remote sensing

Article Application of Geodetic Measuring Methods for Reliable Evaluation of Static Load Test Results of Piles

Zbigniew Muszy ´nski 1,* and Jarosław Rybak 2

1 Faculty of Geoengineering, Mining and , Wrocław University of Science and Technology, 50-370 Wrocław, Poland 2 Faculty of Civil Engineering, Wrocław University of Science and Technology, 50-370 Wrocław, Poland; [email protected] * Correspondence: [email protected]

Abstract: Geodetic measuring methods are widely used in the course of various geotechnical works. The main purpose is usually related to the location in space, geometrical dimensions, settlements, deflections, and other forms of displacements and their consequences. This study focuses on the application of selected surveying methods in static load tests (SLTs) of foundation piles. Basic aspects of the SLT are presented in the introductory section, together with the explanation of the authors’ motivation behind the novel (but already sufficiently tested) application of remote methods introduced to confirm, through inverse analysis, the load applied to the pile head under testing at every stage of its loading. Materials and methods are described in the second section in order to   provide basic information on the test site and principles of the SLT method applied. The case study shows the methodology of displacement control in the particular test, which is described in light of a Citation: Muszy´nski,Z.; Rybak, J. presented review of geodetic techniques for displacement control, especially terrestrial laser scanning Application of Geodetic Measuring and robotic tacheometry. The geotechnical testing procedure, which is of secondary importance for Methods for Reliable Evaluation of the current study, is also introduced in order to emphasize the versatility of the proposed method. Static Load Test Results of Special attention is paid to inverse analysis (controlling of the pile loading force on the basis of Foundation Piles. Remote Sens. 2021, measured deflections, and static calculations by means of standard structural analysis and the finite 13, 3082. https://doi.org/10.3390/ rs13163082 element method (FEM)) as a tool to raise the credibility of the obtained SLT results. The present case study from just one SLT, instrumented with various geodetic instrumentation, shows the results of Academic Editors: a real-world dimensions test. The obtained variability of the loading force within a range of 15% Agnieszka Le´sniak, (depending on real beam stiffness) proves good prospects for the application of the proposed idea Maria Mrówczy´nska,Marta Skiba in practice. The results are discussed mainly in light of the previous authors’ experience with the and Małgorzata Sztubecka application of remote techniques for reliable displacement control. As only a few references could be found (mainly by private communication), both the prospects for new developments using faster Received: 30 June 2021 and more accurate instruments as as the need for the validation of these findings on a larger Accepted: 3 August 2021 number of SLTs (with a very precise definition of beam stiffness) are underlined in the final remarks. Published: 5 August 2021

Keywords: static load test; robotic total station; terrestrial laser scanning; testing appliance deforma- Publisher’s Note: MDPI stays neutral tion; inverse analysis; loading force; reliability; creditability of measured values with regard to jurisdictional claims in published maps and institutional affil- iations.

1. Introduction A fast and reliable detection of the displacements and deformations of civil-engineering structures is one of the biggest challenges posed to contemporary engineering surveying. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. The results of geodetic measurements are commonly used to analyze the efforts of existing This article is an open access article structures (chimneys, cooling towers, silos, and tanks), for which the deformation value distributed under the terms and makes it possible to determine the cross-sectional forces on the basis of the measured conditions of the Creative Commons displacement and estimated stiffness values. The latter factor leads to some doubts about Attribution (CC BY) license (https:// the accuracy of such a procedure because a proper evaluation of stiffness may be difficult creativecommons.org/licenses/by/ in the case of structures that have a number of semi-rigid connections. Backward analysis 4.0/). of measured deformations also makes it possible to determine the impact (pressure) on

Remote Sens. 2021, 13, 3082. https://doi.org/10.3390/rs13163082 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, 3082 2 of 20

structures with a fixed stiffness. In laboratory conditions, remote displacement measure- ment methods are used to assess the stiffness of materials in structures under testing (e.g., bend beams) or to identify the shape of the slip surface in the testing of bulk media (Taylor Schnebeli apparatus). This study presents an unusual example of application of geodetic methods in the study of foundation piles capacity. Static load tests (SLTs) of foundation piles are, due to significant variability of geotechnical parameters, an important research procedure both at the initial design stage [1,2] and at the final inspection of the already made foundation piles [3]. The values determined in the static test are: the force loading the pile under testing and the pile’s displacement (settlement, elevation, and horizontal displacement), recorded at each stage of the test. The issues related to the required reliabil- ity of displacement measurement in a static test and problems with its guarantee (due to the instability of the reference beam for displacement sensors with high accuracy, as used typically) were repeatedly raised [4], and the possibility of ensuring at least the control of these measurements based on geodetic methods was emphasized [4,5]. The reliability of the measurement of the pile loading force is a problem that is signaled less frequently. When hydraulic jacks with known characteristics (i.e., periodically calibrated in accredited laboratories) are used during the test, it is assumed that, in field conditions, the force exerted at the subsequent load stages can be directly determined on the basis of the pressure measured with high accuracy in the hydraulic system (close to the jack in order to avoid pressure losses in the hydraulic system). Static test instructions take into account the obvious phenomenon of a decrease in the loading force due to the extending of the piston of the hydraulic cylinder caused by both the deflection of the beam system transferring loads to the anchor piles, as well as the parallel (continuously, though not uniformly) settlement of the pile. The stability of the loading force is therefore guaranteed by the continuous monitoring of the pressure stability in the hydraulic system. Less often, the problem of the reliability of actuator characteristics (or the entire load system with the pump, pressure gauge, hose, and connector system) is raised, which is determined in laboratory conditions. Paradoxically, this element has the greatest impact on the subsequent assessment of pile , which is the purpose of the test. The problem results from different operating conditions of the actuator in the test stand than in the laboratory. The parallelism of the pressed surfaces (pile head and reaction beam) is virtually impossible to guarantee in field conditions [6]. The use of ball bearings only slightly limits the piston’s pressure to the side surfaces of the cylinder and the inevitable decrease in the force caused by . Reported errors in the assessment of the measured force can reach a dozen or so percent, which directly translates into the determined bearing capacity of the piles under testing. The measurement of force during the static test of a foundation pile can be “to some extent validated” by backward analysis of the deformation of a steel structure intended to ensure the transfer of force to the anchor piles. Knowledge of the beam rigidity and current measurement of its deformation by geodetic methods makes it possible to determine, by means of inverse analysis, the force loading the beam (and, simultaneously, the force loading the pileunder the load). Importantly, the procedure itself is often used in laboratory tests of reinforced concrete elements, wherein their variable stiffness is determined in the conditions of increasing loading force and progressive cracking. This paper presents the results of a static test performed on a specially designed test stand on a natural scale (real-world dimensions test). The presented research is part of a broader research project on the control of closed-bottom pipe-pile (steel pipe with the end closed with a steel plate, vibratory driven to the depth of 8 m). The load-bearing structure consisted of a steel reaction beam (assembled by welding) attached to two anchor piles formed by means of high-pressure injection (jet-grouting). In addition to the classic dial indicators measuring the settlement of the pile under testing, two independent geodetic techniques were used: robotic total station and terrestrial laser scanning. Remote surveying measurements (without the need for direct contact with the loading structure during the test) enabled the determination of: the bending of the reaction beam, lifting of anchor piles, and vertical displacement of the pile under the load in the external reference system not Remote Sens. 2021, 13, x FOR PEER REVIEW 3 of 20

