft clay 150 days after anchor drilling with uncased system was completed. In order to control the pile driving process, Tefera et al. (2013) installed the piezometers in the area of potential failure plains. Moreover, the driven piles were equipped with vertical drains to mitigate the mobilization of pore water pressure.The objective of this paper is to analyze EPWP mobilization due to the installation of controlled modulus columns (CMCs). These are concrete columns formed in the soil with a displacement screw auger. This technology is commonly used for soil improvement in geotechnical engineering involving soft soils (e.g., ASIRI 2012; Basu et al. 2010; Brown 2005; Prezzi and Basu 2005). CMCs are used in commercial building foundations as well as in large infrastructure projects (e.g., Pearlman and Porbaha 2006; Wong and Muttuvel 2013). Full displacement screw augers induce large displacements and modify stress states in the surrounding soil (Larisch et al. 2014; Pfeiffer and Van Impe 1993; Slatter 2000; Suleiman et al. 2015); however, these installation effects are outside the scope of this paper.A very limited amount of research has been undertaken to assess the installation effects related to the monitoring of pore water pressure due to CMC column construction and loading. Larisch (2014) performed a comprehensive set of measurements concerning the development of pore water pressure in surrounding soils due to three different types of screw auger installation. In order to take the measurements in close vicinity of the drilling rig at different stages of a screw auger installation, raked piezocone (CPTU) tests were performed. The pore water pressure was recorded at the CPTU tip at u1 position. Larisch (2014) presented the mobilization of pore water pressure at a distance of 225 mm, equivalent to half of the pile diameter, from the pile shaft during the auger penetration phase, concrete pumping, and auger extraction. Measurements were taken every 5 s and recorded manually. A decline in pore water pressure was observed when the auger tip reaches the level of CPTU tip position. The study, however, was limited to stiff, fine-grained soils. Suleiman et al. (2015) performed comprehensive series of measurements of horizontal stress changes, pore water pressure, and lateral displacement during the construction of CMC columns and static vertical loading tests. They used push-in pressure sensors fitted with a piezometer, allowing for measuring total horizontal stresses and pore water pressure in the soil and shape acceleration arrays to monitor the evolution of soil lateral displacements. These measurements were recorded at different distances from the pile shaft in very soft sandy silt or silty clay. They observed a maximal increase in both lateral stress and pore water pressure when the mandrel passes the sensor level. Meng et al. (2015) recorded lateral soil displacements using inclinometers and pore water pressure in piezometers due to screw pile installation and during subsequent consolidation period in soft clay in Shanghai. The readings were undertaken at four different levels and at varying distances from the pile shaft. They concluded that the area of disturbance due to the installation of a screw piles is mainly concentrated within a circular region of six pile diameters. The pore water pressure distribution near the pile shaft was registered for all phases of pile construction, concreting, and auger withdrawal. The maximum EPWP was recorded at the end of penetration or at the phase of pile concreting. Moreover, they monitored pore water pressure decay in the following 15 days from installation. After that period, the EPWP was practically dissipated.This paper addresses EPWP changes in soft soil surrounding the CMC group during all phases of its construction and during static loading tests. The decay of EPWP mobilized due to CMC group construction is analyzed. Moreover, the pore water pressure buildup and dissipation were also measured in the soft soil near the base of the floating column subjected to the static loading test. The measurements were performed using two types of piezometers with different response times to record short- and long-term changes in pore water pressure evolution. To the best of authors’ knowledge, such research—concerning both installation phase and static load test—was performed only by Suleiman et al. (2015) with pore water pressure registered only near the shaft of a single CMC column at small depths. The aim of this paper therefore is to present: (1) short-term and long-term changes in EPWP during CMC column construction and loading; (2) assessment of the zone that is influenced by CMC installation and static loading; and (3) assessment of the time necessary to dissipate EPWP after installation of the CMC group. The reported data can shed more light on the installation effects induced by CMC columns and the interpretation of static loading of such columns in pullout and compression tests. Other practical aspects of this study are related to the dissipation time of EPWP and its impact on the schedule of static loading tests and the setup effect related to the installation of CMC columns in soft cohesive soils.The Vistula Marshlands Testing SiteGeotechnical Characterization