Soldier Pile Research Articles

Lateral Earth Pressure on Lagging in Soldier Pile Wall Systems

AbstractSoldier pile and lagging is a conventional means of temporary excavation shoring. Timber lagging design has traditionally been based upon the designer’s experience or empirical rules. One such method is the Goldberg Zoino chart used by the Federal Highway Administration. Most of these methods restrict the designer to a consistent soil profile, certain pile spacing, construction grade timber lagging, and limited depth. The methods do not take into account surcharge loads or a variety of other factors that could arise. A designer working outside of ordinary circumstances with unusual loads, varying soil conditions, or alternate lagging materials has difficulty estimating lateral earth pressures.In rigid earth retention systems, lateral earth pressure is generally assumed to be constant along the length of the wall. In soldier pile and lagging systems, the lagging is often considerably less stiff than the steel soldier piles. As the lagging deflects, the soil tends to bridge between the stiffer eleme...

The Breach Problems in the Tunnels of the Boston Central Artery/Tunnel Project (Big Dig)

. The Boston Central Artery/Tunnel (CAT) project consists of some 200 lane-kilometers of bridges, tunnels, and surface roadways. The central part of the project involves constructing two directions of multi-lane roadway to depths exceeding 30 m beneath the existing city by means of the slurry wall technique. The resulting walls, called Soldier Pile Tremmie Concrete (SPTC) walls, held back the in situ soils, supported an existing elevated roadway during construction, and form the final walls of the new tunnels. In September 2004 a breach occurred in one panel in the deepest section of the tunnel, spewing water onto the roadway and creating a crisis of public confidence in the project. Investigation revealed that the breach resulted from a series of shortcomings in construction and inspection, for which the relevant parties took responsibility. Three alternate repair schemes were proposed. Selecting the best procedure involved not only technical considerations but also impacts on neighboring high-rise buildings. The incident has implications for the construction of deep slurry walls in congested areas.

Vertical Stability of Anchored Concrete Soldier–Pile Walls in Clay

The problem of vertical stability in flexible anchored retaining walls is analyzed and the pattern of the behavior under conditions of poor vertical support is described, on the basis of results from case histories, small-scale tests, and numerical modeling. The possibility of shear stress mobilization in the soil-to-wall interface of anchored concrete soldier–pile retaining walls is discussed. A finite element procedure to model excavations supported by soldier–pile retaining walls is described and applied to a numerical case study. Finite element analyses are performed, emphasizing the consequences of vertical instability due to buckling of the soldier piles and the role of interface resistance in vertical equilibrium. The understanding of some results of the numerical analyses, which are highly influenced by the complexity of the interaction between the different parts of the structure, is obtained by reassessing the vertical equilibrium issue in the light of limit analysis. This approach makes it possible to estimate the pile resistance corresponding to the limit situation of excavation collapse. The finite element model is used to confirm this resistance. Some conclusions are drawn.

Three-dimensional pile-soil interaction in soldier-piled excavations

Abstract Strutted excavations, including soldier-piled excavations, are often analysed using two-dimensional (2D) finite element (FE) analyses with properties which are averaged over a certain span of the wall. In this paper, the effects of “smearing” the stiffness of the soldier piles and timber laggings into an equivalent uniform stiffness are examined, based on comparison between the results of 2D and 3D analyses. The ability of the 3D analyses to model the flexural behaviour of the soldier piles and timber laggings is established by comparing the flexural behaviour of various FE beam representations to the corresponding theoretical solutions, followed by a reality check with an actual case study. Finally, the results of 2D and 3D analyses on an idealized soldier piled excavation are compared. The findings show that modelling errors can arise in several ways. Firstly, a 2D analysis tends to over-represent the coupling to pile to the soil below excavation level. Secondly, the deflection of the timber lagging, which is usually larger than that of the soldier piles, is often underestimated. For this reason, the overall volume of ground loss is, in reality, larger than that given by a 2D analysis. Thirdly, a 2D analysis cannot replicate the swelling, and therefore softening, of the soil face just behind the timber lagging. Increasing the inter-pile spacing will tend to accentuate the effects of these modelling errors.

Arching effects behind a soldier pile wall

A wall consisting of anchored steel piles with horizontal timber laggings was selected to support a 16 m deep excavation with a length of more than three kilometres near Cologne in Germany. Vertical holes were bored on the wall line, at 4 m centres, and steel piles were placed within these holes. Good contact between the piles and the surrounding soil was ensured by concreting the remaining space in the holes. In this way the earth pressure behind the wall was transferred to the piles through horizontal arching, so relieving the timber laggings. As a result, lighter timber laggings could be used and the economy of the wall construction could be increased. In situ tests as well as non-linear 3D finite element (FE) analyses were carried out. Horizontal arching was promoted by means of flexible horizontal lagging timbers and solid contact between the piles and the surrounding soil. Vertical arching was induced by high pre-stress forces in the upper ground anchors. FE analyses were based on both the elasto-plastic Mohr–Coulomb model and a more advanced hardening-soil model.