Remote Sens. 2021, 13, 3082 surveying measurements (without the need for direct contact with the loading structure3 of 20 during the test) enabled the determination of: the bending of the reaction beam, lifting of anchor piles, and vertical displacement of the pile under the load in the external reference system not related to the test stand. The measurements of the steel loading structure indi- related to the test stand. The measurements of the steel loading structure indicate both the cate both the possibility of an ongoing control of achieving the assumed maximum force possibility of an ongoing control of achieving the assumed maximum force during the test during the test (in order to ensure the certainty of obtaining the load range required by (in order to ensure the certainty of obtaining the load range required by the standards), the standards), as well as the control of the load-displacement relationship needed to de- as well as the control of the load-displacement relationship needed to determine the pile termine the pile capacity during the final preparation of the static test results. capacity during the final preparation of the static test results. After preliminary analysis, terrestrial laser scanning was chosen for the description After preliminary analysis, terrestrial laser scanning was chosen for the description of geometryof geometry of theof the testing testing appliance, appliance, whereas whereas the the robotic robotic total total station station served served as a as tool a tool in the in fastthe fast and and reliable reliable control control of the of displacement.the displacement.

2.2. Materials andand MethodsMethods 2.1.2.1. Study SiteSite TheThe studystudy sitesite waswas locatedlocated inin BojszowyBojszowy NoweNowe inin southernsouthern Poland.Poland. TheThe geotechnicalgeotechnical profileprofile (Figure(Figure1 1)) was was complex complex with with various various layers layers of of granular granular mineral mineral : soils: mainly mainly fine fine sandssands (FSa)(FSa) and and medium medium sands (MSa), (MSa), and and with with an interbeddingan interbedding layer layer of cohesive of cohesive with soil somewith some organic organic amount amount in the ceilingin the ceiling part. Basic part. geotechnical Basic geotechnical parameters parameters are juxtaposed are juxta- in Tableposed1. in Table 1.

Figure 1. Geotechnical profile at the location of an 8 m steel closed-end pipe-pile under testing. Figure 1. Geotechnical profile at the location of an 8 m steel closed-end pipe-pile under testing.

TableTable 1.1.Basic Basic geotechnicalgeotechnical parameters.parameters.

γ γ ϕφ c c NumberNumber Soil Soil [kN/m[kN/m3] 3] [deg][deg] [kPa][kPa] 1 1- -18.5 18.5 29.829.8 11 2 2FSa FSa 16.6 16.6 30.5 30.5 1 1 3 3MSa MSa 17.7 17.7 36.0 36.0 1 1 4 Or 16.7 18.0 5 4 Or 16.7 18.0 5 5 siCl 19.1 19.2 16 5 6siCl MSa 19.1 17.7 34.7 19.2 1 16 6 7MSa MSa 17.7 17.7 32.7 34.7 1 1 8 MSa 17.7 36.2 1 γ = unit weight, ϕ = friction angle, and c = .

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The pile used in the displacement test was constructed as a closed-end steel pipe, embedded in the ground by means of vibratory driving. Its diameter was 40 cm, total length equalled 8 m, and pipe wall thickness was 10 mm. It may be observed that pile toe is embedded in medium . Its capacity derived from the static calculation and considering the SLT exceeds 1500 kN. Test range was set to 1250 kN due to the limited capacity of anchoring piles and connections in the testing appliance. The testing procedure was defined by the Polish Code of Practice [7] as a maintained load test with a constant rate of loading and maintained time (until settlement stabilization) at every loading step. Two load cycles were performed. The average time between consecutive load steps was 8 min in the first cycle and 11 min in the second cycle (Table2).

Table 2. Schedule of the conducted SLT.

Number Start Time [hh:mm] Stop Time [hh:mm] Pressure [bar] Force [kN] 1 12:43 12:48 0 0 2 12:52 12:56 50 121 3 13:01 13:56 100 243 4 13:10 13:15 150 364 5 13:19 13:24 200 486 6 13:28 13:33 250 607 7 13:37 13:42 300 729 8 13:45 13:50 350 850 9 13:53 13:58 400 971 10 14:03 14:08 450 1093 11 14:13 14:18 480 1166 12 14:22 14:27 500 1214 13 14:31 14:36 350 850 14 14:39 14:44 200 486 15 14:47 14:52 0 0 16 15:17 15:22 50 121 17 15:26 15:31 100 243 18 15:38 15:43 150 364 19 15:49 15:54 200 486 20 15:59 16:04 250 607 21 16:09 16:14 300 729 22 16:20 16:25 350 850 23 16:31 16:36 400 971 24 16:42 16:47 450 1093 25 16:55 17:00 480 1166 26 17:10 17:15 240 583 27 17:19 17:24 0 0

It must be emphasized here that other procedures of SLT, such as constant rate of penetration or constant time of loading at every step, do not change the applicability of the proposed procedure of inverse analysis. The loading scheme, with the reaction beam enabling the transfer of the loads to the anchoring piles (Figure2a), predesignates the possibility of applying remote measurement methods (Figure2b). If the load had to be transferred to a kentledge system, all measurements would be much more complicated due to the limited visibility and access to controlled points. 4 The basic characteristics of the reaction beam are: 2 × HEB450, Jx = 2 × 79,890 [cm ], l = 4.166 [m], and E = 205 [GPa]. The beam was strengthened with 2 additional steel plates in its central part. Increased stiffness was considered in numerical studies of beam deflection. Remote Sens. 2021, 13, x FOR PEER REVIEW 5 of 20

transferred to a kentledge system, all measurements would be much more complicated due to the limited visibility and access to controlled points. The basic characteristics of the reaction beam are: 2 × HEB450, x = 2 × 79,890 [cm4], l Remote Sens. 2021, 13, 3082 = 4.166 [m], and E = 205 [GPa]. The beam was strengthened with 2 additional steel plates5 of 20 in its central part. Increased stiffness was considered in numerical studies of beam deflec- tion.