at the SiteThe Vistula marshlands is a 1,700 km2 plain area located in northern Poland (Fig. 1). The plain started to be formed 6,000 years BP by the alluvial mud brought by the Vistula River. The Vistula marshlands consist of three geological faces: (1) riverbed (sands and occasionally gravels), (2) marsh-swamp-lake (clays, loams, muds and peats), and (3) flood (muds) (Augustowski 1976). A part of the S7 highway that connects Gdańsk and Elbląg was constructed with soil reinforced with CMC columns. The CMC columns considered in this paper were installed at two additional testing fields, near the highway embankment to be constructed. The soil profile with ground water conditions is shown in Fig. 2(a). At the site, two soft soil deposits can be distinguished. The first soft soil layer, spread between 0.7 and 4.05 m below the working platform level, is comprised of loam, clayey mud (organic clay), and peat. The second soft soil deposit, between 7.05 and 14.45 m, consists of organic silt (silty mud), peats, and a mixture of both. The soft soil deposits are separated by a 3-m-thick layer of loose to medium dense sand. Below 14.45 m, a dense Pleistocene sand layer was found. The highest water table level was recorded in the summer period at a depth of 1.7 m below the working platform. However, seasonal variations of the water table level can reach 1.5 m (see Table 1), with the lowest readings observed in winter (about 3.2 m below the working platform level). A detailed description of the soil investigation and geotechnical parameters is given in Konkol et al. (2019).Table 1. Water table changes at the testing siteTable 1. Water table changes at the testing siteMonth, yearPosition below working platform level (m)MethodDecember 20133.3BoringJanuary 20163.1BoringSeptember 20161.7CPTUNovember 20162.7CPTUDecember 20163.2CPTUJune 20171.7CPTUAugust 20171.7CPTUAugust 20171.7PiezometerSeptember 20172.4CPTUAn extensive series of CPTU [Fig. 2(a)] and dilatometer tests (Bałachowski et al. 2018) has shown that the soft soil layer from 7.05 to 12.25 m is quite homogeneous and normally consolidated, with undrained shear strength increasing linearly with depth. It is formed from organic silts with the parameters summarized in Table 2. During piezocone penetration, a considerable excess pore water pressure is generated in soft clays, and the corresponding dissipation curve at 9 m of penetration is given in Fig. 2(b). Monotonic decay—typical for normally consolidated soils—with t50=110  s was observed. Pore water pressure changes during CMC column installation and static loading were recorded in this layer.Table 2. Organic silt geotechnical parametersTable 2. Organic silt geotechnical parametersPropertyValueUnitsGs2.54–2.59—w045.4–49.8%γ015.9–16.9kN/m3γd11.0–11.5kN/m3PL28.3–33.8%LL55.8–60.4%SL25.7–28.0%PI27.5–31.7%LOI4.5–7.1%pH7.0–7.9—Cc0.4026–0.4654—Cs0.0178–0.0252—cv (for in-situ stress)0.34–0.54m2/yearch (for in-situ stress)1.01–1.15m2/yearOCR1–1.47—pc′112.7–126.5kPacu24.2–42.7kPaϕ′31.0°kv (for in-situ stress)1.8 × 10–8m/skh (for in-situ stress)4.2 × 10–8m/sCMCsCMC columns with different lengths were constructed at the testing field. The distribution and lengths of the columns on the testing field fit the program of static loading tests. Column 8 at Trial Field 4 was designed to be a floating type, 11 m long and subjected to pullout static loading followed by compression test. Columns 3, 4, and 7, resting on the bearing layer of compacted sand, were 14.5 m long and designed for further proof testing. The other columns, embedded in the bearing strata, were 18.0 m long and designed as an anchorage system. The construction sequences of the columns at Trial Fields 4 and 5 are given in Fig. 3.InstrumentationThere are two types of piezometers used at the testing site (Fig. 3). The piezometers, denoted P-1 and P-2 [Fig. 4(a)], are drive-point piezometers equipped with vibrating wire pressure transducers. These piezometers are fitted with a low air entry (LAE) filter; they are capable of long-term measurements and well suited to soft clays (Simonsen and Sørensen 2018). According to the manufacturer, they were pushed into soft soil from the bottom of the drilling in the working platform to avoid transducer damage. These piezometers were used during all stages of CMC installation. The P-CPTU piezometer [Fig. 4(b)] is a standard 10-cm2 electric piezocone with pore water pressure filter at u2 position, which allows to record very rapid changes in EPWP due to the high air entry (HAE) filter (Simonsen and Sørensen 2018). Thus, this piezocone with data registered every 1 s can provide measurement of highly variable water pressure for short time periods.Piezometers P-1 and P-2 were pushed into the soil at a rate of 2  cm/s and installed at distances of 0.6 m and 0.9 m from the axis of CMC 8 (see Fig. 3). The piezocone (P-CPTU) was installed at a distance of 1.2 m from the axis of CMC 8 (see Fig. 3). All piezometers were inserted at the depth of 9.0 m. As one can see in Fig. 5, the classic piezometers (P-1 and P-2) are less sensitive due to their LAE filters and have shown only slight EPWP generation during their installation. Piezometer measurements tend towards an average hydrostatic pressure of 73 kPa. The final readings of P-1 and