How to avoid a collision?

[Olivier Vogel]

In the following example, I don't understand why there is a collision at the third measure and I don't know how to avoid it.

Many thanks for your help


\version "2.18.2"

\header { }

\paper {
  %system-count = 2% nombre total de systèmes
  indent = 0\cm
}

global = {
  \key c \major
  \time 4/4\tempo "Allegro vivace"
}


\score {
 \new Staff <<
   \global
   \new Voice {
  \relative c' {
    \stemUp
    c4^"Tutti"\f r8 \tuplet 3/2 { g16( a b } c4) r8 \tuplet 3/2 { g16( a b } |
    c4) r r r8 c'^"Strings" |
    c4. b8 d4. c8 |
    \slurUp g'2( f4) r4 |
  }
 }
 \new Voice {
  \relative c' {
    \stemDown\tieDown
    s1 |
    s2 s4 s8 r8\p |
    { <f d>2 <e c> |
    <d b>~ <d b>4 r4 } |
  }
 }
 \new Voice {
  \relative c'' {
    \stemDown\tieUp
    s1 |
    s1 |
    \once \override NoteColumn.force-hshift = #2 g1~ |
    \tieDown g2~ g4 r4 |
  }
 }
 >>
}