(a) (b)

FigureFigure 2.2. ViewView ofof thethe testtest standstand andand instrumentation:instrumentation: ( a) the SLT appliance with attached dial dial gauges, gauges, prisms prisms for for total total station,station, andand planarplanar targetstargets forfor scanner;scanner; andand ((bb)) thethe positionposition of the laser scanner and tota totall station station right right in in front front of of the the loading loading structures.structures.

During the SLT, two geodetic methods were used. The The first first was was precise precise tacheometry tacheometry (trigonometrical(trigonometrical heighting), the the purpose purpose of of which which was was to to determine determine the the vertical vertical displace- displace- ments of key elements of the test stand (rea (reactionction beam, beam, loaded loaded pile, pile, and and anchor anchor piles). piles). Measurements were performed with a Trimble S3 robotic total station station (Figure (Figure 22b)b) with an angle accuracyaccuracy ofof 0.60.6 mgon mgon and and a a distance distance measurement measurement accuracy accuracy of ±of2 ±2 mm mm + 2+ ppm 2 ppm in thein prismthe prism mode. mode. The The total total station station was was situated situated at a distanceat a distance of 6.85 of 6.85 m and m 8.43and m8.43 from m from the ends the ofends the of reaction the reaction beam. beam. The local The referencelocal reference system system was established was established by three by referencethree reference points (A,points B, and (A, C)B, locatedand C) inlocated the range in the of range 10.16 ÷of11.28 10.16 m ÷ from11.28 them from instrument. the instrument. The arrangement The ar- ofrangement the total of station the total position station and position reference and reference points ensured points aensured favourable a favourable geometry geome- of the referencingtry of the referencing directions directions (Figure3). (Figure The reference 3). The pointsreference were points materialized were materialized by means by of − surveyingmeans of surveying prisms with prisms a diameter with a of diamet 62.5 mmer of and 62.5 prism mm constantand prism of constant30 mm, of mounted −30 mm, in tripodsmounted throughout in tripods thethroughout duration the of theduration measurements of the measurements (Figure2a). The (Figure set of 2a). check The points set of consistedcheck points of (Figureconsisted2a): of (Figure 2a): • • fivefive points (numbers 5–9) located on the re reactionaction beam and and materialized materialized by by geodetic geodetic − prisms with a diameter of 62.562.5 mm and prism constant of −3030 mm; mm; • • two points points (numbers (numbers 2 2and and 3) 3) attached attached to tothe the side side surface surface of the of loaded the loaded pile and pile ma- and materialized by geodetic prisms with a diameter of 25.4 mm and prism constant of terialized by geodetic prisms with a diameter of 25.4 mm and prism constant of −16.9 −16.9 mm; and mm; and • two points (numbers 1 and 4) attached to the left and right anchor piles, respectively, • two points (numbers 1 and 4) attached to the left and right anchor piles, respectively, and materialized by geodetic prisms with a diameter of 25.4 mm and prism constant and materialized by geodetic prisms with a diameter of 25.4 mm and prism constant of −16.9 mm. of −16.9 mm. The total station was remotely controlled by a TSC3 controller, in which the automatic The total station was remotely controlled by a TSC3 controller, in which the auto- series of measurements were defined. In each series, all prisms (A, B, C, and 1–9) were matic series of measurements were defined. In each series, all prisms (A, B, C, and 1–9) measured in two positions of the telescope (face left and face right) using the automatic were measured in two positions of the telescope (face left and face right) using the auto- target recognition system. The first series of measurements was made before starting the matic target recognition system. The first series of measurements was made before start- pile loading. Successive series were performed synchronously to the individual stages of ing the pile loading. Successive series were performed synchronously to the individual pile loading during two loading cycles. stages of pile loading during two loading cycles.

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Figure 3. Arrangement of measuring instruments and reference points. Figure 3. Arrangement of measuring instruments and reference points.

TheThe second second measurement measurement method method was was terrestrial terrestrial laser laser scanning scanning (TLS), (TLS), the the purpose purpose of whichof which was was to obtain to obtain precise precise data ondata the on geometric the geometric shape ofshape the reaction of the reaction beam before beam loading. before Aloading. Leica ScanStation A Leica ScanStation C10 pulse C10 laser pulse scanner laser scanner was used was to used carry to out carry the out measurements the measure- (Figurements 2(Figureb). In the2b). range In the ofrange 50 m, of the50 m, accuracy the accuracy of a single of a single angle, angle, distance, distance, and positionand posi- measurementtion measurement were 3.8were mgon, 3.8 mgon, 4 mm, and4 mm, 6 mm, and respectively. 6 mm, respectively. According According to the manufacturer, to the man- theufacturer, precision the of precision the modeled of the surface modeled was surface 2 mm andwas the2 mm standard and the deviation standard of deviation the target of acquisitionthe target acquisition was 2 mm. was The 2 scanner mm. The position scanner was position located was opposite located to opposite the reaction to the beam reaction and tied-inbeam and to three tied-in reference to three points reference (As, Bs,points and (As, Cs), locatedBs, and inCs), the located vicinity in of the the vicinity test stand of the in thetest range stand of in 10.08 the range÷ 11.83 of 10.08 m (Figure ÷ 11.833). m The (Fig referenceure 3). The points reference were materializedpoints were materialized by circular planarby circular targets planar in blue targets and white in blue colours and white with aco diameterlours with of a 6 diameter inches. The of set6 inches. of check The points set of consistedcheck points of (Figures consisted2a andof (Figures4): 2a and 4): •• threethree points points (number (number 5 5 s, s, 7 7 s, s, and and 9 9 s) s) located located on on the the reaction reaction beam beam and and materialized materialized byby circular circular planar planar targets targets in in blue blue and and white white colours colours with with a a diameter diameter of of 6 6 inches; inches; •• twotwo points points (number(number 22 ss and and 3 3 s) s) attached attached to to the the side side surface surface of of the the loaded loaded pile pile and and materializedmaterialized by by square square planar planar targets targets in in blue blue and and white white colours colours with with a a dimension dimension of of 33× × 3 inches; and •• twotwo points points (number (number 1 s1 ands and 4 s) 4 attacheds) attached to the to leftthe andleft rightand right anchor anchor piles, piles, respectively, respec- andtively, materialized and materialized by square by planar square targets planar in targets blue and in white blue coloursand white with colours a dimension with a ofdimension 3 × 3 inches. of 3 × 3 inches. TheThe laser laser scanner scanner was was controlled controlled by by laptop laptop with with Leica Leica Cyclone Cyclone software software [8]. Before[8]. Before the pilethe loadingpile loading started, started, a detailed a detailed scan scan of the of wholethe whole test standtest stand was was performed. performed. Additionally, Addition- allally, planar all planar targets targets dedicated dedicated for the for scanner the scanne werer measuredwere measured once with once the with total the station. total station. That madeThat itmade possible it possible to assign to aassign common a common coordinate coordinate system to system the measurement to the measurement data from both data instrumentsfrom both instruments at a later stage. at a later During stage. the During subsequent the subsequent loading steps, loading only steps, the planar only targetsthe pla- werenar targets scanned were in detailscanned due in to detail the limited due to th scane limited speed scan (maximum speed (maximum instantaneous instantaneous scan rate isscan 50,000 rate points is 50,000 per second)points per and second) short time and per short load time step per (few load minutes). step (few The minutes). dimensions The describingdimensions the describing distribution the ofdistri thebution controlled of the points controlled on the points test stand on the are test shown stand in are Figure shown5. in Figure 5.