P-2 piezometers included also an increase of pore water pressure due to the weight of the drilling rig on the working platform at the beginning of CMC column construction [Fig. 6(a)].Excess Pore Water Pressure MeasurementsTesting ProgramThe testing program, summarized in Table 3, consisted of several steps. First, two piezometers (P-1 and P-2) were installed 4 days before column drilling. The piezocone (P-CPTU) was installed 1 day before column installation. In the second step, 2 days long, the CMC columns were constructed. In the first day of construction, 10 CMCs were drilled in Trial Fields 4 and 5 CMCs were constructed at Trial Field 5. The next day, eight CMCs were drilled in Trial Field 5. The postinstallation measurements were conducted for 16 days until the full dissipation of the EPWP. Next, the piezometers (P-1 and P-2) were pushed further to the depth of 11 m, 4 days before the static loading test (SLT). The test took 22 h to complete, then the postloading decay of the EPWP was measured during 5 days.Table 3. Test schedule for pore water pressure measurementTable 3. Test schedule for pore water pressure measurementPhase No.DescriptionStart date End date Notes1Piezometers installationAugust 4, 2017August 7, 2017Depth of 9.0 m2CMC drillingAugust 8, 2017August 9, 2017August 8, 2017–trial field no 4 and August 9, 2017–trial field no 53Postinstallation measurementsAugust 8, 2017August 24, 2017Dissipation of EPWP4Piezometer insertion to larger depthSeptember 7, 2017September 7, 2017Insertion to the depth of 11 m5SLTSeptember 11, 2017September 12, 2017Static loading on Column 8 in tension and compression6Post-SLT measurementsSeptember 12, 2017September 19, 2017Dissipation of EPWPControlled Modulus Columns InstallationThe measurements taken were focused on accumulated EPWP generated during construction of CMC group and short-term changes during single column installation. The EPWPs recorded with P-CPTU during the construction of CMC columns are given in Fig. 7. The installation process of CMC consists of auger drilling and concreting with simultaneous auger retrieve. However, during this process some troublesome events may occur, such as drilling pauses, problems with concrete supply, plugging of drilling auger, etc. For longer columns, the operator has to stop the drilling process to mount a special extension to the auger. As a consequence, each column has a unique construction history saved in its log. Examples of EPWP changes for three selected columns, during CMC group construction, are presented in Fig. 7. The generated EPWP is set up with a pile log and the characteristic points of CMC construction are shown. Generally speaking, the pore water pressure decreases rapidly when the auger approaches the piezocone depth, but accumulated EPWP never drops below zero (see Fig. 8). Positive buildup pressure is recorded when the auger position exceeds the level of piezometers. The other events, such as a pause in drilling or concreting, can induce additional changes, unique for each column. Finally, a slight dissipation in the mobilized pore water pressure is observed when the drilling rig moves to the next CMC location.Fig. 8 shows the accumulated EPWP generation due to the construction of 10 CMCs in the Trial Field 4. The changes in accumulated EPWP are visible for each CMC installation, which is clearly seen in all piezometer readings. Buildup pressure response is higher in the P-1 piezometer, the closest to Column 8. One can observe (Fig. 8) that P-CPTU captures the instantaneous PWP changes during installation. The installation of the 10 CMCs at Trial Field 4 increased the accumulated EPWP close to 100 kPa. Almost the full accumulated EPWP is reached during the construction of the first six columns (10, 8, 9, 7, 6, 5) nearest the piezometers. The construction of the remaining columns at Trial Field 4 contributes less to pore water pressure buildup. The rate of the PWP increase due to the installation of columns 3, 2, and 1 is lower than the dissipation rate of the accumulated EPWP at the measurement points.Postinstallation MeasurementsAfter the CMC installation in Trial Field 4, the drilling rig was moved to Trial Field 5, where five columns were drilled on the same day. The remaining columns in Trial Field 5 were constructed the next day. Fig. 9 shows the decay of accumulated EPWP after the construction of the CMC in Trial Field 4. The construction of five CMCs on Trial Field 5 has no visible impact on the accumulated EPWP. However, construction of the remaining eight CMCs the next day slightly increases the piezometer readings. The buildup pressure fully dissipated approximately 14 days after installation of the last CMC on the Trial Field 4.SLTFive days before the SLT, piezometers P-1 and P-2 were pushed further to 11 m depth to measure the EPWP to be mobilized in the neighborhood of the CMC base. The SLT was conducted according to method B (maintained load test) in ASTM D1143 (ASTM 2020). However, some minor modifications were applied, which are allowed according to ASTM D1143. First, the SLT consists of a pullout and compression part (see Table 4). Second, the load increment at the displacement of the column head of 3%D in compression load was kept for 8 h instead of the maximum value of 2 h