Monitoring and analysis of results for two strutted deep excavations using vibrating wire strain gauges

As part of the North East Line Mass Rapid Transit Project in Singapore, Contract 704 included the construction of two underground station structures. The excavation for Serangoon Station was in the residual Bukit Timah granite and the excavation for Woodleigh Station was in the residual Old Alluvium material. The temporary retaining structures for both excavations consisted of driven soldier piles and timber lagging supported by steel struts. The excavation for the construction of both stations was monitored in part using vibrating wire strain gauges on the struts. This paper presents the strut monitoring results. A comparison is made with the apparent earth pressure envelopes recommended by Peck. The need to balance continued development with the land constraints in Singapore means that underground structures will continue to be used. The results presented in this paper are relevant in determining the geotechnical parameters for deep excavation retaining walls constructed in these types of materials. It is concluded that the measured earth pressures relate reasonably well to the apparent earth pressure envelopes recommended by Peck.

Braced Excavations: Temperature, Elastic Modulus, and Strut Loads

Relationships between strut loads, earth pressures, temperatures, and the measurements provided by strain gauges are presented in this paper. A braced excavation up to 20-m deep, 9–20 m wide, and > 650-m long constructed in competent glacially derived sand, silt, and clay soils (including glacial till) provided a significant amount of data for analysis. The excavation was supported by soldier piles and lagging with pipe struts and was covered with decking during construction. A direct correlation between incremental changes in strut load and temperature was observed during the course of the project. The few existing relationships between strut load and temperature were reexamined and were found to produce back-calculated elastic modulus values that were either without comparison or inconsistent with data from field tests and published sources. The relationships derived as a result of this work are supported by limited case-history data from other published sources and are consistent with practical application of elastic deformation concepts and published soil modulus values.

Tieback Walls in Sand: Numerical Simulation and Design Implications

A three-dimensional nonlinear finite-element analysis is used to study the influence of various design decisions for tieback walls. The numerical model simulates the soldier piles and the tendon bonded length of the anchors with beam elements, the unbonded tendon with a spring element, the wood lagging with shell elements, and the soil with solid three-dimensional (3D) nonlinear elements. The soil model used is a modified hyperbolic model with unloading hysteresis. The complete sequence of construction is simulated including the excavation and the placement and stressing of the anchors. The numerical model is calibrated against an instrumented case history. Then a parametric study is conducted. The results give information on the influence of the following factors on the wall behavior: Location of the tendon unbonded zone, magnitude of the anchor forces, embedment of the soldier piles, and stiffnesses of the wood lagging and the piles. The implications in design are discussed.

Use of Soldier Pile-Tremie Concrete Slurry Walls as Permanent Tunnel Walls

The analysis and design of soldier pile—tremie concrete (SPTC) slurry walls for the Massachusetts Highway Department's (MHD) Central Artery/Tunnel (CA/T) Project in downtown Boston are discussed. Unlike most slurry walls, which are used as temporary support for excavation during construction, these SPTC walls are designed also as the permanent wall system for the tunnels. The CA/T includes more than 6.1 km (20,000 ft) of slurry walls. Most of the walls are designed and are to be constructed as SPTC walls. Steel wide-flange soldier piles are installed at 1.21- to 1.83-m (4- to 6-ft) spacings in a slurry trench, and the tremie concrete is placed to form the concrete wall. The steel wideflange piles form the primary support system for the wall. In most of the locations, the concrete is designed to act as lagging spanning between the structural steel members. The concrete base slab is rigidly connected to the SPTC walls, and the composite roof is pin-connected to the walls to form the tunnel. The following ...

Use of Soldier Pile–Tremie Concrete Slurry Walls as Permanent Tunnel Walls

The analysis and design of soldier pile–tremie concrete (SPTC) slurry walls for the Massachusetts Highway Department's (MHD) Central Artery/Tunnel (CA/T) Project in downtown Boston are discussed. Unlike most slurry walls, which are used as temporary support for excavation during construction, these SPTC walls are designed also as the permanent wall system for the tunnels. The CA/T includes more than 6.1 km (20,000 ft) of slurry walls. Most of the walls are designed and are to be constructed as SPTC walls. Steel wide-flange soldier piles are installed at 1.21- to 1.83-m (4- to 6-ft) spacings in a slurry trench, and the tremie concrete is placed to form the concrete wall. The steel wide-flange piles form the primary support system for the wall. In most of the locations, the concrete is designed to act as “lagging” spanning between the structural steel members. The concrete base slab is rigidly connected to the SPTC walls, and the composite roof is pin-connected to the walls to form the tunnel. The following topics are discussed after an introduction to the project: geotechnical conditions, design concept of the SPTC slurry walls, design consideration, underpinning of the existing Central Artery, and low head-room construction.