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Figure 4. Scanned test stand: schematic layout and numbering of check points.

Figure 5. Scanned test stand: distances between structure elements and measurement points. Figure 5. Scanned test stand: distances between st structureructure elements and measurement points. 2.2. Geodetic Techniques for Displacement Control 2.2. Geodetic Techniques for Displacement Control The determination of the displacement of natural objects and anthropogenic struc- The determination ofof thethe displacement displacement of of natural natural objects objects and and anthropogenic anthropogenic structure struc- ture is the main task of engineering geodesy. Depending on the character of the object tureis the is main the main task oftask engineering of engineering geodesy. geodes Dependingy. Depending on the on character the character of the objectof the underobject under scrutiny, there are different requirements in relation to: the dimension of displace- underscrutiny, scrutiny, there are there different are different requirements requirements in relation in relation to: the dimension to: the dimension of displacement of displace- (1D, ment (1D, 2D, and 3D); the coordinate system (absolute or relative); the range of the ex- ment2D, and (1D, 3D); 2D, the and coordinate 3D); the systemcoordinate (absolute system or relative);(absolute the or rangerelative); of the the expected range of displace- the ex- pected displacement and requested accuracy; the availability of object (direct or remote pectedment and displacement requested accuracy;and requested the availability accuracy; ofthe object availability (direct of or object remote (direct measurements); or remote measurements); and the dynamics of displacement changes and duration of the geodetic measurements);and the dynamics and of displacementthe dynamics changesof displace andment duration changes of the and geodetic duration measurement. of the geodetic All measurement. All these factors must be considered when selecting measurement methods measurement.these factors must All these be considered factors must when be cons selectingidered measurement when selecting methods measurement and techniques. methods and techniques. Geodetic methods are primarily aimed at providing an external coordi- andGeodetic techniques. methods Geodetic are primarily methods aimed are atprimarily providing aimed an external at providing coordinate an external system coordi- which nateis independent system which of the is independent object under of survey. the obje Thisct under is an undoubted survey. This advantage is an undoubted but simulta- ad- vantageneously but is associated simultaneously with limited is associated accuracy. with For limited comparison, accuracy. physical For comparison, methods havephysical the methodsaccuracy have of even the a accuracy few orders of ofeven magnitude a few orders higher of butmagnitude they allow higher the but registering they allow of only the registeringrelative displacements. of only relative displacements. The most common applications of geodetic measurements measurements in geotechnics include: • • measurements of of vertical vertical displacements displacements (1D) (1D) of ofbuildings buildings in inthe the vicinity vicinity of deep of deep ex- cavationsexcavations and and vertical vertical displacements displacements of the of thepile pileand andanchor anchor piles piles under under testing testing dur- ingduring the SLT; the SLT; • • measurements of horizontal displacements (2D) of the deep excavation wall and the piles during lateral test loads;

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• measurements of spatial displacements (3D) of the retaining structures and ; and • measurements of deformation of engineering structures (tailing ponds, , and bridges) and the surrounding area. From the surveyor’s perspective, a displacement is a change in the position (coordi- nates) of selected check points representing the object in the assumed time interval in an external reference system, wherein the reference system is independent of the monitored object (“absolute” displacements). We can detect deformations when the mutual position of check points within the object under testing changes additionally. Then, the shape changes and the object does not move like a rigid body. The classic method of determining vertical displacements is precision leveling, which allows for obtaining accuracy in tenths of a millimeter. An example of the application of this method in geotechnical monitoring can be found in [5,9–12]. Nowadays, horizontal displacement measurements are carried out using robotic total stations, sometimes combined with GNSS static measurements on reference points in extensive measurement networks. The common application of total station measurements described in the literature are monitoring [13,14] and monitoring of a deep ex- cavation in the urban space [15], as well as horizontal displacement control during the test of the pile lateral loading [16]. Total stations equipped with high-resolution cameras, called image assisted total stations (IATS), are increasingly used for vibration measure- ments [17,18]. The total station and GNSS receiver can also measure spatial displacements, although the spatial resolution of this measurement is very limited. Additionally, in the case of the GNSS technique, the measurement time is quite long and the accuracy is not very high. Conversely, the advantage is the ability to repeatedly measure the same well-defined points. Nowadays, close-range photogrammetry is a very popular method to obtain 3D models describing the geometry of civil-engineering structures. In recent decades, pho- togrammetry has significantly evolved from the use of analog metric cameras (equipped with professional lenses with low image distortion) to digital cameras (even low-cost, non-metric) that try to compensate the imperfection of lenses with advanced calibration models. The algorithms for digital image processing and the automatic recognition of characteristic points in photos significantly increased the area of application, shortened the development time, and increased the accuracy of the results. The development of the tech- nical capabilities of unmanned aerial vehicles (UAV) extends the field of photogrammetry applications to larger engineering facilities. An example of using UAV to acquire photos and then automatic detection of rail surface defects is presented in [19]. Photogrammetry and digital image correlation are widely used for 3D deformation measurements. Using the structure from the motion technique, we can generate point clouds comparable to those obtained from laser scanners. On the basis of the optimized displacement surfaces, it is possible to evaluate stress distributions using the boundary element method [20]. An example of using a digital image-based method for the deformation-monitoring of a steel truss–concrete composite beam during a static load test is described in [21]. The application of photogrammetry to assess the deformations of rod and plate elements of the steel constructions of hoisting machines is proposed in [22]. For at least a decade, terrestrial laser scanning has been widely used in the measure- ment of displacement and deformation. The continuous development of this technology allows for the increase in the accuracy, range, and speed of measurement. Depending on the type of laser scanner (continuous wave or pulsed) and specific model, the range varies from several dozen to several hundred meters, the accuracy ranges from a few to several millimeters, and the measurement speed can reach a million points per second. Due to the quasi-continuous mapping of the measured surfaces in the form of point clouds, TLS provides much more information about the shape of the object under scrutiny than the point measurement with a total station. The accuracy of the laser scanning in relation to the total station measurements was analyzed among others in [10,23–25]. In the matters Remote Sens. 2021, 13, 3082 9 of 20