according to ASTM D1143. The third modification concerned the time increments during unloading. They were limited to 10–15 min. The aim of these modifications is to capture crucial parts of EPWP generation at the level of pile design load. The SLT program and conducted test is detailed in Table 4 and consists of the following steps: 1.The EPWP during pullout test is investigated. The total column head displacement is assumed to be 12 mm, which is 3%D (D = column diameter).2.The column is unloaded.3.The technical break for rearrangement of the SLT set to the compression load.4.The column is tested in compression to the head displacement equal to 3%D.5.EPWP development (or dissipation) during column design load is kept constant.6.The column is subjected to compression to the ultimate load.7.The column is unloaded. The total time of SLT is chosen to follow EPWP changes with piezometers equipped with LAE filters.Table 4. SLT program and executionTable 4. SLT program and executionPhase noNameProgram descriptionAimsSLT executionNotes1Pullout testPullout test to the column head displacement of 3%D=12  mm (minimum time 3 h)Measure the EPWP during pullout testPullout test to the column head displacement 12.83 mm with 600 kN (time 3 h 20 min) in 12 stepsMaintain each load increment until stabilization (0.25  mm/h) but no less than 15 min2UnloadUnloadingN/AUnloading in three steps (total time=30  min)N/A3PausePause of maximum 2 hTechnical breakPause of 2 h 10 minN/A4Compression testCompression test to the column head displacement of 3%D=12  mm (minimum time 3 h)Measure the EPWP generation due to compression testCompression test to the column head displacement 10.09 mm (time 3 h 55 min)Maintain each load increment until stabilization (0.25  mm/h) but no less than 15 min5PausePause of minimum 8 hCheck the EPWP developmentPause of 8 h 25 minN/A6Compression testCompression test to the 10%D=40  mmMeasure the EPWP until the limit load is reachedCompression test to the 40.36 mm at 784.4 kNMaintain each load increment until stabilization (0.25  mm/h) but no less than 15 min7UnloadUnloadingN/AUnloading in 4 steps (total time 1 h 5 min)N/AThe development of the EPWP in each phase is presented in Fig. 10. As one can see, no EPWP is generated during the pullout test. This suggests no suction developed under the base of the floating column during the pullout test, with all load transferred only by the column shaft. During unloading and the following pause (steps 2 and 3), only a small positive EPWP is generated due to the shear stress level increase and soil movements near the column base. This process is continued during the first part of the compression test (step 4), where the column design load is achieved. During the equalization step, the EPWP slightly increases and then starts to dissipate. In further compression (step 6), a rapid increase in EPWP is observed. When the ultimate load is reached, the column is unloaded (step 7). It is interesting to note that the EPWP still increases in this phase, which could be related to the residual compressive load at the column base.Significant EPWP changes are observed only for the P-1 piezometer. The response of the P-2 sensor is very small and does not exceed 2 kPa. This observation suggests that the water pressure buildup in soft soils during SLT is very local, constrained within the column base vicinity and is practically negligible at 0.9 m from the column axis (0.7 m from the shaft). This is contrary to the observations in the CMC installation phase, where very significant EPWP was recorded in both sensors even at longer distance from the drilled columns. This quite different response of the piezometers could be related to the rate of displacement imposed, which was an order of 2  m/min for column drilling and approximately 0.04  mm/min in SLT in the first stages of the loading test.Postloading MeasurementsPostloading measurements were conducted for 7 days after SLT. As one can see in Fig. 11, the EPWP almost totally dissipated within 96 h (4 days) after SLT. The small variation between the 4th and 7th day was induced by heavy rains. Then, the field measurement of EPWP was completed.Discussion and Interpretation of the DataEPWP Generated during CMC InstallationA continuous increase of EPWP during CMC installation was recorded by piezometers P-1 and P-2. The application of a piezocone as a piezometer allows to describe the instantaneous EPWP changes during auger drilling. The instantaneous EPWP drops significantly just before the auger reaches the sensor level. In case of Column 10, the pore water pressure decreases almost to zero (see Fig. 7). A similar drop in pore water pressure was also observed by Larisch (2014) for full displacement screw augers in stiff overconsolidated clays. The EPWP increased to reach the maximum value almost at the final depth of drilling, which can be related to the stress level increase within the soil mass in the vicinity of the sensor. A small decrease in EPWP was observed during concreting and auger retrieving. Contrary to the observations of Suleiman et al. (2015), no EPWP increase was recorded during mandrel retrieving. This research shows the advantage of using the piezocone (and more generally, piezometers equipped with HAE filters) to