Ground Stabilization for foundation and excavation construction in Florida karst topography

The construction of additional facilities at the Florida Power Corporation's Crystal River Energy Complex required an extensive stabilization program to construct foundations for approximately one half mile of cooling towers. This paper presents an overview of the techniques and methods used in the construction of additional facilities at the Crystal River Generating Station. Excavations for intake and discharge structures were to be more than 20 ft below the water table. Cooling tower foundations were constructed on extensively solutioned Florida limerock located on the central Gulf Coast of Florida. Hybrid ground modification techniques were used to stabilize soft or loose soils and solution features in the underlying limerock, allowing mat and spread footing foundations to be used for most of the facility. The ground modification techniques were used to detect, collapse, densify, and fill solution features below the structure foundations. The bearing capacity after ground modification was verified by field load tests. Subsurface investigations indicated that sheet pile excavations with grouted seals were the optimal technique for cofferdam excavations in the limerock formation. Existing collapsed caverns were encountered during construction of the cofferdam for the intake structure and the adjacent discharge structure, requiring a fast-track design change, considerable field coordination, and daily modifications to accommodate the varied conditions encountered. Tied-back soldier pile cofferdam construction was combined with grouting, underwater excavation, and a massive, unreinforced, concrete tremie slab with grouted anchors, and installed off a barge to provide a dewatered excavation. This project involved the use of hybrid soil-rock anchors to anchor one of the largest continuous placement tremie seals installed in Florida. Other excavations used sheeting and grouting of the underlying limerock formation to provide a dewatered excavation. Ground modification techniques were used to treat a filled cavern discovered in the corner of one discharge structure. The entire construction of this facility was confined to a narrow work area by existing wetlands and the generating station discharge canal. Strict environmental restrictions required by the regulatory agencies to prevent impacts to the surrounding area were imposed on this project.

Use of STABL Program in Tied‐back Wall Design

This review of tied-back wall design examines different factors of safety, and then the effect of free length is demonstrated through wall analyses using the STABL computer program. Many types of searches to determine the most critical failure surface are discussed. It is noted that the FHWA Design Manual Publication DP-68-IR and STABL Program perform compatibly when designing tied-back walls. The number of tiebacks and spacing of soldier piles should be determined from the FHWA Design Manual but the final design load should be obtained using the STABL Program if the slope is unstable. These and other observations are noted.

Soil Arching in Sandy Slopes

Recently soil arching theory has been extended to the study of forces and stresses exerted by a yielding soil mass against discrete piles embedded in a slope and extending into a firm, non‐yielding base. Many uses of discrete pile retaining wall systems exist in current geotechnical practice. In each case, soil arching has been relied upon to some extent as a means of stabilization without concise information about the required spacing of the stabilization piles. This lack of information requires a design engineer to err on the conservative side, and to place the piles closer together than they need be. This study examines the role of soil arching in discrete pile retaining walls and provides an information base for rational design. The present research objective is to experimentally model soil arching action between piles, to determine the influence of and relationships between the essential parameters, and to compare predictions from several new arching theories to experimental model results. The results of the experimental and theoretical comparisons indicate that discrete piles embedded in a slope into a firm, non‐yielding base, can provide significant additional stability to a slope if conditions for soil arching are met. Files can be installed without significantly decreasing slope stability during construction. For retaining walls that rotate vertically about the base between “fixed” side supports, the data obtained by the modified Bransby and Smith numerical method closely approximates experimental results. The stress transfer ability of loose soil is greater than previously thought. A loose deposit does not preclude the possibility of mobilizing soil arching or side friction. In soldier pile and lagging systems, material costs, strengths, and sizes can be optimized to achieve maximum benefit from soil arching.