related to monitoring, various measurement techniques are often combined, which allows for the collecting of a wider range of data and the building of multi-sensor-systems [26,27]. TLS is often used in the monitoring of displacements of landslides [28], in the retaining of structures [29–32], in mechanically stabilized earth walls [33], or in various types of hydrotechnical facilities [10,25,34,35]. Reliable stability analysis of the granite boulder on the basis of TLS data was described in [36]. The study of the deformation of tunnels, as well as of creating deformation models using data obtained from laser scanning, are very popular [37–44]. A slightly different issue that has been recently gaining popularity is the use of TLS to assess the roughness parameters of concrete surfaces interacting with the ground in geotechnical structures. Such analyses are performed with optical scanners on small samples in laboratory conditions [45–47] or with laser scanners on the construction site on a natural scale [29,31,48,49].

2.3. Geotechnical Testing Procedure The constant rate of the load procedure was applied in accordance with the Polish Code of Practice [50]. After the assembling of the testing appliance (pile head, hydraulic jack, reference system for pile head displacement control, reaction beam, steel anchors, and reaction piles) and its instrumentation (dial gauges and controlled points with geodetic prisms and targets), the load was applied in stages to achieve the primarily assumed range. After unloading, the second round of loading was applied to check the pile’s behaviour under a cyclic loading. The main goal of the test according to [50] is to determine the load step, when pile settlements tend to arise and permanent settlements appear (and increase). The second goal of the test is to determine the so-called ultimate capacity, meaning the load that causes unstoppable and infinite settlement. This last value allows for the analysis of pile capacity according to [1]. If ultimate capacity is not used in the course of pile loading, various methods of test extrapolation can be applied; however, it always leads to some risk of inaccuracies. The authors’ previous studies proved that the reliability of extrapolation methods is highly dependent on the accuracy of displacement control at the last steps (stages of loading). The same doubts may arise concerning the accuracy of the force control and for that reason, the applied force range should be somehow controlled using an independent external system. It is worth mentioning that piles used as anchors stabilizing the reaction beam are subject to large pulling forces. Their uplift may significantly change their behaviour (due to residual forces) in the course of standard loading under the building structure. That is why, at least in the Polish Code of Practice [50], the uplift of anchoring piles is limited to 5 mm. If that value is exceeded during the test, the anchoring pile is assumed to have only 80% of this designed load in compression. To avoid such a problem, current control of the anchoring pile uplift must be achieved and relevant modifications of the testing program may be applied. Last but not least, one has to understand the difference between pile behaviour in compression (pushing towards the ground body) and its pulling resistance. In the case of typical axial loading by gravity forces, both pile shaft and base resistances are mobilized at various stages of the pile displacement. First, shaft resistance based on static frictional resistance and adhesion appear between the pile shaft and ground body. The range of pile displacement needed to mobilize this part of its capacity is close to 0.15% of its diameter. Mobilization of bottom resistance is based on a mechanism similar to shallow foundations with an appearance of plastic zones around the pile base. This part of the pile capacity is developed slowly and the full value of the base capacity is assumed to occur when its displacement ranges around 10% of the pile diameter. The pile that is working in tension does not have any reserve of capacity resulting of base resistance. The capacity is achieved relatively suddenly (without warning) and it happens quite frequently that the anchoring pile jumps out of the ground in the course of the test. Such a situation is not acceptable because: (1) it violently stops the testing procedure before the designed range of load is achieved and (2) the anchoring pile that Remote Sens. 2021, 13, x FOR PEER REVIEW 10 of 20

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“withdrawn” from the ground cannot be (in most cases, except precast driven piles) used in further construction. was “withdrawn” from the ground cannot be (in most cases, except precast driven piles) That is why in both cases for the pile under testing and for the anchoring piles, ex- used in further construction. cessive and even redundant measuring (controlling) techniques and systems are neces- That is why in both cases for the pile under testing and for the anchoring piles, exces- sary and should be widely applied. sive and even redundant measuring (controlling) techniques and systems are necessary and should be widely applied. 2.4. Inverse Analysis 2.4. InverseInverse Analysis analysis (or in other words, back-analysis) is a widely known procedure that enablesInverse the proper analysis evaluation (or in other of stresses words, or back-analysis)forces acting on is a a structure widely known on the basis procedure of its deformation.that enables theIn geotechnical proper evaluation engineering, of stresses inverse or forces analysis acting is commonly on a structure used onfor thethe basispur- poseof its of deformation. the so-called In “observational ,method”, which inverse consists analysis of updating is commonly the computa- used for tionalthe purpose model ofon thethe so-calledbasis of the “observational information method”,achieved at which earlier consists stages of loading. updating This the method,computational devised model by Terzaghi on the basis and ofPeck the [51], information was introduced achieved mainly at earlier in stagesthe designing of loading. of deepThis method,excavations devised [52] in by which Terzaghi problems and Peck of the [51], proper was introduced evaluation mainly of earth in pressures the designing create of thedeep need excavations for partial [52 validation] in which and problems calibration of the of proper the computational evaluation of earthmodel pressures at subsequent create stagesthe need of forexcavations. partial validation Earth pressures and calibration acting ofon the soil-shell computational objects modelmade of at subsequentcorrugated sheetsstages may of excavations. also be derived Earth on pressures the basis acting of back-analysis on soil-shell objectsbased madeon corrugated of corrugated sheets’ sheets de- flectionmay also [53]. bederived A similar on procedure the basis of may back-analysis be applied based in tunneling on corrugated works sheets’ in which deflection rock pres- [53]. sureA similar on the procedure lining may may be appliedalso be estimated in tunneling based works on internal in which shape rock deformation pressure on con- the trol.tunnel lining may also be estimated based on internal shape deformation control. ThisThis study study advocates advocates the the inverse inverse analysis analysis for for the evaluation of loading force during thethe SLT SLT procedure. Here, Here, both both geometric geometric charac characteristicsteristics of of the the element element under under load load (e.g., (e.g., constantconstant moment moment of of inertia inertia )J )as as well well as asthe the elasti elasticc modulus modulus of material of material E areE known.are known. ConsiderConsider a well-known a well-known static static equation equation for bend for bend arrow arrowf (1): f (1): ∙F·a2·b2∙ f == (1) 33·E·J·∙l wherewhere F = appliedapplied force; f == bend bend arrow; arrow; aa,, bb == distances distances to to supports; supports; and ll = total total length length of thethe beam, beam, wherein wherein ll == aa ++ bb (Figure(Figure 6).6).