measure the rapid changes of EPWP during drilling together with the driven-point piezometers to record long-term evolution of EPWP. The measurements by Suleiman et al. (2015) and Meng et al. (2015) are not able to catch these sharp changes of EPWP.The mobilization of EPWP during column installation is very complex. It could be described in the auger tip vicinity as a superposition of two phenomena: continuous pore water pressure increase due to auger penetration, which can be modeled with cavity expansion, and pore water pressure changes due to auger rotation. The effect of rotation will be different in the lower and upper part of the auger due to the opposite form of the helix [Fig. 6(b)]. This rotation induces upward soil movement, which decreases the stress level in the soil mass under the advancing tip and reduces pore water pressure close to the lower part of the auger. In certain soil conditions, it could lead to degradation of the soil parameters near the column base. This will produce a negative effect of installation with a reduced base resistance. Reduced cone resistance after auger pile installation in stiff clays was observed by Larisch (2014) and Van Impe et al. (1998). At the upper part of the auger, the soil is displaced outwards, increasing the lateral stress and pore water pressure. This lateral soil displacement and pore water pressure generation can be modeled with cylindrical cavity expansion (Ellouze et al. 2017; Guetif et al. 2007). However, that approach cannot capture the significant vertical component of soil mass movement around the auger. This complex phenomenon of CMC auger penetration can be described using large deformation numerical modeling. For instance, a similar EPWP trend during CMC auger penetration was obtained using coupled Eulerian-Lagrangian formulation and finite-element modeling (Konkol and Bałachowski 2019).Installation Effects and the Extent of Influence ZoneThe installation of CMC columns induces significant changes in pore water pressure up to a distance of 7D from its axis. The contribution of further CMCs (distance between 7D and 15D) to the accumulated EPWP readings is less important (Fig. 8). However, a small effect of the columns drilled on the neighboring testing field (at distance of 30–40D) was still recorded (Fig. 9). This finding confirms the observations of Chandra and Hossain (1993) and Robertson et al. (1990) for driven piles, where the extent zone of 14–35D was observed in normally consolidated (NC) clays and McCabe and Lehane (2003) for pile group in silt. The normalized maximum EPWP recorded with P-CPTU during the installation of columns is presented in Fig. 12 and compared to other measurement data. The figure considers the effect of CMC group installation with the shadow effect of already constructed columns. The comparison to other recorded data thus cannot be straightforward due to different measuring systems applied. However, one can make some general observations that the maximum EPWP is very similar to that recorded by McCabe and Lehane (2003) for the pile group in silt. It is slightly higher than the general tendency observed by Poulos and Davis (1980) for driven piles in low to medium sensibility clays, by Roy et al. (1981) in the vicinity of the base of jacked piles in clayey silt, and by Meng et al. (2015) for a single screw pile in clay. The observed EPWP values for a single pipe pile in slightly overconsolidated soft clay (Xu et al. 2006), closed-ended pile in Young Bay mud (Pestana et al. 2002), and a screw pile in soft sandy silt (Suleiman et al. 2015) are significantly lower than recorded in the present study. These observations make our results consistent with previous measurements. The EPWP distribution in the soil at closer distance to the shaft was not possible to measure during the column installation. The CPTU data connection was lost during the installation of Column 8 (at 2D distance from the P-CPTU). Taking into account the measurements performed on instrumented piles (Gavin et al. 2010; Lehane 1992; Roy et al. 1981), one can expect that EPWP will be higher at closer distance to the column shaft.Static Loading EPWPThe EPWP measurements during pullout tests show no significant EPWP generation near the column base. These results suggests no tensile stress in soft organic silt during the pullout test, which is in agreement with, for example, Han and Ye (2006), who noted no influence of the pile tip in pullout tests in soft Shanghai clay. A very narrow extent of the influence zone during the static compression test was observed, showing that the response of the P-2 piezometer is practically negligible. This suggests that stress changes near the column base during SLT in soft soils are very local and do not extend beyond 1.5–2D distance from the column axis. Taking into account the measurements performed on instrumented piles (Bond and Jardine 1995; Gavin et al. 2010) during static loading, one can expect a higher increase of EPWP closer to the column.References ASIRI (Amélioration des sols par les inclusions rigides). 2012. Recommendations for the design, construction and control of rigid inclusion ground improvements. Paris: Presses des Ponts. ASTM International. 2020. 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