The Cathedral Square substation anchored excavation, Vancouver, British Columbia

The paper describes the design of and construction experiences at a 20 m deep vertical excavation that was required in the heavily built-up downtown Vancouver area. The subsurface conditions may be generalized as an upper 3 m of sand and gravel followed by a sandstone sequence from fine to coarse with increasing depth. Depending on grain size and cementation, the compressive strength of sandstone varied from 6 to 30 MPa.The excavation support system consisted of soldier piles and posttensioned anchors in the overburden and posttensioned anchors and shotcrete in the rock face. Drainage control was provided by a system of deep, vertical perimeter drains connected to a number of near-horizontal drains in the rock face.The paper describes design considerations for the excavation, including an estimate of seismic-induced deformations. The construction aspects described include the use of a rapid cure method to predict grout strength, the use of couplers in anchor bars, and anchor load tests. During construction, a number of failures, attributed to stress corrosion, occurred in high tensile strength steel anchors. The details of the investigation and testing program used to provide an estimate of remaining anchor life are described. Finally, the paper summarizes results from the field instrumentation. Key words: anchors, construction, corrosion, deformation, drainage, excavation, failure, grout, instrumentation, stress.

Tieback Wall Construction—Results and Controls

The tiebacks were used in conjunction with soldier piles and lagging to support the vertical-walled excavation for the Operations Control Center Building of the Washington Metropolitan Area's rapid transit system (Metro). Utilizing wire-strand tendons, the tiebacks were anchored in terrace deposits of clay and sand. Practical testing procedures which were developed on the job to ensure system's safety, while permitting timely execution of the work, are examined in detail. An evaluation is made of three methods used to install tiebacks during the course of construction. Horizontal and vertical movements of soldier piles in each of the four walls are analyzed and depicted graphically, such that movement, type of material, and construction activity can be correlated. Emphasis is place on the importance of establishing a system to monitor the performance of tied-back walls during and after excavation.

Cofferdam for BARTD Embarcadero Subway Station

Inward deformation of cofferdam walls during deep excavation in soft soils are inevitable but steps can be taken to minimize these movements. An excavation 1,160 ft (354 m) long by 55 ft (17 m) wide by 70 ft (21 m) deep in a busy downtown San Francisco street, flanked by major buildings, was performed for the BARTD Embarcadero station successfully in soil strata that contained varying depths of soft clay locally known as recent Bay mud. To provide an impervious rigid cofferdam wall of adequate strength and reasonable thickness a soldier pile and tremie concrete (SPTC) system of sheeting was selected. Successive excavation cuts below prescribed bracing levels were held to a practical minimum. Struts were preloaded to reduce compression deformations. Inclinometers recorded deflections of the walls while strain gages enabled determination of strut loads. The thicker the layer of soft clay the greater the inward movement of the walls notwithstanding the corresponding increases in wall sizes.

Deep Excavations for the Manufacturers Life Centre, Toronto

Geotechnical considerations associated with deep excavations for the Manufacturers Life Centre in Toronto are discussed. The depth to shale bedrock and the necessity to obtain maximum access to the foundation grade resulted in selecting the use of tie backs into the overburden with or without rakers for supporting the soldier pile lagging system. Full scale load tests were carried out to prove the recommended geotechnical design parameters for the shoring system. Adhesion values obtained from full scale field tests on tie backs agreed closely with the shear strength obtained from laboratory quick triaxial tests. However a full scale loading test on an inclined raker footing gave a higher bearing value compared with conventional design procedures.

Slurry Walls for Bart Civil Center Subway Station

Site conditions, laboratory tests, design, construction techniques, ground-water control, and observation and stress measurements made during construction of a subway station in a 78-ft deep excavation in a heavily developed area for the San Francisco Bay Area Rapid Transit System, are described. The excavation was supported by Soldier Pile Tremie Concrete (SPTC) walls consisting of permanent concrete walls cast-in-situ in slurry stabilized trenches and braced at three permanent and two temporary strut levels. The excavation was made in pervious sands, clayey silts, and layers of compressible peat. To minimize excessive settlements of adjacent buildings, the water table outside the excavation was controlled by recharge wells and checked by observations on piezometers. Measured pressures and inward wall deflections during and immediately after completion of the excavation are compared with predicted lateral earth pressures.

Guages used in testing materials

As part of the North East Line Mass Rapid Transit Project in Singapore, Contract 704 included the construction of two underground station structures. The excavation for Serangoon Station was in the residual Bukit Timah granite and the excavation for Woodleigh Station was in the residual Old Alluvium material. The temporary retaining structures for both excavations consisted of driven soldier piles and timber lagging supported by steel struts. The excavation for the construction of both stations was monitored in part using vibrating wire strain gauges on the struts. This paper presents the strut monitoring results. A comparison is made with the apparent earth pressure envelopes recommended by Peck. The need to balance continued development with the land constraints in Singapore means that underground structures will continue to be used. The results presented in this paper are relevant in determining the geotechnical parameters for deep excavation retaining walls constructed in these types of materials. It is concluded that the measured earth pressures relate reasonably well to the apparent earth pressure envelopes recommended by Peck.