FigureFigure 6. 6. StaticStatic scheme scheme of of the the beam beam loaded loaded non-centrically. non-centrically.

AA proper proper approximation approximation of both both the the geometry geometry of of the the beam beam and and its its deflection deflection at at every every stagestage of of loading loading make make it it possible possible to to derive derive the the value value of of the loading force according to EquationEquation (2): (2): 3· f ·E·J·l F = (2) 3∙a2·∙∙∙b2 (2) = 3. Results ∙ The entire test was performed in two cycles and many stages. The pile was loaded and 3.unloaded Results in constant steps. Two independent issues were controlled simultaneously by meansThe of entire dial gauges test was and performed remote geodetic in two methods cycles and in the many course stages. of the The test. pile Pile was settlements loaded andfrom unloaded dial gauges in constant are presented steps. Two in Figure independent7. It may issues be observed were controlled that the pilesimultaneously settlement byrange means is within of dial 30 gauges mm (in and 2 cycles). remote The geodetic expected methods beam deflectionin the course was of also the withintest. Pile the settle- same mentsrange offrom 30 mmdial gauges (both values are presented had to be in pre-assessed Figure 7. It may for proper be observed selection that of the the pile hydraulic settle- mentjack, consideringrange is within its piston 30 mm length, (in 2 cycles). which wasThe equalexpected to 150 beam mm). deflection was also within

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the same range of 30 mm (both values had to be pre-assessed for proper selection of the Remote Sens. 2021, 13, 3082 11 of 20 thehydraulic same range jack, of considering 30 mm (both its values piston hadlength, to be which pre-assessed was equal for to proper 150 mm). selection of the hydraulic jack, considering its piston length, which was equal to 150 mm). 0.00 0.00 -5.00 -5.00 -10.00 -10.00 -15.00 -15.00 -20.00 -20.00 -25.00 -25.00 Settlement [mm] -30.00

Settlement [mm] -30.00 -35.00 -35.00 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 Load [kN] Load [kN]

FigureFigure 7. LoadLoad versus versus settlement settlement curve curve from from the the SLT SLT in two cycles of loading on the basis of dial Figuregauges 7. Loadmeasurement. versus settlement curve from the SLT in two cycles of loading on the basis of dial gauges measurement. gauges measurement. TheThe elaboration elaboration of of the the results results of of geodetic geodetic measurements measurements began with the control of the stabilitystabilityThe elaboration of of reference reference of thepoints. points. results The The of “apparent” “apparent”geodetic measurements displacements displacements began of of the the with points points the controlA, A, B, B, and of theC do stabilitynotnot exceed exceed of reference the the interval interval points. of of 2.5 2.5The times “apparent” in in mean displacements e errorsrrors in their of determination; the points A, therefore, B, and C dothey notfallfall exceed within within the the the interval measurement measurement of 2.5 times noise noise in and and mean the the e refere referencerrors ncein their points determination; can be treated therefore, as a constant they in falltimetime within (Figure the measurement8 8).). On On the the basis basis noise of of the theand data data the from refere from total ncetotal station points station measurements,can measurements, be treated as the a the heightsconstant heights of in the of timethecontrolled (Figure controlled 8). points Onpoints the for for individualbasis individual of the epochs data epochs from were were total calculated calculated station usingmeasurements, using the the trigonometric trigonometric the heights leveling level- of theingmethod. controlled method. Treating pointsTreating the for firstthe individual first epoch epoch as epochs an as initial an were initial measurement, calculated measurement, using the vertical thethe verticaltrigonometric displacements displacements level- of all ingofcontrol method. all control points Treating points were the calculated. were first calculated. epoch Figure as an9 Figure shows initial 9 themeasurement, shows lifting the of anchorlifting the vertical of piles anchor and displacements thepiles settlement and the of settlementofall thecontrol tested ofpoints pilethe tested duringwere pilecalculated. the during SLT. The theFigure beam SLT. 9 The deflectionshows beam the deflection observedlifting of inobservedanchor cycle piles 1 in and cycle and cycle 1the and 2 is settlementcycleshown 2 is in of shown Figure the tested in10 Figurea,b, pile respectively. during10a,b, respectively. the SLT. The beam deflection observed in cycle 1 and cycle 2 is shown in Figure 10a,b, respectively.

Figure 8. “Apparent” vertical displacement (measurement noise) of reference points: A, B, and C during first and second loading cycles with an acceptable range of ±2.5·mA, ±2.5·mB, and ±2.5·mC (two and a half times the mean error of vertical displacement determination for each reference point).

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Figure 8. “Apparent” vertical displacement (measurement noise) of reference points: A, B, and C during first and second loading cycles with an acceptable range of ±2.5·mA, ±2.5·mB, and ±2.5·mC (two and a half times the mean error of vertical displacement determination for each reference point). RemoteFigure Sens. 2021 8. “,Apparent”13, 3082 vertical displacement (measurement noise) of reference points: A, B, and C during first and second12 of 20 loading cycles with an acceptable range of ±2.5·mA, ±2.5·mB, and ±2.5·mC (two and a half times the mean error of vertical displacement determination for each reference point).

Figure 9. Vertical displacement of check points: 1 = left anchor pile, 2 and 3 = loaded pile, and 4= right anchor pile. Figure 9. Vertical displacement of check points: 1 = left anchor pile, 2 and 3 = loaded pile, and 4= right anchor pile.

Figure 9. Vertical displacement of check points: 1 = left anchor pile, 2 and 3 = loaded pile, and 4= right anchor pile.

(a)

Figure 10. Cont. (a)

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(b) (b) Figure 10. Bending of the beam during the SLT on the basis of vertical displacement in control points numbers 5–9 from Figure 10. Bending of the beam during the SLT on the basis of verticalvertical displacement in control points numbers 5–9 from Trimble S3: (a) first loading cycle and (b) second loading cycle. Trimble S3: (a) firstfirst loading cycle andand ((b)) secondsecond loadingloading cycle.cycle. On the basis of the point cloud from laser scanning, the detailed beam geometry data OnOn the the basis basis of of the the point point cloud cloud from from laser laser scanning, scanning, the the detailed detailed beam beam geometry geometry data was obtained, which are shown in Figure 11. waswas obtained, which are shown in Figure 1111..

Figure 11. The dimensions of the reaction beam (top view, front view, and section A-A). FigureFigure 11. 11. TheThe dimensions dimensions of of the the reaction reaction beam beam (top (top view, view, front view, and section A-A).

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The pile settlement is not the main subject of current study as the problem of its cred- ibilityThe was pile discussed settlement in former is not sources the main (includi subjectng ofauthors’ current contributions study as the [4,5,10,16]). problem of Only its credibilitythe relevance was of discussed the load measured in former indirectly sources (including by the control authors’ of oil contributions pressure in the [4 ,hydraulic5,10,16]). Onlysystem the is relevance being checked of the by load back-analysis measured indirectlyof the steel by beam the controldeflection. of oil pressure in the hydraulicLooking system at Figure is being 10, checked we may by observe back-analysis that the of deflection the steel beam raises deflection. at every load step and Lookingreaches its at Figuremagnitude 10, we in may point observe 7, directly that theover deflection the piston. raises As atthe every elastic load modulus step and of reachesthe construction its magnitude steel is in rather point a 7, constant directly value, over the it must piston. be the As geometry the elastic that modulus is potentially of the constructionresponsible for steel the is lower rather displacements a constant value, (deflect it mustions); be see the Figure geometry 11. Let that us isconsider potentially now responsiblethe shape of for the the deflected lower displacements beam at every (deflections); stage of two see cycles Figure of 11 pile. Let loading. us consider Figure now 12 therepresents shape of the the shape deflected of the beam deflected at every beam stage at th ofe last two step cycles (i.e., of the pile highest loading. load Figure value) 12 of representsthe first load the cycle. shape If of we the measure deflected the beam bend at arrow the last by step means (i.e., of the geodetic highest methods, load value) the ofloading the first force load can cycle. be easily If we measureback-calculated the bend and arrow compared by means to the of geodeticforce derived methods, from the the loadinghydraulic force jack can characteristics be easily back-calculated on the basis of and the comparedoil pressure to control. the force We derived can just from compare the hydraulicthe bend arrow jack characteristics in point 7, measured on the basis on the of thebasis oil of pressure remote control.geodetic We measurements, can just compare with thethe bend results arrow of theoretical in point 7, equations measured and on the numeri basiscal of remotestudies geodetic concerning measurements, the precise geometry with the resultsand stiffness of theoretical of bended equations elements. and In numerical this way, studies we may concerning notice that the the precise force causing geometry deflec- and stiffnesstion must of have bended been elements. smaller Inthan this the way, one we de mayrived notice from thatthe hydraulic the force causingactuator deflection character- mustistics haveor that been the smaller beam must than thehave one been derived stiffer from than theassumed hydraulic for the actuator computation. characteristics Figure or13 thatshows the the beam relationship must have between been stiffer the forces than: assumedthe assumed for thevalue computation. (from pressure) Figure versus 13 showsthe force the obtained relationship from between the inverse the forces: analysis. the The assumed result value (blue (fromline) seems pressure) to be versus alarming the forcebut it obtained must be considered from the inverse that the analysis. simplified The Equation result (blue (1) assumes line) seems constant to be stiffness alarming along but itthe must beam. be consideredIn reality, the that beam the simplifiedwas reinforced Equation in its (1)central assumes part with constant an upper stiffness and along lower thesteel beam. plate. In The reality, increased the beam stiffness was in reinforced the central in part its central diminished part with the deflection. an upper and New lower com- steelputation plate. using The the increased dedicated stiffness software in the Robot central Structural part diminished Analysis [54] the for deflection. static analysis New computation(Figures 14–16) using showed the dedicated that the softwaremagnitude Robot of deflection Structural (bend Analysis arrow) [54] may for static be computed analysis (Figuresfrom the 14 following–16) showed equation, that the Equation magnitude (3) (see: of deflection Figure 16). (bend For the arrow) measured may be value computed of dis- fromplacement the following equal to equation,f = 4.82 mm, Equation using the (3) transformed (see: Figure Equation 16). For (4), the we measured achieve a value value of of displacementF = 1397.51 kN. equal to f = 4.82 mm, using the transformed Equation (4), we achieve a value of F = 1397.51 kN. =f = 0.0034490.003449 ∙ ·F (3)(3)

=F = 289.94289.94 ∙ · f (4)(4)

8 7 6 5 4 3 2 1 Measured deflection [mm] 0 0246 Beam length [m]

FigureFigure 12. 12.Simplified Simplified shape shape of of the the deflected deflected beam beam between between supports. supports.

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14001400 yy = = 1.1583x 1.1583x - - 17.603 17.603 12001200

10001000

800800

600600 yy = = 0.8704x 0.8704x - - 13.728 13.728 400400

200200

Force from inverse analysis [kN] 0 Force from inverse analysis [kN] 0 00 500 500 1000 1000 1500 1500 AssumedAssumed forceforce [kN][kN]

Figure 13. Comparison of forces: assumed value (orange line) versus the force from inverse analysis FigureFigure 13. 13.Comparison Comparison of of forces: forces: assumedassumed value value (orange (orange line) line) versus versus the the force force from from inverse inverse analysis analysis in a simplified model (blue line) and in numerical studies with appropriate beam stiffness (grey inin a a simplified simplified model model (blue (blue line) line) and and in numerical in numerica studiesl studies with with appropriate appropriate beam beam stiffness stiffness (grey line).(grey line).line).

FigureFigureFigure 14. 14.14. Bending BendingBending moment momentmoment of ofof the thethe main mainmain beam beambeam with withwith an anan increased increasedincreased stiffness stiffnessstiffness in inin the thethe central centralcentral part. part.part.

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FigureFigure 15.15.Shear Shear forcesforces of of the the beam beam with with an an increased increased stiffness stiffness in in the the central central part. part. Point Point load load equal equal to Figureto 100 kN.15. Shear forces of the beam with an increased stiffness in the central part. Point load equal 100to 100 kN. kN.

Figure 16. Shape of the deflected beam with an increased stiffness in the central part. Point load Figure 16. Shape of the deflected beam with an increased stiffness in the central part. Point load Figureequal to 16. 100Shape kN. of the deflected beam with an increased stiffness in the central part. Point load equal to 100 kN. equal to 100 kN. 4. Discussion 4. Discussion The measured deflections of the steel beam in the first and second loading cycles are presentedThe measured in Figure deflections 10. The values of the of steel the forc beame that in the impose first andsuch second a deflection loading shape cycles at arethe presented in Figure 10. The values of the force that impose such a deflection shape at the

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4. Discussion The measured deflections of the steel beam in the first and second loading cycles are presented in Figure 10. The values of the force that impose such a deflection shape at the subsequent steps of the first cycle are given in Figure 13, versus the load derived from the pressure in the hydraulic system. The apparent load seems to be overestimated within the range of 13% (if we apply the simplified model of the beam, neglecting additional steel plates). The results of calculations considering the increased stiffness in the central part of the beam give surprising results, meaning that the force causing the observed range of deflection must have been higher (within a range of 15%) than the value derived from the oil pressure in the hydraulic system (see Figures 14–16 in which bending moments, shear forces, and finally beam deflections are given for a value of loading force equal to F = 100 kN). Due to the linear dependence between force and bend arrow, similar values may be achieved for other load cycles. However, a more sophisticated analysis based on numerical calculations of deflections shows that the increased stiffness in the central part of the beam (due to the additional steel plates welded-in) may practically have a smaller effect because welding at plate edges does not provide full assembling and its effect may be smaller than it was assumed. If we assume that plates are not fully welded, we receive from numerical calculations that f = 0.0044787·F and consequently F = 224.28·f, which seems to underestimate the value of the force applied. Stress control could be an alternative way to validate the applied force. Both methods (based on stress control and geodetic measurements of shape deflection) can be used simultaneously to provide a higher reliability of results. It is important to realize that the accuracy of both methods depends on proper evaluation of beam stiffness. This study shows just one of the possibilities, namely, the one that allows for providing force control from a certain distance. Through this way or another, numerical analysis proceeded with proper instrumentation of the testing appliance and reliable remote geodetic techniques increased the creditability of results. It is evident that the measured values of displacement and deflection of the reaction beam make it possible to “calculate backwards” the force applied at every load step. The equation given in Figure 13 proves that, in field conditions, the observed increase or decrease of the loading force in comparison to the value computed from the hydraulic jack characteristics on the basis of the measured pressure may reach over 15%. The constant value of approximately 14 kN may be also related to the unexpected shear applied to the piston or the pressure loss between the manometer and the piston. The measurement of force during the SLT of a foundation pile can be “to some extent validated” by the inverse analysis of the deformation of a steel structure intended to ensure the transfer of force to the anchor piles. The presented study shows, however, that a simplified model of the beam should not be used as the inverse analysis procedure results are very much dependent on the real beam stiffness. In the final stage of loading of the pile under testing, achieving the planned (in numerical computations) beam deflection guarantees that the test is carried out in the required load range. It must be emphasized that the underestimation or overestimation of loading force even within a range of 15% does not seriously affect the SLT result (as a quality control procedure of a single pile). However, the SLT can also be carried out just in the way of acquiring information about soil parameters for a further pile design. In that case, any overestimation of geotechnical parameters (soil capacity) may be directly transferred to the design project and badly affect the creditability of the static calculation and reliability of the design. Last but not least, apart from controlling the shape of the deflected beam, a contin- uously controlled uplift of the anchoring piles (Figure9) makes it possible to meet the conditions of the Code [50], concerning the allowable values of anchoring piles’ uplift in the course of the test. This underestimated issue has severe consequences if neither addressed nor dealt with, as the Code [50] admits only 80% of derived capacity for the anchoring pile working under the structure. In the conducted SLT, the lifting of the anchor piles was not Remote Sens. 2021, 13, 3082 18 of 20

uniform. The right anchor pile had lifted a maximum of 3.1 mm and therefore had not exceeded the 5 mm limit. The bending of the load beam was analyzed on the basis of point (discontinuous) data, obtained from the tacheometer measurement. If a faster and more accurate laser scanner was used, it would be possible to obtain a point cloud describing the beam shape from individual loading steps. Such studies are planned for the future.

5. Conclusions This study is a pioneering work concerning the application of remote geodetic tech- niques for loading force control in the course of the SLT of foundation piles, as only a few references could be found (mainly by private communication). The present case study just confirms the suitability of robotic tacheometry and TLS as an additional way of providing creditable information on both values controlled during an SLT: pile head displacement (that is evident as remote control provides higher reliability of measurements, despite slightly lower accuracy in comparison with dial gauges) and loading force (in the condition that proper evaluation of beam stiffness is carried out). The control of the anchoring piles’ uplift is just an added value. The authors understand the complexity of the transmission of geodetic monitoring results to the applied force range in the test. It is inevitable, however, that even if the alert (warning) is not provided in the course of field test, creditable data may help to properly compute pile capacity in further post-processing of field test results. The authors’ earlier studies (placed in the reference list) highlighted the importance of a proper evaluation of the SLT conduct at the last stages of loading that are decisive for proper capacity evaluation. It is a negative coincidence that the faults in the evaluation of settlement (due to the unstable reference system for dial gauges) and load (due to the underestimated or even neglected friction between the piston and actuator body) may appear on the most important final steps of the testing procedure. It must be emphasized that the SLT itself is a very costly testing procedure. All phenomena related to loading and unloading within a range far exceeding the working load of the pile (occasionally reaching the pile’s capacity) make it impossible to repeat the procedure in a short time. A potential failure or damage of the manometer or any other problem in the hydraulic system (e.g., in hydraulic jack or connections of pressure hose) results in a long delay or in lower creditability of the repeated test. The application of geodetic remote measurement techniques helps to overcome this issue. In the same way, it helps to provide information about the pile head settlement in the event of failure of the reference system for measuring the displacement of the pile under testing or the uplift of anchor piles. Prospects for new developments and the need for validation for a larger number of SLTs must be emphasized, and numerous further instrumented pile tests are planned in the near future.

Author Contributions: Conceptualization, Z.M. and J.R.; methodology, Z.M. and J.R.; software, Z.M. and J.R.; validation, Z.M. and J.R.; formal analysis, Z.M. and J.R.; investigation, Z.M.; resources, Z.M. and J.R.; data curation, Z.M.; writing—original draft preparation, Z.M. and J.R.; writing—review and editing, Z.M. and J.R.; visualization, Z.M.; supervision, Z.M. and J.R.; project administration, Z.M. and J.R.; funding acquisition, Z.M. and J.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. 27% of article processing charge (APC) was paid from the budget of the Department of Geodesy and Geoinformatics at the Faculty of Geoengineering, Mining and Geology at Wrocław University of Science and Technology, Wrocław, Poland. Acknowledgments: Authors would like to express their gratitude to PPI Chrobok S.A. for their kind assistance in assembling all testing appliances for field studies. Authors would also like to express their gratitude to Hubert Szabowicz and Jakub Rainer for their kind assistance in numerical modelling used for the current study. Conflicts of Interest: The authors declare no conflict of interest. Remote Sens. 2021, 13, 3082 19 of 20

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